Jack F. Kerr, AIA

In persons with acute giardiasis muscle relaxant reversal agents purchase nimodipine with american express, symptoms develop after agent eradicates infection muscle relaxer 800 mg purchase nimodipine with a visa. Almost all patients respond to therapy and are cured muscle relaxer kidney pain cheap 30 mg nimodipine amex, although Prominent early symptoms include diarrhea spasms just under rib cage order generic nimodipine line, abdominal pain back spasms 40 weeks pregnant nimodipine 30 mg buy overnight delivery, bloating, some with chronic giardiasis experience delayed resolution of sympbelching, flatus, nausea, and vomiting. For many of the latter patients, upper intestinal manifestations such as nausea, vomiting, bloating, and residual symptoms probably reflect delayed regeneration of intesabdominal pain may predominate. Continued infection should be docuusually >1 week, although diarrhea often subsides. Patients chronic giardiasis may present with or without having experienced who remain infected after repeated treatments should be evaluated an antecedent acute symptomatic episode. Diarrhea is not necessarily for reinfection through family members, close personal contacts, prominent, but increased flatus, loose stools, sulfurous belching, and and environmental sources as well as for hypogammaglobulinemia. Symptoms may be continual or In cases refractory to multiple treatment courses, prolonged therapy episodic and may persist for years. Some persons who have relatively with metronidazole (750 mg thrice daily for 21 days) or therapy with mild symptoms for long periods recognize the extent of their discom- varied combinations of multiple agents has been successful. Cryptosporidiosis is acquired by the consumption of oocysts (50% infectious dose: ~132 C. Since oocysts are immediately infectious when passed in feces, personto-person transmission takes place in day-care centers and among household contacts and medical providers. Cryptosporidia are found in the pharynx, stomach, and large bowel of some patients and at times in the respiratory tract. In immunocompetent persons, symptoms develop after an incubation period of ~1 week and consist principally of watery nonbloody diarrhea, sometimes in conjunction with abdominal pain, nausea, anorexia, fever, and/or weight loss. Biliary tract involvement can manifest as mid-epigastric or right-upper-quadrant pain. Diagnosis (Table 254-1) Evaluation starts with fecal examination for small oocysts, which are smaller (4­5 m in diameter) than the fecal stages of most other parasites. Because conventional stool examination for ova and parasites (O+P) does not detect Cryptosporidium, specific testing must be requested. Detection is enhanced by evaluation of stools (obtained on multiple days) by several techniques, including modified acid-fast and direct immunofluorescent stains and enzyme immunoassays. Cryptosporidia can also be identified by light and electron microscopy at the apical surfaces of intestinal epithelium from biopsy specimens of the small bowel and, less frequently, the large bowel. Use of submicron water filters may minimize acquisition of infection from drinking water. Eosinophilia, which is not found in other enteric protozoan infections, may be detectable. The diagnosis (Table 254-1) is usually made by detection of the large (~25-m) oocysts in stool by modified acid-fast staining. Oocyst excretion may be low-level and intermittent; if repeated stool examinations are unrevealing, sampling of duodenal contents by aspiration or small-bowel biopsy (often with electron microscopic examination) may be necessary. The epidemiology of this parasite has not yet been fully defined, but waterborne transmission and food-borne transmission. Some infected patients may be without symptoms, but many have diarrhea, flulike symptoms, and flatulence and belching. The illness can be self-limited, can wax and wane, or, in many cases, can involve prolonged diarrhea, anorexia, and upper gastrointestinal symptoms, with sustained fatigue and weight loss in some instances. The parasite is detectable in epithelial cells of small-bowel biopsy samples and elicits secretory diarrhea by unknown means. The absence of fecal blood and leukocytes indicates that disease due to Cyclospora is not caused by destruction of the small-bowel mucosa. The diagnosis (Table 254-1) can be made by detection of spherical 8- to 10-m oocysts in the stool, although routine stool O+P examinations are not sufficient. Specific fecal examinations must be requested to detect the oocysts, which are variably acid-fast and are fluorescent when viewed with ultraviolet light microscopy. Protozoal Intestinal Infections and Trichomoniasis cryPtosPoridiosis Nitazoxanide, approved by the U. Currently, eight genera of microsporidia-Encephalitozoon, Pleistophora, Nosema, Vittaforma, Trachipleistophora, Anncalia, Microsporidium, and Enterocytozoon-are recognized as causes of human disease. Although some microsporidia are probably prevalent causes of self-limited or asymptomatic infections in immunocompetent patients, little is known about how microsporidiosis is acquired. Encephalitozoon intestinalis in epithelial cells, endothelial cells, or macrophages Polar tubule pierces host epithelial cell, injects sporoplasm E. In symptomatic individuals, the pathology in the bowel- both gross and microscopic-is similar to that seen in amebiasis, with varying degrees of mucosal invasion, focal necrosis, and ulceration. Balantidiasis, unlike amebiasis, only rarely spreads hematogenously to other organs. Blastocystosis Blastocystis hominis remains an organism of uncertain pathogenicity. Iodoquinol (650 mg three times daily for 20 days) or paromomycin (25­35 mg/kg per day in three doses for 7 days) is appropriate for treatment. Trichomonas vaginalis-one of the most prevalent protozoal parasites 1409 in the United States-is a pathogen of the genitourinary tract and a major cause of symptomatic vaginitis (Chap. While the organism can survive for a few hours in moist environments and could be acquired by direct contact, person-to-person venereal transmission accounts for virtually all cases of trichomoniasis. Its prevalence is greatest among persons with multiple sexual partners and among those with other sexually transmitted diseases (Chap. In contrast, infection in women, which has an incubation period of 5­28 days, is usually symptomatic and manifests with malodorous vaginal discharge (often yellow), vulvar erythema and itching, dysuria or urinary frequency (in 30­50% of patients), and dyspareunia. These manifestations, however, do not clearly distinguish trichomoniasis from other types of infectious vaginitis. Although this approach provides an immediate diagnosis, its sensitivity for the detection of T. Metronidazole (either a single 2-g dose or 500-mg doses twice daily for 7 days) or tinidazole (a single 2-g dose) is effective. Alternatives to metronidazole for treatment during pregnancy are not readily available. Most nematodes are free-living, and these species have variably evolved to survive in diverse ecologic niches, including saltwater, freshwater, or soil. Parasitic nematodes have co-evolved with specific mammalian hosts and have no capacity to live their full life cycles in other hosts. Uncommonly, humans are exposed to infectious stages of nonhuman nematode parasites, and the resultant zoonotic nematode infections can elicit inflammatory and immune responses as larval forms migrate and die in the unsuitable human host. Examples include pulmonary coin lesions due to mosquito-transmitted infections with the dog heartworm Dirofilaria immitis; eosinophilic meningoencephalitis