RNA Non-Enveloped Viruses - II Subscribe
Date: 8. February 2018
These viruses are the main cause of the common cold. There are more than 100 serologic types, which explains why the common cold is so common. Replication is similar to that of poliovirus. The cell surface receptor for rhinoviruses is intracellular adhesion molecule 1 (ICAM-1), an adhesion protein located on the surface of many types of cells. There are two modes of transmission for these viruses. In the past, it was accepted that they were transmitted directly from person to person via aerosols of respiratory droplets. The portal of entry is the upper respiratory tract, and the infection is limited to that region. Rhinoviruses rarely cause lower respiratory tract disease, probably because they grow poorly at 37°C. After an incubation period of 2 to 4 days, sneezing, nasal discharge, sore throat, cough, and headache are common. A chilly sensation may occur, but there are few other systemic symptoms. Diagnosis can be made by isolation of the virus from nasal secretions in cell culture, but this is rarely attempted. No specific antiviral therapy is available. Vaccines appear impractical because of the large number of serotypes. Paper tissues impregnated with a combination of citric acid and sodium lauryl sulfate limit transmission when used to remove viruses from fingers contaminated with respiratory secretions. High doses of vitamin C have little ability to prevent rhinovirus-induced colds. Lozenges containing zinc gluconate are available for the treatment of the common cold, but their efficacy remains uncertain.
Caliciviruses are small, nonenveloped viruses with single-stranded RNA of positive polarity. Norovirus is one of the most common causes of viral gastroenteritis in adults both in the United States and worldwide. Norovirus has a non-segmented, single-stranded, positive-polarity RNA genome. It is a nonenveloped virus with an icosahedral nucleocapsid. There is no polymerase within the virion. In the electron microscope, 10 prominent spikes and 32 cup shaped depressions can be seen. There are many serotypes; the exact number is uncertain. Five genogroups have been identified. Most human infections are caused by members of genogroup II. Norovirus is transmitted by the fecal–oral route, often involving the ingestion of contaminated seafood or water. Outbreaks typically occur in group settings such as cruise ships (especially in the Caribbean region), schools, camps, hospitals, and nursing homes. Person-to-person transmission also occurs, especially in group settings. There are many animal caliciviruses, but there is no evidence that they cause human infection. Infection is enhanced by several features of the virus: low infectious dose, excretion of virus in the stool both before the onset of symptoms and for several weeks after recovery, and resistance to inactivation by chlorination and to drying in the environment. It is thought to remain infectious for several days in water, uncooked food, and on environmental surfaces such as door handles.
Norovirus infection is typically limited to the mucosal cells of the intestinal tract. Watery diarrhea without red cells or white cells occurs. Many asymptomatic infections occur, as determined by the detection of antibodies. Immunity following infection appears to be brief, and reinfection can occur. New strains appear every 2 to 4 years and cause widespread infections. Disease is characterized by sudden onset of vomiting and diarrhea accompanied by low-grade fever and abdominal cramping. Neither the emesis nor the stool contains blood. The illness typically lasts 2 to 3 days, and there are no long-term sequelae, except in some immunocompromised patients in whom chronic gastroenteritis can occur. In some outbreaks, certain patient’s manifest signs of central nervous system involvement such as headache, meningismus, photophobia, and obtundation. A PCR-based test on the stool is performed when a specific diagnosis is required. However, the diagnosis is often a clinical one. There is no antiviral therapy or vaccine available. Dehydration and electrolyte imbalance caused by the vomiting and diarrhea may require oral rehydration or intravenous fluids. Personal hygiene, such as handwashing, and public health measures, such as proper sewage disposal and disinfection of contaminated surfaces, are helpful.
REO is an acronym for respiratory enteric orphan; when the virus was discovered, it was isolated from the respiratory and enteric tracts and was not associated with any disease. Rotaviruses are the most important human pathogens in the reovirus family. Rotavirus is a common cause of viral gastroenteritis, especially in young children. Rotavirus has a segmented, double-stranded RNA genome surrounded by a double-layered icosahedral capsid without an envelope. The rotavirus genome has 11 segments. The virion contains an RNA-dependent RNA polymerase. Many domestic animals are infected with their own strains of rotaviruses, but these are not a source of human disease. There are at least six serotypes of human rotavirus. The outer surface protein (also known as the viral hemagglutinin) is the type-specific antigen and elicits protective antibody. Cycle Rotavirus attaches to the cell surface at the site of the β-adrenergic receptor. After entry of the virion into the cell, the RNA-dependent RNA polymerase synthesizes mRNA from each of the 11 segments within the cytoplasm. Capsid proteins form an incomplete capsid around the minus strands, and then the plus strands of the progeny genome segments are synthesized. The virus is released from the cytoplasm by lysis of the cell, not by budding. Rotavirus is transmitted by the fecal–oral route. Infection occurs worldwide, and by age 6 years, most children have antibodies to at least one serotype.
Rotavirus replicates in the mucosal cells of the small intestine, resulting in the excess secretion of fluids and electrolytes into the bowel lumen. The consequent loss of salt, glucose, and water leads to diarrhea. No inflammation occurs, and the diarrhea is non-bloody. It is thought that this watery diarrhea is caused primarily by stimulation of the enteric nervous system. It is likely that intestinal IgA directed against specific serotypes protects against reinfection and that colostrum IgA protects new-borns up to the age of 6 months. Rotavirus infection is characterized by nausea, vomiting, and watery, nonbloody diarrhea. Gastroenteritis is most serious in young children, in whom dehydration and electrolyte imbalance are a major concern. Adults usually have minor symptoms. Although the diagnosis of most cases of viral gastroenteritis does not involve the laboratory, a diagnosis can be made by detection of rotavirus in the stool by using radioimmunoassay or enzyme-linked immunosorbent assay (ELISA). There are two rotavirus vaccines available. Both contain live virus and are given orally. One is a live, attenuated vaccine (Rotarix), which contains the single most common rotavirus serotype (G1) causing disease in the United States. The other is a live reassortant vaccine (Rotateq), which contains five rotavirus strains. Patients with a history of intussusception should not receive either vaccine. The five rotaviruses in the Rotateq vaccine are reassortants in which the gene for the outer surface protein of a human rotavirus is inserted into a bovine strain of rotavirus. A previously approved vaccine (Rotashield) was withdrawn when a high rate of intussusception occurred in vaccine recipients. Hygienic measures such as proper sewage disposal and handwashing are helpful. There is no antiviral therapy.