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Equine Parasite Control

By: Dr. Philip Johnson

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Which Drugs are Available for Deworming Horses?

There are currently a variety of drugs available for the treatment of gastrointestinal parasites. The drugs are divided into a variety of different classifications, including piperazine.

Benzimidazoles

Fenbendazole (Panacur, Safeguard)
Mebendazole (Telmin)
Oxfenbendazole (Benzelmin)
Oxibendazole (Anthelcide EQ)

Macrocyclic Lactones

Ivermectin (Eqvalan, Phoenectrin, Zimectrin)
Moxidectin (Quest)
Ivermectin and Praziquantel( Equimax)

Tetrahydropyrmidines

Pyrantel pamoate (Strongid paste and suspension, Anthelban)
Pyrantel tartrate (Strongid C)

Unless resistance has developed in a parasitic worm population, all these currently marketed compounds have a good level of effectiveness against adults and larval stages in the horse's intestine. Only ivermectin and moxidectin have good effectiveness against larval stages of parasites migrating in the wall of the intestine or in the blood supply to the intestinal tract. High levels of fenbendazole and moxidectin also have effectiveness against the hypobiotic (encysted) cyathastomes in the wall of the intestines.

What is Parasite Resistance?

In time, all species adapt and change to meet the needs of survival. During the preceding 30+ years, equine parasites have changed and the drugs have changed. Parasites now reproduce FASTER than they did in 1966! Over the years, parasites have tended to develop ability to survive different parasite-killing drugs. If a worm parasite develops the ability to survive the effects of a parasite killing anthelmintic drug, it is said to have developed "resistance" to that drug. Broad resistance to many of the older deworming drugs is also now evident. The worms are able to evolve and to pass along resistance mechanisms to their offspring. No single parasite control recommendation should be regarded as perfect and permanent. There does not exist a simple single strategy for minimizing parasites that will work for every horse in every different circumstance. Local factors must be considered. If necessary, local factors must be investigated locally and monitored over time. Fortunately, at this time, parasite resistance has not been reported in the equine parasites for the very effective drugs, ivermectin and moxidectin.

It used to be believed that, by switching between different deworming drugs on successive treatments, the likelihood of parasite resistance could be lessened. The practice of "anthelmintic rotation" was designed to try and reduce the development of parasite resistance and to ensure that, if one deworming treatment "missed" some parasites, they would be killed by the next (different) treatment. Whether or not "anthelmintic rotation" practices truly ever did confer any protection against the development of parasite resistance has never been conclusively demonstrated. There have even been suggestions that this technique might increase the risk for parasite resistance!

If a parasite develops resistance to any single drug in a given category (eg. fenbendazole – in the benzimidazole family), the parasite will also be resistant to all the drugs in the same family. Properly documented parasite resistance to anthelmintic drugs is an important problem and warrants thorough investigation by the veterinarian. Special measures may be needed to provide protection against resistant strains of parasites. The indiscriminate use of anthelmintic drugs without veterinary supervision is an important causative factor for the development of parasite resistance.

The veterinarian will determine which anthelmintic drugs are effective or ineffective on a given farm (there is widespread, but not universal, resistance to benzimidazoles). If resistance to a family of drugs is identified, drugs in that family should not be used. A routine monitoring program should be arranged to ensure that any selected anthelmintic drugs maintain their efficacy on a given premises.

What is the Egg Re-appearance Period?

If an anthelmintic drug is effectively killing adult parasites, the fecal egg count should decrease by more than 90 percent. After a defined period, egg production starts again as encysted (hypobiotic) larvae "emerge" and mature and start producing more eggs. The interval between anthelmintic treatment and resumption of significant egg production is known as the "egg re-appearance period (ERP)". The ERP is different for the different anthelmintic drugs.

Piperazine = 4 weeks
Benzimidazoles = 4 weeks
Strongid suspension or paste = 4 weeks
Ivermectin = 6 to 8 weeks        
Moxidectin = 12 weeks        

Knowledge of the ERP allows the horse owner and veterinarian to know when the horse should be dewormed again if pasture contamination by further parasite eggs is to be prevented. In order to prevent pasture contamination by parasite eggs, horses should not be allowed onto pastures if they are passing parasite eggs in their manure. By strategically deworming horses over a period of 2 to 4 years, the resident population of parasites (encysted larvae) in the horse's intestine and the parasites in the pasture can be progressively depleted.

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