Drug resistance of equine internal parasites

August 10, 2006

The main internal parasites traditionally considered to be important in horses are bots, ascarids, large and small strongyles, and pinworms. Other species, such as tapeworms, stomach worms, and intestinal threadworms also can be of clinical importance. Of the approximately 100 species of internal parasites in horses, about one-half belong to the small strongyle group. The following discussion concentrates on the strongyles and ascarids.

Strongyles live as adults in the large intestine and lay eggs which pass in the feces of horses. In the environment, the egg embryonates into a first-stage larva, which hatches. It then undergoes further development into a second and then into a third-stage larva, which is the infective stage. Grazing horses ingest the third-stage larva after it crawls up on pasture vegetation, hay, or other food. In the horse, the third stage develops through two more stages (the fourth and then fifth, which is the adult). Large strongyles are more pathogenic than the small strongyles because the infective third stage migrates outside the large intestine into blood vessels and may go into internal organs. This may result in blockage of blood vessels and/or other problems that can result in debilitation or even death of the horse. Small strongyles in the infective stage migrate into the wall of the large intestine, where they encyst. These parasites can cause clinical problems (larval cyathostomiasis) and at times even result in killing the horse.
During migration of large strongyles and encystment of the small strongyles, there is development to the fourth stage and, in some instances, fifth stage. During these latter stages, they return to the lumen of the large intestine, mature to egg-laying adults, and the life cycle continues.

Ascarid eggs that are passed in horse feces have a thick shell. Embryonation occurs in the egg in the environment to the infective larval stage; however, the larva does not hatch from the egg until ingested by a horse. These parasites migrate in blood vessels through the liver and then to the lungs, where they undergo further development before being swallowed again. They mature in the small intestine where they can cause problems, including blockage or rupture of the wall. Ascarids can be the cause of death in young horses.

Over the years, several dramatic changes have occurred pertaining to chemical control of internal parasites. These are:

New classes of compounds have not been developed for over 20 years.

Fortunately, at this time, the most pathogenic parasite species (the large strongyles) have been controlled so well that they are essentially endangered and virtually absent in horses on farms with good parasite control programs. There has been no apparent resistance exhibited by these parasites; however, surveillance should be maintained to insure their continued control by antiparasitic products.

Resistance among small strongyle species currently has not been found for ivermectin and moxidectin, but has been documented for all other commercially-available products; i.e., the benzimidazoles (fenbendazole, oxfendazole, and oxibendazole), the tetrapyrimadines (pyrantel pamoate), and piperazine. Recently, there have been indications that ivermectin and possibly moxidectin were much less effective against ascarids in foals than found previously.

Typically, drug-resistance in the small strongyles occurs after a period of usage of anthelmintics. Even though the macrocyclic lactones (ivermectin and moxidectin) still are highly effective on these parasites, it is possible resistance will develop. It is suggested that these compounds be used sparingly to prolong their effectiveness on small strongyles.

Various treatment recommendations are used for parasite control, including daily feeding of pyrantel tartrate and therapeutic administration of compounds. The latter involves periodic treatment — e.g. monthly, bimonthly, and seasonal (spring and summer). One of the most practical and cost-saving measures is to do counts of strongyle eggs per gram of feces (EPGs). Then, only treat horses with EPG counts above a certain level. Some farms use a value of 40 or 100 before administering a dewormer. To determine the efficacy and possibility of resistance to a certain drug, an EPG count should be done on feces from a horse on the day of treatment and again two weeks later to be sure the drug effectively removed the adult egg-laying worms.

Regarding control of ascarids, which commonly occur in young horses, the recent finding of apparent drug-resistance of ivermectin and possibly moxidectin is alarming. This means it is important to determine by fecal examination, as stated above with the small strongyles, if these or other products are still effective in foals. It is recommended to treat foals every six to eight weeks with an effective ascaridicide to remove the worms before they mature. If the ascarids are allowed to mature, which occurs about 10 to 12 weeks after initial infection, then at least two possible life-threatening situations may occur regarding the small intestine of the infected horse: the wall may be ruptured by the action of the ascarids or the lumen may be impacted with dead large ascarids after treatment.

Research on parasite control is a priority in the Classical Parasitology section at the UK Department of Veterinary Science. This involves monitoring the level of parasitism among horses, particularly young animals, on local farms.