Lungworm jumps from rats to snails and human brains
The dreaded rat lungworm Angiostrongylus cantonensis – a parasite with a penchant for rats and snails that occasionally resides in human brains – has become firmly established in the southeastern United States and is likely to continue to spread. This is shown by a study published in the journal Emerging Infectious Diseases.
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The study conducted small-scale monitoring of dead rats at the Atlanta Zoo. Between 2019 and 2022, researchers repeatedly found evidence of the worm. In total, the study identified seven of 33 rats collected (21 percent) with signs of rat lungworm infection. The infected animals were spread throughout the study period, all in different months: one in 2019, three in 2021 and three in 2022, suggesting ongoing transmission.
“While the study is small, it suggests that the zoonotic parasite was introduced and established in a new area in the southeastern United States,” said the authors of the study, led by researchers at the University of Georgia College of Veterinary Medicine, in conclusion.
Given the devastating infection that rat lungworm can cause in humans, the result is worrying. The parasitic roundworms, as their name suggests, are typically found in rats. But they have a complicated life cycle that can be fatal if disrupted.
Sick cycle
Normally, the adult worms live in the arteries of the rat lung – hence the name rat lungworm. There they mate and lay eggs. The larvae of the worm then break out of the lungs, are coughed up by the rat, swallowed and finally excreted again. From there the larvae are picked up by snails or slugs. This can happen when the snails eat the rat feces or when the voracious larvae simply burrow into their soft bodies. The larvae then develop in the snails, which ideally are eventually eaten by rats. Back in the rat, the late-stage larvae penetrate the intestine, enter the bloodstream and migrate to the rat’s central nervous system and brain. There they mature into immature animals and then migrate to the lungs, where they become adults and mate, completing the cycle.
People become accidental hosts in various ways: for example, by eating undercooked snails or accidentally consuming an infected snail that is hiding in unwashed lettuce. Infected snails and slugs can also be first consumed by other animals such as frogs, shrimp, crabs or freshwater crayfish. If people then eat these animals before they have been fully cooked, they can become infected.
Possible routes of infection of the human central nervous system (CNS) by the rat lungworm Angiostrongylus cantonensis
(Image: University of Florida)
When a rat lungworm invades a human, it does what it normally does in rats: it travels to the central nervous system and brain. Sometimes the worms’ migration into the central nervous system is asymptomatic or causes only mild, temporary symptoms. But sometimes serious neurological disorders also occur. These can start with non-specific symptoms such as headaches, sensitivity to light and insomnia and progress to neck stiffness and pain, tingling or burning of the skin, double vision, bowel or bladder problems and seizures. In severe cases, it can cause nerve damage, paralysis, coma and even death.
It is often assumed that the worm is unable to complete its life cycle in humans and ends up wandering idly in the brain for one to two months before being killed by immune reactions. However, there is evidence that adult worms can reach human lungs.
Regardless, there is no specific treatment for lungworm infections. Antiparasitic agents have not been shown to be effective, and there is even evidence that they may worsen symptoms by increasing the immune response to the dying worms. Currently, supportive treatments, painkillers and steroids are usually the only options.
Uncontrolled spread
For all of the above reasons, prevention and control of rat lungworm is critical. The ongoing spread in the USA is therefore alarming. Although rat lungworm has previously appeared in the southeastern United States, cases have been sporadic and have not previously been observed in rats in Georgia. The parasite had previously been detected in captive non-human primates in Florida, Louisiana, Texas and Alabama, and in a red kangaroo in Mississippi. In 2018, a Centers for Disease Control and Prevention study reported six cases in people between 2011 and 2017 that could not be explained by travel.
However, it appears that this worm is spreading quietly, infecting other continents and regions in addition to the central nervous system. Rat lungworm was first described in 1935 in Canton (Guangzhou), China, and for decades thereafter was thought to be limited to affected areas of the Pacific Basin and Southeast Asia. However, with climate change and the human-induced spread of rats and other hosts, particularly giant snails, rat lungworm is rapidly spreading across the globe. It is now found in parts of Africa, the Caribbean and North America. Human cases have already been reported in 30 countries. (A relative of A. cantonensis, A. costaricensis, is also found in Latin America.)
In 2017, Hawaii reported a boom in rat lungworm infections in humans that was linked to the emergence of an invasive “semi-slug” that is particularly adept at absorbing the parasite. Hawaii counted 18 confirmed and three probable human cases that year, a dramatic increase from previous years. A decade earlier, in 2007, the state had recorded just two cases.
The newest frontier for rat lungworm is Europe. Until 2018, the parasite was not considered endemic to this region. But then the worms appeared in hedgehogs on the Mediterranean island of Mallorca. And earlier this year, researchers reported that they discovered it in the city of Valencia on mainland Spain.
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hit the alarm
“If the parasite has gained a foothold in Europe, it could spread further across the continent, possibly also into more temperate regions, as has already happened in Australia and the United States,” the Spanish researchers warned. “In addition, as the climate warms, even more northern parts of Europe may become accessible to A. cantonensis, as is the case in China.”
Given the bleak outlook, it is “imperative that doctors in Europe know more about this parasite and how to diagnose and treat the rare but potentially fatal disease it causes,” the researchers said.
Researchers in Atlanta are similarly sounding the alarm and calling for doctors in the southern United States to keep an eye on rat lungworm. They also call for more surveillance, genetic analysis and modeling, which is “critical to reducing the risk of infection for humans and other animals.”
This article originally appeared on Ars Technica.
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