These RNA viruses are said to cause about 5% of infant gastroenteritis (range, 2%-9%). Symptoms are various combinations of vomiting and diarrhea, lasting 0.5-4.0 days. They originally were identified by EM of stool specimens. Culture is also possible using special procedures. EIA test methods using homemade reagents have been described.
Month: July 2009
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Norwalk Viruses
These are small round RNA viruses. There are some similarities to calciviruses. Infection predominately involves adults, adolescents, and older children. About 55%-75% of adults have antibodies against this virus. Third-world countries have a higher incidence of antibody. Epidemiologically, disease in the United States usually occurs in clusters (outbreaks); reviews found 34%-47% of such gastroenteritis outbreaks (cruise ships, schools, camps) were due to Norwalk viruses. Clinical disease is usually relatively mild and self-limited. Incubation appears to be about 24 hours (range, 10-50 hours). Nausea and vomiting are usually more prominent than diarrhea. The acute phase usually ends in 24-48 hours and most patients do not require hospitalization. The virus appears to affect the jejunum. Diagnosis has been difficult; until recently, stool EM was required. However, EM apparently has sensitivity of only 34%-48%. Even nucleic acid probes with PCR amplification appear to detect less than 85% of cases (using homemade reagents). Enzyme immunoassay methods using monoclonal antibody against Norwalk antigen in stool have also been reported using homemade reagents; these have generally detected less than 50% of cases. The organism has not been cultured to date.
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Enteric Fastidious Adenoviruses
These DNA viruses (unlike other adenoviruses) could not be cultured using standard virus culture systems. The most frequent are types 40 and 41. These enteric adenovirus species are the second most common cause of severe gastroenteritis in young children (after rotavirus), comprising about 10%-15% of cases (range, 5%-52%). It has also been the second or third most common overall cause of infant gastroenteritis. Diarrhea is the predominant symptom. Vomiting may be present but is less prominent than that seen in rotavirus infection. The gold standard for diagnosis has been EM of stool specimens. However, EM cannot differentiate between fastidious and other adenovirus species. Culture can be done in some cases using nonroutine tissue culture cells. Several EIA tests for virus antigen in stool have been described and one commercial kit for types 40 and 41 is now available. Nucleic acid probes have also been used experimentally.
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Rotavirus
These DNA viruses (unlike other adenoviruses) could not be cultured using standard virus culture systems. The most frequent are types 40 and 41. These enteric adenovirus species are the second most common cause of severe gastroenteritis in young children (after rotavirus), comprising about 10%-15% of cases (range, 5%-52%). It has also been the second or third most common overall cause of infant gastroenteritis. Diarrhea is the predominant symptom. Vomiting may be present but is less prominent than that seen in rotavirus infection. The gold standard for diagnosis has been EM of stool specimens. However, EM cannot differentiate between fastidious and other adenovirus species. Culture can be done in some cases using nonroutine tissue culture cells. Several EIA tests for virus antigen in stool have been described and one commercial kit for types 40 and 41 is now available. Nucleic acid probes have also been used experimentally.
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Viruses Predominately Associated with Gastroenteritis
Rotavirus is an RNA virus in the Reoviridae family. It infects many types of mammals and birds as well as humans. Rotavirus is the most frequent cause of infectious diarrhea of infants and young children. Symptoms include diarrhea (65%-100% of cases), fever, and vomiting (48%-92% of cases). Vomiting may precede diarrhea (usually by less than 24 hours) in 34%-55% of patients. Peak infection rates in the United States are in winter months but are equally distributed in tropical areas. Rotavirus has been reported to cause 36%-50% of gastroenteritis severe enough to need hospitalization in nontropical countries. Rotavirus can be identified in the stool of considerable numbers of hospitalized children without diarrhea and some clinically healthy young children, especially in day-care centers or nurseries (about 10%-20%; literature range, 2%-71%). Adults may also become infected (especially those in contact with infected infants or children, 20%-36% in several studies); the majority are asymptomatic, but some develop diarrhea. Diarrhea tends to be more common and severe in the elderly. Some patients with symptomatic rotavirus infection also have respiratory symptoms, either before or concurrent with diarrhea (although this was not present in some studies).
Rotavirus has not yet been cultured. The gold standard for diagnosis is electron microscopy (EM) of stool specimens. Same-day diagnosis can be obtained from stool specimens using ELISA or LA antigen-detection methods. Sensitivity of the various kits available is about 90% (range, 61%-100%), compared to EM. There is a significant difference in sensitivity between some of the kits.
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Arthropod-Borne Viruses (Arboviruses)
As the name suggests, these are viruses transmitted to humans by arthropods (mostly by mosquitos, but some by ticks). There are three groups of diseases: CNS infections (e.g., encephalitis and aseptic meningitis); hemorrhagic fever (e.g., yellow fever and dengue); and nonspecific fever (e.g., dengue and Colorado tick fever). Of encephalitis cases, the most common agent is St. Louis encephalitis, a flavivirus spread by Culex mosquitos. Only about 1% of humans infected develop clinical symptoms. Of these, about 75% have encephalitis. The next most common viral infection is the California bunyavirus group, most often the LaCrosse virus. This is transmitted by an Aedes mosquito. Less common are Western and Eastern equine encephalitis virus disease. Diagnosis can be made by CSF culture or by acute and convalescent serum antibody titers.
