Small mammals are disappearing – are town rats spreading disease?

Final Report.

A report for Kakadu National Park

Ref no: K2010/0030

December 2010

Bethany Jackson*, Simon Ward, Stuart Young and John Woinarski

Biodiversity Conservation Division, NRETAS, PO Box 496 Palmerston 0831, NT. * Murdoch University (WA) and Auckland Zoo (New Zealand).

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Summary

Small native mammals are disappearing in northern Australia, and we don’t know why. At the

same time, town rats (also called black rats) are spreading out in the Top End. Town rats

came to Australia with Europeans in the last 200 years, and we know they can carry diseases

that are new and deadly to Australian animals. Is it possible that the decline of native

mammals is because town rats are spreading diseases? This project looked at this question.

We caught and tested a total of 97 rats; 23 from Kakadu, 69 from Darwin and 5 from

Adelaide River. We killed them and took samples of blood, parasites, genetics and tissue.

We chose 6 diseases we thought might be the biggest problems (for people or native

mammals) if town rats are carrying them and we sent out samples from all the rats to special

laboratories around Australia for analysis. All the tests were negative, which means that

none of the town rats were carrying these diseases.

This is good news, but disease and town rats may still be problems for Kakadu. Diseases we

didn’t test for could still be spread by rats to native mammals or people, or these diseases

could be spread in other ways. Like most other animals, the rats carried parasites; lice were

the most common, and worms in the gut. These can carry new diseases which could be spread

when parasites jump from rats to native mammals. We also caught rats in places many

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kilometres from buildings or communities, in native bush. They shouldn’t be there and we

need action to stop them spreading.

The Problem

Native small mammals are declining across northern Australia – National Parks, Indigenous lands, grazing

properties, coastal and inland plains, rocky country and islands.

Numbers trapped in 1996, 2001 & 2008 Decline is happening across the north of Australia

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Why are they disappearing? Maybe grazing (cattle, buffalo, horses, donkeys), fire regimes, and feral cats.

Maybe disease, but no studies before this one.

Town rats carry diseases and may spread them to native animals (and people).

Rat diseases that might cause problems:

• Leishmania – skin disease that causes ulcers

• Hantavirus – viral disease that causes fever and nausea, leading to kidney failure

• Angiostrongylus cantonensis – ‘lungworm’, also attacks the brain

• Encephalomyocarditis – viral disease that attacks the heart and causes sudden death

• Trypanosomes – parasites that get into your blood and cause various types of diseases

• Leptospirosis – a bacterium that attacks the liver, kidneys and brain

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The Project We trapped for town rats in Kakadu, Darwin, Adelaide River and Kabulwarnamyo. All the town rats we

caught were killed and cut open to get samples for disease testing.

Places we trapped for town rats

a.

b.

c.

Cage traps to catch rats

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Samples: Parts of each town rat taken for disease analyses

brain

glands

ear tip

heart

liver

spleen

lung

stomach

intestine

faeces

fur

kidney

BLOOD

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Rats and disease tests

Taking a blood sample from a rat under anaesthesia,

searching for lice, ticks and mites (orange dots are mites),

post-mortem of a rat to take samples from organs, and

storage of samples in liquid Nitrogen (c. -200°C).

We trapped 23 town rats from 2091 trap nights in

Kakadu, 5 in Adelaide River (140 trap nights) and 69

in Darwin (747 trap nights) – total = 97 rats. The

Kakadu rats came from the Mary River area and

Black Jungle Springs (see maps page 5). We didn’t

catch any rats at Kabulwarnamyo (200 trap nights).

Samples were sent out to 5 specialist laboratories

around Australia for testing. None of the diseases

were found in any of the rats. So town rats are

probably not currently important in spreading these

diseases to native animals or people in the Top End.

This is good news! However, other diseases we

didn’t test for may be contributing to the decline,

and rats may still be important carriers if new

diseases arrive.

The rats we caught were mostly healthy and in good

condition. Like most animals they had parasites like

lice in the fur and worms in the gut, but not in large

numbers likely to cause sickness or death. But

these parasites can carry diseases which can spread

from rats to native mammals or people. More

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studies of disease in small mammals in northern Australia

are needed to look at these questions further.

We are concerned that we caught town rats in near-natural

bush at Black Jungle Springs, and there are more records

of rats in other places in Kakadu. They are obviously

spreading. Even if they don’t carry diseases they can kill

native animals and their eggs or drive them out of these

places. We need to control town rats around communities

and towns and make sure they don’t travel to new areas in

our cars, trucks, boats or other gear.

Thanks to all the rangers who helped us on this project, and to the TOs for permission to trap on their country.

Special thanks to Anne O’Dea at HQ for help organising our

trapping trips, and to Mick at Mary River Roadhouse and

the managers of the tip, Golf Club, ERA Mess, Sports Club,

Gagadju Hotel in Jabiru for letting us trap around their

properties. Thanks also to the people of Kabulwarnamyo,

and everyone who trapped for rats around their houses in

Darwin.

Dr Bethany Jackson (our Vet) releasing a black-

footed tree-rat (a native mammal in trouble) caught

during trapping for town rats.

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Middle

of gut(lumen)

Lining ofgut

Mu

scle

aro

un

d g

ut

1 mm

Slices through a worm

Photo taken with a microscope of a very thin section of the

food pipe of a town rat. The 4 round shapes are slices

through a parasite worm living in the lining of the food pipe.

A near-natural place where town rats should not be,

but now live.