CANBERRA, AUSTRALIA--Potentially fatal rat-borne diseases, such as typhus and leptospirosis, are likely to spread farther around the world, according to research presented here this week at the Archaeological Science Conference 2008.
The findings come from a genetic analysis of the black rat (Rattus rattus), the rodent that spreads the bubonic plague. Researchers led by mammalogist Ken Aplin of Australia's Commonwealth Scientific and Industrial Research Organisation compared DNA sequences from 170 urban and wild black rats from around the world to create a family tree. The data allowed the team to track the rat's prehistoric and modern migrations and to investigate its impact on people. Black rats wreak havoc on agriculture, especially in Asia, and remain a major source of human disease.
The rodent is much more genetically diverse than previously thought, Aplin reported. His team identified six lineages, each of which could turn out to be a separate species. According to the genetic data, the ancestral black rat group first appeared in Southeast Asia about a million years ago.
Since then, the rodent has been on a global march. The study revealed high genetic variation among Indian rats, suggesting that the pest arrived there naturally, long before modern humans evolved. According to Aplin's study, one group of Indian rats invaded the Middle East. The black rat then colonized Europe, probably traveling with the first farmers. Using Europe as a launch pad and sailing ships as transport, the rodent spread to the Americas, Africa, Australia, and the islands of the Indian Ocean and eastern Pacific.
Another two of the six lineages dispersed around Southeast Asia and the western Pacific during prehistoric times and more recently made landfall on other continents, the study showed. One of those lineages lives as a rat ethnic minority, along with the dominant European group, in California. That suggests a second wave of migration to the Americas, perhaps during the gold rush.
The rats haven't stopped invading new territories. "We have evidence that the lineages of black rats are increasingly on the move," Aplin says. "They [continue to] cross oceans and borders in cargo, invading countries."
That could lead to an increasing spread of the diseases that black rats transmit, such as typhus and leptospirosis. The team's comparison of known disease epidemiology and its own genetic results suggest that each group of black rats carries its own disease variants. "Human resistance to newly introduced disease variants may be low," Aplin says. One such assault might have already occurred. Seoul virus, a rat-borne disease well known in Asia, and which causes hemorrhaging, turned up in California in the 1990s, killing several people and taking medical authorities by surprise. "That's the sort of thing that is going to happen increasingly as we see black rats move around the world," Aplin says.
The study will provide the genetic data pest managers need to control the spread of rats in rural areas, says Grant Singleton, a biologist at the International Rice Research Institute in Manila, Philippines, and an expert on rat ecology. The genetic groups are likely to breed in response to differing seasonal cues--behavior that could be exploited in the fight against them. "We now know that we cannot be generic in our management," he says.