What is rat lungworm disease and why are cases rising in dogs in Australia?
Cases of rat lungworm disease have been increasing in dogs on Australia's east coast over the past five years, according to new research from the University of Sydney. A very rare but sometimes deadly condition caused by worm larvae travelling up the food chain, the disease has been found in a small but increasing number of dogs in areas affected by heavy rain, and occasionally also in humans. So why are cases growing and how worried should we be?
The rat lungworm, or Angiostrongylus cantonensis, is a parasite that originated in south-east Asia but has spread to other parts of the world, including North America, Europe and Australia. The parasite's natural host is feral rats, but it can accidentally make its way to other animals through a complicated transmission process involving snails and slugs as intermediate hosts.
When it finds its way into humans and dogs, rat lungworm can cause a disease called neural angiostrongyliasis. In humans, it's diagnosed as a kind of meningitis, or eosinophilic meningoencephalitis. Symptoms in humans include headaches, a stiff neck, tingling or pain in the skin, fever, nausea and vomiting. Symptoms in dogs include hindlimb and tail paralysis, urinary incontinence and abnormal pain responses. The time between eating the infected slug or snail and getting sick is usually between one and three weeks.
University of Sydney doctoral researcher Phoebe Rivory, lead author of the recent paper, explained that it's not the parasite itself that does the damage but the body's attempts to fight it. '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. It is that overt immune response that causes severe headaches and sensations,' Rivory said.
New South Wales Health says it is rare for humans to develop meningitis in response to rat lungworm, and most people who contract rat lungworm have no symptoms at all, while others may have only mild, short-lived symptoms.
Rats infected with adult worms excrete larvae in their faeces, which is then ingested by snails and slugs. Dogs and people are most likely to contract the parasite when they deliberately or accidentally eat a raw snail or slug carrying the larvae, or unwashed vegetables, such as lettuce, contaminated with infected slime. The parasite is not contagious from person to person.
In a well-known case, Australian man Sam Ballard contracted eosinophilic meningoencephalitis in 2010 after being dared by his friends to eat a slug. He fell into a coma for 420 days, and acquired a brain injury. Ballard died in 2018, aged 29. In another well-known case, an American woman was found to have rat lungworm disease after multiple attempts to seek medical help for strange symptoms after a holiday to Thailand, Japan and Hawaii.
The University of Sydney researchers have been examining the prevalence of rat lungworm disease in dogs on the basis that they are a sentinel population for humans – that is, increased rates of the disease in dogs can be an early warning sign of human disease risk.
Researchers analysed 180 clinical samples taken between 2020 and 2024 from dogs suspected to have had the disease, and confirmed 93 cases, all in or around Sydney and Brisbane, and a 'gradual upward trend'. They also discovered a spike of 32 cases in 2022, correlating with high levels of rainfall, which tends to supercharge snail and slug proliferation.
Confirmed cases of the disease in dogs are still very low, restricted to NSW and Queensland, and have fallen since the 2022 spike identified by the researchers, although not to 2019 levels. The researchers suggest that educating communities about how to avoid infection, and what symptoms dog owners should look out for in their pets, may be worthwhile.
Rat lungworm is not a notifiable disease in NSW due to its rarity. In both NSW and Queensland, preventative anti-parasitic treatments for people, especially children, known to have ingested slugs or snails can be accessed through the health system.
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