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Willowview Hill Farm

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Pitfalls of Fecal Checks for Parasites
Eleanor M. Kellon, VMD

The first problem with FECs is they were designed to reduce contamination of the environment, not to make sure the individual is parasite free. Eggs passed in the feces is how parasites spread from horse to horse. However, only adults pass eggs so you know nothing about other life stages, or parasites not laying eggs, or eggs not mixed evenly into the manure.

Any of the following can be undetectable or easily missed:
• Immature or tissue forms of parasites
• Tapeworms
• Bots
• Pinworms

Even small Strongyles, which have the most commonly found egg in the FEC, often decrease or stop egg laying in winter.

There is also a problem with sampling and handling of the sample. Manure samples should be obtained from inside the rectum or immediately after they are passed. They need to be placed in a small sealed container to prevent drying and kept cool (refrigerator temps) until examined to avoid having eggs hatch. Flotation techniques cannot detect hatched larvae. Samples sent through the mail, or even over several hours by carrier without being cooled, are at best inaccurate and could be worthless.

Emphasis has been placed on FECs as a way to determine whether or not a horse should be dewormed. The rationale for this is to help slow the emergence of resistance to the currently used dewormer drugs. Unfortunately, there’s a downside – increasing levels of the large strongyle, S. vulgaris.

S. vulgaris, commonly known as the bloodworm, is the most dangerous parasite. It spends many months in an immature form migrating through the tissues and the blood vessels, often causing irreversible damage. Because this harmful life stage is undetectable on FEC, horses that could have been cleared of the infection are not being treated and the number of infected horses has risen dramatically:

https://pubmed.ncbi.nlm.nih.gov/32904767/
https://pubmed.ncbi.nlm.nih.gov/22703964/

As for all parasites, younger horses, older horses (especially with PPID) and horses living on multi-horse farms are at highest risk of S. vulgaris infection. Several studies have also documented a high infection rate in feral horses.

Only ivermectin and moxidectin can kill the migrating S. vulgaris larvae. In light of emerging evidence that this parasite is making a comeback it would be wise to consider deworming with one of those drugs at least every six months. Include a drug active against tapeworms at least once a year. With PPID horses, even more frequent schedules may be indicated for best protection. Discuss this with your veterinarian.