Experts are warning that cases of a rat-borne disease that can cause lethal infections in humans are growing in dogs in eastern Australia.

Rat lungworm disease, or angiostrongylus cantonensis, is caused by a parasitic worm naturally found in rats and spread by snails and slugs.

It can become a source of disease to humans and dogs.

The disease has caused two known deaths in Australia, including that of Sydney man Sam Ballard, who died after eating an infected slug for a dare in 2010.

University of Sydney veterinary researchers have uncovered a rise in cases in dogs in Sydney and Brisbane over five years.

The number of cases began increasing from 2019, “suggesting escalating human health threats”, and continued to 2024.

The researchers recorded a peak of 32 cases in 2022, likely caused by higher-than-usual rainfall due to La Niña.

“This is another example how the La Niña events with wetter than average periods in Australia lead to increased disease transmission and occurrence,” researcher Jan Šlapeta said.

In comparison, there were about 10 or fewer than 10 cases per year from 2010 to 2018.

The link between the disease and weather conditions has prompted concern that rat lungworm could emerge in humans.

“These snails and slugs, and the infective worm larvae in them, can accidentally be a disease source to us humans and our pet dogs,” Šlapeta said.

“Once in humans or dogs, the worms quickly get to the brain where they cause disease.”

People are typically at risk if they ingest an infected snail or slug, or slime left behind on lettuce and other raw produce.

Most people infected with the disease recover fully without treatment over days, weeks and months.

“In dogs and humans, the parasite enters the brain, but rather than progressing to the lungs like it does in rats, it is killed in the brain by our own immune response,” Phoebe Rivory, who submitted her PhD thesis to the University of Sydney School of Veterinary Science, said.

“It is that overt immune response that causes severe headaches and sensations.”

The disease can cause severe meningitis that requires specialist treatment.

In rare instances, it can be fatal. 

NSW Health regards rat lungworm disease as an “extremely rare infection”. 

It advises people not to consume raw snails or slugs, to wash all fresh produce before eating and to wash their hands after gardening or handling snails or slugs.