Parasitism: Small Strongyles

Feb 09, 2011
Preventative/Preparedness parasites | small strongyles

Small Strongyles (aka Cyathostomes) are currently recognized as the most common equine parasite. Horses can harbor Small Strongyle worm loads in their gastrointestinal tract and show no outward symptoms whatsoever. The larval stages of these parasites are known to burrow into the walls of the small intestine, only emerging into the intestinal lumen at certain times of the year to molt into adult worms and produce eggs that would be recognized on fecal examination! Therefore, horses infected with Small Strongyles may or may not exhibit eggs on fecal exam when, in fact, they may actually be carrying a significant parasite population. Small Strongyles even have the ability to remain encysted in their larval stage for up to 3 years before emergence.

If your horse shows even minimal symptoms of weight loss or seems to be a hard-keeper, you should speak with your veterinarian about the possibility of Small Strongyle infection and have them tested via fecal examination prior to deworming. Small Strongyles have shown incredible resistance to certain classes of dewormers and a FERCT (Fecal Egg Reduction Count Test) where two fecal exams are run – one prior to deworming and one 2 weeks after deworming — is often the only way to determine if your horse’s particular parasite population is resistant to the dewormer you are using.

Strongyle Lifecycle

  • There are at least 56 species of Small Strongyles.
  • They live in the small intestine and cecum.
  • Horses ingest infective Small Strongyle larvae on pasture or in manure from ground.
  • Larvae migrate through intestinal mucosa and usually remain encysted in the walls of the intestine for 2 – 3 weeks before maturing into adult worms and emerging into the intestinal lumen.
  • Larvae can remain encysted in the walls of the intestine for up to 3 years.
  • The time it takes from when a horse ingests an infective larvae to the time when that parasite becomes an adult and produces eggs that are shed in the manure is species dependant and varies anywhere from 5 – 12 weeks.

Signs of Strongyle Infestation

  • Horses may not show any outward signs of infestation. Oftentimes, horses with encysted Small Strongyles will have low or zero fecal egg counts when fecal examination is performed.
  • Complications of Small Strongyle infection include hemorrhagic or fibrinous enteritis (severe inflammation of the intestines), chronic colic, diarrhea and reduced gut motility.
  • Signs of infection can appear slowly or with sudden onset and include profuse diarrhea, severe and rapid weight loss and even death. Chronic infections may cause unthriftiness, poor weight gain, rough hair coat and decreased performance.
  • Severe disease can occur with synchronous emergence of Small Strongyle larvae from the intestinal mucosa and submucosa. This usually happens in late fall, winter or early spring and can cause intermittent diarrhea, dark soft feces with foul odor and/or anemia.


  • It is very common for Small Strongyles to demonstrate resistance to certain classes of dewormers. Many dewormers are not effective at eliminating encysted Small Strongyles and therefore, worm populations will increase in spite of regular deworming.
  • Panacur PowerPac (Fenbendazole) given daily 5 days in a row and Quest (Moxidectin) are the most effective at eliminating encysted stages of Small Strongyles. We recommend one of these therapies at least once a year for normal horses.
  • As Moxidectin is stored in adipose tissue (fat) for up to 3 months, it is important that it not be administered to horses that are underweight, debilitated, too old or too young. It must also not be overdosed in small horses, minis and ponies. If some of the gel drops on the ground, it can kill chickens. Please ask your veterinarian if Quest is appropriate for your horse before administering.
  • Another strategy for preventing encysted Small Strongyles is to put your horse on Pyrantel Tartate daily dewormer (examples include Strongid C2X or Continuex) so that any worms in the lumen of the intestine are killed on a daily basis as they emerge thereby reducing egg shedding into the environment and decreasing the chance of reinfection. Keep in mind that Pyrantel Tartate only kills Small Strongyles in the lumen of the intestine and is not effective at killing the encysted larvae. Normal, adult horses must still be treated with Ivermectin at least twice a year as pyrantel tartate is not effective against Bots or Onchocerca parasites.
Jessica Marsh

Written by Jessica Marsh, DVM