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July 2003 • Volume 37 • Number 7

News

From Africa to North America
Monkeypox Illustrates Emerging Infections Threat
Smallpox vaccine suggested for exposed kids.

Timothy F. Kirn
Sacramento Bureau

Like severe acute respiratory syndrome and West Nile virus before it, the monkeypox virus outbreak in the upper Midwest offered more evidence that emerging infectious diseases no longer respect national, or even continental, boundaries.

Monkeypox has government officials on edge, even though the virus seems to be less contagious than some other viral diseases such as measles and smallpox, and it appears that human populations alone cannot sustain transmission. The basic problem is that there's no telling what can happen when a virus enters a new environment.

“We're always very concerned with any new infection—I mean, this is an era of emerging infectious diseases,” said Dr. Stephen M. Ostroff, deputy director of the Centers for Disease Control and Prevention's National Center for Infectious Diseases. “Whenever you have a new virus being introduced into an ecosystem where it's not been present before, you have to be very concerned about the public health threat.”

That is particularly true when the virus can cause severe illness and can set itself up in a local animal host, as is the case with the West Nile virus, Dr. Ostroff added.

The monkeypox virus was apparently brought to this country by a Gambian giant pouched rat imported by a Chicago-area exotic pets dealer. But the cases of human disease have come from contact with animals indigenous to North America: prairie dogs that were exposed to the rat and, in at least one case, a rabbit. In Africa, the virus was first isolated from a monkey, but the major vector appears to be the squirrel.

As the number of suspected U.S. cases of monkeypox climbed to 87 (in six states), the CDC recommended that persons who have had close or intimate contact with symptomatic individuals or animals confirmed to have monkeypox should receive the smallpox vaccine (including children, pregnant women, and persons with eczema), ideally within 4 days of exposure. Also, bans were imposed on importation and transportation of rodents from Africa and the sale or movement of prairie dogs.

Twenty-eight children at a day care facility in Indiana were potentially exposed to two prairie dogs that later became ill and died; 12 (43%) of the children said they had held or pet the prairie dogs, and 7 (25%) of the children later became ill with symptoms consistent with monkeypox infection. Laboratory evaluation of these children is in progress, according to the CDC.

Any physician who sees a febrile individual or an individual with a rash should ask about exposure to small mammals, particularly prairie dogs or Gambian rats, the CDC recommended. If a suspected patient is seen in a medical setting, infection control personnel should be notified and standard precautions should be taken, including protection against direct contact and airborne transmission. “The possibility of human-to-human transmission in some cases cannot be excluded at this time,” the CDC said at press time.

Those recommendations seem prudent but may be subject to change, Dr. Ostroff noted. With so many open questions about how the U.S. outbreak may unfold, “we have to keep a relatively open mind,” he said. “We don't have all the information.”

Much of what is known about monkeypox comes from two studies in the Democratic Republic of the Congo (DRC, formerly Zaire). The virus, an orthopoxvirus, was first identified in 1970. Since 1983, when smallpox vaccination was halted, sporadic outbreaks of monkeypox have occurred in rain forest regions of Central and West Africa. Smallpox vaccine is considered 85% protective against monkeypox.

The first study, which examined 338 probable cases that occurred between 1981 and 1986, suggested that the case fatality rate was 10% for persons not inoculated with vaccinia vaccine. The secondary attack rate in unvaccinated household members was 9.3%. The researchers concluded that transmission by a person who acquired the infection from another person was rare, implying that the virus cannot be sustained in human populations.

The second study was undertaken in 1997 by experts from the CDC and the World Health Organization, following an outbreak of 71 suspected human cases in the DRC. Initial reports suggested that person-to-person transmission was the predominant source of the cases, and that there might be long chains of transmission. The area, a region of 12 small villages, was beset with famine, and the study had to be halted early, before control subjects could be identified and compared with cases, because of growing civil unrest.

Researchers interviewed residents in nine of the villages about possible monkeypox illness during the preceding 12 months, and collected sera for confirmation from suspected cases. This led to identification of 7 active cases and 81 retrospectively identified cases, for an attack rate of 22 per 1,000 persons, but the range was considerable between villages.

The study showed seasonal variation in the disease, with a clustering of cases during the summer months, followed by a decline, and then a resurgence the next February. The attack rate was similar for males and females, the median age of the cases was 10 years, and all three known fatalities occurred in children less than 3 years old. The children all died within 3 weeks of the onset of their rash, but there was no information to indicate whether the cause was monkeypox or a superinfection that they acquired.

In this study, the overall case-fatality rate was only 4%. The researchers speculated that the mortality rate may have been lower than in the other study because a varicella outbreak in the region that was concurrent with the monkeypox outbreak may have led to some confusion between the two. Still, almost all of the identified cases appeared to have been confirmed in the laboratory from collected sera.

Regarding the features of the illness, 40 of the 78 cases with full information reported that they had had lymphadenopathy. All 75 of the examined individuals had scars from their rash. All seven individuals with active cases had lymphadenopathy, and each had over 100 lesions; five had a rash on the palms and soles, and three had alopecia.

Although the study was interrupted, the researchers did not find evidence of prolonged, human-to-human lines of transmission. There was an 8% secondary attack rate among household members of cases.

Consumption of wild animals was common among the 85 cases for which this factor was evaluated: 71 had eaten squirrel, 79 had eaten monkey, 77 had eaten rat, 64 had eaten porcupine, and 68 had eaten gazelle.