Research: Intestinal Worms Not A Bad Thing — For Tadpoles, Anyway

November 8, 2004

GAINESVILLE, Fla. — Repulsive-looking and seemingly harmful intestinal worms may actually benefit their hosts — at least among tadpoles.

Zoologists at the University of Florida have found that tadpoles infected with a common variety of worms, known as roundworms or nematodes, grow and mature into frogs faster than uninfected tadpoles.

The findings suggest the conventional wisdom that intestinal worms are always detrimental may need reexamination — not only for tadpoles but for other plant-eating animals, including some amphibians, reptiles, and even farm animals, such as cows and sheep. The findings — the first time ever found in animals — come on the heels of other research that appears to show some intestinal worms may help prevent certain diseases in people.

“Typically when people find a nematode in the gut, they immediately think parasite,” said Karen Bjorndal, a professor of zoology at UF. “I think this is going to make a lot of scientists go back to their notes and say, ‘Wait a minute, we may have missed something.’”

A paper about the discovery co-authored by Bjorndal is scheduled to appear early next year in the Journal of Experimental Zoology. The lead author is Greg Pryor, who researched the subject while a doctoral student at UF and is now an assistant professor of biology at Francis Marion University in Florence, S.C.

Frogs and other amphibians are experiencing decline worldwide, with one recent study estimating one-third of amphibians are in jeopardy. Pryor plans to do follow-up research to determine whether declines in nematodes could be a contributing factor. “It’s possible that there are chemicals in the environment or in the water that kill nematodes,” he said.

Wild tadpoles’ lower intestines almost always are infected with the naturally occurring worms, which they acquire early in their lives by ingesting the worms’ eggs, Pryor said. Different varieties of the worms, which look like a squirming grain of rice, infect the digestive systems of many different animals. They’re also closely related to pinworms, which infect humans, causing discomfort but no serious disease.

In a nearly yearlong experiment at UF, Pryor and Bjorndal raised 80 tadpoles, feeding half of the juvenile amphibians plant matter infected with nematode eggs and the others sterilized food.

Infected tadpoles in the experiment, later replicated with similar results, reached the mature frog stage in an average of 191 days, compared with an average of 207 days among the uninfected tadpoles – and both groups were the same size at maturity. That two-week difference is key because tadpoles that become frogs sooner, and get out of the water faster, are more likely to survive and reproduce earlier, Pryor said.

“The longer a tadpole stays in the pond, the more dangers it faces from predators, or from the pond drying or from chemical changes to the water,” Pryor said.

The scientists found the worms appeared to enhance a step in a tadpole’s digestive process known as fermentation. Although the scientists aren’t sure why, the worm-infected tadpoles produced more helpful byproducts during fermentation, essentially wringing more energy from their food. Infected tadpoles got 20 percent of their total daily energy requirement from the fermentation process, while uninfected tadpoles got only 9 percent, they found.

“We think that maybe the nematodes are eating bacteria (already in the gut) and adjusting the fermentation process indirectly,” Pryor said. “Another idea is we found the nematodes are covered in bacteria, and maybe a strain of the bacteria is doing the fermentation.”

Richard Wassersug, a professor of anatomy and neurobiology and a tadpole researcher at Dalhousie University in Halifax, Nova Scotia, called the UF research “fabulously interesting,” saying it appears to clear up a long-standing mystery about how tadpoles — which have long intestines but no stomachs — derive adequate nutrition from their diet of plant matter.

“Often what comes out the back end looks like what went in the front end, so how do tadpoles get anything out of it (food)?” Wassersug asked. “It looks like they depend on worms in their gut.”

Nematodes are known to infect numerous plant-eating fish, amphibians and reptiles, including some turtles and lizards, Bjorndal said. Their frequent manifestation as large, writhing clots in the innards may seem disgusting but is not necessarily damaging, Pryor said.

“You can fill a petri dish absolutely brim full of nematodes from an iguana gut,” Bjorndal said. “We’ve found the same thing in some freshwater turtles — that sometimes the gut is just full of them, yet the turtles and iguanas seem to be completely healthy.”

Biologists have long known that some bacteria residing in animal and human stomachs or intestines are helpful to the digestive process. A University of Iowa researcher argued recently that worms also help prevent diseases, such as inflammatory bowel disease, in people, an idea reportedly borne out in a clinical trial this year. But the UF findings are the first to note a benefit for worms in an animal and suggest that nematodes in other herbivores may also have beneficial effects, Pryor said.

“We know that other types of worms are bad when they’re in the gut, like tapeworms,” Pryor said. “But perhaps we haven’t given nematodes a fair consideration.”