Bug Vanquishes Species

For the first time, scientists have documented an infection wiping out an entire species, in this case a type of land snail. Experts say the finding, reported in this month's issue of Conservation Biology, points up the urgent need to guard against infectious diseases when nursing species off the endangered list.

South Pacific land snails are rare to begin with, but they have taken a hit in the last few decades after residents of Raiatea, in the Society Island chain some 5000 kilometers south of Hawaii, began importing predatory snails from Florida in 1986 to eat another kind of pest snail. Turns out the predators preferred the taste of the native snails, and by 1991 they had driven one species--Partula turgida--to the brink of extinction. Scientists captured the last known P. turgida individuals to try to save them through captive breeding, but 4 years ago the snails began dying off mysteriously. When the population had dwindled from 296 individuals to fewer than 10, parasitologist Peter Daszak of Kingston University in Kingston-upon-Thames, England, and veterinary pathologist Andrew Cunningham of the Institute in Zoology in London set out to find out why.

Before they could solve the puzzle, however, the remaining snails had died. After slicing open the last five bodies, Cunningham noticed something odd: scads of protozoan-like spores in the digestive glands and reproductive tracts, suggesting that a parasite had infected the snails. Daszak put the spores under an electron microscope and spotted spiral tubes--a hallmark feature of Microsporidia, a family of protozoa known to infect aquatic snails. Closer scrutiny revealed the spores to belong to a new species of microsporidian in the genus Steinhausia--persuasive enough evidence for Cunningham and Daszak to conclude that the parasite finished off the snails. Because the apparent killer does not infect other land snails, Daszak says, by killing off P. turgida it may have sealed its own fate.

"It's great that somebody's finally got a concrete example of an infectious disease leading to the extinction of a species," says ecologist Andy Dobson of Princeton University. He and other experts say the finding should serve notice to endangered-species recovery programs that they must closely monitor the cause of death of individuals in their care. "Captive breeding is not always a safe haven," concludes conservation biologist Stuart Pimm of the University of Tennessee, Knoxville. "Species need to be in the wild, not in zoos."