Insecticide and Acaricide (organic) Toxicity: Introduction

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 that label directions accompanying the product always be read and followed.

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. 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 phosphorous fumes). Such measures are generally sufficient to guard against intoxication. Overexposure to chlorinated hydrocarbon insecticides is difficult to measure except by the occurrence of signs of poisoning.

The cholinesterase-inhibiting property of organophosphates may be used to indicate degree of exposure if the activity of the blood enzyme is determined frequently. In humans, serum esterase is usually inhibited first and, in the absence of declining RBC activity, indicates a recent exposure of only moderate degree. Depression of the RBC-enzyme activity indicates a more severe acute exposure or chronic exposure. (Normal cholinesterase activity values vary a great deal in unexposed individuals, and a determination of activity has significance only when compared with the normal value for that individual.)

In addition to their effects on humans, organic pesticides may have 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. Few compounds satisfy all these requirements.

Poisoning by organic insecticides and acaricides may be caused by direct application, by ingestion of contaminated feed or forage treated for controlling 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 the compound sought. A prime example is tetrachlorodiphenylethane (TDE or “Rhothane,” also called 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 humans and dogs and is used to treat certain adrenal malfunctions.

Products stored under temperature extremes or held in partially emptied containers for long periods may deteriorate. Storing a chemical in anything but the original container is hazardous as 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.

A number of carbamate and organophosphate insecticides that bind serine esterases (eg, carbaryl, dichlorvos, methiocarb, carbofuran, paraoxon, mevinophos, aldicarb, and monocrotophos) are also immunotoxic. Impaired macrophage signalling through interleukin I and II 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.