The Canadian Peregrine Foundation

Canadian Cooperative Wildlife Health Centre: Wildlife Health Centre Newsletter
Spring & Summer 2006
pp. 10-11: Ontario Region

Common Toxicological Problems of Ontario Wildlife

Although numerous causes of death exist, poisoning is often the first suspicion when an animal is found dead. Every year the CCWHC laboratory in Guelph diagnoses a considerable number of poisoning cases involving wildlife. As there is no all-purpose toxicological test available that will detect all toxic compounds in a sample, it is usually necessary to first have some rough guess as to what poison is likely involved in order to obtain a diagnosis of poisoning. Some methods, such as mass spectrophotometry and gas chromatography, have greatly increased the chances of detecting a toxic exposure, but in most instances, pathologists still need to have a suspicion of the specific material involved in each particular case and thus order the appropriate test that will detect it.

This article will outline a sampling of cases involving wildlife in Ontario and some of the more common toxicological exposures that we have detected in recent years.

Cholinesterase-inhibiting insecticides

Pesticides such as organophosphates and carbamates exert their action by inhibiting the action of enzymes that break down the neurotransmitter acetylcholine. In this manner, nerves which use this neurotransmitter will continue to fire and stimulate the tissues which they serve. This will affect many tissues, including the heart, brain and smooth muscle of the gut, resulting in the rapid death of the animal.

Poisoning with these compounds is detected in one of two ways: the compound itself may be detected in the stomach contents of the animal, or, more commonly, the activity of the cholinesterase enzyme in the brain is measured. If enzyme activity is reduced below 80% of normal, it is likely that the animal has been exposed to a cholinesteraseinhibiting pesticide. If activity is below 50% of normal, pesticide exposure is the likely cause of death. The difficulty is that many laboratories do not have normal values for species of wild birds, making it difficult to tell whether or not a measured value is normal.

Each year we have a few cases of waterfowl, typically Canada Geese, that consume one of these pesticides, often Diazinon. Exposure usually occurs while grazing on grass that has recently been treated for insect pests. These events typically happen during June and July when lawns are being treated and groups of geese are grazing on the affected area. In each of the years of 2004 and 2005, there were 3 incidents of this type, involving 6-17 birds per incident.

For many years there has been concern about the possibility of songbirds being exposed to pesticides in either agricultural or urban/suburban locations where these insecticides are in use on agricultural crops or lawns and gardens. Cases of individual birds found dead are rarely tested for insecticide exposure, but when groups of birds are found dead in one location, there is a higher index of suspicion that some form of poisoning could be responsible. In the past two summers, the lab has diagnosed pesticide poisoning in 3 incidents of this sort, one involving red-winged blackbirds, one with pigeons and one with crows.

A review of cases from previous years revealed a number of similar events, involving a range of species including; grackles, starlings, cowbirds and ring billed gulls. In many of these, the cholinesterase testing was suggestive of poisoning, but not so depressed as to make the diagnosis definitive.

In addition, there have been 3 cases involving peregrine falcons in which there was significant brain cholinesterase depression. Cholinesterase levels in these 3 birds ranged from 2.0 – 13.0 IU/g/min compared with levels of 17.9-30.5 in 6 other peregrines. The suspicion is that the peregrines were intoxicated through the consumption of songbirds that had been exposed to insecticide. In all 3 of these birds, the ventriculus contained the remains of birds that had been consumed as prey.

Cholinesterase-inhibiting insecticides are occasionally employed in malicious poisonings involving wildlife. Recent case records include 3 cases in bald eagles in which brain cholinesterase levels were 25-50% of those of the single reference bird tested. In all of these cases, the birds had material in their upper digestive tracts which might have been bait used to deliver the poison. However, a specific compound was not identified in any of these cases, leaving them as suspicious or presumptive diagnoses. More recently, a marten and two ravens were found dead close to one another, and a bag containing white powder was also found in the vicinity. All 3 animals showed depressed brain cholinesterase levels and the powder was identified as a carbamate insecticide.


Avitrol (4-aminopyridine) is a compound used in nuisance bird control and is delivered through bait, typically whole or chopped corn. It is described as a frightening agent that exerts its main affect on the brain of the bird. Under the influence of avitrol, birds behave erratically and emit alarm calls; this in turn causes the bird’s flockmates to become alarmed and disperse, and in this way the flock is frightened away. Although avitrol is not intended to be lethal, birds can become disorientated resulting in accidents and death. Furthermore, birds can die of overdose if the corn bait is inadequately or improperly mixed, resulting in a lethal dose of the agent.

Since 1989, the Ontario region of the CCWHC has diagnosed 69 separate incidents involving avitrol poisoning. The number of birds involved in these incidents has ranged from single birds, which is the most common occurrence, to hundreds of birds seen sick or dead due to avitrol exposure. Pigeons, as a target species for bird control, are the birds most commonly involved in avitrol poisonings, but a range of other species, including mourning dove, crow, grackle, blue jay and, surprisingly, trumpeter swan and Canada goose, have been diagnosed. Most of these birds are likely poisoned accidentally when bait is put out for pigeons, but we have now seen 3 separate larger scale incidents of poisoning of grackles or crows, which were possibly deliberate, off-label uses.

