Toxicity of Anthelmintics

Dr. Dan Miller

(Reprinted from Pygmy Goat WORLD magazine with permission)

Q Can I deworm my pregnant doe with no ill side effects?|
A The following answers how both the doe and unborn fetus might be affected by various deworming medications

Anthelmintics not only affect nematodes (worms), but also may have effect on the host itself. Some of these are related to the same biochemical mechanism that operates against the parasite and some of them are peculiar to the host. They can affect some hosts and some host species and not others. And, although most of them are dose dependent, not all are.

There are three main families of anthelmintics on the market: avermectins, along with the milbemycins (levamisole and pyrantel/morantel), and the benzimidazoles. They are classed by their chemical structure and by their method of action. Therefore, within each group most members act similarly and have similar effect, but because of small chemical differences, not all members will act exactly the same.

Avermectins & Milbemycins 

Starting with the avermectins and milbemycins, it can be said that they are safe in just about any host at any time. Their effect is to interfere with certain types of communication between two nerve cells. This particular type of communication exists in nematodes and arthropods (insects, ticks, mites), but in vertebrates it only exists in the central nervous system. Fortunately these drugs do not penetrate into the brain and spinal cord so no damage is done. The known exception to this statement are collies, possibly Old English Sheepdogs, Red Tick Hounds, and turtles. Don't treat your pet turtle with ivermectin; use something else.

Research has shown that overdosing with ivermectin and the other members of this group, even in pretgnant animals, causes no damage. I am acquainted with a Pygmy goat that accidentally received the whole contents of a tube of ivermectin paste designed for a 1200-pound horse and showed no noticeable effects. There is also no effect on semen production or on fertility in the female. Thus, one could safely use ivermectin, moxidectin or duramectin in pregnant goats.

Levamisole, Morantel & Pyrantel

The group that includes levamisole is somewhat more toxic. These chemicals also affect transmission of neural impulses, but by a different mechanism. They do affect the host but in doses higher than necessary to kill the parasite. For this reason it is very important to carefully calculate the dose used according to the body weight. An overdose of levamisole by 4X will cause the animal to show clinical signs such as salivation and tremors. A 10X dose will cause death. It is not difficult to overdose goats, especially if you are using a product and delivery system intended for cattle use. In addition, subcutaneous injection results in blood levels 4X higher than from oral administration, so injection is more likely to result in toxicity than oral administration.

Most of the effects of levamisole are acute and act directly on the animal treated. Because of its method of action (among other things, causing smooth muscle contraction), there is the supposition that it can cause a stretched uterus to start contracting. This is not a good thing to happen in an animal that is fairly close to kidding. In real life, however, this is not seen very often.

The other members of the group, morantel and pyrantel, differ chemically from levamisole, but they have the same mechanism of action, attaching at the same receptor and causing the same effect. Since both are given orally,they are less often associated with toxicity than levamisole, and pyrantel especially is relatively nontoxic because it is not well absorbed into the bloodstream.


The benzimidazoles are a varied group of compounds with a relatively wide range of toxicity. The newer benzimidazoles that are relatively insoluble are so safe that it is difficult to get enough into an  animal to cause acute damage. Most of them can be greatly overdosed with little effect on the animal, but a few of them are associated with pathological changes. For instance, in dogs the use of mebendazole has been associated with liver damage. Cambendazole has caused deformations in foals when given during the first two months of gestation. Parbendazole, which is more likely to cause toxic effects in the host than most other benzimidazoles, can cause deformation in lambs. This effect is on the development of the leg bones and occurs when the treatment is given close to day 17 of gestation. Other benzimidazoles that are reported to have the same effect are exfendazole, albendazole and febantel. Fenbendazole, oxibendazole and mebendazole do not appar to cause congenital problems. It should be emphasized that outside of the danger period during the first third of pregnancy, there is no problem with using these other benzimidazoles in pregnant animals.

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