In this video, Dr. Martin Nielsen gives us tips on deworming senior horses. Nielsen started as a Professor at Denmark’s Aarhus University, Department of Animal and Veterinary Sciences, in December 2024. Previously, he worked at the University of Kentucky’s Gluck Equine Research Center from 2011 and was a full Professor at the end of his time there. Nielsen is a veterinarian with a PhD in veterinary parasitology.

(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.)

Click on the above image to watch the video on deworming horses.

Senior Horses and Parasites

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 the contribute more to the overall infection pressure within that herd.”

He said owners with senior horses in their herds should make sure to have them tested regularly for internal parasites go 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.

Caution With Seniors

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 you do any other horse, advised Nielsen. However, sometimes aged horses can be in the ‘high shedder’ category 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.

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