NEW YORK (Reuters Health) - Some thirty years after authorities doled out the last dose of smallpox vaccine, the world faces another multiplying menace: monkeypox.

The monkeypox virus in an undated micrograph. REUTERS/CDC

A new study suggests that the monkeypox virus, which the smallpox vaccine also grants immunity against, is now at least 20 times as common as it was shortly after victory over smallpox had been declared.

“The eradication of smallpox was one of the greatest achievements known to man,” lead researcher Anne Rimoin of the University of California, Los Angeles School of Public Health told Reuters Health. “But a consequence of ceasing smallpox vaccinations is that now the world’s population is vulnerable to other (related viruses) such as monkeypox.”

While the infection is somewhat less serious than smallpox, it can still scar and even kill its victims. And in contrast to its cousin, monkeypox is not only able to jump between humans, but can infect through contact with small animals that harbor the virus. As a result, its control could be all the more challenging, warned Rimoin.

Converging political, social, economic and environmental factors make African nations -- in particular, the Democratic Republic of the Congo -- especially vulnerable to the infection, she explained. The virus’s favorite animal hosts such as squirrels and monkeys are endemic there, and civil war has forced many people to rely heavily on hunting wildlife for sustenance. Some have even migrated deep into the animals’ forest habitats to seek refuge from the violence.

“The virus has probably been on the rise for years, but the country lacked surveillance,” Rimoin noted. “To find disease, you have to look for it.”

So she and her colleagues, who included many local Congolese, did just that. Using Chinese bicycles like pack mules to transport supplies, and with funding from the U.S. National Institutes of Health, they surveyed nine local health zones for signs of monkeypox between November 2005 and November 2007. They identified 760 cases of laboratory-confirmed monkeypox.

Compared to similar surveillance conducted in the 1980s, Rimoin’s team found a 20-fold increase in monkeypox cases -- far more than they ever expected to find. In a single health zone, the average number of yearly cases rose from less than 1 to roughly 14 per 10,000 people.

Most of the victims were born after smallpox vaccination was officially discontinued in 1980. Vaccinated individuals were more than five times less likely to become infected with monkeypox compared to those without the vaccine’s protection, the researchers report in the Proceedings of the National Academy of Sciences.

“What we’re seeing is a harbinger of things to come,” said Rimoin. She warned that the virus could grow more widespread with further deforestation, continued movement of people from rural to urban areas, bushmeat trafficking and importation of exotic pets.

“And every new infection provides the virus with the opportunity to evolve into a more serious or transmissible virus,” she added.

It’s already clear that the Democratic Republic of the Congo isn’t the only home for the virus. The Republic of the Congo and Sudan also reported cases in recent years. And in 2003, monkeypox arrived in the U.S. Midwest with imported African rodents, before spreading among prairie dogs and sickening 90 people.

Experts fear an even more virulent and efficient virus could return to the western world.

“The higher the rate of new infections, the greater the chance that travelers from the U.S. will be exposed, and that the disease will be imported into the U.S. -- possibly establishing itself in U.S. rodent populations,” Dr. Dan DiGiulio of Stanford University School of Medicine in California, who was not involved in the study, noted in an email to Reuters Health.

So what can be done to keep the virus at bay? Rimoin suggested that behavioral interventions may be the most effective strategy at this point, including teaching people at risk of infection what animals may be most likely to carry monkeypox and how to handle them to avoid infection, as well as isolating infected individuals.

Continued active surveillance is also important to better identify the animal reservoirs and rates of animal-to-human versus human-to-human transmission. “Once we understand more about this virus and what it may mean for us,” she said, “we may be able to consider specific interventions, perhaps vaccinating groups that are at significant risk of infection.”

DiGiulio added the need for animal importation policies, and research into effective antiviral treatments and vaccine development.

“Three decades after the eradication of smallpox, pox viruses still deserve our close attention,” said Rimoin. “And we shouldn’t only worry about its accidental introduction but also as a deliberate terrorist release.”

SOURCE: link.reuters.com/juz48n Proceedings of the National Academy of Sciences, online August 30, 2010.