CDC Information on Monkeypox Virus

In response to an increasing number of persons in North America infected with the monkeypox virus, the Centers for Disease Control and Prevention (CDC) has released an interim case definition and measures for infection control. The information is available online atwww.cdc.gov/ncidod/monkeypox/index.htm, and is evolving as more is learned about the disease and effective treatment.

Monkeypox is a rare viral disease found mostly in the rainforest countries of central and west Africa. The disease is called “monkeypox” because it was discovered in laboratory monkeys in 1958. Studies of animals in Africa later found serologic evidence of infection in ground squirrels, as well as rats, mice, and rabbits. In 1970, monkeypox was identified as the cause of a smallpox-like illness in humans in remote African locations.

In early June 2003, monkeypox was reported among residents in Wisconsin, Illinois, and Indiana. On the basis of preliminary investigations, it appeared that most of the patients became ill after having contact with infected prairie dogs purchased as pets. Some patients may have been infected through contact with other infected animals, including a Gambian giant rat (purchased as an exotic pet) and a rabbit. The possibility of human-to-human transmission in some cases cannot be excluded.

This is the first evidence of community-acquired monkeypox infection in the United States. There have been no deaths related to the outbreak. The number of cases and states involved in the outbreak will likely change as the investigation continues.

There is not enough information to determine with certainty how monkeypox arrived in the United States. One hypothesis is that the prairie dogs contracted the virus from infected Gambian giant rats housed in the same animal-holding facility or pet shop. The rats were likely imported from Africa. Studies have shown that Gambian giant rats and other rodents in Africa have evidence of monkeypox virus infection. The federal government has banned all sales of prairie dogs and halted the import of African rodents.

People can get monkeypox from an infected animal through a bite or direct contact with the infected animal's blood, body fluids, or lesions. The disease also can be spread from person to person, but it is much less infectious than smallpox. The virus is thought to be transmitted by large respiratory droplets during direct and prolonged face-to-face contact. In addition, monkeypox can be spread by direct contact with body fluids of an infected person or with virus-contaminated objects, such as bedding or clothing.

The disease is caused by Monkeypox virus, which belongs to the orthopoxvirus group of viruses. Other viruses in this group that can cause infection in humans include variola (smallpox), vaccinia (used in smallpox vaccine), and cowpox viruses.

In humans, the clinical features of monkeypox are similar to those of smallpox, except that swelling of lymph nodes is associated with monkeypox. About 12 days after exposure, the illness begins with fever, headache, muscle aches, backache, swollen lymph nodes, a general feeling of discomfort, and exhaustion. Within one to three days (sometimes longer) after onset of fever, the patient develops a papular rash (i.e., raised bumps), often first on the face but sometimes initially on other parts of the body. The lesions usually develop through several stages before crusting and falling off. The illness typically lasts two to four weeks.

In Africa, monkeypox is fatal in up to 10 percent of people who get the disease; the case fatality ratio for smallpox was about 30 percent before the disease was eradicated.

As of June 12, 2003, there is no proven, safe treatment for monkeypox. Smallpox vaccine has been reported to reduce the risk of monkeypox among previously vaccinated persons in Africa. The CDC is recommending that persons investigating monkeypox outbreaks and involved in caring for infected people or animals should receive a smallpox vaccination to protect against monkeypox. Persons who have had close or intimate contact with people or animals confirmed to have monkeypox also should be vaccinated. These persons can be vaccinated up to 14 days after exposure. The CDC is not recommending pre-exposure vaccination for unexposed veterinarians, veterinary staff, or animal control officers, unless such persons are involved in field investigations.