THE MERCK VETERINARY MANUAL
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Overview of Insecticide and Acaricide (Organic) Toxicity

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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 four groups: 1) organophosphates, 2) carbamates, 3) organochlorines, and 4) pyrethrins and pyrethroids. Because of worldwide use, these chemicals pose health risks to nontarget species, including people, 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 due to 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. Overexposure to chlorinated hydrocarbon insecticides is difficult to measure except by the occurrence of overt signs of poisoning.

Organophosphate and carbamate insecticides produce their toxicity by inactivation of acetylcholinesterase (AChE) enzyme at the synapses in nervous tissue and neuromuscular junctions, and in RBCs. Therefore, the cholinesterase-inhibiting property of organophosphates or carbamates may be used to indicate degree of exposure if the activity of the blood/RBC-AChE is determined during an early period of exposure.

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 maximum 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.

Chemical synthesis rarely yields 100% of the product of interest, and normally there are, in variable proportions, structurally related compounds that have biologic effects different from those of the compound sought. A prime example is dichlorodiphenylethane (DDD): the p,p′-isomer is an effective insecticide of low toxicity for most mammals; the o,p′-isomer causes necrosis of the adrenal glands of people and dogs and is used to treat certain adrenal malfunctions.

In general, products stored under temperature extremes or held in partially emptied containers for long periods may deteriorate. But 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 several fold. Similar impurities can be formed and potentiate the toxicity of another organophosphate insecticide, phenthoate. Storing a chemical in anything but the original container is hazardous, because in time its identity may be forgotten. Accidental contact with animals or people 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 organophosphate insecticides can result in potentiation of malathion toxicity by a hundredfold.

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

Last full review/revision August 2014 by Ramesh C. Gupta, DVM, MVSc, PhD, DABT, FACT, FACN, FATS

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