No raccoon boom after vaccination program.

Garbage can owners in Scarborough, Ontario, can sleep as peacefully as ever.

Vaccinating the city's raccoons against canine distemper in the early 1990s successfully reduced the prevalence of the disease in these urban scavengers. Even better news to some residents, the shots do not seem to have triggered the population boom that some feared would create bigger, healthier hordes of raccoons to rattle garbage cans all night, report researchers at the University of Guelph in Ontario.

The raccoon shots raise the deceptively simple question of how to determine the effects of a disease on a population, observes Guelph wildlife ecologist Thomas D. Nudds. He and his colleagues used the raccoon test to provide data for a rare large-scale analysis of a wildlife disease. Their results appear in the May Ecological Applications.

Nudds bemoans the tendency to assume that a disease which kills many animals necessarily holds the population in check. "They might have died anyway," he says. Canine parvovirus, for example, decreases the survival of wolf pups, yet a detailed analysis has shown that it does not regulate the wolf population.

Without a strict analysis, says Nudds, limited wildlife management dollars may be wasted trying to prevent a disease that doesn't menace a population. "You can spend a lot of time running down the wrong path," he warns.

The question of population-level effects also comes up in discussions of vaccinating wild animals against rabies or other diseases to keep the creatures from posing a public health hazard. Would this strategy trigger booms in nuisance animals in cities? As Nudds puts it, "Nobody wants rabies, but nobody wants skunks either. So what are you going to do?"

To study such problems, he and his colleagues monitored an Ontario Ministry of Natural Resources' project that trapped, vaccinated, and released skunks and raccoons in certain areas of the city. Because no rabies outbreak has since struck Scarborough, the researchers can't draw conclusions about rabies' role in controlling animal numbers.

In 1991, the project added canine distemper shots to the regimen. Canine distemper provides a good model for studying whether a disease regulates a population, Nudds notes.

After comparing animals in the vaccination zone to those in another part of the city, the researchers concluded that the vaccine was indeed keeping disease prevalence down--about 1.4 percent of raccoons were infected rather than 8.3 percent. Yet the program did not change overall growth trends in the raccoon population.

Nudds cautions against applying these results directly to rabies or to skunks, but he does note similarities between the way rabies and canine distemper spread and the way skunks and raccoons adapt to city living.

Wildlife veterinarian Elizabeth S. Williams of the University of Wyoming in Laramie says she hopes that publication of the raccoon study will inspire more researchers to tease out the population-level effects of diseases. "These studies are hard to do," she comments.