The togaviruses are enveloped, icosahedral viruses that contain a positive-sense, single-stranded RNA genome and generally three structural proteins. The capsid (C) protein encloses the viral RNA, forming the nucleocapsid, and the two other proteins (E1 and E2) are glycoproteins that form the hemagglutinin-containing viral spikes that project from the lipid bilayer. The family Togaviridae is divided into two genera: Alphavirus and Rubivirus.

A. Alphavirus

 The alphaviruses, of which there are approximately 26, are arthropod-borne viruses (arboviruses), which are transmitted to humans and domestic animals by mosquitoes. All alphaviruses share a common group antigen. Some arboviruses were initially isolated from horses, hence, the word “equine” in their names.

1. Epidemiology and pathogenesis: Alphaviruses have a broad host range, being able to replicate in organisms that are widely separated phylogenetically, such as mosquitoes and humans. Following inoculation of an Alphavirus by a mosquito, the patient is observed to have a viremia, following which the virus may be seeded in various target organs (for example, the CNS in the encephalitis viruses).

 2. Viral replication: Following attachment to the cell surface, the virus is internalized by receptor-mediated endocytosis. Like the picornaviruses, genome replication is as described for Type I RNA viruses .

3. Clinical significance: Several different clinical syndromes are associated with Alphavirus infections of humans. These include: 1) acute encephalitis (Eastern and Western equine encephalitis viruses), 2) acute arthropathy (Chikungunya virus), and 3) a febrile illness with a flulike syndrome (Venezuelan equine encephalitis virus). However, the majority of infections are subclinical and can be diagnosed only by the demonstration of an immune response.

4. Laboratory identification: This is generally accomplished by the demonstration of a rise in antibody titer (that is, comparing acute and convalescent sera). The virus can also be isolated from CSF, blood, or tissue. IgM antibody specific for the pathogen can be detected in the CSF of patients suffering from acute infection.

5. Prevention: The most important measure for prevention of infections caused by Alphavirus is control of the mosquito vector population. A Venezuelan equine encephalitis vaccine is available.

B. Rubivirus

 The sole member of the Rubivirus genus is rubella virus. The structure and replication of rubella virus are basically as described for the alphaviruses. Respiratory secretions of an infected person are the primary vehicles for rubella virus transmission. Rubella causes a mild clinical syndrome that is characterized by a generalized maculopapular rash and occipital lymphadenopathy. [Note: This is known as “German measles,” not to be confused with “measles” (rubeola), caused by the measles virus.] In most cases, these symptoms may be hardly noticeable, and the infection remains subclinical. For this reason, the only reliable evidence for a prior infection with rubella virus is the demonstration of antirubella antibodies. The clinical significance of rubella lies not in the primary infection described above but, rather, in the severe damage possible to the developing fetus (especially in the first trimester) when a woman is infected during pregnancy (congenital rubella). This dam age can include congenital heart disease; cataracts; hepatitis; and abnormalities related to the CNS, such as mental retardation, motor dysfunction, and deafness (Figure 1). Fetal damage resulting from rubella infection is preventable by use of the live attenuated rubella vaccine that is included with the routine child hood vaccinations. This vaccine, which has few complications, is effective in preventing congenital rubella because it reduces the reservoir of the virus in the childhood populations and also ensures that women reaching childbearing age are immune to rubella infection. The vaccine should not be given to women who are already pregnant or to immunocompromised patients, including babies. In the United States, rubella outbreaks often begin among infected persons from countries where rubella is not included in routine immunizations.

Fig1. Pathology of rubella virus infection.

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مواضيع ذات صلة

Pathogenesis and clinical significance of HIV

Transmission of HIV

HIV replication

Hepatitis D

Prevention and Treatment of hepatitis B

Differentiating Acute and Chronic Hepatitis and the Chronic Carrier State

Flaviviridae

Caliciviridae

Picornaviridae

Laboratory Assays of Hepatitis B

Epidemiology of Hepatitis B

Hepatitis A