News in Science

Christmas Island rats wiped out by disease

A new study has concluded that native rats found on Australia's Christmas Island more than 100 years ago, fell prey to "hyperdisease conditions" caused by a pathogen that led to its extinction.

The study, published in the latest issue of the journal PLoS One, presents the first evidence for extinction of an animal entirely due to disease.

The researchers say it's possible for any animal species, including humans, to die out in a similar fashion, although a complete eradication of Homo sapiens would be unlikely.

"I can certainly imagine local population or even citywide 'extinction,' or population crashes due to introduced pathogens under a condition where you have a pathogen that can spread like the flu and has the pathogenicity of the 1918 flu or Ebola viruses," says co-author Assistant Professor Alex Greenwood of Old Dominion University, Virginia.

The 1918 flu killed millions of people, while Ebola outbreaks have pushed gorillas close to extinction.

Deathly ill

For the Christmas Island study, Greenwood and his colleagues collected DNA samples from the island's now-extinct native rats, Rattus macleari and R. nativitatis, from museum-housed remains dating to both before and after the extinction event, which occurred between 1899 and 1908.

Co-author Ross MacPhee, a curator of vertebrate zoology at the American Museum of Natural History in New York, explains that Charles Andrews of the British Museum documented at the time that black rats were first brought to the island via the SS Hindustan in 1899.

The ship-jumping black rats then carried a protozoan known as Trypanosoma lewisi. A related organism causes sleeping sickness in humans.

"Fleas are the intermediate host for one of the developmental stages of Trypanosoma, and the only likely method (of disease spread) is infected fleas crossing from black rats to endemic rats," says MacPhee.

After the Hindustan's arrival, the native island rats were observed staggering around deathly ill on footpaths. Shortly thereafter, they were never seen again.

The museum DNA samples showed that Christmas Island native rodents collected before the black rats invaded the island were not infected with the protozoan, but six out of 18 collected post-contact were infected.

"Not every rat would have to be infected," Greenwood explains. "If you push a population down to an unsustainable number then it will collapse. In addition, if a substantial number of reproducing individuals became infected and ill, even if they survived the infection, their reproduction rate may be lowered and lead to a population crash."

Island species

Scientists are currently concerned about the future of Tasmanian devils, which have been dying in record numbers due to devil facial tumor disease, a contagious cancer for which the carnivorous marsupials appear to have no immunity.

Island species seem to be more vulnerable to extinction by disease. In a prior study, MacPhee determined that at least 80% of all species-level losses during the past 500 years have occurred on islands.

"The general explanation for islander susceptibility would presumably be that island denizens live in a sort of bubble, protected by water barriers from diseases prevalent on mainlands or elsewhere," says MacPhee. "But when the bubble is broken - think measles epidemics in Iceland in the 19th century - the mortality can be extreme."

Associate Professor Karen Lips, of Southern Illinois University, says that the new research was "well done and convincing, despite the limited number of samples available."

She also pointed out that island-like conditions exist within mainland areas.

"I work up on mountaintops, another kind of island with high endemism, which is greatly affected by emerging infectious disease," she says.

Elk in North America, for example, have suffered worrisome population losses due to wasting diseases induced by prions. Various South Pacific fruit bats and amphibians are also under threat now due to infectious diseases.

"What can be done?" asks MacPhee.

"Probably nothing other than captive conservation," he says. "Most wildlife biologists are hoping that such diseases, although severe, will eventually accommodate and the species will pull through."