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A study of spiny lobsters conducted by researchers at Old Dominion University and the Virginia Institute of Marine Science shows for the first time that animals in the wild shun their neighbors who have contracted an infectious disease.

The researchers report in the May 25 issue of the prestigious international journal, Nature, that avoidance begins even before an infected lobster shows symptoms of the disease.

The Nature article was written by Mark Butler, ODU professor of biological sciences; Donald Behringer, ODU postdoctoral student; and Jeffrey Shields, an associate professor at VIMS.

According to the authors, the lobsters probably are prompted by their sense of smell to avoid infected creatures, and the tactic may limit disease transmission in the wild.

Butler, Sheilds, and Behringer, who obtained the National Science Foundation grant that funded the team's research, have been studying viral disease in Caribbean spiny lobsters (Panulirus argus) for more than five years. A lethal pathogenic virus called PaV1 that strikes juvenile lobsters and is transmitted mainly by physical contact was the focus of the most recent study.

Because spiny lobsters are social and share communal dens, such a virus could have devastating consequences if there were no mechanism to check its spread, the authors write. As it is, surveys in Florida waters of the juvenile Caribbean spiny lobster population reveal a steady PaV1 infection rate of less than 7 percent. This rate is far less than would be predicted by the social nature of the lobsters and the highly infectious nature of the virus.

During underwater surveys, the researchers found that infected lobsters only rarely shared dens with healthy mates. To test whether healthy lobsters were avoiding the infected ones, or vice versa, laboratory tests were conducted. These showed that healthy lobsters avoided dens containing infected fellow crustaceans, but that infected lobsters did not discriminate between healthy and diseased lobsters.

The researchers reported that lobsters inoculated with PaV1 developed symptoms of the disease after six weeks and became infectious after eight weeks. Most of the healthy lobsters began to avoid the infected ones before symptoms developed, and all were avoiding the infected ones within eight weeks.

Butler said the research shows that spiny lobsters, though more primitive than many other animals, have evolved the biological machinery necessary to distinguish their diseased neighbors, and that their ability to do so is finely tuned to click in before the disease becomes infectious. "It's unlikely that lobsters are unique in this way, and I suspect that other species have a similarly rich behavioral repertoire that permits them to detect and minimize the risk of exposure to pathogens," he said in an interview.

He noted that other scientists have long speculated that behaviors of this kind confer such an obvious survival advantage that the behaviors should have evolved.

Research over the past half century has shown that social animals have evolved behaviors to decrease the probability of infection. For example, something as simple as diet change can accomplish this. But there are many other effective behaviors. Twitching by some mammals can drive off disease carrying mosquitoes. Grooming by licking can remove parasites and reduce the probability of wounds becoming infected. A dust or mud bath, or a long sit in the sun, and fastidious nest management, all are behaviors that reduce risk.

"There is no record, however, of social animals avoiding diseased individuals of their own species in the wild," the authors write.

Added Butler, "Given the previous lack of evidence for behavioral avoidance of disease, most epidemiological models of the spread of disease in nature have assumed it does not exist. Our results suggest that we should revisit this assumption and consider how behavioral interactions among individuals may alter the spread of disease."

He said the research team now will try to determine: if lobsters detect disease via chemical or visual signals, how infection alters a lobster's ability to detect whether another lobster is infected and how many ways the PaV1 virus can be transmitted.

This article was posted on: May 24, 2006

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