Large Strongyles in Horses: Bloodworm Parasites and Colic Risk

Large strongyles are intestinal parasites of horses, often called bloodworms. The most important species is Strongylus vulgaris, because its immature stages migrate through arteries that supply the intestines. That migration can inflame and damage blood vessels, a process linked to reduced intestinal blood flow and colic.

Adult worms live in the large intestine and shed eggs into manure. Horses become infected when they graze or eat feed contaminated with infective larvae from the environment. Once swallowed, the larvae do not stay put. They move through tissues and blood vessels before returning to the intestine to mature.

This matters because a horse may carry large strongyles before obvious digestive signs appear. In some cases, the first clue is poor thrift, intermittent colic, or a more serious abdominal emergency. Even though large strongyles are less common than they once were in well-managed herds, they still matter because the damage comes from the migrating larvae, not only the adult worms your vet may suspect from manure testing.

Large strongyles spread through the fecal-oral route. Eggs passed in manure hatch and develop into infective larvae on pasture. Horses pick up those larvae while grazing, eating hay placed on contaminated ground, or living in areas where manure is not removed regularly.

Risk rises when many horses share pasture, stocking density is high, manure management is limited, and deworming plans are based on habit instead of testing. Horses newly arriving to a farm can also introduce parasites if they are not evaluated as part of a herd parasite-control plan.

The most serious disease is tied to the life cycle of Strongylus vulgaris. After ingestion, larvae migrate in arteries that supply the intestines, especially the cranial mesenteric artery. This can cause inflammation, clot formation, and reduced blood flow. That is why a parasite problem can turn into a circulation problem inside the abdomen.

Not every horse exposed will become sick in the same way. Age, pasture exposure, herd management, and the effectiveness of previous deworming all influence risk. Your vet can help build a plan that fits your horse, your farm, and your region.

Diagnosis usually starts with a physical exam, history, and fecal egg count. A fecal egg count can confirm that a horse is shedding strongyle-type eggs, but it has important limits. It cannot distinguish large strongyles from small strongyles, and it does not detect the migrating larval stages that cause the most dangerous blood-vessel damage.

Because of that, your vet may combine fecal testing with the horse’s age, deworming history, pasture exposure, and any signs of colic or poor condition. In horses with abdominal pain or concern for vascular damage, your vet may recommend bloodwork, abdominal ultrasound, or referral evaluation. In some cases, diagnosis is based on a combination of parasite evidence plus clinical signs rather than one single test.

For herd management, your vet may also use fecal egg count reduction testing to check whether a dewormer is working well on your farm. That helps guide future treatment choices and supports a more targeted parasite-control plan.

If your horse has recurrent colic, it is especially important not to assume parasites are the only cause. Colic has many possible triggers, and your vet may need to rule out impaction, displacement, ulcers, enteroliths, or other intestinal disease.

Prevention works best when it combines targeted deworming with manure and pasture management. Current equine parasite guidelines support moving away from automatic year-round deworming every few months. Instead, your vet may recommend fecal egg counts once or twice yearly in adult horses, baseline treatment for key parasites, and more focused treatment for horses that shed more eggs.

Pasture hygiene matters. Removing manure regularly, avoiding overstocking, rotating grazing areas when possible, and not feeding directly on contaminated ground can all reduce exposure. New horses should be discussed with your vet before joining the herd so they do not quietly bring in parasites or resistant worms.

It also helps to remember that fecal egg counts are management tools, not perfect disease detectors. A low or moderate count does not fully rule out migrating large strongyles. That is one reason your vet may still recommend strategic treatment for large strongyles even when you are trying to reduce unnecessary deworming.

The goal is not to eliminate every parasite from every horse at all times. The goal is to lower disease risk, reduce pasture contamination, and preserve dewormer effectiveness over time. A farm-specific plan with your vet is usually the safest and most sustainable approach.