Limiting summer’s bombardment of flies is a goal for all horsemen — something we want to accomplish with the least amount of expense, effort and mess. Feed-through fly-control agents sound like just the ticket. What could be easier' Easy, that is, until you read the product-label precautions. As one reader put it, “Why would I feed something to my horse that I can’t touch with my bare hands'”
Feed-through fly-control products contain the active ingredient tetrachlorvinphos, trade name Rabon oral larvicide. This chemical is a member of the organophosphate (OP) family, which includes such unfriendly relatives as chemical warfare nerve gas. There is also evidence that Rabon causes liver cancer in experimental animals.
Tetrachlorvinphos is one of the OPs mandated for review by the Food Quality Assurance Act of 1996, a federal law ordering the Environmental Protection Agency (EPA) to take a closer look at what, if any, levels of chemicals are truly “safe.”
On July 21, 1999, the EPA published final rules revoking 367 tolerances for 38 pesticides in foods, including uses for tetrachlorvinphos. This is a result of the Delaney Clause of the Federal Food, Drug and Cosmetics Act, which prohibits the establishment of any tolerance levels when the chemical involved has been shown to cause cancer in humans or experimental animals, such as the liver cancer Rabon may cause.
Any level of tetrachlorvinphos/Rabon remaining on foods is considered unsafe for human consumption. For that matter, you don’t have to look any further than the label to wonder about the wisdom of feeding Rabon to your horse. There are mandatory precautions about even getting the product in contact with your skin. If Rabon and other organophosphates are not safe to touch, how could they be safe to feed your horse'
Is It Absorbed'
The safety of the tetrachlorvinphos in the Rabon oral larvicide form hinges on the chemical being bound to a molecule of vinyl (yes, the same vinyl as in upholstery). Vinyl itself is a highly toxic substance, with profound hormonal effects. However, if the tetrachlorvinphos-vinyl compound truly remains intact as it passes through the horse, there would be no problems. It certainly doesn’t sound like the type of thing the horse’s digestive tract would easily break down and absorb.
We checked scientific literature, searched through Biomednet’s Evaluated Medline, including access to the National Institute of Health Medline, from 1966 to the present, asked both the manufacturers of the feed-through fly-control products and the company that manufactures the Rabon itself if there are studies documenting the level of absorption in horses. We couldn’t find any.
Stephanie Padilla, a biochemist at the EPA and an expert in OPs, told us, “I’ve looked at the structure of tetrachlorvinphos. It would be highly lipid soluble and could be absorbed.” She didn’t feel the vinyl would make much difference.
Absorption has been looked at in beef and dairy cows, where there are concerns about residues in meat and milk for human consumption. These show a low level of absorption. The level of absorption in laboratory animals is also unknown. Toxicity studies look only for short-term effects of various dosages, not how much the animal has actually absorbed.
When we found out there were no actual absorption studies done on horses, we looked for studies that would give blood-chemistry indications the Rabon is absorbed.
Organophosphates are processed in the liver to metabolites that inactivate an enzyme in the blood, tissues and red blood cells called acetylcholinesterase. This enzyme breaks down acetylcholine, a body chemical involved in nerve activation. This is the mechanism of OP toxicity (see sidebar). The ability to irreversibly inactivate acetylcholinesterase is unique to the organophosphates and a related group of chemicals, the carbamates.
Blood and red-blood-cell levels of the enzyme will be depressed for up to two weeks with carbamate poisoning, up to four weeks with organophosphate poisoning. Since the effect is limited to these two types of chemicals (and a handful of obscure, injectable prescription drugs), blood testing for acetylcholinesterase levels can be used to diagnose such poisonings and is easier to perform than testing for the actual chemical in the tissues.
We could find no studies that look at acetylcholinesterase levels in horses being fed Rabon oral larvicide. In dogs, long-term feeding studies (one for three months, one for a year) showed depressed cholinesterase activity only at very high intakes (3,160 mg/kg of diet and 2,000 mg/kg of diet, respectively).
We received equine safety studies from the manufacturer of Rabon (Boehringer Ingelheim). One fed five times the usual dose to two breeding stallions for a 60-day period and reported no adverse effects on semen quality compared to two control stallions not receiving the chemical. Another fed twice the recommended amount to broodmares — one pregnant before the study started, the three others conceiving after the study started — for a 12-month period, with four control mares not receiving it.
