Dewormers Are Safe, But Not Foolproof

Modern dewormers might be called wonder drugs. They’re nonprescription, so horseowners can take this aspect of horse care into their own hands, unlike the old days of tube worming.

The excellent efficacy of modern dewormers has dramatically decreased parasite-related problems in horses, particularly colic. And, since most are available in an easy-to-use paste formula, they’re not difficult to administer properly.

But all this ease can make us forget one important fact: Dewormers are definitely drugs — and the use of any drug dictates caution.

Early Days
Deworming your horse used to be a big deal, done by your vet via stomach tube. Fasting was often recommended, and it wasn’t unusual for the horse to go off feed. Owners were cautioned against stressing the horse immediately before or after deworming. Laminitis was always a concern.

The first larvicidal deworming treatment — tubing your horse with 10 times the regular dose of thiabendazole — carried a high risk of the horse going off feed, colic, nonspecific “toxicity” symptoms and laminitis. Pretreating the horse with antihistamines, bute or Banamine was routine when a heavy burden of immature worms was suspected. These precautions were not related to the higher dose of thiabendazole per se, but rather to the fact that the horse could have a bad reaction to the en masse killing of parasites.

Modern Dewormer Reactions
When it comes to eliminating parasites, today’s dewormers definitely are more powerful than the older drugs, but they are also safer in terms of potential side effects from the drug itself. That’s why they’re available over the counter. However, that doesn’t mean they are problem-free.

The FDA’s drug approval process includes submission of studies that demonstrate the drug’s effectiveness, effective dose, and the safety of the recommended dose. Safety studies usually use foals, since they are considered the most sensitive group for drug reactions. If the drug is intended for breeding stock, specific safety studies must be done in these as well. Other safety data comes from dosage and efficacy studies, which list any adverse reactions encountered in the adult test animals used to determine dosage.

By the time a drug reaches the market, several hundred adult horses usually have been studied, as well as smaller numbers (often 10 or fewer) of foals and breeding animals.

If a drug makes it to the marketplace, you can be sure it has not had any significant safety issues. However, the FDA realizes that these studies, which involve only small numbers, may not tell the whole story. In fact, this is one of the reasons the FDA maintains a database of Adverse Drug Experiences (ADE).

Manufacturers are required by law to report to the FDA any information they receive concerning a possible negative reaction to their product — including a report of treatment failures. In addition, vets and owners also can report bad experiences with a product to the FDA (see sidebar at end of story).

The FDA’s ADE database is open to the public. In 1998, the FDA received 630 reports about horses. Moxidectin accounted for over half of these (366 reports, see chart on page 9). Moxidectin was also the second-most frequently reported drug in adverse-event reports for all species, all drugs, coming in second only to the anti-arthritis medication carprofen (Rimadyl) used in dogs.

The most commonly reported adverse reactions to dewormers involve abdominal/digestive-tract problems and neurological reactions.

Interpreting The Numbers
The FDA’s ADE database is obviously based on the assumption that “where there’s smoke, there’s fire.” It’s also likely that these numbers represent a fraction of the actual number of adverse drug reactions, since many are never reported. However, there are also several caveats to bear in mind with these numbers.

Even if a product only has one death or a handful of other reported problems, your immediate reaction is likely to be “not for my horse.” However, these numbers need to be weighed against how many times a year the drug is administered to get a perspective on the risk.

Older classes of dewormers, such as the benzimadazoles and piperazine, with little or no larvicidal activity, aren’t as widely used as other dewormers. On the other hand, several million doses are sold each year of broad-spectrum dewormers like moxidectin and ivermectin. In this context, the mathematical odds of your horse having a problem are low.

Products that are new to the market may also be more likely to be reported to the FDA, since vets and owners are more alert for negative reactions and may be more likely to blame problems on the new product.

It’s also important to remember that these reports represent only suspected, not proven, adverse reactions. Some of the reports also represent product packaging problems and reports that the drug was ineffective.

For example, the primary complaint against pyrantel in 1998 (15%) was that it was not effective. The reports didn’t differentiate between daily-dose Strongid or purge-dose (paste) Strongid.

