Parasite Protection for Your Herd

Parasite protection using an anthelmintic or dewormer can be challenging due to developing resistance and lack of new deworming options. Timing of and selective adminstration is a key factor.

By Dr. Aaron Tangeman 

Q:I have been using a schedule to rotate dewormers in my livestock. My neighbor recently said that deworming on a schedule is no longer advised. What should I do to protect my herd from parasites?

A: Anthelmintics, which are vital to maintaining the health of livestock and companion animals, act by killing or expelling internal parasites.

An anthelmintic, or dewormer, is considered to be resistant once it fails to reduce the fecal egg count (FEC) by 95 percent.

They join antibiotics as a classification of essential medications in veterinary practice that are developing resistance to the microorganisms they were designed to control.

Pharmaceutical companies have generally had little incentive to spend large amounts of money researching and developing new anthelmintics that can result in a low return on investment.

The first new anthelmintic to be developed in more than 25 years, an amino acetonitrile derivative (AAD), is in clinical trials sponsored by Novartis and Cambria Bioscience. The synthetic anthelmintic shows promise for use against all sheep and cattle gastrointestinal nematodes.

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Without new, approved, effective anthelmintics, ways must be found to effectively manage parasites without rendering the available medications impotent.

The presence of parasites in livestock is inevitable. Anthelmintics do not totally eliminate parasites but, when needed, can decrease loads and help producers maintain thriving livestock herds and improve their financial returns.
Conventional thinking held that administering dewormers on an established rotation schedule was an effective method to control parasites and prevent resistance from developing.

However, recent—and ongoing—research has called this practice into question. Parasitologists on a worldwide basis are alarmed by the increasing occurrence of parasite gene mutations that render existing anthelmintics ineffective.
They are conducting extensive research to develop innovative methods to control and manage parasites in ruminant and equine populations.

Administering an anthelmintic should decrease the host’s parasitic load. Despite the process of deworming, some parasites, in various stages of their life cycle, will survive either within a host animal or on pasture, and thus remain susceptible to the anthelmintic.

These surviving parasites are considered to be in refugia, and will compete with genetically mutated parasitic strains, resulting in decreased numbers of the resistant forms.

Because susceptible strains survive, the life of the anthelmintic is effectively prolonged.

Anthelmintics are best delivered during seasons when refugia populations are larger and can better withstand a drawdown in numbers, typically during rainy seasons such as spring and fall.

It is advisable to deworm only your livestock that demonstrate an excessive parasitic burden.

Signs that a parasitic load is becoming burdensome might include:

  • An animal in poor nutritional condition despite the availability of appropriate feed 
  • Rough hair coat
  • Diarrhea
  • Paleness of the conjunctiva or gums that may indicate anemia
  • Bottle jaw in small ruminants

Diagnostic tools include performing a fecal egg count (FEC) and fecal egg reduction count test (FERCT). If resistant parasites are identified on follow-up, your veterinarian can test for larval identification.

Because prevention and treatment modalities are continually being evaluated and revised, you should consult your veterinarian for the latest updates.


Dr. Aaron Tangeman received his Doctorate of Veterinary Medicine from the Ohio State University in 1998 and practices in Northeast Ohio.

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