Merck Manual

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Professional Version

Overview of Insecticide and Acaricide (Organic) Toxicosis in Animals


Ramesh C. Gupta

, DVM, PhD, DABT, FACT, FACN, Toxicology Department, Breathitt Veterinary Center, Murray State University;

Robin B. Doss

, Breathitt Veterinary Center, Murray State University

Reviewed/Revised Aug 2022 | Modified Nov 2022

Insecticides are any substance or a mixture of substances intended to prevent, destroy, repel, or mitigate insects. Similarly, acaricides are substances that can destroy mites. A chemical can exert both insecticidal and acaricidal effects. Based on their properties, these chemicals can be classified into several groups: organophosphates, carbamates, pyrethrins and pyrethroids, neonicotinoids, phenylpyrazoles, triazapentadiene, oxadiazine, isoxazolines, and others.

Because of worldwide use, these chemicals pose health risks to nontarget species, including humans, domestic and companion animals, wildlife, and aquatic species. In large animals, poisoning is often due to inadvertent or accidental use, whereas in small animals (particularly dogs) poisoning is often a result of malicious intent.

Pesticide labels must carry warnings against use on unapproved species or under untested circumstances. These warnings may pertain to acute or chronic toxicity, or to residues in meat, milk, or other animal products. Because labels change to meet current government regulations, it is important to always read and follow all label directions accompanying the product.

Each exposure, no matter how brief or small, results in some of the compound being absorbed and perhaps stored. Repeated short exposures may eventually result in intoxication because of cumulative effect. Every precaution should be taken to minimize human exposure. This may include frequent changes of clothing with bathing at each change, or if necessary, the use of respirators, rain gear, and gloves impervious to pesticides. Respirators must have filters approved for the type of insecticide being used (eg, ordinary dust filters will not protect the operator from organophosphorus insecticide fumes). Such measures are generally sufficient to guard against intoxication.

Organic pesticides are known to exert deleterious effects on fish and wildlife as well as on domestic species. In no event should amounts greater than those specifically recommended be used, and maximal precautions should be taken to prevent drift or drainage to adjoining fields, pastures, ponds, streams, or other premises outside the treatment area.

The safety and exposure level of these compounds in target species has been carefully established, and application recommendations and regulations must be followed. Individuals, including veterinarians, have been prosecuted for failure to follow label directions or to heed label warnings and for failure to warn animal owners of the necessary precautions.

An ideal insecticide or acaricide should be efficacious without risk to livestock or persons making the application and without leaving residues in tissues, eggs, or milk. Only a few compounds may meet all these requirements.

Poisoning by organic insecticides and acaricides may be caused by direct application, by ingestion of contaminated feed or forage treated for control of plant parasites, or by accidental exposure. This discussion is limited to only those insecticides or acaricides most frequently hazardous to livestock or likely to leave residues in animal products.

In general, products stored under temperature extremes or held in partially emptied containers for long periods may deteriorate. However, during storage, malathion produces isomalathion, which is many times more toxic than malathion. In addition to isomalathion, two other technical impurities of malathion (malaoxon and trimethyl phosphorodithioate) can be formed and can potentiate the toxicity of malathion by severalfold. Similar impurities can be formed and potentiate the toxicity of another organophosphate (OP) insecticide, phenthoate. Storing a chemical in anything other than its original container is hazardous, because in time its identity may be forgotten. Accidental contact with animals or humans may then have disastrous consequences. Consumer-mixed and unapproved combinations can be very dangerous and should never be used. For example, simultaneous administration of two OP insecticides can result in potentiation of malathion toxicity by 100-fold.

A number of cholinesterase-inhibiting carbamate and OP insecticides (eg, carbaryl, dichlorvos, methiocarb, carbofuran, paraoxon, mevinophos, aldicarb, and monocrotophos) are also immunotoxic. Impaired macrophage signaling through interleukin-1 and -2 appears to be involved, and the insecticide levels that cause this effect are very low. In addition, these insecticides are neurotoxic. This can lead to subtle but damaging influences on the health of exposed animals.

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