Smallpox caused by variola virus has been eradicated from the world in 1980 following an intensive worldwide campaign by the World Health Organization (WHO). However, the possible use of smallpox virus as an agent of bioterrorism is the main concern now.
Variola virus shows following features:
· The variola virus is a large, brick-shaped virus measuring 300 3 200 3 200 nm, almost visible by light microscopy.
· The virion consists of protein (90%), lipid (5%), and DNA (3%).
· The viral genome consists of a large, double-stranded, linear DNA that is fused at both ends. The DNA measures 130–375 kbp in size and has a terminal loop.
· The extracellular virion possesses two envelopes, while the intracellular virus has only one envelope. The outer envelope that encloses the extracellular virion consists of host cell lip-ids and several virus-specific proteins including hemagglu-tinins. These virions consist of a large number of proteins (more than 100), at least 10 of which show enzymatic activity needed for replication of the genome.
· The virus consists of many enzymes that facilitate in the syn-thesis, polyadenylation, and methylation of viral messenger RNA (mRNA).
Poxvirus replication is complex. Among the DNA viruses, they are unique in that the complete replication cycle of the virus occurs in the cytoplasm of the host cell. The virus encodes the enzymes required for mRNA and DNA synthesis essential for genetic replication. The virus encodes and carries all proteins necessary for synthesis of mRNA. The virus also encodes pro-teins for other functions, such as DNA synthesis and immune escape mechanisms. Finally, the virions are assembled and acquire their envelopes by budding from the cell membrane as they are released from the cell.
Susceptibility to physical and chemical reagents: Variolavirus is most stable at low temperature and low humidity. It remains viable for months at room temperature, if protected from sunlight and in the cold or when freeze-dried for years. It is resistant to action of 50% glycerol and 1% phenol. Even though enveloped, the virus is not susceptible to ether, hence is not inactivated by ether. It is susceptible to ultraviolet light and other irradiations and is also readily inactivated by forma-lin and oxidizing disinfectants.
Antigenic properties: Smallpox virus has a single serotype, whichwas responsible for successful vaccination against the disease. All poxviruses including vaccinia viruses have a common nuclear pro-tein antigen. These poxviruses contain at least 20 different anti-gens, such as LS antigen, agglutinogen, and hemagglutinin.