Australasian Science: Australia's authority on science since 1938

Historical treasures in a modern pest: the Black Rat story

By Angela Lush

The genome of the Black Rat will provide a clearer picture of its role in spreading disease, and will help policymakers prepare for possible outbreaks in the future.

They scuttle under houses and along fences. Their beady eyes peer out from behind leafy fronds and they often draw screams from the faint-hearted.

Black Rats are a common sight around Australian homes, but many residents may not know these pests hold the scientific key to unlocking the causes of historic and future disease around the world. A South Australian Museum team has embarked on a project to map the genetic story of the Black Rat. The data will provide a clearer picture of the rat's role in spreading disease and this information will help policymakers prepare for possible outbreaks in the future.

Rattus rattus – the Black Rat – is the world's worst agricultural and urban animal pest. They can spread a range of diseases to humans and wildlife and severely damage agricultural crops, stored food and native fauna. In the late Middle Ages, it is estimated that one in three people in Europe was killed by the Bubonic Plague – a disease spread by fleas on Black Rats. The rats have been destructive in modern times too. Those introduced onto Christmas Island caused the extinction of the native Christmas Island Rat in just nine years. The Museum's research could help to predict what may happen to local fauna if the rats' range expands into new areas.

Professor Steve Donnellan and his team at the South Australian Museum's Evolutionary Biology Unit are discovering how rats moved around the globe in the past to help us understand how they adapted to different environments. He says this will help scientists predict the threats they pose in the future.

"Black rats are introduced into many parts of the world – including Australia – and we think they originally came from southern India and south-east Asia. Once humans began to move around and trade, Black Rats started to accompany people and expand their range," says Prof Donnellan.

In south-east Asia alone, many hundreds of millions of people suffer from illness or die due to rat-borne diseases. The Bubonic Plague still causes health problems in places like Asia, and other diseases such as typhus are found worldwide, and are a persistent and costly public health issue.

The South Australian Museum team are using genetic information to 'capture history' from specimens of Black Rats held in museums all around the world. Just a few hairs from a Black Rat are all it takes to tell scientists about the rat's biological profile and its role in history. Researchers compare DNA profiles from rats in their native habitats with those where the rats have been introduced. This tells them where the introduced rats originally came from and eventually which genes are involved in helping them adapt to the new environment.

"It's not just a matter of saying the species was there because we found its remains - Understanding their history will help us to identify the disease risks associated with current Black Rat populations and how these rats may adapt to environmental changes in the future," Prof Donnellan says.

Since the study began in 2009, six different types of Black Rats have been identified. Describing the genetic make-up of the different rat types is be an important step towards developing new rat control methods, particularly those involving viruses or parasites.

The team is hoping to gain access to rat collections in other museums to fill in gaps in the Black Rat story. These collections, such as those held in the Natural History Museum in London, not only contain rats from around the world, but throughout history as well. When combined with advances in DNA technology, scientists will develop the clearest picture yet of the story of this infamous pest.

"We can look at populations that were sampled by museum collectors 100 to 200 years ago and see the turnover in genetic information – how the genes have changed with climate or environment. We have been able to push the historical perspective back beyond 1000 years ago by examining rat bones from archaeological digs in caves. "

"Now we can understand something about their genetic makeup and not just tick off the appearance of a species at a particular point in time. If specimens are preserved in the right way, we can also gain information on their diet, diseases and parasites and find out a bit more about their ecology.''

Australia has both the Asiatic and Indian Black Rat and they are widespread throughout south eastern Australia. Professor Donnellan and his team will employ their genetic detective work to see if the populations are acting in a similar way or if they pose slightly different risks, like carrying a different suite of diseases.

"We can now gain a completely new perspective that we couldn't even dream about before. Early collectors like Charles Darwin would never have thought that the specimens of black rats that they collected could be used in this way."

South Australian Museum