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Poxviruses
VIRUS:- Poxviruses (Orthopoxviruses): Smallpox
GENERAL CONCEPTS:
- A passage from T. B. Macaulay's The History of England from the Accession of James II, volume IV, serves to preface this page describing the disease smallpox:
"The smallpox was always present, filling the churchyards with corpses, tormenting with constant fears all whom it had stricken, leaving on those whose lives it spared the hideous traces of its power, turning the babe into a changeling at which the mother shuddered, and making the eyes and cheeks of the betrothed maiden objects of horror to the lover."
- The Poxviruses contain a dsDNA genome, have a complex morphology and replicate in the host cytoplasm.
- Poxviruses cause localized and generalized infections in humans and animals.
- Smallpox was the major human disease agent but it was officially declared eradicated by the World Health Organization (WHO) in 1980.
- Smallpox is transmitted via the respiratory route. In the body, it is spread by a transient viremia to internal sites, and then a second viremia to the skin where the characteristic lesions erupt.
- The lack of an animal reservoir made eradication of human disease possible.
DISTINCTIVE PROPERTIES:
- Poxviruses are large and brick shaped having a complex symmetry (not icosahedral or helical) and small surface tubules.
- The genomic DNA has a molecular weight of 100-200 x 106 daltons.
- The virion contains a DNA-dependent RNA polymerase (DDRP, transcription enzyme) that is required for infection. Other enzymes in the virion complete the uncoating process and initiate early replication, which occurs in acidophilic intracytoplasmic inclusions.
- Poxviruses are antigenically complex but may share common internal antigens.
PATHOGENESIS:
- Smallpox is a systemic infection with a very characteristic rash.
- Transmission of disease occurs via the respiratory tract. Following a 12 day incubation, lesions first appear in the mouth and throat and then on the skin. The rash develops in a synchronous and centrifugal manner with lesions first taking the macular form, then papular, vesicular, pustular and finally as crusts.
- Two forms of smallpox exist: the more serious form, variola major, has about a 50% mortality rate; the less severe form, variola minor results in less than 1% death.
HOST DEFENSES:
- Antiviral antibody is protective. Hyperimmune serum can prevent disease. The cell-mediated response is also important, however, since individuals with hypogammaglobulinemia still recover.
EPIDEMIOLOGY:
- Before eradication, Asia and India were endemic for variola major; South America for variola minor.
DIAGNOSIS:
- Clinical: The characteristic lesions with their centrifugal and synchronous development are diagnostic.
- Laboratory: Electron microscopy of crusts can reveal the intracytoplasmic inclusions.
CONTROL:
- Sanitary: Quarantine of diseased individuals.
- Immunological: Vaccination of all individuals.
- Chemotherapeutic: None.
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