Smallpox

The last case of smallpox in the United States occurred in 1949. In 1972, routine immunization with smallpox vaccine was discontinued in the United States. In 1978, the last case in the world occurred in Somalia. Two years later, the World Health Organization declared that smallpox had been eradicated. Only two laboratories, one in the United States and one in Russia, still have stocks of smallpox virus, which have been maintained for research purposes. There have been reports that the Russians manufactured large quantities of virus for possible wartime use. The question is whether or not some of the Russian supplies might have fallen into other hands, though there is no evidence at present that this happened. Because of recent concern that smallpox may recur, though there have been no cases for more than 20 years, the illness should be reviewed.

The incubation period for smallpox ranges from 7-to-17 days (usually 12-to-14 days), which is relatively long for a viral illness. Initial symptoms consist of high fevers, malaise, headache, and backache, followed by the appearance of small raised red spots (maculopapular rash) that begin inside the mouth and on the face and forearms, then spread sequentially to the legs and trunk. The skin lesions evolve into small blisters, then pustules. Scabs develop, which eventually separate, leaving pitted scars. In its early stages, the rash may resemble that of chickenpox, with two key differences. (1) In smallpox, the lesions are densest on the face and outermost extremities, whereas in varicella, the greatest number of lesions are on the trunk. (2) In smallpox, all lesions are at the same stage of development, whereas in varicella, the lesions are at different stages of evolution, i.e. some may be blisters while others are scabbing.

A small percentage of patients develop flat-type smallpox, characterized by large, flat lesions with a soft, velvety texture, or hemorrhagic smallpox, characterized by severe toxicity, diffuse ecchymoses (bleeding into the skin), and high mortality. Partially immune individuals (including those who were vaccinated many years ago) may develop a milder illness with an abbreviated course.

Complications include arthritis, osteomyelitis, keratitis, encephalitis, and pulmonary edema.

Skin lesions should be cultured to confirm the diagnosis. Specimens must be referred to a specialized high-containment laboratory. Once the presence of smallpox is established, additional cases do not require laboratory confirmation.

Smallpox patients must be confined to negative pressure isolation rooms. Visitors and staff must wear gowns, gloves, and masks at all times. In a large outbreak, patients who are less ill should be cared for at home. Patients do not become infectious until the rash appears and remain infectious until all scabs separate. Those who have been exposed to smallpox but are asymptomatic do not need to be quarantined, but must be closely observed for 17 days after their last exposure to the disease. If they become febrile during this time, they must be immediately quarantined, preferably at home, until the fever has resolved and it is certain that they do not have smallpox. The ability to identify and quarantine potential smallpox cases before they become infectious is of great assistance in controlling an outbreak.

There is no known treatment for smallpox. Historically, overall mortality was 30% in unvaccinated patients. However, there has never been a smallpox outbreak in a country with modern hospital and ICU facilities. Given the substantial advances in supportive care, outcomes would probably be considerably better if an outbreak were to occur today in an industrialized country. However, mortality would be high in a developing nation.

Smallpox vaccine consists of live vaccinia virus, an organism that is closely related to the virus which causes smallpox, but which typically causes only a mild, localized skin infection when injected into humans. Rare complications of smallpox vaccine include progressive or generalized vaccinia infection, postvaccinial encephalitis, and eczema vaccinatum, characterized by vaccinial infection of current or previous eczematous skin lesions. The duration of protection from smallpox vaccine is thought to be about ten years, so it is presumed that those who were vaccinated before the 1970's are no longer immune. The Centers for Disease Control has stockpiled approximately 12-15 million doses of smallpox vaccine, which were produced in the 1970's and may have lost their potency. Current supplies are being reserved for use in a possible outbreak situation. Current supplies are being reserved for use in a possible outbreak situation. Studies are in progress to determine if the vaccine will retain its effectiveness when diluted, to produce more doses if needed. New vaccines are under development, but will not be available for the foreseeable future.

From the Centers for Disease Control (CDC)

Smallpox