Organic Phosphorus/Phosphonomethyl Amino Acid Herbicides

Glyphosate, a broad-spectrum herbicide, is widely used by the general public on residential lawns and gardens. It has herbicidal activity against a vast range of annual and perennial weeds and is generally regarded as practically nontoxic to mammals and aquatic and avian species (Malik, Barry, and Kishore, 1989). Therefore it is unlikely that ingestions, even if directly from the container, will result in illness. Some dogs, however, experience vomiting, hypersalivation, and diarrhea after ingestion of glyphosate directly from the container. This is usually attributed to the inactive surfactant found in liquid formulations (Smith and Oehme, 1992). Dogs with protracted vomiting and/or diarrhea can be treated with antiemetics, antidiarrheals, and intravenous fluids, replacing electrolytes as needed based on serum electrolyte concentrations. These clinical effects usually resolve rapidly, and the prognosis is very good.

Phenoxy Acid Derivative Herbicides

The phenoxy acid derivatives are broad-spectrum herbicides used extensively for broad-leaf weed control. The chlorophenoxy herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is commonly used around the home and in agriculture. Experimental data suggest that dogs are sensitive to toxicity, leading to gastrointestinal signs and myotonia when given doses of 175 or 220 mg/kg (Beasley, 1991). A case report involving an intact male weimaraner demonstrated clinical evidence of myotonia after an accidental (nonexperimental) exposure (Chen, Bagley, and Talcott, 2010). Toxicity after oral exposure leads initially to gastrointestinal signs of vomiting, diarrhea, and abdominal pain, although absence of these signs does not rule out exposure. Prolonged gastrointestinal signs may lead to fluid and electrolyte imbalances. Neurologic signs occur after high doses or chronic exposures and are characterized by myotonia. Sustained muscle contractions occur after stimulation because of a failure of the muscles to relax. Muscle stiffness, extensor rigidity, and a stiff-limbed gait are observed. Although reports of myotonia are rare following accidental ingestions of 2,4-D, treatment should still be undertaken, including decontamination with oral activated charcoal and supportive care, based on the clinical signs observed. Treatment with intravenous fluids helps to replace fluids and electrolytes lost due to gastrointestinal signs and may help to excrete the herbicide and its metabolites. Keeping the dog cage-rested while minimizing stimulation may help relieve signs of myotonia.

An association between lymphoma in dogs and the use of herbicides has been postulated. A case-control study in 1991 found an association between the use of 2,4-D on yards and the development of canine malignant lymphoma, with an odds ratio of 1.3 (Hayes et al, 1991). Additionally, the report documented a twofold increased risk of canine lymphoma in homes that used 2,4-D for 4 or more years. In 2001 another case-control study found an association between dogs living in an industrial area and the development of canine lymphoma. This same study also found an association between the use of chemicals by owners, specifically paints or solvents, and canine lymphoma (Gavazza et al, 2001). Additionally, a third case-control study reported in 2004 demonstrated an increased risk of transitional cell carcinoma of the urinary bladder in Scottish terriers associated with the use of phenoxy herbicides (Glickman et al, 2004). However, epidemiologic or observational studies such as those cited cannot demonstrate actual cause and effect. Moreover, depending on the study design, additional risk factors may or may not be included in the analysis. Nevertheless the development of neoplasias associated with chronic environmental exposure to phenoxy herbicides is a topic that will continue to be debated and investigated. Although these chronic environmental-type exposures do not require emergency veterinary care, clinicians need to be aware of them and educate owners of potential risks. At the least dog owners should be cautioned to limit access to lawns soon after herbicide applications if such applications cannot be avoided.

Similar to the debate about neoplasia in dogs, there has been speculation about an association between the use of herbicides, fertilizers, or plant pesticides and the development of hyperthyroidism in cats. One case-control study in 1988 found an association, but another case-control study in 2000 did not (Scarlett, Moise, and Rayl, 1988; Martin et al, 2000). Although a definitive connection requires further investigation, it may be prudent to inform owners that a risk may be present and have them limit cats from access to lawns soon after these applications.