How safe are anthelmintics (dewormers)?


Any time a drug is given to an animal, there is concern about its safety. Is the drug safe for all animals at the therapeutic (or labeled) dose and what happens if the drug is overdosed? How much is too much? The safety of young animals and pregnant females is of particular concern.

There are three broad classes of dewormers for sheep/goats (in the US):  benzimidazoles, macrocyclic lactones, and cell depolarizers. Benzimidazoles (white dewormers) have the widest margin of safety. Safety indexes range from 3 to 20, mostly >10 (Parasitipedia.net). Fenbendazole (Safe-Guard®) has an especially low risk of toxicity.

Albendazole (Valbazen®) has a teratogenic effect, meaning it can disturb development of the embryo or fetus. For this reason, it should not be given during the first 45 days of pregnancy or 45 days after the male is removed from the flock/herd. The label states this.

The macrocyclic lactones are usually well tolerated. They can usually be administered up to 10x the recommended dose without causing serious side effects. In fact, the safety margin for ivermectin is about 30x for sheep. Moxidectin is not quite as safe as ivermectin, though safety studies showed no adverse effects when sheep were given 5x the recommended dose.

Though reproductive safety studies have not been done in the US, moxidectin is considered to be safe for pregnant females. Claims of abortion due to administration of macrocyclic lactones are unsubstantiated. While moxidectin has been proven to be safe for young lambs (< 1 month), it is only labeled for sheep/lambs over 4 months of age (in the US).

The safety margin of levamisole (Prohibit®, Leva-Med®) is the lowest of any of the dewormers. It ranges from 2 to 6, depending on formulation. Plus, you have to be careful with dosage, as it can be mixed using different dilutions. Some veterinarians advocate diluting the drug further to ensure a wider margin of safety, especially if small doses of the drug are being given.

Though uncommon, some animals may be hyperactive for a few minutes after receiving levamisole. There are anecdotal reports that levamisole causes late term abortion, especially in goats. However, the only scientific paper I could find showed that a dewormer dose (7.5 mg/kg) of levamisole had no effect on pregnancy in sheep. It has been postulated that the stress of handling during late pregnancy could contribute to the claim of abortion (in goats). Regardless, it's probably best to avoid using levamisole in late pregnancy (in goats).

While in the same drug category, morantel tartrate has a higher safety margin than levamisole, about 20x (in sheep). However, the two drugs should never be given at the same time. Similar to levamisole, dosing is tricky, as products containing morantel contain different concentrations of the drug. In fact, the same can be said if different formulations (other than drenches) of any drug are used.

It is now recommended that clinically-parasitized animals be given combination treatments. There are no known additional risks when giving more than one dewormer at the same time. The precautions that exist for giving a drug singly exist when giving the drug as part of a combination treatment.

Overall, fewer safety studies have been done with goats. On one hand, goats require higher dosages of dewormers than sheep, due to their higher rate of metabolism. On the other hand, they are more sensitive to higher dosages, especially kids.

To avoid overdosing, it is recommended that dosing be based on actual weights. This is especially important when deworming young or pregnant animals. If you don’t have a scale, you should use a weigh tape. However, if you are dosing a group of animals, you should calibrate the drench gun for the heaviest animals in the group, as underdosing can accelerate dewormer resistance. 

Ideally, you should fill the syringe based on the weight of each individual animal. It’s also important to avoid unnecessary stress when handling and deworming animals. Have a handling system or crowding pen. Practice low stress handling. Don’t handle ewes/does too close to their due dates.

Compared to older drugs and many old-time remedies, today’s dewormers are not only more powerful, but they are safer. At the same time, it’s important to give an accurate dose of the drug, minimize stress when deworming, and practice caution when treating sick or stressed animals.


Additional reading
Anthelmintic safety: Merck Veterinary Manual
Parasitipedia.net: parasites of dogs, cats & livestock: biology and control