IVM low efficacy against inhibited stages
While the adult worms are easily removed by a wide range of anthelmintics, as long as the population hasn’t developed resistance to the chosen drug, encysted small strongyle larvae are much more difficult to remove. The cysts protect the parasite within the intestinal wall and prohibit the drugs from reaching the larvae.
There are three classes of anthelmintic used in the treatment of small strongyles:
- Benzimidazoles – e.g. fenbendazole and oxfendazole
- Macrocyclic lactones (ML) – e.g. ivermectin and moxidectin
- Tetrahydrophyrimidines – e.g. pyrantel salts
Fenbendazole at recommended doses (5 mg/kg daily for 5 days) has been shown to control adult and developing larval stages of small strongyles and a higher dose (10 mg/Kg daily for 5 days) has been found effective during the encysted stage. However, increasing resistance to fenbendazole among populations is limiting effectiveness and toxicity at higher doses must be cautioned.
While ivermectin is extremely effective for removing small strongyles in adult, luminal larval, and developing stages, it has low efficacy against inhibited stages. https://www.succeed-vet.com/educational-resources/disease-library/colitis/parasitic-colitis/
