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Parasite resistance in Ruminants- What do we do now?

By Jane Naramore, LVT

In the last 35 years, we have benefited from the use of many anthelminitics for the prevention and control of nematode parasites in ruminants. Anthelminitics have evolved from thiabendazoles and levamisoles into benzimidazoles and are now found as avermectins and milbemycins. The efficiency of these delivery systems, along with the simplification of scheduling in these products, have made the drugs popular within the agricultural community;however, anthelmintic treatment is not the only effort needed to control parasitism. Drug resistance to nematodes in sheep and goats is a “wakeup call” to all livestock producers. Furthermore, research and development of these drugs is stagnant, and many believe that anthelminitcs have reached their maximum efficiency. As livestock producers, we must learn to examine the use of existing drugs to ensure their effectiveness and strategically develop management programs to control parasite loads.

Along with the concerns of parasite drug resistance, environmental toxicity and tissue residues are also a concern to the livestock producer. Current studies are looking for alternatives to developing expensive anthelmintic drugs. Some of these include vaccines, biological control agents, such as fungi, alternative chemicals and parasite growth regulators.

In the meantime, we can make changes in management to help us keep parasite damage to a minimum and use existing anthelminitics judiciously to supplement our control program.

The following list includes some of these management recommendations:

  1. Use fecal egg counts (McMaster egg counts) to determine the effectiveness of the drug/control methods you use for parasite control and to identify the parasites you are dealing with.
  2. Use alternate grazing between cattle, horses, and sheep/goats. Most parasites are species specific- sheep, goats, camelids will share many parasites so don’t intermingle these if possible!
  3. Use rotational grazing- a pasture is never “clean” just less infective at appropriate intervals and times of year. This method also improves pasture growth if used properly.
  4. Dilution is the solution- all livestock will have some parasites; you don’t want to get rid of all parasites, you want animals that can remain healthy with reasonable levels of parasitism.
  5. Feed hay/grain off the ground away from wet areas. For small ruminants use a cleanable platform for feeding. (ie.Trailer bed)
  6. Keep areas around watering systems well drained and clean.
  7. Select parasite resistant animals for breeding stock. Cull those that are consistently “wormy”.
  8. Use anthelmintics at turnout especially for young stock.
  9. Use appropriate anthelmintics at low parasite larval availability. Check small ruminants regularly using the FAMACHA eye mucousal color chart. Anemia caused by Haemonchus contortus can quickly become lethal.
  10. Check with your veterinarian to devise a specific parasite control plan for your farm.