Toxoplasma gondii is a protozoan parasite of the domestic cat. According to Professor Alan Johnson of the Molecular Parasitology Research Group, reproduction of the parasite occurs in the cat's gut, and the infective stage of the parasite, the oocyst, is passed in the cat's faeces.

"Humans can contract the parasite in either of two ways," said Professor Johnson, "Directly, via oral ingestion of oocysts when digging in the garden or cleaning out cats' litter trays; or indirectly, via infected sheep meat or pork.

"If sheep or pigs ingest the oocyst, the parasite can form tissue cysts in the skeletal muscles. Humans can then become infected by eating this undercooked meat."

Swelling of the lymph nodes is the most common clinical manifestation of toxoplasmosis in otherwise healthy people. Central nervous system impairment is the most characteristic feature of toxoplasmosis in immuno-deficient people, such as AIDS patients.

Toxoplasma gondii is very common in most human populations. About 30% of the adult Australian population show evidence of past exposure to the parasite, and it has been cultured from our local meat supply.

"If first contracted during pregnancy, T. gondii can cause stillbirth or foetal abnormalities," said Professor Johnson. "In Australia about 0.2-0.5% of women of child bearing age show evidence of acute toxoplasmosis.

"Three out of four congenitally infected babies asymptomatic at birth will develop problems such as mental retardation or hearing defects later in life, and as many as 90% will suffer eye problems as they grow older," he said.

As well as the loss and reduction in quality of human life, toxoplasmosis causes great financial loss in the agricultural industry, being a major cause of abortion and stillbirth in sheep flocks in Britain, Australia and New Zealand. Toxoplasmosis has also been identified as a cause of abortion in Angora goats in both Tasmania and New South Wales.

It is also very common in Australian wildlife, which means that many endangered native species are at risk, and that the consumption of under-cooked kangaroo meat may be another cause of infection in humans.

Members of the Molecular Parasitology Research Group are attempting to differentiate between two lineages of the parasite, to determine if they have different lifecycles and hosts.

"In addition, we are investigating the immune response of host animals to the different lineages of the parasite in order to learn more about potential vaccination strategies for toxoplasmosis," said Professor Johnson.

"Toxoplasmosis poses significant threats to our human and animal populations. Research into new treatments and controls will result in significant medical and economic benefits."