Poxviridae
المؤلف:
Cornelissen, C. N., Harvey, R. A., & Fisher, B. D
المصدر:
Lippincott Illustrated Reviews Microbiology
الجزء والصفحة:
3rd edition , p269-271
2025-08-24
350
Poxviruses belong to a family of large, genetically complex viruses having no obvious symmetry. Members of this family are widely distributed in nature. The agent of previous medical importance to humans, variola virus, was the cause of smallpox, the first infectious disease to be declared eradicated from the Earth. Among the factors that led to this success are: 1) the availability of an effective, attenuated vaccine; 2) variola’s antigenic stability (that is, only a single antigenic type existed); 3) the absence of asymptomatic cases or persistent carriers; 4) the absence of an animal reservoir; and 5) the emotional effect of this highly lethal, disfiguring disease, which helped to galvanize public support of and cooperation in the eradication efforts. The highly effective poxvirus vaccine contains live vaccinia virus (which causes cowpox), and the viral genome is currently being used in attempts to construct vectors carrying immunizing genes from other infectious agents. Finally, the poxvirus, molluscum contagiosum virus (MCV), causes small, wart like tumors (not to be confused with true warts caused by papilloma virus).
A. Structure and classification of the family
The genome is a single linear molecule of double-stranded DNA, with a coding capacity for more than 200 polypeptides. The virion contains enzymes that are involved in early steps of replication. The vertebrate poxviruses are related by a common nucleoprotein anti gen but are otherwise quite distinct. Humans are the natural host for variola and MCV, but monkeypox, cowpox, and several other animal poxviruses can also cause human disease.
B. Replication of the poxviruses
Poxviruses follow the basic replication pattern for DNA viruses, with a few notable exceptions. The most striking of these is that the entire replication cycle takes place in the cytoplasm, the virus providing all of the enzymes (including a viral DNA-dependent RNA polymerase) necessary for DNA replication and gene expression. Final maturation by acquisition of a lipoprotein envelope occurs as the virus buds from the cell. The replication cycle is rapid and results in early shut-off of all cell macromolecular syntheses, causing the death of the cell.
C. Epidemiology and clinical significance
The stages of smallpox are illustrated in Figure 1. Although naturally occurring smallpox is no longer a threat, the mutation of one of the animal poxviruses to a form more virulent for humans has continued to be of concern. Human infections with monkeypox are clinically similar to smallpox and, although somewhat less severe, nevertheless have a mortality rate of about 11 percent. Such infections have only been observed where the human population comes into close contact with infected animals. In its natural state, monkey pox is not readily transmitted among humans. MCV infection occurs only in humans, causing benign wartlike tumors on various body surfaces. Usually spread by direct contact, the virus can be spread among adults via sexual contact.
Fig1. Time course of smallpox.
D. Laboratory identification
The unique cellular localization of poxvirus replication has enabled rapid diagnosis by observation of DNA-containing intracytoplasmic inclusion bodies in cells scraped from skin lesions.
E. Treatment and prevention
Although immunization with vaccinia is no longer done routinely, it is still carried out in certain groups, such as the military and laboratory workers. Although one of the safest vaccines in healthy recipients, individuals with eczema may develop a generalized vaccinia rash covering the surface of the body. Immunocompromised patients are likely to develop progressive vaccinia, which has a high mortality rate. Postvaccinal encephalitis, with a mortality of 40 percent, is a rare secondary hazard accompanying vaccination.
F. Smallpox as a biologic weapon
Smallpox is potentially a devastating biologic weapon because it is highly contagious and has a high case fatality rate—more than 30 percent among unvaccinated persons. In 1972, the United States stopped routine vaccination of civilians against smallpox. As a result, more than 40 percent of the population is now susceptible to smallpox infection, with the percentage increasing each year. As a result of Project Bioshield, the United States supported the development of a new-generation smallpox vaccine that would be administered in the event of a bioterrorism attack. The new vaccine (called MVA for modified vaccinia Ankara) contains a mutant form of the vaccinia virus, which cannot replicate in humans. The vaccine is safe, even in immunocompromized individuals, and protective against monkeypox in a primate infection model.
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