Deworming by fecal egg count (FEC) is advocated as a way to avoid overexposure of parasites to deworming drugs. It's important to understand the effect of deworming by FEC is primarily to minimize environmental contamination, not optimally protect the individual.
Young horses, PPID horses, and seniors, in general, are at high risk for parasitism. Even some young adult horses have poor parasite resistance. Much damage is done by immature forms that are not even laying eggs yet.
The problem with FEC is their limited capacity to detect parasites.
- They only reflect adults that are actively laying eggs at the time of the test
- Egg-laying activity may vary seasonally
- Tapes are often missed because eggs are laid in packets/segments, not evenly mixed in the manure
- Can't detect tissue forms or immature stages in the lumen but these can be the most harmful for the host
- Bots missed
- Pinworms missed (lay their eggs on the perianal skin)
- Strongyloides in adults not detected
None of that matters in term of herd health because small Strongyles are the major threat and the only mode of transmission is the contamination of the environment by fecal shedding, but it sure matters to the individual.
The other problem is technical limitations. Samples sent through the mail are useless. Samples not collected as soon as they are passed and then kept cold are likely inaccurate. The reason is that eggs hatch and flotation methods do not pick up larvae.
As for resistance, helminths come equipped with drug metabolizing enzymes which in some individuals transform the dewormers to harmless compounds. Obviously those genetically fortunate enough to possess those enzymes are the most likely to become resistant by upregulating enzyme activity when they are exposed to the drug - if it doesn't kill them first.
Any parasitologist will tell you the major way resistance develops is underdosing. (Ditto for antibiotics and bacteria). What doesn't kill them makes them stronger. Frequent exposure can cause a dewormer to go from a 20% failure rate to 80% or higher.
It's also true they can't be fully resistant to a drug they have never been exposed to so there is an argument in favor of not automatically exposing parasites to a variety of dewormer drugs (rotation). On the other hand, dead parasites do not develop resistance or pass it on to their descendants.
In any case, it's wise not to attempt to deworm with drugs that are known to have widespread resistance in the species you are treating. This includes the following reported resistances:
- Roundworms (Ascarids): Ivermectin, moxidectin, pyrantel
- Strongyles: Fenbendazole, pyrantel, albendazole
- Pinworms: https://wp.me/p2WBdh-G5
Last but far from least, there is growing evidence that the practice of only deworming by FEC is leading to the reemergence of Strongylus Vulgaris - "bloodworms".
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9034621/
These are the large Strongyles which do significant damage to the intestine and arterial system when they are migrating. Back when I was starting out as a veterinarian we still had to deal with many horses with S. Vulgaris damage and it wasn't pretty - severe recurrent colic, intestinal infarction, peritonitis, debilitating lameness, even to the point of euthanasia. If you wait to see the eggs in feces, it may be too late. Regular deworming with ivermectin or moxidectin had all but wiped them out.
Deworming strategies have gone from one extreme to another - from frequent, regularly scheduled treatments to deworming only on the evidence of egg-laying adults on FECs. Neither is ideal. Speak to your vet about developing a program that is optimal for your individual horse.
-Eleanor Kellon, VMD