Fertility, gestation length and delivery were normal in all four mares, and the foals also were reported to be normal. In a shorter trial, 11 pregnant mares were fed the recommended dosage of Rabon for 40 days and 14 mares fed the recommended dosage for 60 days, with no reported adverse effects. Fifteen horses of various ages, sexes and breeds, including one pregnant mare, were fed either 90 mg/kg of diet or 450 mg/kg of diet for 30 days with no obvious adverse effects.
Tissue levels of the chemical were not studied. Cholinesterase levels were not studied. Despite the small number of horses involved in these safety studies and what we believe was a short duration of feeding, the EPA accepted these studies as sufficient evidence that Rabon was safe to feed to horses for feed-through fly control.
We also questioned some major manufacturers of Rabon-containing products for horses. Farnam, who manufactures Equitrol, initially told us the only complaints they had received about their product were related to it being less than 100% effective. This statement was later modified. They further told us they have sold over 30,000,000 individual daily doses (about 166,666 six-month treatments or about 10,000 a year) since 1983.
United Vet Equine (Equi-Fly and Equi-Fly Plus) yearly sells about 450,000 individual daily doses (2,500 six-month daily treatments). They had a report from a woman who had three broodmares founder after receiving one of these products for three days but on investigating further also found she had not purchased it directly from them and the product was outdated, clouding the issue.
Several other companies sell straight Rabon (no fillers/base) for use as a premix in making bulk treated feeds, but sales figures pertaining to horses alone are not available.
A total of 12,500 horses a year on feed-through fly control sounds like a lot but only amounts to 0.125 to 0.2% of the six to 10 million horses in this country. Still, you would expect that somebody would have reported a problem by now beyond founder in horses being fed an outdated product. They have.
Suspected Rabon-Related Problem
On March 1, 2000, an open-to-the-public docket to receive comments concerning the safety of tetrachlorvinphos was opened by the EPA as part of a new Pilot Public Participation Process in the reassessment of potentially toxic chemicals. Within days, the EPA received a detailed account of experiences with horses on a Texas farm receiving feed-through fly control from 1994 to 1998.
All of the eight mares, all with previously normal reproductive histories, experienced problems over those years, including five premature deliveries, one dysmature foal, one absorption, one abortion, three dystocias, three premature lactations and one stillbirth. One mare also f oundered (did not have retained placenta).
Hormone levels on the mares, cultures and pathological examination of the placentas were negative. Other clinical signs in these mares, as well as geldings and colts on the premises, included muscle spasms, loss of muscle tone/mass, poor exercise tolerance, tearing, and abnormal thyroid testing.
Over the next three years, an exhaustive search was made for the cause:
• A county agricultural extension agency expert examined the ranch for the presence of toxic plants to no avail.
• The manufacturer of the feed-through fly-control product (Farnam’s Equitrol) was contacted but replied the product could not be causing any problems since it is not absorbed.
• Samples of bedding, moss, trail bark and hay were sent to Auburn University for testing for toxins.
• Well water and samples from the overhead fly-spray system using pyrethrins were sent out for testing for contaminants.
• The county reviewed records of mosquito spraying (primarily pyrethrins used) but found no spraying had been done within the spray range of the ranch and none of the neighbors had spraying done on their land.
• Several toxicologists and reproductive experts were consulted.
Finally, in July 1998, blood from a mare who had begun lactating at 90 days and aborted a stillborn and decaying fetus at 97 days was submitted for toxin screening. Her serum cholinesterase level was extremely low (5.4 compared to a low normal of 32). Immediate testing of other mares and a colt was also done. All also showed depressed cholinesterase levels from 3.7 to 7.3, classical for organophosphate or carbamate toxicity.
Having exhausted the other possible sources, attention was directed to management aspects these horses had in common, which boiled down to their diet of Purina feeds, dehydrated alfalfa and a coastal Bermuda hay. All were dewormed every two months with ivermectin, one on Strongid-C. The only common source of OPs located was the feed-through fly control.
Feed-through fly control was stopped immediately and cholinesterase levels rechecked in 14 days. Although higher, they were still not normal, ruling out carbamate toxicity (depressed cholinesterase levels return to normal in hours to days with carbamates). Accompanying the narrative history of the horses were copies of all laboratory tests on the horses, veterinary hospital records, pathology reports and materials, water, etc. that had been submitted for toxicology testing, as well as purchase records documenting the use of the feed-through fly control.
