DEFORMED FROG MYSTERY
By Sean Henahan, Access Excellence
Oneonta, NY (Nov. 21, 1996)
Reports of an unusually high number of deformed frogs
from all parts of the country first led to suspicions that the
mutations were the result of pollution, pesticides, or
UV-induced genetic changes. However, it is more than likely that
these changes can be attributed to parasitic worm infection,
according to experts.
Caption: Frog with extra Leg. (c)J. Helgen and the Minnesota
Pollution Control Agency
Stan Sessions, professor of biology at Hartwick College,
did an analysis of similar deformities discovered in populations
of frogs and salamanders from northern California [Sessions and
Ruth, 1990: "Explanation for naturally occurring supernumerary
limbs in amphibians," The Journal of Experimental Zoology 254:
38-47]. The study showed that the limb deformities in these
amphibians were induced by cysts of a parasitic flatworm called
a digenetic trematode.
Sessions spoke about his research on limb deformities in
natural populations of amphibians at the recent "Central North
American Amphibian Deformities" conference hosted by the U.S.
Environmental Protection Agency. Deformed frogs have now been
found at more than 100 sites in 54 of Minnesota's 87 counties,
and they have also turned up in Wisconsin, Quebec, and Delaware.
The EPA is concerned that the widespread nature of this
phenomenon suggests a major environmental problem that could
have negative impacts on amphibians, with potential implications
for human health.
Trematodes have a complex life cycle that involves several
hosts, in this case including snakes, pond snails, and
amphibians. The trematodes increase dramatically in number while
in the snail hosts by a process called "polyembrionic
amplification." During a second larval stage, free-swimming
flatworm larvae are released from the snails and attack frog
tadpoles in which they form cysts called metacercariae. Once
they find an amphibian host, the cercariae burrow into tissues
around the cloaca and form metacercarial cysts.
The developing hind legs of frog tadpoles are located
right next to the cloaca and are almost always invaded by the
cysts. Sessions hypothesized that the cysts caused massive
mechanical damage to the developing limb, causing it to
reorganize into numerous smaller limb buds and resulting in the
outgrowth of up to 10 extra hind limbs. He confirmed this idea
by implanting tiny plastic beads into the developing limb buds
of captive tadpoles; this caused extra limbs to develop. These
results showed that the mechanical effects of the parasite cysts
are sufficient to cause the limb deformities in frogs, although
they do not rule out other factors.
These results strongly suggest that parasites account for
many of the recent reports of deformed frogs from the Midwest
and elsewhere. But why the sudden increase? One possibility,
suggested by Sessions at the conference, is that there are
simply more people looking for deformed frogs and this happened
to be a good year for trematodes (or especially pond snails). On
the other hand, it could indicate a new and more serious
Other possible causes or contributing factors suggested at
the Minnesota conference
include various kinds of environmental pollution, pesticides,
and UV-induced genetic mutation. Some commonly used pesticides,
for example, may contain residues that can potentially interfere
with basic biochemical pathways in developing cells. One outcome
of the conference was the recognition of the need for a
realistic program of scientific research that is broad enough to
take into account both the role of parasites and the possibility
of chemical pollutants or other factors.
Sessions' future research plans include an analysis of the
kinds of environmental factors, such as fertilizer run-off in
agricultural areas, that can encourage large increases in pond
snail populations, which in turn would generate enormous
population explosions of trematode cercariae through the process
of polyembryonic amplification.
"Basic natural history research has been neglected in recent
years, but it is this kind of information that is required in
order to make realistic assessments of possible environmental
threats to humans," he notes.
Related information on the
Deformed Frogs: Search
for the Causes
Frogs, UV Light