due to ingested eggs of the raccoon ascarid Baylisascaris procyonis; and eosinophilic meningitis due to ingestion of larvae of the rat lungworm Angiostrongylus cantonensis. Nematode parasites of humans include worms that reside in the intestinal tract or localize in extraintestinal vascular or tissue sites. Depending on the species, fertilized females release either larvae or eggs containing larvae. Nematodes have five developmental stages: an adult stage and four sequential larval stages. These parasites characteristically are surrounded by a durable outer cuticular layer. Nematodes have a nervous system; a muscular system, including muscle cells under the cuticle; and a developed intestinal tract, including an oral cavity and an elongated gut that ends in an anal pore. Adults may range in size from minute to >1 meter in length (with Dracunculus medinensis, for example, at the long end of this spectrum). Humans acquire infections with nematode parasites by various routes, depending on the parasitic species. Ingestion of eggs passed in human feces is a major global health problem with many of the intestinal helminths. In other species, infecting larvae penetrate skin exposed to fecally contaminated soil. Some nematode infections are acquired by consumption of specific animal-derived foods. Adult tapeworms are elongated, segmented, hermaphroditic flatworms that reside in the intestinal lumen or, in their larval forms, may live in extraintestinal tissues. Tapeworms include a head (scolex) and a number of attached segments (proglottids). The worms attach to the intestinal tract via their scolices, which may possess suckers, hooks, or grooves. Tapeworms do not have a functional gut tract; rather, each tapeworm segment passively and actively obtains nutrients through its specialized surface tegument. Mature proglottids possess both male and female sex organs, but insemination usually occurs between adjacent proglottids. When ingested by an intermediate host, an egg releases an oncosphere that penetrates the 255e-1 255e Introduction to Helminthic Infections Peter F. Helminthic worms are highly prevalent and, depending on the species, may exist as free-living organisms or as parasites of plant or animal hosts. The parasitic helminths have co-evolved with specific mammalian and other host species. Accordingly, most helminthic infections are restricted to nonhuman hosts, and only rarely do these zoonotic helminths accidentally cause human infections. Helminthic parasites of humans belong to two phyla: Nemathelminthes, which includes nematodes (roundworms), and Platyhelminthes, which includes cestodes (tapeworms) and trematodes (flukes). Helminthic parasites of humans reside within the human body and hence are the cause of true infections. In contrast, parasites of other genera that reside only on mucocutaneous surfaces of humans. Helminthic parasites differ substantially from protozoan parasites in several respects. First, protozoan parasites are unicellular organisms, whereas helminthic parasites are multicellular worms that possess differentiated organ systems. Second, helminthic parasites have complex life cycles that require sequential stages of development outside the human host. Thus, most helminths do not complete their replication within the human host; rather, they develop to a certain stage within the mammalian host and, as part of their obligatory life cycle, must mature further outside that host. During the "extra-human" stages of their life cycle, helminths exist either as free-living organisms or as parasites within another host species and thereafter mature into new developmental stages capable of infecting humans. Thus, with only two exceptions (Strongyloides stercoralis and Capillaria philippinensis, which are capable of internal reinfection), increases in the number of adult helminths. This requirement is germane both to the consideration of helminthic infections in individuals and to ongoing global efforts to interrupt and/or minimize the acquisition of helminthic infections by humans. Third, helminthic infections have a predilection toward stimulation of host immune responses that elicit eosinophilia within human tissues and blood. The many protozoan infections characteristically do not elicit eosinophilia in infected humans, with only three exceptions (two intestinal protozoan parasites, Cystoisospora belli and Dientamoeba fragilis, and tissue-borne Sarcocystis species). The magnitude of helminth-elicited eosinophilia tends to correlate with the extent of tissue invasion by larvae or adult helminths. For example, in several helminthic infections, including acute schistosomiasis (Katayama syndrome), paragonimiasis, and hookworm and Ascaris infections, eosinophilia is most pronounced during the early phases of infection, when migrations of infecting larvae and progression of subsequent developmental stages through the tissues are greatest. In established infections, local eosinophilia is often present around helminths in tissues, but blood eosinophilia may be intermittent, mild, or absent. Humans acquire infection by ingesting animal tissues that contain cysticerci, and the resultant tapeworms develop and reside in the proximal small bowel. Alternatively, if humans ingest eggs of these cestodes that have been passed in human or animal feces, oncospheres develop and can cause space-occupying extraintestinal cystic lesions in tissues; examples include cysticercosis due to T. Nutrients are obtained both through their integument and by ingestion into the blind intestinal tract. Flukes are hermaphroditic except for blood flukes (schistosomes), which are bisexual. Eggs are passed in human feces (Fasciola, Fasciolopsis, Clonorchis, Schistosoma japonicum, S. Expelled eggs release miracidia-usually in water-that infect specific snail species. Depending on the species, cercariae can penetrate the skin (schistosomes) or can develop into metacercariae that can be ingested with plants. The health impacts of many helminthic infections are varied and are based on the frequent need for repeated exposures to increase the worm burdens in infected humans. Ongoing global mass-treatment programs are currently aimed at diminishing the local prevalences of specific helminths and their consequent impacts on the health of local populations. Parasitic nematodes of medical significance may be broadly classified as either predominantly intestinal or tissue nematodes. This chapter covers the tissue nematodes that cause trichinellosis, visceral and ocular larva migrans, cutaneous larva migrans, cerebral angiostrongyliasis, and gnathostomiasis. All of these zoonotic infections result from incidental exposure to infectious nematodes. Although most infections are mild and asymptomatic, heavy infections can cause severe enteritis, periorbital edema, myositis, and (infrequently) death. Life Cycle and Epidemiology Eight species of Trichinella are recognized as causes of infection in humans. After human consumption of trichinous meat, encysted larvae are liberated by digestive acid and proteases. After ~1 week, female worms release newborn larvae that migrate via the circulation to striated muscle. Although host immune responses may help to expel intestinal adult worms, they have few deleterious effects on muscle-dwelling larvae. Human trichinellosis is often caused by the ingestion of infected pork products and thus can occur in almost any location where the meat of domestic or wild swine is eaten. Human trichinellosis may also be acquired from the meat of other animals, including dogs (in parts of Asia and Africa), horses (in Italy and France), and bears and walruses (in northern regions).

