Yes, and they can kill you. But there’s more to the story…
I was walking through my garden in suburban Brisbane recently, my dog loyally trotting by my side, when I noticed an arresting sight. A big, white slug with a bright red triangular pattern on its back was inching its way up a palm tree. I paused to study it and thought that its geometric marking looked like a warning sign. Perhaps it was poisonous? I steered my curious pooch away from it, just in case.
But subsequent research revealed it was a harmless red triangle slug, Australia’s largest native land slug. They can reach sizes of up to 14 centimetres in length, and I reckon this one could have been a record breaker.
Queensland Museum honorary research fellow, Dr John Stanisic, recently told ABC News that finding native slugs, such as the Triboniophorus graeffei (scientific name for the red triangle slug), was a sign of a healthy ecosystem.
With more introduced slugs in Australia than native ones, it felt like a treat to have spotted this beauty in my own backyard.
Slugs are essentially snails without the shell and are more broadly referred to as molluscs. And while often harmless inhabitants of our backyards, they can play host to a deadly parasite.
The threat of Angiostrongylus cantonensis (rat lungworm) in Australia
Ever heard of Angiostrongylus cantonensis, commonly referred to as rat lungworm? Didn’t think so. I hadn’t either until a friend’s dog was thought to be infected by this parasitic roundworm and suffered severe, ongoing neurological symptoms as a result. Found in Southeast Asia, the Pacific Islands and Australia, the rat lungworm can pose significant health risks to both humans and animals.
The Angiostrongylus cantonensis parasite primarily infects rats, hence its name “rat lungworm”. In its lifecycle, the adult worms reside in the pulmonary arteries of rats, where they reproduce and release larvae. These larvae are then excreted in the rat’s faeces, contaminating the environment. Molluscs such as snails and slugs ingest the larvae, which develop further within these intermediate hosts. Humans and other animals can become accidental hosts by consuming infected molluscs or contaminated produce.
Health implications of rat lungworm in Australia
In humans, rat lungworm usually only causes mild illness which resolves itself. But in rare cases, it can cause eosinophilic meningitis, a severe condition characterised by the inflammation of the brain and its surrounding membranes. Symptoms can range from mild headaches and neck stiffness to severe neurological impairments such as paralysis, coma, and even death. The onset of symptoms typically occurs within one to three weeks after ingesting the larvae, but the disease can progress rapidly.
Cases of rat lungworm in humans in Australia
According to the authors of an article published in the Paediatric Infectious Disease Journal:
“From 1971-2018, 28 Australian cases have been reported, with acquisition restricted to Southeast Queensland and New South Wales. Ages ranged from 10 months to 45 years, most were male, and most likely acquired infection from consuming unwashed lettuce or vegetables. The mortality rate was 18%; most fatalities occurred in children <1-year-old. Long-term neurological deficit was reported in 14% of cases and a full recovery in 57% of cases.”
A real-life case study of human rat lungworm infection in Australia
A report in the Medical Journal of Australia, co-authored by Dr John Walker from the Department of Medicine at Sydney University, examines the case of a young man from Sydney diagnosed with human eosinophilic meningitis:
“Repeated questioning revealed that the patient had ingested, 5 weeks earlier, for a dare, two slugs from a garden in a Sydney suburb,” Dr Walker said.
Treatment included repeated cerebrospinal fluid drainage and regular administration of corticosteroids. The patient was finally discharged from hospital 17 days after diagnosis. He continued taking medication for four weeks after discharge.
“After 5 months, he successfully returned to full-time studies and competitive sport,” Dr Walker said.
Angiostrongylus cantonensis is the most common infectious cause of eosinophilic meningitis worldwide and is endemic in South-East Asia and the Pacific Basin.
The first human infection with the A. cantonensis parasite reported in Australia was from Brisbane in 1971. Since then, a fatal case occurred in a child who ingested molluscs in a suburban Brisbane garden.
Humans become accidental hosts when they ingest the laval stage in raw or undercooked molluscs or crustaceans or in fresh vegetables contaminated by infected molluscs,” Dr Walker said.
“In our patient, the acute febrile gastrointestinal illness six days after consuming the slugs may have been caused by invasion of the parasite through the intestinal wall.
“Initial entry into the meninges, and subsequent migration through brain parenchyma, caused the clinical picture of meningitis followed by encephalitis.
“Angiostrongylus meningitis is usually mild and resolves spontaneously over six weeks,” Dr Walker said.
Keeping things in perspective: the chances of rat lungworm infection
While the presence of rat lungworm in Australia is something to be aware of, the overall chances of becoming infected are relatively low. The small number of reported cases over decades in a country with a population exceeding 26 million indicates that the infection rate remains quite small. It’s important to remember that not everyone infected by the parasite goes on to develop eosinophilic meningitis. Most will experience only a mild illness or no symptoms at all. It’s also good to know that people infected with rat lungworm cannot pass the infection on to others.
How to avoid rat lungworm infection in Australia
Here are four key strategies to minimise the risk of infection:
Avoid eating raw or undercooked snails and slugs: These molluscs are the primary intermediate hosts for the parasite. Avoid consuming them raw or undercooked, as this can lead to ingestion of the larvae. (Personally, I would never eat one, even if well cooked!).
Wash fresh produce thoroughly: Fresh vegetables, particularly leafy greens, can be contaminated with larvae from infected snails and slugs. Washing produce thoroughly under running water and using a produce brush can help remove potential contaminants.
Practice good food hygiene: In addition to washing produce, ensure that any utensils and surfaces that come into contact with raw snails or slugs are cleaned and disinfected properly.
Practice good gardening hygiene: Do not touch snails or slugs, supervise children and always wash your hands after gardening or playing outdoors.
Where are dangerous slugs and snails located in Australia?
Currently, the parasite that uses snails and slugs as its host seems to be limited to the states of Queensland and New South Wales, but experts believe it could travel further afield. If you suspect you have come into contact with the parasite, seek medical attention promptly. Early diagnosis and treatment can mitigate the severity of the infection. Additionally, reporting suspected cases to public health authorities can help track and manage the spread of the parasite.
Final word: amazing slugs and snails of Australia
The last thing I want to do is demonise the average garden snail or slug. Australia has some incredibly weird and wonderful creatures and our home-grown molluscs are among them. If you want to read more about our unique and colourful native slugs, check out this Australian Geographic article: Eight bizarre Australian slugs.