Deformed Frogs: What Are the Causes?

Reports of worldwide declining amphibian populations have concerned scientists, because amphibians are considered indicator species. As such, amphibians are particularly sensitive to environmental changes, which can suggest future problems for humans.

In addition to amphibian declines, there has been a recent increase in reports of deformed frogs, particularly those with multiple or missing hindlimbs. Frog deformities have been attributed to DNA damage from too much UV exposure, as well as chemical contamination. But it is now believed that most frog deformities are caused by infestation of a common parasite.

Frogs are an important part of the lifecycle of this particular parasite. This flatworm resides within snails, and periodically sheds offspring into the water. These larval parasites (now called cercaria) find their way to tadpoles, and usually burrow in around where the hindlimbs will form. After burrowing into the tadpoles the parasites form cysts around themselves, which disrupt the tadpole's developing limb causing it to split into several limbs as it goes through metamorphosis. (Incidentally, this is the same parasite that burrows into human skin causing swimmers itch in people swimming in freshwater lakes and ponds.)

Developing frogs with multiple limbs are particularly vulnerable to predation. While this is detrimental for the frogs, it is ll part of the parasite's tidy strategy. Animals such as raccoons and aquatic birds (known as hosts) will more easily capture deformed frogs than healthy ones.

When the predator eats a deformed (infected) frog, they themselves become infected and will shed the parasite in their feces. The host will generally defecate in the water, releasing parasites back into the pond where they can infect snails, then tadpoles again, creating more deformed frogs. Thus the cycle is complete.

It is suspected that environmental conditions can exacerbate the extent to which parasites can thrive. Increased contamination from pesticides has been alleged to make the tadpoles more vulnerable to infestation. Rising nitrogen contents from cattle grazing in farm ponds can make populations of snails thrive which increases the numbers of parasites present.

Multiple limbs are not the only type of deformity observed in frogs. Limbs can be bent, small, or missing. Many times there are explanations for these other types of frog deformities. Most, but not all, times when amphibians are missing part of all of their limbs, it is due to an unsuccessful predator attack. Fish and carnivorous insect larvae, like those of the dragonfly, will often miss when they attack a tadpole, snapping off only the tip of the leg. When the injury heals, it often appears as if the limb formed that way unless the limb is x-rayed.

It is unclear as to why there seems to be increase in the number of reported deformed frogs. Some believe our environment is becoming increasingly polluted making the frogs weaker and more vulnerable to deformities. Others would state that the number of deformed frogs is not increasing, but only that the reports of incidents are increasing due to rising public awareness.