These epizootics/epidemics are the result of high-level viremia in horses and mules muscle relaxant xylazine discount 30 mg nimodipine, which transmit the infection to several types of mosquitoes muscle relaxant food nimodipine 30 mg buy cheap. Humans also have high-level viremia muscle relaxant tinidazole purchase nimodipine australia, but their role in virus transmission is unclear muscle relaxant indications order nimodipine 30 mg mastercard. Epizootics of Venezuelan equine fever occurred repeatedly in South America at intervals of 10 years from the 1930s until 1969 muscle relaxant gaba cheap nimodipine 30 mg visa, when a massive epizootic spread throughout Central America and Mexico, reaching southern Texas in 1971. No further epizootic disease was identified until 1995, when additional epizootics took place in Colombia, Venezuela, and Mexico. This finding suggests that active evolution and selection of epizootic viruses are under way in South America. During epizootics, extensive human infection is the rule, with clinical disease in 10­60% of infected individuals. Most infections result in notable acute febrile disease, while relatively few infections (5­15%) result in neurologic disease. The most recent large epizootic of Venezuelan equine fever occurred in Colombia and Venezuela in 1995; of the more than 85,000 clinical cases, 4% (with a higher proportion among children than adults) included neurologic symptoms/signs, and 300 cases ended in death. Humans can be protected by immunization with similar vaccines prepared from Everglades virus, Mucambo virus, and Venezuelan equine encephalitis virus, but the use of the vaccines is restricted to laboratory personnel because of reactogenicity, possible fetal pathogenicity, and limited availability. Equids and humans become infected, and both suffer encephalitis without amplifying the virus in nature. Louis encephalitis virus is transmitted in a similar cycle in the same regions harboring western equine encephalitis virus; disease caused by the former occurs about a month earlier than that caused by the latter (July through October). Large epidemics of western equine encephalitis occurred in the western and central United States and Canada during the 1930s through 1950s, but in recent years the disease has been uncommon. This decline in incidence may reflect in part the integrated approach to mosquito management that has been employed in irrigation projects and the increasing use of 1317 agricultural pesticides. The decreased incidence of western equine encephalitis almost certainly reflects the increased tendency for humans to be indoors behind closed windows at dusk-the peak biting period by the major vector. After an incubation period of ~5­10 days, western equine encephalitis virus causes a typical diffuse viral encephalitis, with an increased attack rate and increased morbidity among the young, particularly children <2 years old. In addition, the lethality rate is high among the young and the very elderly (3­7% overall). Infants <1 year old-particularly those in the first months of life-are at serious risk of motor and intellectual damage. Twice as many males as females develop clinical encephalitis after 5­9 years of age. This difference in incidence may be related to greater outdoor exposure of boys to the vector but may also be due in part to biologic differences. A formalin-inactivated vaccine has been used to protect laboratory workers but is not generally available. Many of the numerous viruses listed in Table 233-1 probably cause at least a few cases of this syndrome, but only some of these viruses have prominent associations with the syndrome and are of biomedical importance. The fever and myalgia syndrome typically begins with the abrupt onset of fever, chills, intense myalgia, and malaise. Physical findings are minimal and are usually confined to conjunctival injection with pain on palpation of muscles or the epigastrium. The duration of symptoms/signs is quite variable (generally 2­5 days), with a biphasic course in some instances. Although pharyngitis or radiographic evidence of pulmonary infiltrates is found in some patients, the agents causing this syndrome are not primary respiratory pathogens. The fever and myalgia syndrome is often described as "influenza-like," but the usual absence of cough and coryza makes influenza an unlikely confounder except at the earliest stages. Complete recovery is the general outcome for people with this syndrome, although prolonged asthenia and nonspecific symptoms have been described in some patients, particularly after infection with lymphocytic choriomeningitis virus or dengue virus types 1­4. Efforts at prevention of viral infection are best based on vector control, which, however, may be expensive or impossible. For mosquito control, destruction of breeding sites is generally the most economically and environmentally sound approach. Emerging containment technologies include the release of genetically modified mosquitoes and the spread of Wolbachia bacteria to limit mosquito multiplication rates. Depending on the vector and its habits, other possible approaches include the use of screens or other barriers. Lymphocytic choriomeningitis virus is transmitted to humans from the common house mouse (Mus musculus) by aerosols of excreta and secreta. The virus is maintained in the mouse mainly by vertical transmission from infected dams. The vertically infected mouse remains viremic and sheds virus for life, with high concentrations of virus in all tissues. Infections among scientists and animal caretakers can occur because the virus is widely used in immunology laboratories as a model of T cell function and can silently infect cell cultures and passaged tumor lines. In addition, patients may have a history of residence in rodent-infested housing or other exposure to rodents. An antibody prevalence of ~5­10% has been reported among adults from Argentina, Germany, and the United States. Lymphocytic choriomeningitis/meningoencephalitis differs from the general syndrome of fever and myalgia in that the onset is gradual. Conditions occasionally associated with the disease are orchitis, transient alopecia, arthritis, pharyngitis, cough, and maculopapular rash. An estimated one-fourth of patients (or fewer) experience a febrile phase of 3­6 days. These patients virtually always recover fully, as do the rare patients with clear-cut signs of encephalitis. During the initial febrile phase, leukopenia and thrombocytopenia are common, and virus can usually be isolated from blood. The pathogenesis of lymphocytic choriomeningitis/meningoencephalitis is thought to resemble that following direct intracranial inoculation of the virus into adult mice. The onset of the immune response leads to T cell­mediated immunopathologic meningitis. Infection should be suspected in acutely ill febrile patients with marked leukopenia and thrombocytopenia. In pregnant women, infection may lead to fetal invasion with consequent congenital hydrocephalus and chorioretinitis. Many of these viruses cause individual infections and usually do not result in epidemics-e. In the relevant orthobunyaviral Bunyamwera serogroup (Bunyamwera, Batai, Cache Valley, Fort Sherman, Germiston, Guaroa, Ilesha, Ngari, Shokwe, and Xingu viruses), Ngari virus recently has been implicated in a large epidemic in Africa. These viruses cause acute febrile disease and are transmitted by mosquitoes in neotropical forests. Explosive epidemics involving thousands of patients have been reported from several towns in Brazil and Peru. Iquitos virus, a recently discovered reassortant and close relative of Oropouche virus, causes disease that is easily mistaken for Oropouche virus disease; its overall epidemiologic significance remains to be determined. Sandfly fever is caused by at least six distinct phleboviruses of the phlebovirus sandfly