Keeping an Eye on Poxviruses

Abstract

On the basis of descriptions of the skin lesions, there were two major candidates for the diagnosis. The first candidiate was chickenpox, which is caused by a highly contagious human-adapted herpesvirus, and is endemic in central Africa, where immunization rates are low. The second candidiate was monkeypox, a zoonosis caused by an orthopoxvirus maintained in local rodents, which usually occurs as single cases, but can also produce short chains of person-to-person transmission. Although monkeypox has been observed with increasing frequency in the neighboring Democratic Republic of the Congo over the past four decades, it has only been reported once previously in the Republic of the Congo in a 2003 outbreak described by the same group of investigators. 2–4 In the present instance, MacNeil and others found that serum samples from approximately 40 of the villagers showed evidence of recent chickenpox, but their test results also identified three people who may have been exposed to monkeypox virus. This report reflects the increasing sophistication of efforts to detect the occurrence of poxviral zoonoses, which also include cowpox virus infection. Why is it important to monitor these diseases, which are barely known to most of the public health community? Their relative obscurity in part reflects the fact that, before the eradication of smallpox, vaccination with vaccinia virus largely prevented their occurrence, by crossprotecting against other members of the genus Orthopoxvirus . 5 However, now that most of the world’s population has never been vaccinated, there is no longer an immune barrier to keep these agents from crossing over from their reservoir species into the human population. Monkeypox is the disease of greatest concern because it closely resembles smallpox, but it is less contagious and has a lower case-fatality rate. 2 Although it has presumably occurred in central Africa for millenia, monkeypox was only discovered in 1970 during the course of the smallpox eradication campaign in Zaire (the present Democratic Republic of the Congo), when a smallpox-like illness developed in an infant in a rural village that could not be linked to any chain of person-to-person transmission. Because all other members of her family and most of the surrounding population had been vaccinated, it was initially feared that the child had contracted smallpox directly from an animal reservoir, potentially dooming efforts to eradicate the disease. 6,7 However, the agent isolated from her skin lesions proved to be monkeypox virus, which had been discovered 12 years earlier when it caused an outbreak of smallpox-like disease in monkeys in a holding facility in Copenhagen, Denmark. Field studies over the subsequent decade identified some 150 additional cases of human monkeypox in central and west Africa, principally in Zaire, and indicated that squirrels and perhaps other rodents were the natural reservoir. Because the disease appeared to pose little threat to the local population and none to the outside world, vaccination was halted in Zaire in 1980. 7