A Pox on Monkeypox

Medical Author: Frederick Hecht, M.D.
Medical Editor: Barbara K. Hecht, Ph.D. and Leslie J. Schoenfield, M.D., Ph.D.

June 18, 2003 -- Whoever had heard of monkeypox until this month? Monkeypox became news in the American media because of an outbreak of the disease in the Upper Midwest.

The outbreak is thought to have begun because of a Gambian giant rat. This exotic animal, although ill with monkeypox, was imported to Wisconsin. There, in a pet store, the Gambian rat transmitted the monkeypox virus to a prairie dog. The prairie dog, unfortunately, proved to be a super-transmitter of this virus and infected at least 18 people. And so the disease spread.

Monkeypox seems new but it is really not. It has been known for over 40 years. Monkeypox was identified in monkeys in 1959. Its ability to infect people, however, was not recognized until 1970. Scientists subsequently realized that the usual hosts of the virus are rodents. In fact, the main hosts are African squirrels. Thus, infections in monkeys and people are accidental. People are infected from handling sick squirrels. (Squirrels are eaten in some parts of Africa.)

Monkeypox is usually said to be "milder" than smallpox. Still, monkeypox is not all that mild. The death rate with smallpox in Africa was 30% (before smallpox was eradicated). By comparison, the death rate with monkeypox in Africa is as high as 10%.

The monkeypox virus belongs to the family of pox viruses. The pox viruses are the most complex and largest of all known viruses. They are so big they can be seen under the ordinary light microscope. (Most viruses can only be seen under the electron microscope.)

The best known pox virus is vaccinia, the cause of smallpox. Other members of the pox family of viruses capable of infecting humans include cowpox and molluscum contagiosum. Cowpox causes ulcerative lesions (sometimes called "milkers nodules") on the hands of dairy workers. It is also of great historic importance since in 1796 Edward Jenner innoculated people with cowpox to protect them against smallpox. Molluscum contagiosum causes warty bumps on the skin with a central indentation (an "umbilicus"). The virus is transferred by direct contact, sometimes as a veneral disease.

How about chickenpox? Its name is misleading. You see, the varicella virus that causes chickenpox is not a pox virus. It is actually a member of the herpesvirus family.

Even without chickenpox, the pox virus family is a prolific one. It has members that infect a considerable range of animals, causing diseases like buffalopox, camelpox, canarypox, goatpox, juncopox, mousepox, pigeonpox, rabbitpox, raccoonpox, sheeppox, skunkpox, and swinepox. And there are lots of other poxes.

Monkeypox has many lessons to teach us. The danger of importing exotic animals, for one. Another lesson is that bioterrorism is not exclusively the province of people. Nature knows something about bioterrorism, too. Witness the outbreaks first of SARS and now of monkeypox. What will be next?

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