fever serocomplex (Chagres virus, sandfly fever Cyprus virus, sandfly fever Naples virus, sandfly fever Sicilian virus, sandfly fever Turkey virus, and Toscana virus). Sandfly fever Naples virus, sandfly fever Sicilian virus, and Toscana viruses are the most important human pathogens of this group. Phlebotomus sandflies transmit the viruses, probably among small mammals, and infect humans by bites. Sandfly fever is found in the circum-Mediterranean area, extending to the east through the Balkans into parts of China as well as into western Asia. Sandflies are found in both rural and urban settings and are known for their short flight ranges and their small sizes; the latter enables them to penetrate standard mosquito screens and netting. A common pattern of disease in endemic areas consists of high attack rates among travelers and military personnel and little or no disease in the local population, who are protected after childhood infection. Punta Toro virus is a phlebovirus that is not part of the sandfly fever serocomplex but that, like the members of this complex, is transmitted by sandflies. Punta Toro virus causes a sandfly fever­like disease in the Latin American tropical forest, where the vectors rest on tree buttresses. Epidemics have not been reported, but antibody prevalence among inhabitants of villages in endemic areas indicates a cumulative lifetime exposure rate of >50%. Flaviviruses the most clinically important flaviviruses that cause the fever and myalgia syndrome are dengue viruses 1­4. In fact, dengue is probably the most important arthropod-borne viral disease worldwide, with 50­100 million infections occurring per year. Year-round transmission of dengue viruses 1­4 occurs between latitudes of 25°N and 25°S, but seasonal forays of the viruses into the United States and Europe have been documented. Through increasing spread of mosquitoes throughout the tropics and subtropics and international travel of infected humans, large areas of the world have become vulnerable to the introduction of dengue viruses. Thus, dengue and severe dengue (see "Viral Hemorrhagic Fevers," below) are becoming increasingly common. Bursts of dengue cases are to be expected in the southern United States, particularly along the Mexican border, where containers of water may be infested with A. Closed habitations with air-conditioning may inhibit transmission of many arboviruses, including dengue viruses 1­4. Dengue begins after an incubation period averaging 4­7 days, when the typical patient experiences the sudden onset of fever, frontal headache, retroorbital pain, and back pain along with severe myalgias. These symptoms gave rise to the colloquial designation of dengue as "break-bone fever. The illness may last a week, with additional symptoms and clinical signs usually including anorexia, nausea or vomiting, and marked cutaneous hypersensitivity. Near the time of defervescence on days 3­5, a maculopapular rash begins on the trunk and spreads to the extremities and the face. Epistaxis and scattered petechiae are often noted in uncomplicated dengue, and preexisting gastrointestinal lesions may bleed during the acute illness. Laboratory findings of dengue include leukopenia, thrombocytopenia, and, in many cases, elevations of serum aminotransferase concentrations. Virus is readily isolated from blood in the acute phase if mosquito inoculation or mosquito cell culture is used. Reoviruses Several orbiviruses (Lebombo, Kemerovo, Orungo, and Tribec viruses) and coltiviruses (Colorado tick fever, Eyach, and Salmon River viruses) can cause fever and myalgia in humans. With the exception of Lebombo and Orungo viruses, all of these viruses are transmitted by ticks. Several hundred patients with this disease are reported annually in the United States. The infection is acquired between March and November through the bite of an infected ixodid tick, the Rocky Mountain wood tick (Dermacentor andersoni), in mountainous western regions at altitudes of 1200­3000 m. The most common presentation is fever and myalgia; meningoencephalitis is not uncommon, and hemorrhagic disease, pericarditis, myocarditis, orchitis, and pulmonary presentations have also been reported. The most important differential diagnostic considerations since the beginning of the twentieth century have been Rocky Mountain spotted fever (although Colorado tick fever is much more common in Colorado) and tularemia. Colorado tick fever virus replicates for several weeks in erythropoietic cells and can be found in erythrocytes. This feature, detected in erythroid smears stained by immunofluorescence, can be diagnostically helpful and is important during screening of blood donors. The causative agents are hantaviruses of a distinct phylogenetic lineage that is associated with the cricetid rodent subfamily Sigmodontinae. Several other related viruses (Anajatuba, Andes, Araraquara, Araucária, Bayou, Bermejo, Black Creek Canal, Blue River, Castelo dos Sonhos, El Moro Canyon, Juquitiba, Laguna Negra, Lechiguanas, Maciel, Monongahela, Muleshoe, New York, Orán, Paranoá, Pergamino, Río Mamoré, and Tunari) cause the disease in North and South America, 1319 but Andes virus is unusual in that it has been implicated in human-tohuman transmission. Each type of rodent has its own particular habits; in the case of deer mice, these behaviors include living in and around human habitation. Severe prodromal symptoms/ signs may bring some patients to medical attention, but most cases are recognized as the pulmonary phase begins. Typical signs are slightly lowered blood pressure, tachycardia, tachypnea, mild hypoxemia, thrombocytopenia, and early radiographic signs of pulmonary edema. During the next few hours, decompensation may progress rapidly to severe hypoxemia and respiratory failure. A specific diagnosis is best made by IgM antibody testing of acute-phase serum, which has yielded positive results even in the prodrome. Hemoconcentration, hypoalbuminemia, and proteinuria should also be sought for diagnosis. Patients with severe illness also have acidosis and elevated serum lactate concentrations. During this period, hypotension and shock with increasing hematocrit invite aggressive fluid administration, but this intervention should be undertaken with great caution. Because of low cardiac output with myocardial depression and increased pulmonary vascular permeability, shock should be managed expectantly with pressors and modest infusion of fluid guided by pulmonary capillary wedge pressure. Mild cases can be managed by frequent monitoring and oxygen administration without intubation. Extracorporeal membrane oxygenation is instituted in severe cases, ideally before the onset of shock. Lethality rates remain at ~30­40% even with good management, but most patients surviving the first 48 h of hospitalization are extubated and discharged within a few days with no apparent long-term residua. The antiviral drug ribavirin inhibits hantaviruses in vitro but did not have a marked effect on patients treated in an open-label study. An assault, direct or indirect, on the microvasculature leads to increased permeability and (particularly when platelet function is decreased) to actual disruption and local hemorrhage (a positive tourniquet sign). Cutaneous flushing and conjunctival suffusion are examples of common, observable abnormalities in the control of local circulation. In most patients, hemorrhage is an indication of widespread vascular damage rather than a life-threatening loss of blood volume. However, in all of these diseases, generalized circulatory disturbance is critically important. In some viral infections, direct damage to the vascular system or even to parenchymal cells of target organs is an important factor; in other viral infections, soluble mediators are thought to play a major role in the development of hemorrhage or fluid redistribution.