The owners had recontacted Farnam, initially speaking with a research veterinarian there. Attempts were made to set up a meeting with the company to present them with the information and to discuss a possible independent university study, with the EPA to do the cholinesterase testing on the study horses. A university had already agreed to do this study. However, Farnam declined a meeting, and the owners were told all future communications would have to go through the legal department.
Late in 1999, a representative from Farnam’s legal department told the owners they had already commissioned such an independent university study and results would be available in “a few months.” Although Farnam confirmed they are doing a study, they would not give us any details about when results would be available.
We also questioned Farnam about a statement they made to us in their official reply to our inquiry regarding any adverse reaction reports they may have had from their customers, in which they used the phrase: “without any adverse reactions relating to any reproductive complications or abnormalities being reported to Farnam.” When confronted with the account above, Farnam retracted this, stating they had meant to say except for this one incident.
Both Farnam and the manufacturer of Rabon are calling this farm’s detailed, documented case histories “anecdotal,” saying they were not “controlled studies.” Is this reasonable' How many horsemen carefully plan to control all other variables and make sure a new supplement is fed only to some of the horses, while keeping others as controls in the event a problem arises'
They also repeatedly referred back to the number of doses of product that have been sold, implying this proves it is safe. We find this problematic. Anyone using feed-through fly control has been told it is “totally safe” and “passes quickly through the horse’s digestive system without being absorbed” (quotes from the HorseHealth USA’s Equimart 1999 catalog).
With this strong an assurance of safety, few people, or vets for that matter, are going to even consider the possibility that the product is causing problems, let alone do the type of in-depth investigation and cholinesterase testing needed to document a connection. One of the owners of the Texas ranch holds a Ph.D. in chemistry.
Also included in the report to the EPA are the stories of eight other individuals who experienced similar problems with feed-through fly-control products. Two are veterinarians, one a breeding farm manager and two more were professional breeders. However, none of them had the data to back up their suspicions as the Texas ranch did, especially the classical depression of cholinesterase activity in blood from affected horses.
Padilla told us the information provided on the Texas horses was “meticulously documented” and points strongly to the feed-through fly control as the source and OP poisoning as the problem.
Products containing Rabon in a palatable base have an estimated shelf life of 18 months, if the product is stored in a cool environment and without exposure to high humidity. That 18 months begins the day the product is packaged.
However, our packages didn’t carry an expiration date or a date of manufacture, making it impossible for the consumer to know how old it is. “Middle-man” distributors, such as equine-supply houses, are left to their own devices when it comes to rotating stock and making sure nothing becomes outdated. Consumers should carefully confirm that any distributor’s/retailer’s stock was manufactured well within that 18-month window.
Perhaps breakdown of the product — either because it is outdated or after opening and exposing it to an extreme environment — generates by-products that are more toxic than the original compound.
We asked for details of breakdown products from the manufacturer of Rabon but did not get them. Dr. John Riner of Boehringer Ingelheim told us they were “of no concern.” The weather conditions in Texas were also such that the product was being used nearly year round, in contrast to what would occur in more temperate areas of the country.
Since organophosphates are cumulative — that is, the drug slowly builds up in the body fat over time — such continuous feeding over a prolonged period may have contributed to the chance of toxicity developing. We don’t know if this is true, since studies looking into it have never been done.
Another possibility is the interaction between the Rabon and other substances being used. Ivermectin can enhance the toxicity of some organophosphates because it increases the activity of the liver enzymes that convert the Rabon into its toxic metabolites in the body.
There is also evidence that pyrethrin fly sprays and some organophosphates used in combination can increase toxicity. We don’t know if this holds true for Rabon in horses or not because, yet again, studies haven’t been done.
Feed-through fly control, when used with other good fly-management measures, is effective. But is it safe' We believe the jury is still out.
It certainly wouldn’t be the first time that problems have been linked to a chemical that was previously thought to be safe. This happens because of limited safety studies, d ata extrapolated between species, or simply because it took more experience with the use of a chemical for the problems to surface. One thing is clear: Rabon is absorbed in other animal species. Why not in horses'
Whether the problems in the Texas horses can be linked directly to Rabon or to Rabon interacting with other substances remains to be proven, but the cholinesterase data alone seems to be sufficient reason for any rational person to conclude that use in horses needs further study.
Also With This Article
Click here to view "Efficacy Of Feed-Through Fly Control."
Click here to view "Report Any Suspected Problems."
Click here to view "Organophosphate Toxicity."
Click here to view "Diatomaceous Earth As Feed-Through Fly Control."