November 2000

ANIMAL HEALTH ALERT

PESTICIDE INTOXICATION IN ARIZONA HORSES

The Arizona Veterinary Diagnostic Laboratory (AZVDL) would like to advise Arizona equine veterinarians and horse owners of several recent cases of pesticide intoxication in horses in the Tucson area. The animals were all being fed Timothy hay which reportedly was sold through a Tucson hay dealer.

All horses involved had subnormal whole blood acetylcholinesterase levels and two hay samples tested at the AZVDL contained the acetylcholinesterase-inhibiting pesticide chloropyrifos. Chloropyrifos is an organophosphate insecticide that is commonly marketed under the trade names Lorsban® and Dursban® . The presence of chloropyrifos in the hay lots was subsequently verified by two other laboratories, including the Arizona Department of Agriculture Laboratory in Phoenix.

Clinical signs consistently reported by owners of affected horses were lethargy and runny eyes. In addition, drooling and foaming at the mouth while chewing was seen by one owner who also reported that two horses underwent exploratory surgery after they developed severe colic. Another horse owner reported that a filly being fed Timothy hay began stumbling and had watery eyes and a few loose stools. The stumbling stopped about a week after the owner stopped feeding Timothy hay. In another unconfirmed case where the affected horse was being fed Timothy hay, the owner reported muscle tremors, staggering, grinding of the teeth, repeated washing of the mouth in the water trough, and an elevated body temperature in addition to the lethargy and runny eyes.

In past years the AZVDL has confirmed equine intoxication from contamination of alfalfa hay with the pesticide methamidophos.

If intoxication from pesticide-contaminated hay is suspected we recommend the following:

1. Stop feeding suspect hay and retain the lot for pesticide analysis.

2. Submit a sample of whole blood in an EDTA tube for acetylcholinesterase testing. This is an inexpensive, rapid test, and if the result is subnormal, it confirms that a horse has been exposed to an acetylcholinesterase-inhibiting compound.

3. Submit samples of the suspect hay for pesticide analysis if indicated by acetylcholinesterase testing. Preferably several bales of the suspect hay should be sampled with a hay core sampler. If this is not possible, submit at least a 4-inch thick flake of the suspect hay for pesticide testing.

If pesticide residues are detected in the hay, the case will be referred to the Arizona Department of Agriculture for verification and further investigation.

If you have further questions please feel free to contact us at the AZVDL at (520) 621-2356. Check our website (http://microvet.arizona.edu/AzVDL/index.htm, click on the "Animal Health Alert" button) for any updates on this alert.