These infections most often follow a breach in the mucosal barrier resulting from appendicitis spasms near heart generic nimodipine 30 mg buy, diverticulitis spasms side of head purchase genuine nimodipine, neoplasm muscle relaxant herbs order nimodipine from india, inflammatory bowel disease muscle spasms xanax withdrawal generic nimodipine 30 mg online, surgery spasms 1983 imdb buy nimodipine with visa, or trauma. On average, four to six bacterial species are isolated per specimen submitted to the microbiology laboratory, with a predominance of enteric aerobic/facultative gram-negative bacilli, anaerobes, and streptococci/enterococci. Other anaerobes commonly isolated from this type of infection include Peptostreptococcus, Prevotella, and Fusobacterium species. The dominance of four to six bacterial species out of the more than 500 colonic mucosal species is related both to the virulence factors of these species and to the inability of clinical laboratories to culture many other species residing in the colonic mucosa. Disease originating from proximal-bowel perforation reflects the microbiota of this site, with a predominance of aerobic and anaerobic gram-positive bacteria and Candida. Patients usually present with fever; abdominal pain, tenderness, and distention; and watery diarrhea. The primary pathogen is thought by some authorities to be Clostridium septicum, but other clostridia and mixed anaerobes have also been implicated. More than 50% of patients developing early clinical signs can benefit from antibiotic therapy and bowel rest. In case­control studies of children with undiagnosed diarrheal disease, enterotoxigenic B. Pelvic Infections the vagina of a healthy woman is a major reservoir of anaerobic and aerobic bacteria. In the normal microbiota of the female genital tract, anaerobes outnumber aerobes by a ratio of ~10:1 and include anaerobic gram-positive cocci and Bacteroides species (Table 201-1). Anaerobes are frequently encountered in pelvic inflammatory disease, pelvic abscess, endometritis, tubo-ovarian abscess, septic abortion, and postoperative or postpartum infections. These infections are often of mixed etiology, involving both anaerobes and coliforms; pure anaerobic infections without coliform or other facultative bacterial species occur more often in pelvic than in intraabdominal sites. Septic pelvic thrombophlebitis may complicate the infections and lead to repeated episodes of septic pulmonary emboli. Anaerobic bacteria have been thought to be contributing factors in the etiology of bacterial vaginosis. This syndrome of unknown etiology is characterized by a profuse malodorous discharge and a change in the bacterial ecology that results in replacement of the Lactobacillus- 1099 dominated normal microbiota with an overgrowth of bacterial species including Gardnerella vaginalis, Prevotella species, Mobiluncus species, peptostreptococci, and genital mycoplasmas. Pelvic infections due to Actinomyces species have been associated with the use of intrauterine devices (Chap. Skin and Soft Tissue Infections Injury to skin, bone, or soft tissue by trauma, ischemia, or surgery creates a suitable environment for anaerobic infections. These infections are most frequently found in sites prone to contamination with feces or with upper airway secretions-. Moreover, anaerobes have been isolated from cutaneous abscesses, rectal abscesses, and axillary sweat gland infections (hidradenitis suppurativa). The deep soft-tissue infections associated with anaerobic bacteria are crepitant cellulitis, synergistic cellulitis, gangrene, and necrotizing fasciitis (Chaps. The most frequently isolated organisms include Bacteroides, Peptostreptococcus, Clostridium, Enterococcus, and Proteus species. These infections usually involve a combination of Peptostreptococcus species and S. Necrotizing fasciitis, a rapidly spreading destructive disease of the fascia, is usually attributed to group A streptococci (Chap. The most frequently isolated anaerobes in these infections are Peptostreptococcus and Bacteroides species. Many patients with osteomyelitis due to anaerobic bacteria have evidence of an anaerobic infection elsewhere in the body; most commonly, infected adjacent soft-tissue sites are the source of the organisms involved. Examples are diabetic foot ulcers and decubitus ulcers that may be complicated by mixed aerobic-anaerobic osteomyelitis. Prevotella and Porphyromonas species are detected in infections involving the maxilla and mandible, whereas Clostridium species have been reported as anaerobic pathogens in cases of osteomyelitis of the long bones following fracture or trauma. In patients with osteomyelitis, the most reliable culture specimen is a bone biopsy sample free of normal uninfected skin and subcutaneous tissue. Unlike anaerobic osteomyelitis, anaerobic pyoarthritis in most cases is not polymicrobial and may be acquired hematogenously. Anaerobes are important pathogens in infections involving prosthetic joints; in these infections, the causative organisms (such as Peptostreptococcus species and P. Bacteremia Transient bacteremia is a well-known event in healthy individuals whose anatomic mucosal barriers have been injured. These bacteremic episodes, which are often due to anaerobes, have no pathologic consequences. The incidence of anaerobic bacteremia decreased from the 1970s through the early 1990s. This change may have been related to the administration of antibiotic prophylaxis before intestinal surgery, the earlier recognition of localized infections, and the empirical use of broad-spectrum antibiotics for presumed infection. A study from the Mayo Clinic compared three periods (1993­1996, 1997­2000, and 2001­2004) and found a 74% increase in the mean incidence of anaerobic bacteremia; this finding contrasts with a 45% decrease in incidence from 1977 to 1988 at the same institution. In contrast, a report from Switzerland compared two periods (1997­2001 and 2002­2006) and found decreases in both the number of anaerobe-positive blood cultures and the proportion of all blood culture isolates that were anaerobes. The majority of anaerobic bacteremias are due to gram-negative bacilli-mainly the B. Other organisms causing bacteremia include Clostridium species (10%), Peptostreptococcus species (10%), and Fusobacterium species (5%). Debilitating diseases such as malignancies, diabetes, organ transplantation, and abdominal and pelvic surgeries are among the predisposing factors for anaerobic bacteremia. In a retrospective nested case­control study, diabetes was identified as a risk factor for anaerobic bacteremia when the source of bacteremia was unknown. When bloodstream invasion occurs, patients can become extremely ill, with rigors and hectic fevers. The clinical picture may be quite similar to that seen in sepsis involving aerobic gram-negative bacilli. Reported case­fatality rates are high, ranging from 25% to 44%, and appear to increase with the age of the patient (with reported rates of >66% among patients >60 years old), with the isolation of multiple species from the bloodstream, and with the failure to surgically remove a focus of infection. However, anaerobic streptococci, which are often classified incorrectly, are responsible for this disease more frequently than is generally appreciated. Anaerobes can reach the pericardial space by hematogenous spread, by spread from a contiguous site of infection. Specimens must be collected by meticulous sampling of infected sites, with avoidance of contamination by the normal microbiota. Specimens appropriate for anaerobic culture include sterile body fluids such as blood, pleural fluid, peritoneal fluid, cerebrospinal fluid, and aspirates or biopsy samples from normally sterile sites. As a general rule, liquid or tissue specimens are preferred; swab specimens should be avoided. Because even brief exposure to oxygen may kill some anaerobic organisms and result in failure to isolate them in the laboratory, air must be expelled from the syringe used to aspirate the abscess cavity, and