Wisconsin state epidemiologist Jeff Davis said monkeypox virus had not yet been confirmed in the worker, but it was suspected and experts were testing tissue specimens. The worker is ill and has been isolated, state health officials said.
Monkeypox is typically transmitted from animal to animal, but in rare cases it can pass from human to human, Davis said. A monkeypox outbreak in the Congo in 1996 and 1997, for example, grew through person-to-person transmission, he said.
"In this case, there was no animal exposure," Davis said of the health care worker. "The only contact was with a human."
The announcement came a day after U.S. government moved aggressively to contain the first outbreak of monkeypox in the Western Hemisphere by prohibiting imports of African rodents, banning the sale of prairie dogs - believed to have spread the virus to humans - and recommending smallpox shots for people exposed to monkeypox. The government also expanded its search for infected prairie dogs to 15 states.
The smallpox vaccine can prevent monkeypox, an exotic African disease, up to two weeks after exposure to the virus but is most effective in the first four days.
"We're optimistic we can deliver the vaccine to these people in time to do good," said Dr. David Fleming, the Center for Disease Control and Prevention's deputy director for Public Health and Science.
As of Wednesday, health officials had confirmed a total of nine human cases of the disease - four in Wisconsin, four in Indiana and one in Illinois. Fifty-four possible cases had been reported - 25 in Indiana, 17 in Wisconsin, 11 in Illinois and one in New Jersey, CDC spokesman Tom Skinner said. No one in the United States has died of the disease.
Fleming said he is confident the outbreak will be controlled.
"Monkeypox is a disease that is potentially transmissible from person to person but at a fairly low level," he said. "I don't anticipate the same kind of problem that we anticipate from SARS."
The smallpox vaccine is 85 percent effective against monkeypox, Fleming said. The vaccine is widely available because states stocked up on it out of fear of bioterrorism. More than 37,000 health workers in the United States have been vaccinated.
"State health departments have been actively involved in planning and preparing for the possibility of a bioterrorist event," Health and Human Services Secretary Tommy Thompson said. "We are now seeing that this level of preparation can also assist in unexpected, natural outbreaks."
The CDC said health care workers, veterinarians and family members who have cared for or had close contact with infected people or animals should get vaccinations. The agency also warned veterinarians and doctors to be on the lookout for the symptoms, especially in owners of prairie dogs or exotic rodents from Africa.
CDC officials didn't know how many people would have to be vaccinated, but Fleming said he expected the number to be modest.
About 40 out of every million people given the smallpox vaccine for the first time will face a life-threatening injury, and one or two will die. Still, the CDC is recommending vaccinations even for pregnant women, children and people with eczema - for whom the vaccine is usually discouraged - who have been exposed to infected prairie dogs.
"Because of the real risk here ... we're recommending a somewhat aggressive approach of who should get the vaccine," Fleming said.
Monkeypox-infected prairie dogs distributed from Phil's Pocket Pets of Villa Park, Ill., may have been sold to numerous buyers in 15 states since April 15, according to a Department of Agriculture emergency warning issued Wednesday.
Monkeypox, which produces pus-filled blisters, fever, rash, chills and aches, is a milder relative of smallpox. It has a mortality rate of 1 percent to 10 percent in Africa, but U.S. officials believe better nutrition and medical treatment here probably will prevent deaths.
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Symptoms can be accompanied by diarrhea, swollen lymph nodes, a sore throat and mouth sores. In some cases, a victim may experience trouble breathing. Symptoms are at their worst for 3 to 7 days, after which the fever lessens and blisters begin to form crusts.
Monkeypox is a viral disease with a clinical presentation in humans similar to that seen in the past in smallpox patients. Smallpox no longer occurs, following its worldwide eradication in 1980, whereas monkeypox is still seen as a sporadic disease in parts of Africa.
The virus responsible for monkeypox is related to the virus that used to cause smallpox (both are orthopoxviruses). Vaccination against smallpox (no longer necessary) also gave protection against monkeypox. Before the eradication of smallpox, vaccination was widely practiced and protected against both diseases. However, children born after 1980 have not been vaccinated against smallpox and are likely to be more susceptible to monkeypox than older members of the population. The death rate from monkeypox is highest in young children, reaching about 10%.
Most cases occur in remote villages of Central and West Africa close to tropical rainforests where there is frequent contact with infected animals. Monkeypox is usually transmitted to humans from squirrels and primates through contact with the animal's blood or through a bite.
Following reports of ongoing cases of human monkeypox in the Democratic Republic of the Congo ("DRC", formerly Zaire) representing a new pattern of the disease, the Ministry of Health in DRC and the World Health Organization (WHO) organized two investigations in February and October 1997.
In the past, an outbreak of monkeypox in DRC would have been limited to the village and would not last long because it did not spread extensively after the first patients recovered. However, the present study indicates that monkeypox disease is changing its pattern of infection in humans. The outbreak had a much higher rate of person-to-person transmission than seen previously, and spread through many generations of transmission, thus maintaining the outbreak for more than a year.
Previous studies over a twenty-year period had shown that the rate of transmission of monkeypox within households was low, suggesting that the disease had a low potential for transmission from person to person. Outbreaks were generally self-limiting after one or two sequential transmissions. However, the two recent WHO studies have to-date shown that:
- The outbreak in DRC presents the largest cluster of monkeypox cases - albeit with less severe clinical disease ever reported: 511 suspect cases were identified with onset of disease between February 1996 and October 1997; some of these suspect cases are thought to be chickenpox and laboratory confirmation studies are underway.
- The proportion of suspect cases who were 16 years of age or older (21%) was higher than previously reported (8%). Young children had mostly been affected in previous outbreaks.
- The percentage of suspect cases from person to person transmission (78%) was higher than previously reported (30%). This was associated with the clustering of cases in household compounds and prolonged chains of transmission from person to person.
- Secondary attack rates among contacts of primary cases (those thought to be infected by animal contact) have been calculated, on a preliminary basis, at 8%, which is similar to secondary attack rates of 4-12% estimated in Zaire in the early 1980s.
- The proportion of deaths (2%) was lower than previously reported (10%); all were aged under eight years and died within three weeks of disease onset.
The ending of vaccination programs against smallpox in the late 1970's has probably led to an increase in susceptibility to monkeypox and could explain the larger size of the most recent outbreak, the higher proportion of patients aged 15 and over, and the spread through many generations of transmission.
WHO is concerned that monkeypox could pose a public health problem in this region of DRC and therefore vigilance must be maintained by strengthening detection systems for monkeypox and completely investigating future outbreaks.
Further WHO studies are planned in the region to determine the need for additional risk-reduction measures.
In May 1996, the 49th World Health Assembly decided that the last remaining stock of smallpox virus held in two research centers in the Russian Federation and the United States of America should be destroyed as the last step in the complete and final global elimination of smallpox. It also decided that WHO would keep 500 000 doses of smallpox vaccine (which is also effective against monkeypox). The smallpox vaccine seed virus (vaccinia virus strain Lister Elstrea) will be maintained in the WHO Collaborating Centre on Smallpox Vaccine at the National Institute of Public Health and Environmental Protection in Bilthoven, Netherlands, so that new stocks of vaccine can be produced if needed.