Overview of Herbicide Poisoning in Animals

Herbicides are used routinely to control noxious plants. Most of these chemicals, particularly the more recently developed synthetic organic herbicides, are quite selective for specific plants and have low toxicity for mammals; other, less-selective, compounds (eg, sodium arsenite, arsenic trioxide, sodium chlorate, borax) were formerly used on a large scale and are more toxic to animals. The time recommended before treated vegetation is grazed ("grazeout period") or used as animal feed is available for a number of products.

Vegetation treated with herbicides at proper rates, followed by appropriate grazeout periods, is not typically hazardous to animals that consume it. Particularly after the herbicides have dried on the vegetation, only small amounts can be dislodged. Excessive application or incorrect compound use is often evident by damage to lawns, crops, or other foliage.

The residue potential for most of these agents is low. However, runoff from agricultural applications and entrance into drinking water cannot be excluded. Inadvertent administration of herbicide concentrates in livestock watering systems is rare but can occur if leftover product is stored in containers that lack accurate labeling or are labeled as water medications. In addition, failure of watering equipment resulting in possible exposure to animals has been anecdotally reported. The possibility of residues should be explored if food-producing animals are exposed to herbicides in water.

Most health problems in animals result from exposure to excessive quantities of herbicides because of improper or careless use or disposal of containers. When herbicides are used properly, poisoning problems in veterinary practice are rare. With few exceptions, it is only when animals gain direct access to the product that acute poisoning occurs.

Acute clinical signs usually will not lead to a diagnosis, although acute GI signs are frequent. All common differential diagnoses should be excluded in animals with sudden onset of clinical signs of disease or sudden death. The case history is critical. Sickness after feeding, spraying of pastures or crops adjacent to pastures, a change in housing, or direct exposure may lead to a tentative diagnosis of herbicide poisoning.

Generally, the nature of exposure is hard to identify because herbicides can be stored in mis- or unlabeled containers. Unidentified spillage of liquid from containers or powder from torn or damaged bags near a feed source, or visual confusion with a dietary ingredient or supplement, may cause the exposure. Once a putative chemical source has been identified, an animal poison control center should be contacted for information on treatments, laboratory tests, and likely outcome.

Chronic disease caused by herbicides is even more difficult to diagnose due to the numerous products to which animals can be exposed, the difficulties in singling out a specific compound and establishing actual exposure times, and often the lack of specificity of clinical signs. A history of herbicide use in proximity to the animals, their feed, or a water source, or a gradual change in the animals’ performance or behavior over a long period of time, can suggest chronic disease. Occasionally, manufacture or storage of herbicides nearby is implicated.

Samples of possible sources (eg, contaminated feed and water) for residue analysis, as well as tissues from exposed animals taken at necropsy, are essential. Months or even years might be required to successfully identify a problem of chronic exposure.

To recognize whether an animal has been exposed to herbicides or accidental poisoning, standardized analytical procedures for diagnostic investigation of biological materials have been established and are subsumed under the term biomonitoring. Accurate biomonitoring is an important tool to evaluate exposure to herbicides by measuring the levels of chemicals, their metabolites, or altered biological structures or functions in biological materials (eg, urine, blood or blood components, exhaled air, hair or nails, and tissues). The use of urine is advantageous because of ready availability. As such, in order to assess exposure and health risk to exposed animals, urine has been used for biomonitoring of several herbicides:

• 2,4-D

• 2,4,5-T

• MCPA (2-methyl-4-chlorophenoxyacetic acid)

• atrazine

• diuron

• alachlor

• metolachlor

• paraquat

• diquat

• imazapyr

• imazapic

• imazethapyr

• imazamox

• imazaquin

• imazamethabenz-methyl

If a poisoning is suspected and biomonitoring is desired, practitioners should reach out to laboratories to confirm whether testing for such agents and metabolites is possible, because testing is limited at many diagnostic laboratories.

If poisoning is suspected, the first step in management is to halt further exposure. Animals should be separated from any possible source before attempting to stabilize and support them. If there are life-threatening clinical signs, efforts to stabilize animals by general mitigation methods should be started.

If an intoxication is suspected, the involvement of a veterinarian is encouraged. Practitioners should consider reaching out to a poison control center or a veterinary toxicologist to get additional information regarding clinical signs associated with intoxication and potential treatments. Specific antidotal treatments, when available, can help to confirm the diagnosis.

As time permits, a more detailed history and investigation should be completed. The owner and veterinarian should be made aware of the need for full disclosure of facts to successfully determine the source of poisoning, eg, unapproved use or failure to properly store a chemical.