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Creutzfeldt-Jacob (C-J) Disease
This disease is also known as spongiform encephalopathy (describing the typical microscopic changes in affected brain tissue). It is transmitted by a protein agent known as a prion (proteinaceous infectious agent) that resembles one of the genes in structure. About 90% of cases are sporadic and 5%-15% are hereditary with autosomal dominant transmission. In the hereditary form a gene with point mutation appears to be the cause. The sporadic cases do not show a detectable gene mutation and the mechanism of disease is not known. A similar disease in sheep is called scrapie. Another similar disease in New Guinea tribesmen was known as kuru. In C-J disease, most patients develop symptoms at age 40-60 years (so-called presenile dementia). Symptoms resemble those of Alzheimer’s disease to some degree, but the disease progresses much more rapidly and 90%-95% of patients die within one year. Besides mental changes there is ataxia and myoclonal muscle contractions. The major brain area affected is the cerebral cortex frontal lobe; occasionally patients have occipital lobe or cerebellar involvement. There is neuron death with vacuolization of their cytoplasm accompanied by proliferation of astrocytes and fibrosis but no inflammatory cell response. CSF usually does not show any abnormalities. Diagnosis is made by brain biopsy.
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Jc Virus Infection
The JC virus belongs to the polyoma virus group of the papovavirus family, which are double-stranded DNA viruses without an envelope. BK virus is also in the polyoma virus group. It appears that infection by both viruses occurs during childhood or adolescence, with about 50% of the population demonstrating antibody before adulthood, rising later to 80%-90%. The JC virus localizes to and remains latent in the kidney, from whence it occasionally may reactivate. If a patient becomes immunosuppressed, especially during AIDS, reactivated JC virus can infect lymphocytes, be carried to the brain, infect oligodendroglia glial cells, and produce a demyelinating disease called progressive multifocal leukoencephalopathy. This occurs in about 4% of patients with AIDS. Diagnosis is made through biopsy using immunologic stains containing antibody against polyomavirus. One report applied a homemade nucleic acid probe with PCR amplification to urine of JC virus patients and obtained an excellent detection rate.
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Hantavirus Pulmonary Syndrome
This disease first appeared in a small epidemic among New Mexico Navaho Indians in mid1993 as a fatal respiratory infection with resemblance to influenza or mycoplasma disease. It subsequently affected some Europeans and Hispanics. Symptoms began as an upper respiratory infection with fever and myalgias, after which acute respiratory failure developed. Thrombocytopenia was present in about 70% of cases, and leukocytosis with mild neutrophil immaturity was frequent. The reservoir of infection was identified as the deer mouse.
Diagnosis is possible using serologic tests for hantavirus antibody. Nucleic acid probe with PCR amplification can be attempted on peripheral blood leukocytes, although lung biopsy tissue yields twice as many positive results.
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Rabies
Human disease from rabies virus infection is very uncommon in the United States. The number of human cases in the United States is usually less than five per year, and only nine were reported from 1980 to 1987. However, there are always questions about the disease, and several thousand cases of animal rabies are confirmed each year (6975 in 1991). Until 1990, wildlife rabies was most common in skunks; beginning in 1990, raccoon cases have been most frequent, followed by skunks, bats, and foxes in order of decreasing numbers. Actually most animal bites are from caged rodent pets such as rabbits, gerbils, or mice. This is not a problem since rodents (including squirrels) very rarely are infected by the rabies virus even when wild. Interestingly, of the nine human rabies cases mentioned above, six did not give a history of bat or other animal contact when they were hospitalized. Non-bite transmission of rabies from human to human (e.g., contact with saliva or CSF) has not been proven in the United States to date.
The standard procedure for suspected rabies in domestic dogs or cats is to quarantine the animal under observation of a veterinarian for 10 days. The incubation period in humans is 1-3 months (although inoculation through facial bites may have an incubation as short as 10 days), which provides enough time for diagnosis of the animal before beginning treatment in the person who was bitten. Animal rabies will produce symptoms in the animal within 10 days in nearly all cases.
For wild animals, if the animal was defending itself normally, it might be captured and quarantined. If it was thought to be exhibiting abnormal behavior, the animal is usually killed and the head is sent for rabies examination of the brain. The head (or brain) should be kept at refrigerator temperature (not frozen) and sent to a reference laboratory (usually a public health laboratory) as soon as possible. If the specimen will be received within 1 day, it should be sent refrigerated with ordinary ice; it if is to be stored longer than 1 day, dry ice should be used.
Diagnosis. Laboratory diagnosis consists of stained impression smears of the brain, mouse inoculation, and serologic tests. Impression smears from Ammon’s horn of the hippocampal area in the temporal lobe stained with Seller’s stain is the traditional method for diagnosis. The smears are examined microscopically for Negri body inclusions in neurons. Use of Seller’s stain has approximately 65% sensitivity, with sensitivity reported to be somewhat greater than this in dogs (75%-80%) and somewhat less in skunks and bats. Fluorescent antibody stains on the smears have more than 95% sensitivity and currently are the standard method for diagnosis. Mouse inoculation with fresh brain extracts also has more than 95% sensitivity but may eventually be replaced by tissue culture. Saliva from animals or humans can be used for mouse inoculation, but the sensitivity is not as great as brain testing. All specimens should be taken with sterile instruments, which should be immediately decontaminated by autoclaving after use. Serologic tests (ELISA method) for rabies antibody in serum or CSF can be done if the patient has not been immunized against rabies. For serum, this requires two specimens drawn at least 1 week apart. For CSF, a single specimen positive result is diagnostic.