the needle must be capped with a sterile rubber stopper. It is also important to remember that prior antibiotic therapy reduces the cultivability of these bacteria. When infections occur in proximity to mucosal surfaces normally harboring an anaerobic microbiota, such as the gastrointestinal tract, female genital tract, or oropharynx, anaerobes should be considered as potential etiologic agents. Although these odors are nearly pathognomonic for anaerobic infection, the absence of odor does not exclude an anaerobic etiology. The presence of gas in tissues is highly suggestive, but not diagnostic, of anaerobic infection. Sometimes these organisms have morphologic characteristics associated with specific species. When cultures of obviously infected sites or purulent material yield no growth, streptococci only, or a single aerobic species (such as E. Any anatomic breach must be closed promptly, closed spaces drained, tissue compartments decompressed, and an adequate blood supply established. Other factors influencing the selection of antibiotics include need for bactericidal activity and for penetration into certain organs (such as the brain), toxicity, and impact on the normal microbiota. Antibiotics active against clinically relevant anaerobes can be grouped into four categories based on their predicted activity (Table 201-2). Nearly all the drugs listed have toxic side effects, which are described in detail in Chap. Because of the slow growth rate of many anaerobes, the lack of standardized testing methods and of clinically relevant standards for resistance, and the generally good results obtained with empirical therapy, there has been limited interest in testing these organisms for antibiotic susceptibility. However, one study of antibiotic-treated patients with Bacteroides isolates from blood found mortality rates of 45% among those whose isolates were deemed resistant to the agent used and 16% among those whose isolates were deemed sensitive. It is accepted that testing is important for patients with serious or prolonged infections or in cases in which antibiotics have not had an impact. Testing is also helpful in monitoring the activity of new drugs and recording current resistance patterns among anaerobic pathogens. The antibiotics with the greatest activity against nearly all anaerobic bacteria include carbapenems, -lactam/-lactamase inhibitor combinations, metronidazole, and chloramphenicol. In recent years, the activity of clindamycin, cefoxitin, cefotetan, and moxifloxacin has decreased against B. The cephamycins cefoxitin and cefotetan display greater activity against this group, but rates of resistance have increased, with current figures at ~10% in the United States and higher in Argentina (28%) and Europe (17%). Rates of resistance to -lactam agents among anaerobes other than Bacteroides are lower but are highly variable. Although resistance rates reported from most countries are still low, several studies have documented nonsusceptibility to ampicillin/sulbactam in 0. Carbapenems (ertapenem, doripenem, meropenem, and imipenem) are equally active against anaerobes, with <1% of B. Higher rates of carbapenem nonsusceptibility are being reported from some countries (5% in Germany, 8% [to doripenem] in Canada, and 7­12% in Taiwan). Resistance to metronidazole is more common among gram-positive anaerobes, including P. Resistance to clindamycin among non-Bacteroides anaerobes is much less common (<10%). Tigecycline is active against some anaerobic bacteria, including Peptostreptococcus, Propionibacterium, Prevotella, Fusobacterium, and most Bacteroides species. Its efficacy for treatment of intraabdominal infections was comparable to that of imipenem in two phase 3 double-blind clinical trials. This drug is therefore recommended as single-agent treatment for complicated intraabdominal infections, but resistance (~6%) among Bacteroides and non-Bacteroides species has been reported. Fluoroquinolones such as moxifloxacin have shown potential in the treatment of mixed aerobic-anaerobic infections. A survey in the United States found a 38% rate of resistance to moxifloxacin among the B. Despite excellent in vitro activity against all clinically important anaerobes, chloramphenicol is less desirable than other active drugs for the treatment of anaerobic infection because of documented clinical failures. For infections originating below the diaphragm, aerobic gram-negative coverage is essential. Metronidazole also is not active against Actinomyces, Propionibacterium, or other gram-positive non-spore-forming bacilli. Category 1 (Nearly Always Active) Carbapenems (imipenem, meropenem, doripenem) Metronidazolea -Lactam/-lactamase inhibitor combination (ampicillin/sulbactam, ticarcillin/clavulanic acid, piperacillin/tazobactam) Chloramphenicolb Category 2 (Usually Active) Tigecycline High-dose antipseudomonal penicillins 1102 If a patient fails to respond to one of the category 1 or category 2 drugs (Table 201-2), consideration should be given to alternative therapy and to determination of the resistance patterns among Bacteroides isolates. The duration of therapy also depends on the infection site; the reader is referred to specific chapters on sites of infection for recommendations. Infections above the diaphragm usually reflect the orodental microbiota, which does not include the B. Up to 60% of clinical isolates classified as Prevotella or Porphyromonas species, non­B. Because most of these infections have a mixed etiology that includes microaerophilic and aerobic streptococci, antibiotics that cover both aerobic and anaerobic bacteria are recommended. The recommended regimens include clindamycin, a -lactam/-lactamase inhibitor combination, or metronidazole in combination with a drug active against microaerophilic and aerobic streptococci. Bronchoscopy in lung abscess is indicated only to rule out airway obstruction and does not enhance drainage; in any event, it should be delayed until the antimicrobial regimen has begun to affect the disease process so that the procedure does not spread the infection. Surgery is almost never indicated because of the danger of spilling the abscess contents into the lungs. However, penicillin G and metronidazole also cross the blood­brain barrier and are bactericidal for many anaerobic organisms. Specifically, a drug from category 1 (Table 201-2) must be included for broad-spectrum coverage. Single agents suitable for this purpose include the carbapenems, cefoxitin, and -lactam/-lactamase inhibitor combinations. A two-drug regimen is an alternative, with one drug active against coliforms and the other against anaerobes. Although clindamycin and cefotetan were previously considered acceptable options for intraabdominal infections involving anaerobes, these drugs are no longer recommended because of escalating rates of resistance in the B. Ampicillin/sulbactam is not recommended because of high rates of resistance among community-acquired strains of E. Cases of anaerobic osteomyelitis in which a mixed flora is isolated from a bone biopsy specimen should be treated with a regimen that covers all isolates. When an anaerobic organism is recognized as a major or sole pathogen infecting a joint, the duration of treatment should be similar to that used for arthritis caused by aerobic bacteria (Chap. Therapy includes the management of underlying disease states, the administration of appropriate antimicrobial agents, temporary joint immobilization, percutaneous drainage of effusions, and (usually) the removal of infected prostheses or internal fixation devices. Surgical drainage and debridement procedures such as sequestrectomy are essential for the removal of necrotic tissue that can sustain anaerobic infections. The outcome of anaerobic bacteremia is significantly better in patients either initially given or switched to appropriate therapy on the basis of known antibiotic susceptibilities. The possibility of drug resistance must be entertained; if resistance is involved, repeat cultures may yield the pathogen.

