PARASITE RESISTANCE TO DEWORMERS - AN IMPENDING CRISIS We currently have three basic classes of dewormers for use in horses in the United States, Benzimidazoles, Tetrahydropyrimadines, and the Macrocyclic Lactones.  Familiar brand names of these dewormers are Panacur (fenbendazole), Strongid (pyrantel pomoate), Quest (moxidectin), and Zimecterin (ivermectin).  Praziquantel is often used in combination with moxidectin and ivermectin to treat for tapeworms.  Strains of small strongyles that are resistant ot the Benzimidazoles and the Tetrahydropyrimidines have been documented as fairly widespread, and some strains have been found to be resistant to the ivermectins.  Resistant strains have evolved due to the repeated and/or frequent use of a single class of dewormer.  If the same class of dewormer is used repeatedly, and some of the parasites on the farm are resistant to the dewormer, you ae eliminating the susceptible strains of the parasite.  After a few parasite generations, the primary parasite population is the resistant strain, rendering the class of dewormer ineffective on that farm.   Small stongyles may cause mild and chronic colic, weight loss, diarrhea, loss of appetite, por coat condition, and intestinal ulcers.  Small strongyles are the parasite of primary interest in the treatment of adult horses.  Round worms, or ascarids, are of the greatest concern for foals and young horses.  Adult horses usually develop immunity to roundworms, but they can still carry enough adults to shed eggs onto the pasture.  Adult roundworms can form clusters in the small intestine, sometimes resembling a wad of spaghetti noodles.  These clusters can result in impaction and colic, which may result in rupture of the gut and death.  Tapeworms (anoplocephala perfoliata) are a risk to any horse grazing on pasture.  Heavy loads of tapeworms may cause colic, bowel obstruction, and intestinal inflammation.   Fecal egg counts can be used to monitor the adult parasite population in an individual horse's gut.  In a herd, most adult horses will shed a few eggs, and a few animals will shed a lot of eggs.  A few horses are responsible for most of the parasite eggs distributed onto the pasture, and these horses usually continue to shed high egg counts throughout their life.  Fecal egg counts will identify the heavy egg shedders in the herd.  Egg counts can also be used to determine the eficacy of the dewormer used if you do an egg count just prior to deworming and again two weeks after deworming.  High egg count animals should be dewormed more frequently, and low egg count animals should be dewormed less frequently.   How can we protect our horses and slow the development of resistance to the few classes of dewormers we currently have?  First, we must do fecal egg counts to monitor product efficacy and to determine the parasite load of individual horses.  A strategic deworming program that does not try to eliminate all worms by treating all horses all of the time must be adopted.  Products must be selected that will work on the individual farm and that meet the needs of the age of the horse.  Deworming programs must be tailored to the individual horse and the individual farm according to the infomration derived from fecal egg counts.

Site Map