Red-worm-of-trout fish parasite initially discovered in NSW now found in Queensland, Victoria

The invasive parasite species, which resembles a worm, was found burrowed in the flesh of native freshwater fish in New South Wales. (Supplied: Shokoofeh Shamsi)

A little-known freshwater fish parasite which can infect humans could be far more widespread than previously thought.

The parasite Eustrongylides excisus, commonly known as red-worm-of-trout, can eat through the walls of intestines in fish and anything that eats the fish including, in very rare cases, humans.

Red-worm-of-trout can infect humans and other mammals when they eat uncooked or undercooked fish caught in fresh water and can be lethal to some animals.

Charles Sturt University veterinary parasitologist Shokoofeh Shamsi previously found the parasite in fish in New South Wales but said it has now been found in southern Queensland and Victoria.

She wants to conduct a full study to find out just how widespread it is.

"My burning question is 'what is the distribution map of these parasites?'" she said.

"Are they [in] continuous distribution from Queensland to Victoria, or are there areas for whatever reason where these parasites are more abundant and other areas where fish are healthy and safe?

"I think that's something people want to know."

Professor Shamsi discovered and documented red-worm-of-trout in Australia in 2023 but said not much was known about how widespread it is or its impact in Australia.

"The reason [we are noticing the parasite now] could be changes in the way we manage our river system. [Or] Is it because we have more population of the parasite or more awareness of the parasite?" she said.

"We don't know."

The parasite's life cycle goes through three animals.

It infects an invertebrate in the water, which is then eaten by a fish which becomes infected.

Finally, a bird eats the infected fish and becomes infected.

The parasite then burrows into the bird's intestines and when it defecates the parasite spreads its larvae to more invertebrates.

While it is rare for the parasite to infect humans, Professor Shamsi said pets fed raw, infected fish could be affected.

"Obviously this can be very painful and cause serious issues, but a lot of the time when humans feel pain [they] go to the doctor so it doesn't kill them. But if we give raw fish to our pets it can kill them," she said.

Human infection is very rare but there have been at least six confirmed cases.

Three US fishermen in Maryland were infected in 1982 after they ate live minnows.

They later reported to hospital with abdominal pain where the worms were discovered.

A 17-year-old boy was also infected in New Jersey when he ate raw minnows.

In 1989, a 24-year-old man in the USA reported abdominal pain after eating sushi prepared at a friend's home in New York. The friend had bought unfrozen fish of an unspecified species at a local market.

Medical staff extracted a 4.2-centimetre-long worm from his intestines.

In the most recent case, in 2013, two women in South Sudan each had a worm burrowing their way out of their skin, with the worms measuring 7 centimetres and 8.5 centimetres.

This is the only case of worms emerging from human skin.

Brandon Ruehle, an assistant professor of biology with Peru State College in Nebraska, said as long as people practised good food safety the parasite shouldn't infect them but that even if it did, it likely wouldn't live long enough to do much harm.

"It's actually very rare for a parasite like this nematode to get into a host, that's not the right host, and then wiggle around and do damage, normally our stomach acids will take care of that," he said.

However, Professor Ruehle said while he thought it was unlikely humans would be affected, it was important to investigate the spread and impact of red-worm-of-trout.

"At this stage anything that we say along the lines of 'oh it's not a big issue' is based almost entirely on the fact that we don't know enough. It might be a huge problem, it might not be. You need to look and find out," he said.

However, Professor Shamsi said the public should not fear eating seafood.

"It's really important not to be scared of eating seafood, I for one can't have a week without eating seafood, but like a lot of types of food. We just can't let our guard down," she said.