Clinical Manifestations Both endemic trachoma and adult inclusion conjunctivitis present initially as conjunctivitis characterized by small lymphoid follicles in the conjunctiva spasms with stretching order nimodipine canada. In regions with hyperendemic classic blinding trachoma muscle relaxant drugs medication nimodipine 30 mg low price, the disease usually starts insidiously before the age of 2 years muscle relaxant indications nimodipine 30 mg fast delivery. The cornea becomes involved muscle relaxant dogs cheap nimodipine online mastercard, with inflammatory leukocytic infiltrations and superficial vascularization (pannus formation) muscle relaxant depression cheap nimodipine online. As the inflammation continues, conjunctival scarring eventually distorts the eyelids, causing them to turn inward so that the lashes constantly abrade the eyeball (trichiasis and entropion); eventually the corneal epithelium is abraded and may ulcerate, with subsequent corneal scarring and blindness. Destruction of the conjunctival goblet cells, lacrimal ducts, and lacrimal gland may produce a "dry-eye" syndrome, with resultant corneal opacity due to drying (xerosis) or secondary bacterial corneal ulcers. In such areas, the active infectious process usually resolves spontaneously in affected persons at 10­15 years of age, but conjunctival scars continue to shrink, producing trichiasis and entropion with subsequent corneal scarring in adults. Recurrent eye infections develop most often in patients whose sexual partners are not treated with antimicrobial agents. The clinical diagnosis of endemic trachoma should be confirmed by laboratory tests in children with relatively marked degrees of inflammation. Follicular conjunctivitis in European or American adults living in trachomatous regions is rarely due to trachoma. Simultaneous treatment of all sexual partners is necessary to prevent ocular reinfection and chlamydial genital disease. Topical antibiotic treatment is not required for patients who receive systemic antibiotics. Infectious forms of the organisms are shed from both symptomatic and apparently healthy birds and may remain viable for several months. Psittacosis in humans may present as acute primary atypical pneumonia (which can be fatal in up to 10% of untreated cases); as severe chronic pneumonia; or as a mild illness or asymptomatic infection in persons exposed to infected birds. A pandemic of psittacosis was once stopped by banning shipment or importation of psittacine birds. As a result of quarantine of imported birds and improved veterinary-hygienic measures, outbreaks and sporadic cases of psittacosis are now rare. Endocarditis, hepatitis, and neurologic complications may occur, and fatal cases have been reported. Any antibody titer above 1:16 is considered significant evidence of exposure to chlamydiae, and a fourfold titer rise in paired sera in combination with a clinically compatible syndrome can be used to diagnose psittacosis. Seropositivity usually is first detected at school age, and rates generally increase by ~10% per decade. About 50% of individuals have detectable antibody at 30 years of age, and most have detectable antibody by the eighth decade of life. It begins in the upper respiratory tract and, in many persons, persists as a prolonged asymptomatic condition of the upper respiratory mucosal surfaces. As mentioned above, epidemiologic studies have demonstrated an association between serologic evidence of C. The organism has been recovered in culture from atheromatous plaque-a result indicating the presence of viable replicating bacteria in vessels. Antimicrobial treatment of the infected animals reverses the increased risk of atherosclerosis. In humans, two small trials in patients with unstable angina or recent myocardial infarction suggested that antibiotics reduce the likelihood of subsequent untoward cardiac events. However, larger-scale trials have not documented an effect of various antichlamydial regimens on the risk of these events. This organism has been reported to account for as many as 10% of cases of community-acquired pneumonia, most of which are diagnosed by serology. Any antibody titer above 1:16 is considered significant evidence of exposure to chlamydiae. There are no official recommendations for diagnosis of chronic infections, although many research studies have used high titers of IgA as an indicator. Recommended therapy consists of 2 g/d of either tetracycline or erythromycin for 10­14 days. Viruses can replicate only within cells because their nucleic acid does not encode many enzymes necessary for the metabolism of proteins, carbohydrates, or lipids or for the generation of high-energy phosphates. Virusoids are nucleic acids that depend on cells and helper viruses for packaging their nucleic acids into virus-like particles. Prions cause neurodegenerative diseases such as Creutzfeldt-Jakob disease, Gerstmann-Straüssler disease, kuru, and human or bovine spongiform encephalopathy ("mad cow disease"). The capsid is composed of 162 capsomeres: 150 with sixfold and 12 with fivefold axes of symmetry. Viruses of a single family have similar structures and may be morphologically indistinguishable in electron micrographs. Subclassification into genera depends on similarity in epidemiology, biologic effects, and nucleic acid sequence. Most viruses that infect humans have a common name related to their pathologic effects or the circumstances of their discovery. In addition, formal species names-consisting of the name of the host followed by the family or genus of the virus and a number-have been assigned by the International Committee on Taxonomy of Viruses. This dual terminology can cause confusion when viruses are referred to by either name-e. Viral nucleic acid is usually associated with virus-encoded nucleoprotein(s) in the virus core. Viral nucleic acids and nucleoproteins are almost always enclosed in a protein capsid. Because of the limited genetic complexity of viruses, their capsids are usually composed of multimers of identical capsomeres made up of one or a few proteins. Icosahedral capsid structures approximate spheres and have two-, three-, or fivefold axes of symmetry, whereas helical capsid structures have only a twofold axis of symmetry. The nucleic acid, nucleoprotein(s), and protein capsid together are called a nucleocapsid. Other viruses are more complex and have an outer phospholipid, cholesterol, glycoprotein, and glycolipid envelope that is derived from virus-modified infected cell membranes. Cell nuclear, endoplasmic reticulum, Golgi, or plasma membranes that become parts of the viral envelope have usually been modified during infection by the insertion of virus-encoded glycoproteins, which mediate contact of enveloped virus with uninfected cell surfaces. Matrix or tegument proteins may fill the space between the nucleocapsid and the outer envelope of the virus. Enveloped viruses are usually sensitive to lipid solvents or detergents that can dissolve the envelope, whereas viruses with protein nucleocapsid exteriors may be somewhat detergent resistant. Structures of prototypical pathogenic human viruses are described in Table 214e-1. Infection is frequently initiated by weak electrostatic or hydrophobic interactions with the cell surface. Subsequent stronger, more specific