In June 2000, we expressed concerns about using feed-through fly-control products containing Rabon, citing problems on a Texas ranch that appeared to be linked to organophosphate (OP) poisoning. Rabon, which is the brand name for tetrachlorvinphos, is the active ingredient in equine feed-through fly-control supplements and an organophosphate. The Texas horses had been receiving a Rabon-containing product. The biochemical evidence of OP poisoning — depressed levels of the cholinesterase enzymes in the blood — gradually returned to normal after the Rabon supplement was stopped.
When the owners and managers of a sport horse farm in California read about the Texas case, they noted they were experiencing similar problems, including reproductive difficulties, hyperexcitability and other neurological signs, stunted growth and orthopedic problems, laminitis, thyroid disorders, suspected poor immune function, exercise intolerance/weakness, GI problems. They tested blood cholinesterase levels on affected horses and found them to be depressed. The levels slowly returned to normal after the chemical was stopped.
The farm, Cottonwood, sued Farnam, the manufacturer of Equitrol, which was the feed-through fly-control product they were using, and was awarded over $1 million by a jury in the U.S. District Court for the Central District of California, Santa Ana. Farnam is appealing the verdict.
Research And Data
Jury decisions are not the same thing as scientific proof, of course, but the research data appears to be mounting as well.
Dr. John Madigan of the University of California, Davis, has completed a controlled trial of Equitrol feeding. This study shows marked depressions in blood cholinesterase levels in these horses when fed Equitrol and that the horses were noted to be significantly more “spooky”/easily excited than control horses.
In addition to the cholinesterase data and case information supplied by Cottonwood and the owners of the Texas farm, the EPA (Environmental Protection Agency) has on file the results of a controlled study commissioned by Farnam, completed in March 2000 by Midwest Veterinary Services, confirming suppression of cholinesterase levels in horses fed Equitrol.
Another study was done by an Arizona veterinarian whose own horses had problems with colic while on Equitrol. Again, a significant depression of cholinesterase levels was noted in the treated horses but not in untreated controls.
Another incident involved a horse owned by a veterinarian that developed lethargy, coordination problems, increased tearing, decreased appetite and hair loss while on Equitrol. Cholinesterase was tested during this time and found to be severely depressed, gradually returning to normal over one to two months when Equitrol was stopped. This data and reference to other incident reports appear in an FDA memorandum dated July 23, 2002.
On December 18, 2002, a notice appeared in the Federal Register that states: “EPA has determined that labels for tetrachlorvinphos feed-through products for horses must state that the product is a chlolinesterase inhibitor, describe signs of cholinesterase inhibition in horses, caution against the use with other cholinesterase inhibiting compounds, and direct horse owners to consult a veterinarian before using products containing tetrachlorvinphos on debilitated, aged, breeding, pregnant or nursing animals.” The EPA is in the process of implementing the changes.
Farnam will appeal the decision of the California jury. “We stand by the product’s safety and effectiveness reflected by its long-standing tenure in the marketplace,” said Chris Jacobi, president of Farnam Horse Products in Arizona. “All of our products undergo extensive testing before they can become available to consumers and are designed to help horses and their owners. Indeed, Equitrol has been recently re-registered by the EPA.”
Farnam said that the chemical is absorbed by the horse but suggested that the depressed cholinesterase levels are only an indicator of exposure, not necessarily toxicity. They stated: “Farnam does not agree that there are any legitimate medical studies or any other medical literature demonstrating that the use of Equitrol on a pregnant or nursing mare would be harmful. However, the company recommends that horse owners discuss with their personal veterinarians its use with specific horses and their particular situation.”
While Equitrol is the brand-name at the center of this lawsuit, other feed-through fly-control products also contain tetrachlorvinphos at similar levels. As we stated four years ago, this chemical does effectively reduce flies. The question remains at what cost. We strongly advise readers to check the ingredient labels and recommend caution if you use them. We suggest that you consider other methods of minimizing flies.