Currently the most popular management strategy for the control of intestinal parasites is to deworm based upon fecal egg count results. This developed because of concerns that the prior approach of routinely deworming at regular intervals was increasing the number of parasites with drug resistance.
The rationale is that if you decrease exposure to deworming drugs you will slow the appearance of resistance. An integral part of the new approach is to not deworm horses with only low fecal egg counts, to keep unexposed parasites in the population. Theoretically this will slow the spread of resistance but there may be a price to pay.
from University of Pennsylvania
For one thing, there are reports that large strongyles are making a come back. These parasites, aka “bloodworms”, were a very significant scourge in the days before ready availability of paste dewormers. Their immature forms migrate extensively for months in the gut wall and the blood vessels, often causing damage that results in chronic colic for the life of the horse, acute colic from obstructed blood flow or crippling clots in arteries feeding the hind legs. Now that many horses are only being dewormed once or twice a year, they have a better chance to do damage in that untreated interval.
A more immediate concern is the small strongyles (cyathostomes); recognized as the major intestinal parasite of today’s horses. Infective larvae hatch from the eggs. Inside the horse, the larvae may mature immediately or enter the wall of the intestine to go into a dormant state.
We assume the fecal egg counts are an accurate reflection of parasite burden but that is just not the case with small strongyles. The encapsulated dormant forms do not lay eggs. Even the adults have highly variable egg laying activity. Research has shown that egg laying picks up shortly in advance of the time of year when larvae will have the best survival chances on pasture and drops off or stops when environmental conditions are either very cold or hot and dry.
Dormant larvae also seem to adjust the timing of their emergence and maturation to coincide with favorable conditions for the larvae. Cyathostomiasis is a condition of colic, loss of appetite, weight loss, intestinal ulceration and loss of blood proteins into the intestinal tract. It is caused by a mass emergence of small strongyle larvae and can actually be fatal. This is typically observed in late winter in areas that have freezing temperatures over the winter.
Your horse probably doesn’t have enough larvae for a full blown cyathostomiasis episode but could very easily be harboring enough to cause a puzzling colic episode. Parasite burdens can also delay shedding.
To avoid these issues have a talk with your veterinarian about the timing of fecal exams and drugs used to deworm. Only moxidectin will reliably remove the dormant forms since there is currently widespread resistance to the five day, double dose fenbendazole (Panacur). Adults can be killed by moxidectin or ivermectin but if your fecal is taken at a time of year when egg laying could be low you may see a deceptively low egg count.
Don’t be outfoxed by a worm!
Eleanor Kellon, VMD