A wide variety of pesticides cause toxicity in wildlife. This disease description will focus on cholinesterase-inhibiting pesticides. These pesticides are known to inhibit an important chemical transmitter (cholinesterase) in the nervous system. This group of pesticides includes 2 large classes of chemicals: organophosphates and carbamates. These pesticides are widely used because they are toxic to many species of insects, parasites, plants, and fungi that are considered pests. They are also toxic to many animal species, and when these pesticides are applied over large areas, non-target plants, fish, and wildlife can become poisoned.

Pesticide use can result in sporadic deaths in wildlife, particularly birds, and at times can result in the death of large numbers of birds. This chemical poisoning has been diagnosed in raptors including Barn Owls in Pennsylvania, who apparently died from the accumulation of these compounds after ingestion of small mammals with elevated levels of pesticides. Cholinesterase-inhibiting compounds can also cause severe illness and even death in humans who have increased exposure.

Cholinesterase-inhibiting pesticides are toxic to many vertebrate species, but birds seem to be particularly sensitive. Over 100 species of birds have been reported with cholinesterase toxicity. In the United States it is more common in waterfowl, songbirds, and raptors.

Cholinesterase-inhibiting pesticides are used throughout the world in many different habitats including forests, agricultural land, rangeland, wetlands, residential areas, and commercial areas. Wild bird mortalities due to these chemicals have been reported throughout United States in these habitats. These pesticides do not persist very long in the environment, so mortality events usually occur shortly following pesticide application.

Birds can be exposed to pesticides by consuming seeds or vegetation treated with the chemicals. Pesticides in granular form may be mistaken as food or grit and consumed by birds. Birds may also consume dead or struggling poisoned insects. Raptors are usually intoxicated by feeding on carcasses of animals killed by pesticides or preying on live animals containing the toxins. Birds can also be exposed to these chemicals by way of inhalation, absorption through the skin, or drinking water contaminated by pesticide runoff.

Cholinesterase-inhibiting pesticides disrupt the nervous system and prevent normal transmission of nerve impulses. Different wildlife species vary in their sensitivity to different compounds. Birds that ingest high doses of pesticide may start showing clinical signs within minutes, while it may take hours to a day for clinical signs to appear following the ingestion of a more moderate dose. Affected animals often exhibit excess salivation, tearing, urination, diarrhea, and vomiting. Birds and mammals alike may exhibit convulsions, lethargy, paralysis, tremors, blindness, and other neurological signs. Animals may also develop difficulty breathing and often die due to paralysis of the respiratory muscles. Many animals are found dead without observation of any clinical signs.

A diagnosis is reached by laboratory analysis of pesticide levels in the blood or organs of affected animals or by other laboratory tests.

If treatment is attempted administration of certain drugs can help to alleviate the symptoms of cholinesterase-inhibiting pesticide poisoning. Stomach and skin decontamination is also important to prevent further absorption of the chemicals.

Pesticides should be applied properly by trained and licensed pesticide applicators and diluted according to the labels in order to prevent wildlife intoxication. When possible, pesticides should not be used in areas with high concentrations of wildlife, especially during the reproductive season. People should make an effort to use pesticides that are more specific to the pests being targeted and less toxic to wildlife. When die-offs occur due to pesticide toxicity, birds should be prevented from entering the contaminated area and any pesticide in bags or treated seed or grains should be removed. Carcasses of poisoned animals should be removed promptly. People investigating wildlife die-offs due to pesticide toxicity should wear protective clothing such as impermeable gloves, rubber boots, and respirators if there is a risk of toxin inhalation.

Fairbrother, A. 1996. Cholinesterase-inhibiting Pesticides. Pages 52-60 in A. Fairbrother, L. N. Locke, and G. L. Hoff, editors. Noninfectious Diseases of Wildlife, Second Edition. Iowa State University Press, Ames, Iowa, USA.