attachment to a cell plasma membrane protein, carbohydrate, glycolipid, heparan sulfate proteoglycan, or sialic acid enables stable binding to a specific cell surface "receptor" that mediates fusion with the cell plasma membrane (see Table 145e-1). Receptor binding is often augmented by a viral surface protein interaction with more than one cell surface protein or co-receptor. Receptors and co-receptors are important determinants of the species and cell type that a virus can infect. Prototype viruses of each family that cause human disease are listed in Table 214e-1. Influenza virus has an outer-membrane hemagglutinin glycoprotein that binds to sialic acid on respiratory tract cell plasma membranes. The hemagglutinin mediates adsorption to cell membranes, receptor aggregation, and endocytosis. As the endosome pH decreases in the cell cytoplasm, the influenza hemagglutinin conformation changes, enabling hydrophobic helices, which are initially at the base of the hemagglutinin, to extend, interacting and fusing with the endosome membrane and thereby releasing the viral genome into the cell cytoplasm. The influenza virus M2 membrane channel protein has a key role in lowering endosome pH and permitting virus and cell membrane fusion. Hydrophobic interactions required for fusion can be susceptible to chemical inhibition or blockade. Enfuvirtide binds to gp41 and prevents conformational changes required for fusion. To produce infectious progeny, viruses must produce proteins necessary for replicating their nucleic acids as well as structural proteins necessary for coating their nucleic acids and for assembling nucleic acids and proteins into progeny virus. The medically important viruses in this group are rotaviruses and Colorado tick fever virus. The herpesviruses include eight viruses that cause a wide range of inflammatory and malignant diseases in humans. After receptor binding and fusion with plasma membranes or endocytic vesicle membranes, herpesvirus nucleocapsids are released into the cytoplasm with tegument proteins and are transported along microtubules to a nuclear pore. Transcription of immediate-early genes requires the viral tegument protein and cell transcription factors. Heat shock or other cell stresses can cause these cell factors to enter the nucleus, activate viral gene expression, and initiate replication. Instead, their early genes have upstream enhancing elements that bind cell transcription factors. Herpesviruses also encode enzymes that increase the deoxynucleotide triphosphate pools. However, endogenous human retrovirus replication has not been documented or associated with any disease. These porcine retroviruses are a potential cause for concern in xenotransplantation because retrovirus replication could cause disease in humans. The assembly and egress of mature infectious virus mark the end of the eclipse phase of infection, during which infectious virus cannot be recovered from the infected cell. In general, the capsid proteins of viruses with icosahedral nucleocapsids can self-assemble into densely packed and highly ordered capsid structures. Excess viral membrane glycoproteins are synthesized to saturate cell receptors and facilitate separation of the virus from the infected cell. Some viruses encode membrane proteins with enzymatic activity for receptor destruction. Oseltamivir and zanamivir are neuraminidase inhibitors that are used to treat or provide prophylaxis for influenza virus infection. Herpesvirus nucleocapsids acquire an initial envelope by assembling in the nucleus and then budding through the nuclear membrane into the endoplasmic reticular space. The initially enveloped herpesvirus is then de-enveloped and released from the cell either by exocytosis or by re-envelopment at the plasma membrane. Nonenveloped viruses depend on the death and dissolution of the infected cell for their release. Many mature-appearing virions are imperfect and have only incomplete or nonfunctional genomes. Despite the inefficiency of assembly, a typical virus-infected cell releases 10­1000 infectious progeny. Adenovirus packaging is notoriously inefficient, and a high ratio of particle to infectious virus may limit the amount of recombinant adenovirus that can be administered for gene therapy since the immunogenicity of defective particles may contribute to adverse effects. Mutations resulting in less efficient viral growth, or fitness, may be detrimental to the virus. Nevertheless, mutations resulting in evasion of the host immune response or resistance to antiviral drugs are preferentially selected in patients, with the consequent perpetuation of infection. Viral genomes can also be altered by recombination or reassortment between two related viruses in a single infected cell. Although this occurrence is unusual under most circumstances of natural infection, the genome changes can be substantial and can significantly alter virulence or epidemiology. Reassortment of the avian or mammalian influenza A hemagglutinin gene into a human influenza background can result in the emergence of new epidemic or pandemic influenza A strains. More complex viruses of the poxvirus or herpesvirus family encode many proteins that serve these functions. Some of these viral proteins have motifs similar to those of cellular proteins, while others are quite novel. These strategies often provide unique insights into the control of cell growth, cell survival, macromolecular synthesis, proteolytic processing, immune or inflammatory suppression, immune resistance, cytokine mimicry, or cytokine blockade. The importance of host range factors is illustrated by the effects of specific host determinants that limit the replication of influenza virus with avian or porcine hemagglutinins in humans. These viral proteins have adapted to bind avian or porcine sialic acids, and spread of avian or porcine influenza viruses in human populations is limited by their ability to infect human cells. Apoptosis is the expected consequence of virus-induced inhibition of cellular macromolecular synthesis and viral nucleic acid replication. Although the induction of apoptosis may be important for the release of some viruses (particularly nonenveloped viruses), many viruses have acquired genes or parts of genes that enable them to forestall infected-cell death. Adenoviruses and herpesviruses encode analogues of the cellular Bc12 protein, which block mitochondrial enhancement of proapoptotic stimuli. In all these situations, infection begins on an epithelial or mucosal surface and spreads along the mucosa and into deeper tissues. Infection may spread to cells that can enter blood vessels, lymphatics, or neural circuits. The dependence of smallpox virus and poliovirus infections on interhuman transmission makes it feasible to eliminate these viruses from human circulation by mass vaccination. Herpesviruses also survive by interhuman transmission but may be more difficult to eliminate because they establish persistent latent infection in humans and continuously reactivate to infect new and naïve generations. Animals are also important reservoirs and vectors for transmission of viruses causing human disease. Insect vectors can mediate parenteral transfer of viruses that reach high titers in animal or human hosts.

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