The Pox Diseases

The reason for including smallpox in the first edition was that there was a possibility that like the diseases already mentioned, one of the animal pox diseases could become serious in humans. Smallpox vaccination probably lasts for 10 years, possibly up to 30 years, but since smallpox vaccination was stopped in 1979, there is a decreasing number of people with any remaining immunity.

Monkey pox is a rare zoonosis, significant because it produces a disease similar to smallpox, localized to tropical rain forest areas of west and central Africa. Most cases have been reported from Congo (formerly Zaire). Although it is a disease of monkeys, it occasionally affects humans. It has a comparable case-fatality rate to smallpox although the secondary attack rate is much lower (15%).

The characteristics of smallpox and monkey pox are:

• clear-cut prodromal period of sudden onset of fever, headache and prostration;

• peripheral distribution of the rash (including soles and palms);

• lesions pass through the same stages at the same time;

• fever intensifies as the rash progresses to the pustular stage;

• lesions are deeply seated flat-topped and centrally depressed.

These features should be compared with chickenpox (Section 12.1).

Smallpox vaccination (vaccinia virus) also protects against monkey pox, so the waning level of vaccination poses the theoretical possibility of it spreading. However, the well-defined distribution, and the low secondary attack rate in close contacts, makes it unlikely to develop into such a serious disease and smallpox vaccination can be used to prevent it.

There are also other animal pox virus diseases which have infected or could infect humans. Examples are cowpox, camelpox, tanapox, yabapox, buffalopox and goatpox. There is very little evidence to suggest that any of these diseases can cause an infection in people that is likely to be spread from one person to another to any marked extent, but their importance is to recognize that they can occur and to differentiate them from smallpox.

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