Effective Equine Parasite Control


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Best practices – and mistakes to avoid – in internal parasite control in horses.

If there is one thing related to parasites in horses that Martin K. Nielsen, DVM, Ph.D., Gluck Equine Research Center, University of Kentucky wants to make very clear, it’s this – parasite infection cannot be prevented. “That is a very common misconception – that horses can be kept parasite-free,” he said. “They cannot, and they should not. It is just as normal for horses to harbor parasites in their intestinal tract as it is for them to harbor bacteria there.”

As the chair of the AAEP Parasite Control Subcommittee, Nielsen authored the AAEP Internal Parasite Control Guidelines. He provided Veterinary Advantage some best practices and noted several misconceptions to avoid with effective internal parasite control in horses.

Start with drug efficacy

Of veterinary practitioners doing parasite control in horses well, Nielsen said the starting point is to test drug efficacy. “If they are not doing this, they are fumbling in the dark, he said. “So this is where it starts. We need to routinely evaluate treatment efficacy every year. There are so many ineffective treatments being administered because people never test. So this is key and has to happen.”


Fiction vs. reality

Nielsen listed several misconceptions distributor reps should be aware of:

Rotation between dewormers does not prevent drug resistance. “So many people believe this is the case. The reality is that there is so much drug resistance out there that there often is nothing to rotate between.”

There is no point in deworming at the first frost. “The external stages of these parasites do great in freezing conditions, so there is really no such thing as the killing frost. Besides, we want to deworm when we can actually affect parasite transmission, and that is during the warmer times of the year.”

Keeping horses stalled and confined for days following deworming is pointless. “This is so common but makes no sense,” he said. “The worms that may be coming out following deworming are dead. And the infective stages are not adult worms, but rather infective larvae, which develop from eggs being produced by the adult worms.” Horses being dewormed may have a few eggs in their feces for a couple of days following deworming, but that is completely insignificant compared to the many thousands of eggs produced over the weeks and months before deworming.

There is no good all-around dewormer. “That belongs in the past. We have a portfolio of products that each work against some parasites but not others. The products need to be chosen accordingly.”

The information on the drug labels is outdated, incorrect, and misleading. “Drug label information represents historical information from when the products were first introduced. For the majority of equine products, this was more than 20 years ago. The labels do not reflect current efficacy profiles and do not mention findings of drug resistance. So drug label information is not useful. The only way to obtain useful information is through efficacy testing.”

There is no value to deworming “wormy” horses with a half dose of dewormer. “There is just no evidence that this does anything to protect the horse. Half a dose of dewormer does not translate to killing half the worms. That is just not how it works. In fact, a half dose kills just about as many worms unless resistance has developed. Furthermore, adverse reactions to deworming are not proportional to the number of worms killed. We have evaluated adverse reactions to deworming in heavily parasitized horses in several studies and not found anything.”

Questions to ask

Nielsen recommended distributor reps discuss the monitoring plan in place on the farm. “Without testing, there is no strategy.” So the questions should be:

What is your resistance profile?

  • For which parasites?

When did you last test treatment efficacy?

  • And when are you doing it next?
  • For which products?

Veterinary Advantage readers can visit Nielsen’s YouTube Channel for more information about equine parasite control youtube.com/channel/UC_pDqB6sRvi1XJ8AHUapPCw. Or follow him on Twitter at @MartinKNielsen.

Parasite Control Programs

Considerations for mature horses:

Focus on control of cyathostomins. Depending on climatic conditions, one or two yearly treatments are sufficient to prevent large strongyles. Consider including a treatment effective against encysted cyathostomins at a time when the mucosal burden is at its peak. Typically, this is more likely to occur toward the end of the grazing season (i.e., fall in northern climates and spring in the more tropical and subtropical climates.) Include a cestocide at least annually if they
are a problem in your region.

Deworming programs for adult horses should be designed with the following principles in mind:

  • Evaluate the efficacy of the dewormers used on each farm at least every year using the FECRT.
  • All further treatments should be targeting horses with a high strongyle contamination potential.
  • A basic foundation of anthelmintic treatments should be considered for all horses. This should consist of one or two yearly treatments to target large strongyles, tapeworms, bots, and spirurid nematodes responsible for causing summer sores (Habronema spp. and Draschia spp.). In most cases, one or two yearly treatments will achieve this goal.
  • Focus on anthelmintic treatments during peak transmission seasons (usually spring and fall when pasture refugia are at their highest).
  • Perform FECRT yearly to evaluate the efficacy of anthelmintics against strongyles and ascarids.

Considerations for foals, weanling, yearlings:

  • Targeted treatments (selective therapy) based on FEC are NOT recommended in this age group. However, FECs are still important in this age group for other purposes, and the following considerations should be made.
  • During the first year of life, foals should receive a minimum of four anthelmintic treatments. First deworming should be carried out at about 2-3 months of age, and a benzimidazole drug is recommended to ensure efficacy against ascarids. Second deworming is recommended just before weaning (approximately 4-6 months of age). An extra treatment can be justified before weaning if the two treatments’ time period exceeds 3 months. At weaning, FECs are recommended to determine whether worm burdens are primarily strongyles or ascarids to facilitate the right choice of drug class. Third and fourth treatments should be considered at about 9 and 12 months of age, respectively, and treatment should primarily be targeting strongyles. In areas where tapeworms are present, a tapeworm-directed treatment should be included in the 9-month treatment or before the end of the first calendar year. The AAEP also recommends FEC monitoring in these age groups to check for ascarids.
  • Perform FECRT yearly to evaluate the efficacy of anthelmintics against strongyles and ascarids.
  • Strongyloides westeri is rarely a cause of diarrhea in young foals, thanks to the advent of benzimidazoles and macrocyclic lactones. It is widely used to deworm mares around foaling to prevent the lactogenic transmission of this parasite. However, there is no evidence to support this procedure.

“The Handbook of Equine Parasite Control, 2nd edition” is recommended for further reading.

Source: AAEP Internal Parasite Control Guidelines

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