Worm Control News from Horsemen's Laboratory

Worm Control News
November, 2009

In this section of the newsletter, we'll feature an equine parasite each month, with information such as the health consequences for horses, the parasite's lifecycle, and/or appropriate dewormer medications.

This month: Large Strongyles

Last month we looked at small strongyles; this month is about their larger relatives, the Strongylinae, also known as large strongyles or large bloodworms. Historically, large strongyles have been the most dreaded and feared of parasites, due to the extreme health issues they can cause in a horse, including death.

Like small strongyles, large strongyles go from horse to horse via eggs in passed manure. The eggs hatch outside the horse, and can go through their three larval development stages in as little as a week under ideal conditions. This is why it's important to remove manure from pasture areas several times weekly.

In the third stage, large strongyle larvae develop a protective coating, allowing them to pass through the horse's acidic stomach without being killed. This is the stage when they are infective to the horse, since the larvae are able to climb up blades of grass, and horses ingest them while grazing. Once ingested, the large strongyle larvae migrate throughout a horse's internal organs, such as the arteries, pancreas, and liver, often for months at a time before returning to the gut as adult strongyles, to lay eggs and repeat the cycle.

Large strongyles have mouths with sharp teeth, and they latch onto internal tissue and organs, sucking blood from those tissues. During their journey through a horse, they might relocate several times, causing a wide range of problems including weight loss, anemia, localized hemorrhage, internal organ inflammation, and restricted blood flow. But, that's not all; in some cases large strongyles can cause blood clots, arterial rupture, and colic due to restricted intestinal blood flow. Several species of large bloodworms are well-known, including Strongylus vulgaris, Strongylus edentatus and Strongylus equines; each type has its own migration path and associated health risks.

Current broad-spectrum dewormers are effective against adult large strongyles that are in their egg-laying phase of life, but not necessarily against the migrating larvae. To kill large strongyle larvae, only the macrocyclic lactone (e.g., ivermectin and moxidectin) and some benzimidazoles (at specific dosages) classes of dewormers are effective.

Since large strongyles can be very hardy in pasture situations, care should be taken in terms of pasture management, such as limiting pasture dragging to hot, dry weather in order to expose and kill eggs. Stalled horses are unlikely to suffer from large strongyle infestation, however keeping a horse contained to avoid parasite infestation can result in other problems, such as behavior issues.

While large strongyle infestations are no longer the issue they were several decades ago, they can still create problems in a horse population. The best cure in this case is a bit of preventive management.

To read the entire November issue of Worm Control News, click here.