Dangerous Diseases: Rat Parasite Colonizing Florida Is Alarming Researchers

Updated | Microscopic parasites lurk in all manner of places. Pond sediment, mosquitoes, pets, unwashed fruit—all of these places can be safe harbors for these potentially dangerous organisms, which take many forms. Rat lungworm, also known as Angiostrongylus cantonensis, is a parasitic nematode—tiny organisms that look like worms but are not actually worms—that can wreak havoc on the human nervous system. Specifically, it can cause meningitis, a swelling of the protective membranes surrounding the brain and spinal cord. According to a recent study, this parasite is currently spreading across Florida.

Researchers at the University of Florida collected wild rats (scientific name Rattus rattus), rat fecal samples and snails from 18 counties throughout the state. Of the 171 rats collected, 39 tested positive for the nematode. It was also found in six of the 37 fecal samples. Among the 1,437 snails collected, 27—from different collection sites—were harboring the parasite. The finding is surprising because rat lungworm is typically found in more tropical regions, such as Hawaii.

The Florida counties where the parasite was found include Leon, Alachua, Saint Johns, Orange and Hillsborough. All of these are in the north or central region of the state.

"This study indicates that A. cantonensis is established in Florida," the authors write in their study, published in PLOS ONE in May. "The ability for this historically subtropical nematode to thrive in a more temperate climate is alarming." The authors note that climate change and rising average temperatures will expand the habitable range for snails, leading to the continued spread of this parasite in temperate areas.

"The parasite is here in Florida and is something that needs to be taken seriously," Heather Stockdale Warden, an infectious disease researcher at the University of Florida and lead author of the study, said in a statement. "The reality is that it is probably in more counties than we found it in, and it is also probably more prevalent in the southeastern U.S. than we think."

Human infections with rat lungworm are rare. Such infections occur when people eat snails containing the nematode larvae. Sometimes, A. cantonensis infects freshwater shrimp, frogs and flatworms. The nematode cannot complete its lifecycle in these hosts, but it can live there for a time. Such colonizations can also lead to human infections, though these are even rarer.

The parasite was first identified in 1935 in rats from China. As the name implies, rat lungworm lives in rats during part of their lifecycle. But although these rodents are the predominant host for A. cantonensis, it also spends a portion of its life in snails or slugs (rats eating snails allows the parasite to get around). If humans eat a snail hosting the parasite, it can mature inside the brain, instead of in a rat. The difference is that the brain-dwelling parasite will die there, causing eosinophilic meningitis, whereas a rat-dwelling parasite will return to the animal's bloodstream.

Larval worms are released into the world via rat feces. Several species of rats are involved in this job. Snails or slugs may eat the feces or the parasite can seep into the body walls of these gastropods. Once inside, the larvae mature and eventually enter the guts of rats that eat the snails or slugs. The parasites move through the walls of the rat intestine into its bloodstream, and some will reach the rat brain, where they attain their nearly final form. This "sub-adult" nematode then returns to the rat circulatory system, wending its way to the right ventricle and pulmonary arteries, where it completes its growth. Male and female nematodes mate and the female lays her eggs in the rat bloodstream. The eggs are carried to the lungs, where they hatch. The tiny larvae then burst out of the lungs and enter the trachea. The rat swallows them and soon they are expelled in rat feces, where they await a hungry snail.

After the parasite was found in China in the 1930, other reports noted its presence in Fiji, Tahiti, Guam and Hawaii, among other places. The parasite has also been found in Egypt, India, South Africa, Madagascar, Cuba, Brazil, Haiti and the southeastern United States. Currently, the parasite is prevalent in southeast Asia and tropical Pacific islands, according to the Centers for Disease Control and Prevention.

According to researchers, the parasite first arrived in the continental U.S. in the 1980s. Researchers believe it was carried here by rats arriving on ships bound for New Orleans, Louisiana. In 1995, a Louisiana child who ate a snail became infected. More recently, a mysterious fever in two children in Texas was traced back to A. cantonensis. They hadn't eaten snails, but one of the children was known to snack on lettuce leaves. The parasite has a much longer history in Hawaii, where it has existed for more than 50 years. This report marks the first time researchers have found the parasite outside of a subtropical region in the U.S.

The three gastropod species that tested positive for the parasite in the Florida study had never been documented as hosts before. Two of them have ranges far beyond Florida; one is found throughout temperate North America. The parasite has already been found in cotton rat in Oklahoma and was the cause of at least one bird death in California.

In China and southeast Asia, eating raw or undercooked snails or slugs is a known source of human infections by A. cantonensis. Twenty-three people in Jamaica have been infected since 2000, and food contaminated with snails was involved in about 15 of them. Two of these cases resulted in death and four resulted in lasting neurological damage.

To date, no human infections in Florida have been reported. But the researchers also note that some infections may be misdiagnosed because the larvae are hard to detect.

This story has been updated with additional information about the history of A. cantonensis in the United States.