There has long been a concern over the possibility of secondary poisonings in which a predator might be exposed from eating an avitrol-intoxicated bird. Since 2001, we have tested 19 peregrine falcons for avitrol exposure. In 2 of these birds, trace amounts were detected, while quantifiable amounts were found in 5 other birds, all of whom had died of traumatic injuries. In addition, 1 merlin has been found with detectable avitrol in tissue. Although these birds are not exposed to directly lethal amounts of the compound, it seems reasonable to believe that any slight degree of disorientation could be fatal at the speeds achieved by birds of prey in flight.

Ethylene Glycol

Ethylene glycol, which is the main ingredient of antifreeze, has long been known as a common cause of poisoning of humans and domestic animals, but has not commonly been reported in wildlife. The metabolic breakdown of ethylene glycol in the body produces toxic metabolites which exert effects on many organs, including brain, kidney and heart, commonly resulting in death.

Domestic animals have commonly been exposed inadvertently, as for example when a person is changing the antifreeze in a vehicle and leaves some of the compound available in an open container or drainpan. However, it seems questionable whether wild animals are being exposed by this route, or whether the poisoning may instead be deliberate. In our case records, going back to 1994, we have 19 cases involving ethylene glycol poisoning. The majority involve either raccoons or skunks (12 raccoons, 4 skunks). There is also one case involving an opossum. While most of these cases have been from urban areas, there have been two unusual cases deriving from rural locations; one a turkey vulture and the other a black bear.


The anticoagulant rodenticides, of which warfarin is the oldest, are some of the most commonly used poisons. Unlike warfarin, which required several exposures to be effective, the more recently developed rodenticides are more potent, requiring only a single exposure to be fatal. Although these compounds are designed and licenced to be used within buildings, they are sometimes used out of doors.

In our case records, we have a small number (10) of confirmed poisonings involving anticoagulant rodenticides and a great many more suspected cases. The laboratory test available to us is not highly sensitive, and many cases test negative when the post-mortem evidence leads to a high suspicion of poisoning. Of these confirmed cases, 5 involved Bromodialone, 3 were Brodifacoum and 2 were Warfarin.

Six of the cases involved grey squirrels, which also made up the bulk of the suspected, but unconfirmed, cases, and one case involved a number of short-tailed shrews. There has long been concern involving the potential for secondary poisoning with these compounds, in which a predator or scavenger might be exposed through eating a poisoned rodent. In some localities such poisonings have been documented in raptors. In our case records, there is one raccoon and 2 crows, all of which may have been poisoned after consuming the carcass of an affected rodent. We have never documented anticoagulant rodenticide in a raptor.

Zinc phosphide is a rodenticide commonly used in outdoor pest control. We have had 4 cases of zinc phosphide poisoning: 3 of these involved groups of Canada geese, the fourth a group of wild turkeys. Two of the episodes involving geese were on landscaped properties at an industrial park, while the third was at a golf course. The turkeys were poisoned in an apple orchard.


Lead is a well-known and long-standing potential source of toxic exposure for wildlife. The use of lead in shot used for waterfowl hunting has now been banned, and there are localized restrictions on its use in fishing tackle. Thus it may be expected that the number of birds poisoned with lead will decline over time. However, it remains one of the most common poisonings diagnosed at our laboratory.

In the period 2000-2005, there were 52 confirmed incidents of lead poisoning. Trumpeter swans are the birds most commonly involved (27), with common loons (10), Canada geese (5), other waterfowl (4), bald eagles (5) and a peregrine falcon making up the remainder of the cases. Waterfowl typically are poisoned by picking up spent lead shot from hunting, and some popular hunting areas are likely badly contaminated from years of hunting activity. Common loons typically are poisoned from ingesting lead fishing weights.

The source of lead for bald eagles is largely unknown, but in the past it was thought that they obtained lead from scavenging injured waterfowl. with the ban on lead shot for waterfowl hunting, this should no longer have been the case.

The peregrine falcon is even more puzzling. The case involved a young of the year bird from Burlington. Tissue levels in liver and kidney were markedly elevated (23 and 28 ppm wet weight, respectively), indicating a substantial exposure.

Submitting cases for toxicology testing

Communication is an important part of the laboratory component of an investigation of a suspected poisoning of wildlife. The location, circumstances, time of year, observed behavior of the animals involved, all of these aspects are important in evaluating the possible causes of death. Even if poisoning is high on the list of possibilities, the pathologist will usually do a complete necropsy examination to eliminate other causes of death. It is important to remember that many tests are quite specific, and it is necessary to have a particular toxin in mind in order to have any chance of a positive result.

If poisoning is suspected in a wildlife mortality incident, it is a good idea to begin with a phone call or email to the regional laboratory, in order to discuss the possible causes, and perhaps to obtain suggestions on sample collection and handling.

Doug Campbell (CCWHC Ontario/Nunavut Region)

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