least in Europe.
The Black Rat Cometh
By Cheryl Jones
ScienceNOW Daily News
6 February 2008
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.
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