AS IF West Nile fever, SARS and BSE weren’t bad enough, North America now has another new disease to contend with: monkeypox. More than 37 people may have caught the virus from pet prairie dogs, though only four cases have been confirmed so far. If wild  rodents have been infected, the disease could become endemic, in the same way that West Nile has.

The prairie dogs, native American rodents, caught monkeypox from a sick Gambian giant rat at an animal dealer near Chicago. It is not yet clear if the rat was imported from Africa, where this species often carries the virus. Officials are trying to trace any infected animals.

Monkeypox is a close relative of smallpox. In Africa, infected animals occasionally infect humans, causing a pustular rash, cough and fever similar to smallpox. But the virus is far less dangerous than smallpox, with a 10 per cent death rate at most, and it rarely spreads from person to person. Even if it does, it usually dies out after being passed on once or twice.

However, some virologists fear monkeypox will evolve into a more dangerous form. If the virus somehow becomes established in wild American rodents, it will certainly face new selective pressures.

In the Congo in 1997, World Health Organization investigators found that the virus was spreading more readily among people, suggesting it is already evolving into a more virulent form. But the increased rate of spread could simply be due to people’s heightened susceptibility to monkeypox now smallpox vaccination has ended.