Parasitism in Horses

Over the past several years, there has been an increased interest in the role of small strongyles in disease production in horses.  In contrast to their larger cousins, it was previously held that small strongyles were of low pathogenicity to horses; however, it is now accepted that these parasites often result in severe disease.

Small strongyles, also referred to as cyathostomes and the disease they cause as larval cyathostomosis, are a group of parasites that inhabit the colon and cecum of horses.  Up to 40 different species of small strongyles have been identified and they represent the most common parasites of horses on pasture.  Adult worms living in the lumen of the intestine produce eggs that pass into the environment.  There the eggs hatch and develop into infective larvae and horses ingest the larvae as they graze.  Once inside the gastrointestinal tract of the horse, the larvae penetrate the large intestine and become encysted.  It is this process that can cause disease.

Normally, after a period of one to two months of further development, the larvae emerge and return to the lumen of the intestine where they mature into adults.  If low numbers of larvae are involved in this process, the effects on the horse are minimal.  If, however, a large number of larvae enter a period of arrested development (which can happen under certain conditions) problems can ensue.  Various factors can trigger simultaneous emergence of large numbers of larvae with disease resulting from the associated inflammation and damage to the intestine.  Signs that are observed with cyathostomosis can include diarrhea, colic, weight loss, weakness, and edema.  Diagnosis can be difficult and it is often made only at necropsy.  Fecal examination for eggs is associated with inconsistent results; however, demonstration of larvae in the feces can be helpful in arriving at a diagnosis.

In central Kentucky, where management practices are typically excellent, there have still been 45 cases of larval cyathostomosis diagnosed at the University of Kentucky Livestock Disease Diagnostic Center over the past 6 years.  Yearly totals ranged from 4 to 11 cases.  Although the ages of affected horses were from 6 weeks to 33 years, our results indicate that this is a disease affecting primarily adult horses.  Twenty-three horses were 1 to 5 years of age, 15 horses were greater than 5 years old, and only 7 were less than one year of age.

In approximately 50% of the cases it was believed that the small strongyle infestation was the primary cause of death, while in the other cases the parasites represented a secondary problem.  Signs of disease described in the horses submitted to the diagnostic laboratory were similar to other reports and included primarily weight loss and diarrhea, with fewer showing weakness, colic, or death with no prior signs of illness.  Gross lesions seen at necropsy ranged from no observable change to loose content, pinpoint-sized nodules in the mucosa of the cecum and colon that were sometimes hemorrhagic, and edema of the cecum or colon.  Microscopically, cyathostomosis typically was characterized by encysted larvae within the mucosa and submucosa with edema and accumulation of inflammatory cells around the cysts.

Control of small strongyle infestation can pose several problems.  Treatment after the onset of clinical signs can be difficult due to rapid onset and severity of disease.  Many dewormers are only effective against adult small strongyles.  The most effective control is achieved by the regular use of anthelmintics that are efficacious against the tissue stage.  Several products are available that have been shown in studies to have significant efficacy against encysted cyathostome larvae.

Dr. Neil Williams, (606) 253-0571
Livestock Disease Diagnostic Center.