Life

One of humanity's greatest scourges - the black rat - may help health experts track the spread of disease.

New work probing Rattus rattus's origins and historical movements should help health officials track its ongoing dispersal - and might also explain anomalies in its spread of diseases such as typhus and plague.

Ken Aplin at CSIRO Sustainable Ecosystems in Canberra, Australia, and colleagues have analysed the mitochondrial DNA (mtDNA) of 170 black rats from 76 regions in 32 countries. They also surveyed other Rattus species to probe the animals' genetic links and origins.

The team found six lineages within an expanded R. rattus group, which includes a few other species. These lineages all originated in Asia - in India and the Himalayan region, Thailand, the Mekong Delta of Vietnam, Indonesia, and East Asia.

Human association

The work reveals four major episodes of global rat expansion, all associated with human migration or trade.

About 20,000 years ago, one lineage moved from western India to the Middle East and from there to Europe. This was probably associated with the growth of trade networks during the Neolithic and Bronze Age. This lineage then spread to Africa and the Americas with European explorers.

The East Asian lineage, meanwhile, arrived in Micronesia, from Taiwan via Japan, the Philippines and Indonesia, only 3500 years ago.

The new DNA dataset should not only help researchers investigating the history of human migration and trade, but also aid in the monitoring and management of the spread of black rats - and the diseases they carry - today, says Aplin.

Separate species?

It has been unclear why some human diseases associated with R. rattus are more common in some places than others, but it is possible that different lineages of the black rat each carry a different set of pathogens, Aplin adds.

The work does suggest that some of the rat lineages should in fact be recognised as separate species, the team says.

Aplin will present his research at the Archaeological Science Conference at the Australian National University, Canberra, next week.

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