Nutrient pollution drives frog deformities by ramping up infections

High levels of nutrients used in farming and ranching activities fuel parasite infections that have caused highly publicized frog deformities in ponds and lakes across North America, according to a new study led by the University of Colorado at Boulder.

The study showed increased levels of nitrogen and phosphorus cause sharp hikes in the abundance and reproduction of a snail species that hosts microscopic parasites known as trematodes, said Assistant Professor Pieter Johnson of CU-Boulder's ecology and evolutionary biology department. The nutrients stimulate algae growth, increasing snail populations and the number of infectious parasites released by snails into ponds and lakes. The parasites subsequently form cysts in the developing limbs of tadpoles causing missing limbs, extra limbs and other severe malformations, Johnson said.

"This is the first study to show that nutrient enrichment drives the abundance of these parasites, increasing levels of amphibian infection and subsequent malformations," said Johnson. "The research has implications for both worldwide amphibian declines and for a wide array of diseases potentially linked to nutrient pollution, including cholera, malaria, West Nile virus and diseases affecting coral reefs."

Johnson is the lead author of a study on the subject published online the week of Sept. 24 in the Proceedings of the National Academy of Sciences. Co-authors include Jonathan Chase from Washington University, Katherine Dosch, Richard Hartson, Daniel Sutherland and Stephen Carpenter from the University of Wisconsin, Jackson Gross from the Southern California Coastal Water Research Project and Don Larson from University of Alaska. The National Science Foundation funded the work.

Deformed frogs first gained international attention in the mid-1990s when a group of Minnesota schoolchildren discovered a pond where more than half of the leopard frogs had missing or extra limbs, he said. Since then, reports of deformed amphibians have become widespread in the United States, leading to speculation they were being caused by factors like pesticides, increased ultraviolet radiation or parasitic infection.

While parasite infection is now recognized as a major cause of such deformities, the environmental factors responsible for increases in parasite abundance had largely remained a mystery until the study was undertaken, Johnson said.

"One of our main goals was to understand how parasites are going to respond to land-use changes and ecosystem alterations," he said. "What we found is that nitrogen and phosphorus pollution from agriculture, cattle grazing and domestic runoff have the potential to significantly promote parasitic infection and deformities in frogs."

The trematode has a complex life cycle that involves three host species, he said. In addition to the infectious stage in snails and the cyst stage in frogs, the parasites rely on predators including wading birds to complete their life cycle by consuming infected frogs and spreading the parasite back into the ecosystem through defecation.

The research team built 36 artificial ponds in central Wisconsin similar to farm stock tanks -- a common breeding site of frogs and salamanders -- and stocked each with selected numbers of snails and tadpoles of the green frog. In addition to adding nutrients, the researchers took on the role of birds in the trematode life cycle by adding parasite eggs to the tanks, then measuring the subsequent ecological responses.

In ponds with added nutrients, snail biomass increased by 50 percent and the snails increased parasite egg production by up to eight-fold, he said. The infection rate in frogs rose by two- to five-times in those tanks, Johnson said.

As few as 12 trematode larvae, known as cercariae, can kill or deform a single tadpole by burrowing into their limb regions and disrupting normal leg development, he said. A single infected snail can produce more than 1,000 cercariae in one night. Frogs that become deformed rarely survive long in the wild, he said.

"We were able to watch nutrient pollution move through the life cycle of the parasite as it cascaded through the food web," he said. "Since most human diseases involve multiple hosts, understanding how increased nutrient pollution affects freshwater and marine food webs to influence disease is an emerging frontier in ecological research."

A recent study of more than 6,000 species of amphibians worldwide concluded that 32 percent were threatened and 43 percent were declining in population. While the causes range from habitat loss to emerging disease, the researchers are now exploring how nutrient pollution and limb malformations contribute to the pattern, Johnson said.

Source: University of Colorado at Boulder

Related stories:

Best way to treat malaria: Avoid using same drug for everyone, scientists say
(PhysOrg.com) -- A team of scientists employing a sophisticated computer model pioneered at Princeton University and Resources for the Future has found that many governments worldwide are recommending the wrong kind of malaria treatment.
Leishmaniasis parasites evade death by exploiting the immune response to sand fly bites
Cutaneous leishmaniasis, a disease characterized by painful skin ulcers, occurs when the parasite Leishmania major, or a related species, is transmitted to a mammalian host by the bite of an infected sand fly. In a new study from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, scientists have discovered L. major does its damage by not only evading but also by exploiting the body's wound-healing response to sand fly bites, as reported in the August 15 issue of Science.
White Blood Cell Uses DNA 'Catapult' to Fight Infection
(PhysOrg.com) -- U.S. and Swiss scientists have made a breakthrough in understanding how a type of white blood cell called the eosinophil may help the body to fight bacterial infections in the digestive tract, according to research published online this week in Nature Medicine.
Research exposes new target for malaria drugs
The malaria parasite has waged a successful guerrilla war against the human immune system for eons, but a study in this week's Journal of Biological Chemistry has exposed one of the tricks malaria uses to hide from the immune proteins, which may aid in future drug development.
Plant Parasite 'Wiretaps' Host
A parasitic plant that sucks water and nutrients from its plant host also taps into its communications traffic, a new report finds. The research could lead to new ways to combat parasites that attack crop plants.
A bee's future as queen or worker may rest with parasitic fly
(PhysOrg.com) -- Strange things are happening in the lowland tropical forests of Panama and Costa Rica. A tiny parasitic fly is affecting the social behavior of a nocturnal bee, helping to determine which individuals become queens and which become workers.
Study shows parasites outweigh predators
In a study of free-living and parasitic species in three estuaries on the Pacific coast of California and Baja California, a team of researchers from the University of California, Santa Barbara, the United States Geological Survey, and Princeton University has determined that parasite biomass in those habitats exceeds that of top predators, in some cases by a factor of 20. Their findings, which could have significant biomedical and ecological implications, appear in the July 24 issue of the science journal Nature.
New indicator uncovered that can predict coral health
A new indicator of coral health has been discovered in a community of microscopic single-celled algae called dinoflagellates. The study, released in the July 8th edition of the journal Proceedings of the National Academy of Sciences, reveals that a particular type of these algae renders corals more susceptible to disease.

News discussion:

General Science news