Camelpox and smallpox viruses called very close relatives

May 6, 2002 (CIDRAP News) – The camelpox and smallpox viruses are more closely related to each other than to any other virus, which suggests that camelpox could become a threat to humans through natural processes or genetic engineering, according to researchers who sequenced the camelpox virus.

Caroline Gubser and Geoffrey L. Smith of Oxford University in the United Kingdom compared camelpox virus with other orthopoxviruses. Writing in the­­ Journal of General Virology, they state, "Each comparison gave the same conclusion: CMPV [camelpox virus] is the closest known virus to variola virus, the cause of smallpox."

Camelpox virus causes a severe and economically important disease in camels but has rarely, if ever, caused disease in people, according to the report. Likewise, camels are not susceptible to smallpox, though camels immunized with smallpox virus resist subsequent infection with camelpox, the authors say. They sequenced the genome of a virulent camelpox strain called CMS, which was isolated from Iran in 1970, and also the termini of camelpox virus strain 903 from Somalia. Only a few of the virus's individual genes had previously been sequenced.

In comparing CMPV CMS and the BSH strain of variola virus (VAR-BSH), the investigators found that their genomes are colinear except in the lengths of the inverted terminal repeats and the presence of four insertions of 1.5 to 2.9 kilobases (kb) in CMPV. The authors also compared the percentage of nucleotide identity in the central 110 kb of CMPV, VAR-BSH, and vaccinia virus (VV) strain COP. Nucleotide identity in the region was found to be at least 91%, but CMPV and VAR are more closely related (an average of 98% identity) than either CMPV and VV or VV and VAR. Further, a separate computerized analysis of the viruses' DNA showed that the genetic distance between CMPV and VAR is lower than that between the other two pairs of viruses.

All the comparisons showed that camelpox and smallpox are genetically closer to each other than to any other virus. The authors speculate that the two viruses evolved from a common ancestor, possibly a rodent virus, probably after the advent of intensive agriculture about 7,000 years ago.

Gubser and Smith say the growth in the percentage of people who are "immunologically naïve" for orthopoxviruses increases the danger that these viruses will emerge or re-emerge as a threat to human health. In addition, the growth in the number of people whose immunity is suppressed by HIV infection poses a risk that the orthopoxviruses such as camelpox will jump species and adapt to humans.

"Finally," the authors write, "it is unclear whether all, only a few, or just one of the differences between the CMPV and VAR genomes are responsible for the inability of CMPV to cause human disease. Consequently, genetic modification of CMPV to delete genes that are present in CMPV but absent in VAR might be unwise."

Recent news reports have said that Iraqi officials revealed to United Nationls weapons inspectors in 1995 that Iraqi scientists were working with camelpox, ostensibly in the hope of using it as a weapon to which Iraqis would be immune, while foreign troops would be susceptible.

Gubser C, Smith GL. The sequence of camelpox virus shows it is most closely related to variola virus, the cause of smallpox. J Gen Virol 2002;83(4):855-72
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