We've come to assume that parasites are the main cause of frog deformities. Research suggests otherwise.
What's happening with the deformed frogs?
The widespread idea that parasites are the natural cause of frog abnormalities has made the public complacent and has deflected research away from chemical contaminants, the more likely cause. Yet research now demonstrates that parasites don't trigger the majority of deformities, specifically shortened or missing legs. Nor can they cause misshapen jaws, missing or misplaced eyes, or spinal column defects. The suspect parasite, nicknamed "Rib" for Ribeiroia, is now known to cause only multiple or branched limbs in frogs.
In 1995, middle-school students found deformed frogs in a pond east of Henderson, Minn., and the Minnesota Pollution Control Agency began an investigation. By fall 1996, my MPCA colleague and I had received more than 170 reports of deformed frogs in more than 50 counties. Our pictures of grotesquely misshapen frogs went everywhere in the media. TV news showed eager students collecting frogs and expressing their concerns. Everyone became alarmed.
For me personally, examining the pathetically abnormal frogs -- most with shortened or missing legs, a few with extra or branched limbs, some missing one eye -- was frankly horrifying. What went wrong? We worried: Might something harm other species, including humans? We all wanted the answer, and soon.
Scientists had four ideas about causes of deformities: ultraviolet radiation, chemical pollutants, parasite infections and trauma from predation. Early on, a few biologists ardently touted the parasite hypothesis, even before anyone had demonstrated directly that the Rib parasite caused extra legs to grow in developing tadpoles in lab exposures. A recent study suggesting that fertilizers could increase snail populations (a host for parasites) and thus trigger more frog deformities received a huge splash in the media. Strangely, a significant study based at Yale University did not.
The Yale study, led by David Skelly, clearly rules out parasites as a cause of leg deformities in frogs in Vermont. First, Skelly's team found no Rib parasites in a large number of snails and frogs from dozens of ponds, many known to have deformed frogs. Second, the majority of the deformities in Vermont frogs, as in Minnesota, were shortened or missing legs, not branched or extra limbs.
Skelly has ruled out the Rib parasite, not just in Vermont but for most of the United States, because of Rib's patchy distribution and because reduced limbs comprise the majority of deformities nationally. Ultraviolet radiation is ruled out because the deformities it caused in lab tests were not like ones seen in natural populations. Predation is ruled out because most studies find very few limbs bitten off by predators.
What about in Minnesota? The suspect Rib parasite has been found here, but its distribution is erratic. Lots of Rib were found in frogs in three of the ponds we called hot spots for their high percentages of deformed frogs. But Rib was missing, or was present only in low levels, in several other frog hot spots. In Minnesota, the predominant deformity is shortened or missing legs, not branched or multiple limbs, another indication that Rib is not widely involved here.
This leaves us with chemical pollutants as the likely cause of deformities, suggests David Skelly, who found that frogs in ponds near agricultural land were more than twice as likely to have deformities than frogs away from farmland. Researchers in Canada also have reported finding more deformed frogs in ponds surrounded by crops.
Skelly advocates new field-based research on chemical toxicants in ponds with deformed and normal frogs, coupled with lab studies on suspect agents. Some factor not yet measured, or perhaps a mixture of chemicals, may be involved.
The MPCA ceased investigating deformed frogs in 2001, and no one that I know of is looking at frogs for deformities, let alone analyzing pollutants in frog habitats. So how are the frogs? Does anyone know?
Judy Helgen is a retired MPCA research scientist who conducted work on deformed frogs and wetlands.
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