PROJECT
Impact of Agricultural Pesticides on the Health of Amphibian Populations

National Program: Water Surveillance (Pesticide Science Fund)
Section Involved: River Ecosystems Research

Objective

The aim of this project is to determine if the pesticides used in agricultural areas of the St. Lawrence Valley, specifically in the Boucherville Islands and in a few tributary rivers, weaken amphibian immune systems, thus making them more vulnerable to agents of disease.

The Decline of the Leopard Frog, Rana pipiens

Amphibian populations in many different parts of the world are in decline, especially in western North America and Australia. It is a phenomenon from which Canadian species have therefore not escaped unscathed. For several decades, mass die-offs have led to the disappearance of entire populations and even to the extinction of some species. A number of these declines can clearly be attributed to the loss or degradation of wetlands, while others could result from the combined influence of many different factors, including climate change, increased ultraviolet radiation, introduction of exotic species, overharvesting (for human consumption), pollution, etc.

Recent studies have pointed to another culprit: infectious disease. The latest research suggests that pathogenic organisms like viruses, bacteria or fungi could play an important role in the decline of amphibians, especially in the case of mass die-offs in North America. Although several different hypotheses have been advanced, it is thought that the toxic substances being discharged to the environment might be weakening the immune systems of amphibians and eroding their capacity to ward off disease and keep parasites at bay.

Do Pesticides Affect the Capacity of Frogs to Withstand Disease?

This question is now being studied by a team of researchers at Environment Canada. The frog selected for this study, the Leopard Frog (Rana pipiens), is the amphibian model for the development and standardization of toxicity tests in Canada.

The Leopard Frog was once found throughout North America. Like many other members of the amphibian family, however, its numbers have dwindled recently, especially in the western portion of its range. Die-offs associated with epidemics of infectious disease, particularly red leg disease, have been documented.

Range of the Leopard Frog

The floodplains of the St. Lawrence River and its tributaries are important breeding grounds for the Leopard Frog. In areas of intensive agricultural activity, tadpoles are exposed to the pesticides carried into rivers by farm runoff and drains. Froglets and adults alike can also be directly exposed to these contaminants when they venture onto sprayed farm fields.

In the Lab

To evaluate the combined effect of agricultural pesticides and parasites, Leopard Frog tadpoles were raised in captivity until their metamorphosis into young frogs. They were then exposed to a mixture of the pesticides atrazine, metribuzin, aldicarb, dieldrin, endosulfan and lindane for 21 days. Some of these insecticides and herbicides are still used in agriculture, while others persist at trace levels in the aquatic environment even though their use has been banned for many years. One of the main constituents of the mixture, atrazine, is a herbicide that has been used in corn farming since the mid-1950s.

After exposure to the pesticides, the frogs were placed in infection chambers alongside infectious larval parasites of the nematode Rhabdias ranae. Certain aspects of the frogs’ immune responses were also assessed before and after the infection experiment.

Life cycle of Rhabdias ranae in the lungs

Life cycle of Rhabdias ranae (Rhabditidae). The cycle includes a parasitic stage inside the frog and a free-living stage in the soil from which infective larvae arise. When in contact with a frog, infective larvae penetrate the skin (1) and migrate to the lungs (2) where they establish and become hermaphroditic adults (3). they then produce eggs which pass up the trachea, enter the gut (4) and hatch into larvae in the large intestine before they are released in feces (5). In the soil, these larvae molt into male and female adults that then mate (6). Ovoparous larvae that escape from degenerating females are infective (7).

Main Findings

Our results show that pesticides have no effect on the efficiency with which parasites are able to penetrate frog skin. In frogs exposed to the highest concentrations of pesticides, however, Rhabdias ranae did become established in the lungs in much less time and in greater number, suggesting the accelerated migration and faster maturation of the parasite and thus its possible increased virulence. Additionally, certain components of the immune response are suppressed in animals subjected to pesticides at equivalent concentrations measured in a number of St. Lawrence tributaries in summer.

Frog health may be affected by an increase in the parasitic load. Studies have found that an increased number of nematodes in the lungs of toads may slow growth and make difficult those intense physical activities requiring a continual supply of oxygen to the muscles. A weakened respiratory capacity may be critical for young Leopard Frogs during their seasonal migration toward hibernating grounds. A high number of nematodes in the lungs may also compromise the survival of frogs during their first winter.

In the Natural Environment

The Environment Canada research team will be assessing the state of health of natural populations of Leopard Frogs exposed to agricultural contaminants in the St. Lawrence Valley, particularly in the Boucherville Islands and in a tributary of the Yamaska River. Launched in 2001, the primary aim of this field study was to compare the level of parasitic infection in frogs in areas of intensive corn production with frogs in reference areas. Corn fields are known to require vast quantities of pesticides, including herbicides like atrazine.

Location of Leopard Frog sampling sites

Young Leopard Frogs were captured at five sites exposed to pesticides and at two reference ponds.

At each site, water samples are taken at different times in the summer to trace the evolution of the pesticides. Concentrations of pesticides in surface water vary tremendously throughout the year. Maximum values are generally found in June and July after a heavy rainfall. This is also the period when Leopard Frog larvae develop, and when it is impossible for them to avoid being exposed to the toxic substances present in the aquatic environment.

Collaborators

Concordia University

INRS – Institut Armand-Frappier

McGill University