Predicting a Potential Terrorist Threat

Study calculates extent of infection if smallpox broke out

WEDNESDAY, Dec. 12, 2001 (HealthDayNews) -- If smallpox broke out, each person initially infected could pass the disease on to 12 other people, a new mathematical study says.

But that number would decrease as doctors recognize the disease they're dealing with and public health plans kick in, says a report in the Dec. 13 issue of Nature.

The findings have a special urgency in the aftermath of the terrorist attacks Sept. 11 and the 18 mysterious anthrax-related infections of people from contaminated mail.

Smallpox, a disease declared eradicated by the World Health Organization in 1979, is at the top of the U.S. government's list of potential bioterror weapons. The last known cases -- one death and a limited outbreak -- were in England in 1978 and were caused by a lab accident.

But cultures of the virus are known to be in an undisclosed storage area handled by the U.S. Centers for Disease Control and Prevention in Atlanta, and at a facility in Novosibirsk, Russia.

"As the virus still exists, it still poses a threat," says senior investigator Steve Leach from the Centre for Applied Microbiology and Research in Salisbury, U.K.

Smallpox is a highly contagious virus that has been documented for at least 3,000 years, according to WHO. People can catch it through personal contact and saliva drops from an infected person's breath. The virus enters the body through the respiratory system, then goes into the blood. After an incubation period of about 12 days, initial symptoms of fever, headache, vomiting and even delirium begin.

Two to three days later, the smallpox rash appears, starting as small pink spots and progressing to blisters deep in the skin's surface. After several days, these blisters dry up, forming crusts on the skin that fall off, leaving pockmarks.

There is no cure for smallpox once the illness has started, and over the course of history it has killed tens of millions of people. If a person is vaccinated within four days of being exposed, however, that usually works to head off the worst of the disease. Without any treatment, smallpox causes death in up to 30 percent of cases, usually within two weeks of the rash appearing.

Using data from several historical outbreaks of smallpox, including in Boston in 1721, in London around 1836 and in Kosovo in 1972, Leach and a colleague at the British research center used mathematical modeling to predict what might happen during a modern outbreak.

The researchers calculated that since the global smallpox immunization program ended in 1972, the general immunity of the Western population to smallpox has dropped to approximately 18 percent, which they say will continue to fall with time.

That's far below the 70 percent to 80 percent level that is considered necessary to eradicate smallpox in industrialized countries.

Combining this poor level of immunity with the average number of smallpox patients infected by each initial case of the disease, the researchers estimated that up to 12 people would become infected by the first people who contracted the disease. That number would decrease to six as health authorities recognized what they were dealing with and took appropriate steps.

Leach says the models could not precisely predict global infection or death rates from such an outbreak. But, he notes, "in past epidemics, mortality rates tended to be higher in the very young, the elderly and in some with underlying medical conditions."

Although infectious diseases like chicken pox or measles have a much higher transmission rate, Leach says that the emergence of smallpox could lead to a significant epidemic.

Thomas Reilly, an assistant professor of biology at Ball State University in Muncie, Ind., says that family physicians at the front lines of such an outbreak would probably have difficulties recognizing the symptoms of smallpox in the initial cases.

But he casts doubt on concerns that terrorists might hold a stock of smallpox intended for use as a bioterror weapon. Only Russia and the United States have the facilities capable of maintaining smallpox cultures at the specific temperatures and conditions they require, he says. And those facilities, he adds, are heavily guarded.

"You'd have to have a Ph.D. with a vendetta to get these things up and going again," he says. "And that Ph.D. would have to, at some point in time, be vaccinated against the virus if he's going to work with it."

Reilly says that vaccination is a useful weapon against smallpox. "I'm for vaccination, if we can do it safely and effectively," he says. There are risks, however -- smallpox vaccination can cause reactions that range from mild rashes to rare, fatal encephalitis.

But Leach says that in the event of an outbreak, immunization is not the only tool at the disposal of health authorities.

"Public health responses to infectious disease threats have always been judged against the perceived risks of the proposed interventions," says Leach. "In the past, immunization was only one part of successful public health responses [to] smallpox."

"In the latter stages of the eradication program for smallpox, expanding ring vaccination [around outbreaks], contact tracing, patient isolation and movement restrictions were all successfully employed to limit outbreaks," he adds.

About half the U.S. population has never been vaccinated for smallpox, according to the CDC. The CDC has an emergency supply of vaccine, and last year it awarded a contract to a company to make more.

What to Do: For more information on smallpox, check out the Web sites for the Centers for Disease Control and Prevention, the World Health Organization or the National Institutes of Health.

SOURCES: Interviews with Steve A. Leach, Ph.D., scientific leader, Microbiological Risk Assessment, Centre for Applied Microbiology and Research, Salisbury, U.K.; Thomas J. Reilly, Ph.D., assistant professor, Department of Biology, Ball State University, Muncie, Ind.; Dec. 13, 2001, Nature
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