Of the moxidectin adverse reactions reported in horses in 1998, 149 were overdosed, some because of packaging problems. However, whether these overdoses actually exceeded the reported safety margin of moxidectin is not clear.

“Internet furor has falsely lead to conclusions of causality whenever something bad happens after Quest has been given,” said Dr. Tom Overbay, director of professional services at Fort Dodge, which manufactures Quest (moxidectin brand name).

“No causality is assigned in these (FDA) reports. It is purely defined by the association in time between the administration of the product and the undesirable event,” said Overbay.

“Our internal controls assure that every case we receive is not filtered in any way, but reported to the FDA per their guidelines. . . . I also believe that to present the (FDA) data as you suggest falsely represents a product which research has shown to be very safe and more efficacious than other products,” said Overbay.

High-Risk Horses
Deworming risks basically break down into two divisions: 1) Toxicity from the drug itself, and 2) Reactions to deworming in general.

To reduce toxicity problems from the drug itself, you must consider the reported safety margins for each drug and be sure you remain within those guidelines when using a dewormer. Special consideration also needs to be given to horses receiving other medications or who are ill/debilitated (see sidebar page 8). These factors can interfere with a drug’s metabolism in the body and could lead to symptoms of overdose even with a recommended dose.

In addition, horses with abnormally low or high amounts of body fat may metabolize drugs differently. Normal liver function is essential to the proper metabolism of the drugs.

As a general rule, don’t mix the use of dewormers with:

• Tranquilizers or other drugs that affect the nervous system,
• Other dewormers,
• Drugs affecting the intestinal tract,
• Eye drops that affect the pupil.

If an adverse effect is related to the pharmacology of the drug itself, you will likely see neurological signs with most of the modern-day dewormers.

Side effects/adverse reactions may also occur, apparently, as a result of large numbers of parasites dying off at once. It is common knowledge that horses harboring a large number of parasites are at greater risk of a “bad reaction” to deworming.

Of course, it’s also a “damned if you do, damned if you don’t” situation wh en horses that don’t respond to less-powerful dewormers become candidates for the more potent ones.

The most common adverse effects with deworming are decreased appetite and depression, but the feeling generally passes in a day or so. More severe problems include diarrhea, colic, laminitis, even shock, clotting abnormalities, collapse and in some cases, possibly death. The precise mechanism behind colic and more severe toxicities following deworming is not really known, but both the likelihood and severity of the reactions seems to be linked to how heavily the horse was parasitized. The reactions are likely of an allergic or toxic nature.

It isn’t only horses in poor body condition that are high risk. Horses that have been regularly dewormed but using only agents that primarily remove adults may harbor large numbers of immature parasite larvae.

Periodically removing the adults controls the commonly recognized signs of parasitism — weight loss, poor coat, pot-bellied appearance — so that these horses may appear generally in good flesh and healthy. However, they may be at high risk for an adverse reaction to a potent dewormer that will kill off large numbers of immature larvae.

Foals, weanlings and yearlings are always considered high-risk for deworming. This is because even the most aggressive deworming schedules are never as effective in young horses as in adults. Effective parasite control requires a healthy, fully functional local immune response in the gut, which is not well developed in younger horses.

Even a horse that has been maintained on the newer larvicidal dewormers and has no apparent risk factors could have an adverse dewormer reaction if he picked up a large burden of larvae between dewormings, which could occur at a show or grazing on a trail ride. However, the likelihood is much lower.

Bottom Line
Modern dewormers are more powerful than older drugs. They have a broader range of activity against many different parasites, more parasites overall and more worm life stages.

However, this “power” comes with a price — an increase in possible reactions and even serious toxicity. When in doubt about the safety of a deworming drug or your horse’s status as a high-risk horse, get the advice of your veterinarian. Dewormers may be as easy to purchase as horse treats, but they are still drugs and shouldn’t be taken lightly.

Also With This Article
Click here to view “Report Those Adverse Reactions.”
Click here to view “What You Can Do.”
Click here to view “Drug Brand Names.”
Click here to view “How Safe Is Your Deworming Drug'”
Click here to view “Just Say No.”