My Senior Horse Podcast: Equine Parasites

There are many myths and truths about equine parasites in senior equids. In this podcast, we talk to one of the world’s top equine parasitologists to learn what is fact and what is fallacy. Martin Nielsen is a DVM, PhD (parasitology), DVSc (doctor of veterinary science), DACVM (American specialist in parasitology), and DEVPC (European specialist in veterinary parasitology). He is the Schlaikjer Professor of Equine Infectious Disease at the University of Kentucky’s Gluck Equine Research Center.

Click on the above player to listen to the podcast.

Senior Horses as ‘Shedders’

Horses are exposed to pretty much the same parasite species all over the world. But, Nielsen said, we do see some particular patterns in senior horses that are characteristic of that age group.

First, he said, many senior horses acquire some endocrine metabolic diseases that have been shown to interfere to some degree with the immune response to parasites. “We also see a tendency to have higher egg counts [in senior horses],” said Nielsen. “Parasite egg counts in those horess doesn’t mean that they necessarily have more parasites. Those parasites are able to crank out more eggs.”

That means the senior horse’s immune system is less able to hold that egg production in check. More eggs means the infection pressure from the number of infective parasites on grass pasture can be higher with this age group.

“So, from the standpoint of parasite control, these horses may be what we refer to as higher shedders,” he said. “We have some specific recommendations to treat them more than we would treat a horse that has a lower egg count because those egg counts tend to be consistent across time.”

If those horses are ‘pumping out’ more parasite eggs, that can increase the parasite numbers in your whole herd.

PPID and Parasites

Nielsen works with Dr. Amanda Adams of the University of Kentucky. Adams focuses on the health and care of senior horses. Adams established UK’s Aged Horse Research Program. She heads the Linda Mars Aged Horse Care and Education Facility at the University of Kentucky. This facility sits on about 120 acres with about 60 senior horses who will help lead the way on breakthroughs in care for older equids.

Adams serves on the Executive Advisory Council for My Senior Horse.

They worked on parasite research that looked at normal senior horses, those with PPID (pituitary pars intermedia dysfunction, or Cushing’s disease), those with insulin dysregulation (ID), and horses that had both PPID and ID.

“The good news is that ivermectin was very effective in all four groups,” stated Nielsen. “It knocked those egg counts down to 0 within two weeks.”

They also looked at how quickly the egg counts returned to higher levels. They did see some differences in these groups of senior horses.

Horses with PPID and ID had shorter egg reappearance (parasite egg counts came back quicker than in normal adult horses).

“So, there’s something there that maybe just allows the parasites to regain function more quickly,” Nielsen said. “That’s one aspect to think about with parasites in senior horses.”

Tapeworms

The equine tapeworm “is the only intestinal parasite that the horse doesn’t appear to be able to really develop any immunity to,” said Nielsen. “So, a senior horse is just as likely to harbor these parasites as is a yearling or very young horse.”

Nielsen said the most important risk factor for any horse to have tapeworms is amount of time spent at pasture.

“What’s concerning is that in just the last couple of years, we’ve demonstrated multi-drug-resistant parasites among the tapeworms,” he stressed. “That’s why it is important to test for tapeworms.”

He said the two different dewormers on the market with label claims for treatment of equine tapeworms are not working well today.

How are Dewormers Holding Up?

When asked how well in general the dewormers in the marketplace are working today, Nielsen said, “I feel like we’re just watching the final implosion as we speak.”

He said researchers are seeing strongyles showing resistance to all three classes of dewormer that are available. And as said above, tapeworms are having increasing resistance to dewormers.

Ascarids, which are normally only seen in younger horses, are being found in some senior horses. They also have some drug resistance in ivermectin/moxidectin classes, but sometimes in the other two classes of dewormers.

Pinworms have a lot of drug resistance, he said. “Many people claim that nothing seems to work,” noted Nielsen.

He said researchers have documented ivermectin/moxidectin resistance in pinworms.

“I think this is a result of really not having seen anythignnew on this market since 1981, when ivermectin was a new thing and a fantastic rug for us in veterinary medicine and in human medicine.”

Nielsen said it’s not for lack of effort on the side of the pharmaceutical companies. ‘It’s very expensive and tough screening for new compounds,” he added.

What Can Horse Owners Do?

“It’s important to test whether what you are doing [with your deworming program] is actually working,” said Nielsen. “And by testing, I mean parasite fecal egg counts and testing after deworming.”

He said owners should test the same horses before deworming and 2 weeks after deworming. This will let you know how effective the dewormer is in your animals.

“If you’re not doing this, I think now is a very good time to start,” emphasized Nielsen.

He said there are still ways to deworm your horses. “First, you need to find out what parasites you have and what might work against the parasites inside your horses.

“In many cases, there still are effective products,” he said. “So, it’s jut a matter of finding out which ones are the ones in your case.”

The testing also is aimed at reducing the treatment intensity. “Deworming less than we have traditionally means going way from deworming every six weeks or every eight weeks or whatever people have been doing for many, many decades,” Nielsen said. “Then treating more on an as-needed basis. Treating the ones that are high shedders a little bit more. And treating the ones that aren’t high shedders a couple of times a year, if that.”

Management Against Parasites

Neilsen reminded horse owners that environmental management is still very useful.

“The gold old husbandry virtues that we use are things like pasture management, mixed and alternate grazing with ruminants, and even resting pastures,”he said.

He gave the example of one farm in Central Kentucky that had one group of horses that had substantially lower parasite egg counts over and over on multiple occasions. He said the treatment efficacy was also better in those horses than for the rest of the horse operation.

The farm manager said this group of horses was turned out on pastures that the year before were used for making hay. “So, the pasture had been rested for a whole year,” Nielsen said.

“I know that not everyone has the luxury of resting a pasture for an entire year, but this taught us a lesson,” he said. “This was more effective than all of the dewormers we had been using on these horses.”

That farm also switched up rotating broodmares and yearlings on pastures. “The broodmares were adult horses (and some of them may have been seniors), were taking off pasture and yearlings were put on,” Nielsen noted.

He said since healthy adults usually have lower parasite egg shedding rates, the pastures had lower infection pressure. “The number of infective larvae per kilo or pound of grass that they eat is lower,” he explained.

He reminded owners that equine internal parasites do like environments with moisture and do well in cold temperatures, but they don’t do well in hot, dry climates.

Editor’s note: You can learn more from Nielsen through videos on his YouTube channel.