Q: I dewormed my eight-month-old weanling for the first time recently, using a Fenbendazol-based product. A lot of small thin, red worms came out.
Is this indicative that I should follow it up with any drench with a different active ingredient?
Katie, Queensland, Australia
A: Hi Katie. The worms you saw are likely to be small strongyles (cyathostomins). They likely came out because the drug killed them. As all horses acquire small strongyle infection, it is not surprising to find worms in the feces during the days after deworming. Your observation, however, does not give us any information about the efficacy of the drug.
We know it killed some worms, but we do not know how many worms may have survived inside the horse. Small strongyle populations have been shown to be widely resistant to fenbendazole the world over, and it is advisable to check how well it works on your premises.
The only way to do this is to perform paired fecal egg counts; one at the time of treatment, and another one 14 days later. Your veterinarian can help you interpret the results, but overall fenbendazole should reduce the egg counts 90% or more. If the reduction is less than 80% it is strong evidence for resistance.
In your case, it is too late to perform the pretreatment egg count. But if you run an egg count about two weeks after the deworming and find moderate to high numbers of eggs (ie in the range of 100 and above), it may indicate resistance.
Other possible drug choices would be pyrantel (morantel) and ivermectin, but I still recommend checking for resistance.
Performing egg counts will also provide you with important information about which parasite types are present in your horse. An eight-month old is likely to harbour the large roundworm Parascaris equorum, which requires different treatment considerations. The eggs of the roundworm can easily be identified with the egg count techniques.