The senior horses in your herd might require extra attention when it comes to deworming. Although older horses typically have lower parasite egg counts, those with pituitary pars intermedia dysfunction (PPID, also known as equine Cushing’s disease) and other health conditions often become high shedders who increase the overall parasite burden within the herd.

In this video, Martin Nielsen, DVM, PhD, DACVM, offers tips for deworming senior horses. Nielsen is a professor at Aarhus University’s Department of Animal and Veterinary Sciences in Denmark. Prior to starting this position in December 2024, he was a professor at the University of Kentucky’s Gluck Equine Research Center.

(Editor’s note: The content of this video is the expert’s approach to the topic. Please consult with your practitioner if you have questions.)

Parasite Egg Counts in Senior Horses

Nielsen said that in large studies of horse populations, senior horses are usually the ones least likely to have parasite eggs in their fecal samples. “They have the lowest counts and the largest proportion of zeros (no eggs detected),” he said. “Senior horses typically don’t have a lot of parasite infections.

“However,” he noted, “senior horses can have other health issues that I’m sure many of you are aware of. These include endocrine disorders and metabolic conditions that can suppress the immune system of the horse, which can affect the parasite burden.”

Nielsen said horses with PPID and other conditions tend to have higher strongyle parasite egg counts.

“That doesn’t necessarily mean they have more parasites, but they are high shedders,” explained Nielsen. “That means they contribute more to the overall infection pressure within that herd.”

He said owners with senior horses in their herds should test them regularly for internal parasites to guide deworming strategies.

“We’ve also shown recently that some of these metabolic conditions that horses can have when they get older can somehow interfere with the anthelmintic treatment (deworming),” he noted. “It’s not a whole lot, but it seems like the response to treatment can be a little less compared to horses that don’t have some of these conditions.”

He mentioned that insulin dysregulation can particularly interfere with anthelmintic treatments.

The Senior Horse’s Immune Response to Parasites

Nielsen said most healthy senior horses have a good immune system and immune response to parasites since they have been exposed their entire lives. “But sometimes there are these specific conditions that can interfere with egg count levels and treatment efficacies.”

How to Deworm Senior Horses

Approach deworming of senior horses the same as with any other horse, advised Nielsen. However, some aged horses are “high shedders” of parasite eggs. These high shedders are the ones that “we might want to treat a little bit more during grazing season,” said Nielsen.

“In the AAEP Parasite Control Guidelines that we just published, we operate on a two-prong approach,” said Nielsen. “We have a baseline approach that should be considered for all horses. Then we add on top of that based on what we find in our fecal samples. If we find out a horse is a high shedder, that horse needs to get additional treatments on top of that baseline for all horsses.

“Sometimes, your senior horse could be in that category,” he said.

He added that the types of dewormers and the threat of resistance are the same no matter the age of the horse.

Further Content

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