What Dewormer Works Best? Part 1

Posted on August 1, 2018 by Jerrilee.
Categories: health, history, therapy.

Article Written by Donald H Bliss, Ph.D  midamericaagresearch.net

Equine Dewormers:
Equine dewormers currently on the market in the USA can be classified into three separate classes of compounds based on the mode of action.
These three major classes are: the benzimidazoles and pre-benzimidazoles (febantel, fenbendazole, oxibendazole, mebendazole and oxfendazole), the macrocyclic lactones (avermectin and moxidectin families), and the tetrahyo-pyrimidines (pyrantel). The mode of action is different for each class of compounds. The benzimidizoles are non-soluble compounds that destroy the metabolism of the parasites by interfering with the cell functions in the parasites and by preventing the uptake of food thus starving the worms to death. The macrocyclic lactones are very soluble compounds and affect the nervous system killing the parasites causing a non-spastic paralysis while the pyrimidines kill the parasites by acting on the nervous receptors causing a spastic paralysis.
All three classes of compound have excellent efficacy against the adult parasites, but each dewormer class has a defined mode of action with a different level of activity against various developing and encysted larvae. The time it takes for larvae missed by treatment to develop into an adult parasite following treatment depends upon what larval stage the product is efficacious against. It takes longer for late L3 larvae to develop into an adult parasite than it will for late L4 larva. This difference can be measured in the time it takes for worm eggs to reappear in the feces following treatment. The longer it takes for eggs to reappear the more effective the product is against both the developing and encysted larvae.
Using products correctly and understanding their characteristics can help keep all classes of products viable. Fenbendazole, for example, is an excellent product when used in a strategic deworming schedule. However, if parasite contamination is allowed to develop in the environment and parasite levels increases in the animals until a high population of encysted larvae are present in high numbers, the efficacy of fenbendazole at the recommended dose is drastically reduced.
Two key issues have been identified with fenbendazole that can affect its efficacy. The first issue is that this compound is not very soluble in liquids such as gastric juices or blood. The second issue is that it kills the parasite by destroying it’s ability to metabolize food. Encysted larvae are in an arrested state with reduced metabolism and reduced absorption of nutrients. Because of fenbendazole’s low solubility and reduced metabolism of the encysted larvae, the product needs direct physical contact to kill these encysted parasites. When fenbendazole is given at 10 times the recommended dose spread over a five-day period it is successful against both developing and encysted larvae (10 mg/kg given daily for five days). By flooding the astrointestinal tract with molecules of fenbendazole, direct contact is made with the encysted larvae successfully killing them.

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