Poxviruses

            Poxviruses are the largest viruses that infect vertebrates. This group include the human viruses – Variola (Small Pox), Molluscum contagiosum, animal viruses, bird viruses and insect viruses.

Variola virus is the causative agent of small pox. The vaccinia virus was used as the small pox vaccine. Vaccinia virus is unique in that it is an artificial virus and does not occur in nature as such. It is employed as a vector for the development of recombinant vaccines. The genome can accomodate about 25,000 foreign bps, but it is not suitable as a vector for human use due to its pathogenic effects.

Morphology: Poxviruses measure about 230 X 300 nm and are ovoid to brick- shaped. Nucleocapsids of these viruses do not conform to either of two types of symmetry found in most other viruses, hence are known as complex viruses. An outer membrane and envelope enclose the core and core membrane. Inside to core membrane is a biconcave nucleoid containing the double stranded (ds) DNA. On either side of the nucleoid is a lens shaped structure called lateral body. It also contain many enzymes and proteins. The genome of vaccinia virus consists of 86,000bps. Paschen in 1906 developed a staining technique for the virus particles and demonstrated the elementary bodies, ie., Paschen bodies in smears from small pox lesions. The virion contains many enzymes. The entire multiplication of viruses takes place in the cytoplasm of the infected cell.

Poxviruses are stable and if protected from sunlight may remain viable for months at room temperature. They are resistant to 50% glycerol and 1% phenol, but arereadily inactivated by formalin and oxidising disinfectants.

Antigenic Structure: All poxviruses share a common nucleoprotein (NP) antigen. It also contain LS antigen- a complex of heat labile L and heat stable S antigens, agglutinogen and hemagglutinin, which is responsible for the agglutination of erythrocytes of fowls.

Cultivation: Variola and Vaccinia grow on the CAM of 11-13 day old chick embryo producing pocks in 48-72 hours. Variola pocks are small, shiny, white convex, non-necrotic, non-hemorrhagic lesions. Vaccinia pocks are larger, irregular, flat, greyish, necrotic lesions, some are hemorrhagic. Both these viruses can be grown in tissue cultures of monkey kidney, HeLa and chick embryo cells. Cytopathic effects are produced by Vaccinia in 24-48 hours and more slowly by Variola. Eosinophilic inclusion bodies- Guarnieri bodies- can be demonstrated in stained proportions. The inclusion bodies consist of viral aggregates in a matrix. They can also be grown in animals experimentally.

Pathogenicity

Small Pox- In 1980 the WHO officially proclaimed that small pox had been eradicated world wide. It was an exclusively human infection with no animal reservoir. There were no carriers as the virus was eliminated completely from the patient on recovery. The source of infection was a patient in the early phase of the disease, though infectivity extended from the appearance of buccal mucosal lesions to the disappearance of all the skin lesions. Infection usually occured only in close contacts.

Virus entered the body by inhalation through throat and respiratory tract. The incubation period was 12 days. During this period it infects the phagocytic cells and later blood cells. The infection spreads to skin cells, causing pus filled vesicles. Acute systemic symptoms begin with fever, backache, headache. Vesicles appear first appear in the mouth and throat, then rapidly spread to the face, forearms, hands and finally the trunk and legs. Vesicles becomes opaque and pustular and encrust within about 2 weeks. Death is most likely to occur 10-16 days after onset of the first symptoms. Scrapings from lesions are used to differentiate small pox from chicken pox, syphilis.

Small pox could be diagnosed in the laboratory by detection of viral antigen or by isolation of the virus from the blood in the early phase or from the eruptive lesions. Rapid diagnosis was possible by electron microscopy. Small pox vaccine was a live preparation of Vaccinia virus propogated.

           

Cow Pox – caused by vaccinia virus, causes lesions at abrasion sites, inflammation of lymph nodes and fever. This infection is obtained from cows. Cow pox lesions are seen on the udder and teats of cows and may be transmitted to humans during milking. The disease is associated with some fever.

Molluscum contagiosum – This virus elicits only a slight immune response. Although infected cells cease to synthesize DNA, the virus induces neighbouring uninfected cells to divide rapidly. It affects only humans and is distributed worldwide. It causes pearly white to light pink, painless, tumor-like growths scattered over the skin. The disease usually affects children and young adults, and it can persist for years. It is acpuired by personal contact or from gym equipments, swimming pools. Treatment generally involves the removal of growths by chemicals or localized freezing.