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These viruses cause croup, laryngitis, bronchiolitis, and pneumonia in children and a disease resembling the common cold in adults. The genome RNA and nucleocapsid are those of a typical paramyxovirus. The surface spikes consist of hemagglutinin (H), neuraminidase (N), and fusion (F) protein. These viruses are transmitted via respiratory droplets. They cause disease worldwide, primarily in the winter months. These viruses cause upper and lower respiratory tract disease without viremia. A large proportion of infections are subclinical. Parainfluenza viruses 1 and 2 are major causes of croup. Parainfluenza virus 3 is the most common parainfluenza virus isolated from children with lower respiratory tract infection in the United States. Parainfluenza virus 4 rarely causes disease, except for the common cold. Parainfluenza viruses are best known as the main cause of croup in children younger than 5 years of age. Croup is characterized by a harsh cough and hoarseness. Most infections are diagnosed clinically. The diagnosis can be made in the laboratory either by isolating the virus in cell culture or by observing a fourfold or greater rise in antibody titer. PCR assay can also be used. There is neither antiviral therapy nor a vaccine available.
Coronaviruses are an important cause of the common cold, probably second only to rhinoviruses in frequency. Coronaviruses have a non-segmented, single-stranded, positive-polarity RNA genome. They are enveloped viruses with a helical nucleocapsid. There is no virion polymerase. The virus adsorbs to cells via its surface spikes (hemagglutinin), after which it enters the cytoplasm, where it is uncoated. Coronaviruses are transmitted by the respiratory aerosol. Infection occurs worldwide and occurs early in life, as evidenced by finding antibody in more than half of children. Outbreaks occur primarily in the winter on a 2- to 3-year cycle. Coronavirus infection is typically limited to the mucosal cells of the respiratory tract. Approximately 50% of infections are asymptomatic, and it is unclear what role they play in the spread of infection. Immunity following infection appears to be brief, and reinfection can occur. Pneumonia caused by SARS coronavirus is characterized by diffuse edema resulting in hypoxia. The common cold caused by coronavirus is characterized by coryza (rhinorrhea, runny nose), scratchy sore throat, and low-grade fever. The diagnosis of the “common cold” is primarily a clinical one. If SARS or MERS is suspected, antibody-based and PCR-based tests can be used. There is no antiviral therapy or vaccine available. A combination of ribavirin and steroids has been tried in the treatment of life-threatening cases of SARS, but their efficacy is uncertain.
This virus causes rubella and congenital rubella syndrome. Congenital rubella syndrome is characterized by congenital malformations. Rubella virus is a member of the togavirus family. It is composed of one piece of single-stranded RNA, an icosahedral nucleocapsid, and a lipoprotein envelope. Because knowledge of rubella virus replication is incomplete, the following cycle is based on the replication of other togaviruses. The virus is transmitted via respiratory droplets and from mother to fetus transplacentally. The disease occurs worldwide. In areas where the vaccine is not used, epidemics occur every 6 to 9 years. Initial replication of the virus occurs in the nasopharynx and local lymph nodes. From there it spreads via the blood to the internal organs and skin. Natural infection leads to lifelong immunity. Rubella is a milder, shorter disease than measles. After an incubation period of 14 to 21 days, a brief prodromal period with fever and malaise. Rubella virus can be grown in cell culture, but it produces little cytopathic effect (CPE). There is no antiviral therapy. Prevention involves immunization with the live, attenuated vaccine. The vaccine is effective and long-lasting (at least 10 years) and causes few side effects, except for transient arthralgias in some women. To protect pregnant women from exposure to rubella virus, many hospitals require their personnel to demonstrate immunity, either by serologic testing or by proof of immunization.
This virus causes rabies, an encephalitis. Rabies virus is the only medically important member of the rhabdovirus family. It has a single-stranded RNA enclosed within a bullet-shaped capsid surrounded by a lipoprotein envelope. The virus is transmitted by the bite of a rabid animal that manifests aggressive, biting behavior induced by the viral encephalitis. The virus is in the saliva of the rabid animal. The virus multiplies locally at the bite site, infects the sensory neurons, and moves by axonal transport to the central nervous system. The incubation period varies, according to the location of the bite, from as short as 2 weeks to 16 weeks or longer. Rapid diagnosis of rabies infection in the animal is usually made by examination of brain tissue by using either PCR assay, fluorescent antibody to rabies virus, or histologic staining of Negri bodies in the cytoplasm of hippocampal neurons. There is no antiviral therapy for a patient with rabies. Only supportive treatment is available. In the United States, the rabies vaccine contains inactivated virus grown in human diploid cells. There are two approaches to prevention of rabies in humans: pre-exposure and post-exposure immunization. In the United States, an alternative vaccine used in dogs and cats contains live canarypox virus genetically engineered to contain the gene for the envelope protein of rabies virus.
A cancer called adult T-cell leukemia/lymphoma and a neurologic disease called HTLV-associated myelopathy (also known as tropical spastic paraparesis or chronic progressive myelopathy). HTLV and HIV are the two medically important members of the retrovirus family. Both are enveloped viruses with reverse transcriptase in the virion and two copies of a single-stranded, positive-polarity RNA genome. The stability of the genes of HTLV is much greater than that of HIV. The replication of HTLV is thought to follow a typical retroviral cycle, but specific information has been difficult to obtain because the virus grows poorly in cell culture. HTLV is transmitted primarily by intravenous drug use, sexual contact, or breast feeding. Transplacental transmission has been rarely documented. Transmission by blood transfusion has greatly decreased in the United States with the advent of screening donated blood for antibodies to HTLV and discarding those that are positive. HTLV causes two distinct diseases, each with a different type of pathogenesis. One disease is adult T-cell leukemia/lymphoma (ATL) in which HTLV infection of CD4positive T lymphocytes induces malignant transformation. Infection with HTLV is determined by detecting antibodies against the virus in the patient’s serum using the ELISA test. There is no specific antiviral treatment for HTLV infection, and no antiviral drug will cure latent infections by HTLV. ATL is treated with anticancer chemotherapy regimens. Antiviral drugs have not been effective in the treatment of HAM. Corticosteroids and danazol have produced improvement in some patients. There is no vaccine against HTLV. Preventive measures include screening donated blood for the presence of antibodies, using condoms to prevent sexual transmission, and encouraging women with HTLV antibodies to refrain from breast feeding.