Tuesdays with Tony

As a regular reader of my weekly words of wisdom, I’m sure you’re aware of my strong feline feelings on deworming. However, I shall summarize it briefly. Deworm less. A lot less. Like, deworm-once-a-year less. This is accompanied by, do fecal egg counts to determine if any horse needs more deworming, and even then it’s only going to be a total of twice yearly. Where did all my purrfectly amazing feline knowledge come from? Science. The Gluck Center at University of Kentucky, and more specifically the hard work of a prodigious parasitologist, Dr. Martin Nielsen and the other humans who work in his lab. You may have seen a recent paper from his lab that looked at parasite egg shedding and numbers in an incredibly detailed way in a herd of horses who haven’t been dewormed in 40 years! This week, let’s talk about the work of Dr. Dr. Ashley E. Steuer. She’s the lead author on this paper. And no, I didn’t type Dr. twice on accident. She’s a DVM and a PhD! Also, cats don’t make mistakes.

What They Looked At

Gluck is a super cool place to geek out on horse scientific stuff. They’ve got research going on all sorts of things. As part of that research, they have a herd of horses maintained on pasture that never get dewormer, and hasn’t since 1979. 

For this paper, they asked the following questions: 

  1. Do horses shed more parasite eggs at different times of year? 
  2. Do young horses shed more than mature or aging horses? 
  3. What species do they shed when? 
  4. When do foals start shedding? 
  5. Do pregnant mares shed more eggs right before foaling? 

If you want to read the entire paper, which I recommend, you can click here. For the rest of you, I’ll give you my synopsis below.


Some work has been done on the question, do horses have a season to their egg shedding? Apparently that work was mostly done long ago in a land far, far away (England). This work was done in a very different climate: Kentucky. The horses were monitored for a full twelve months, and Kentucky, being Kentucky, was kind enough to have four seasons. (My native land of Florida only has Summer, and a brief not summer, then back to Summer.) This allowed for evaluation in a truly hot season, and a for-real cold season, with nice middle of the road seasons as well. Drum roll please: no real difference was seen in the adults based on time of year. This is different from what the British studies found, and may be a reflection of different types of climates or a change in parasites. Well, that’s interesting!

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Young Horses and their Shedding

Boys are ickier than girls. That was my fine feline conclusion. Foals shed a combination of Parascaris, and Strongylid sp. Young foals shed predominantly S. westeri as expected and boys do it earlier and more; girls are less and longer. Boys were just plain higher than girls on the other two. There was a hypothesis put forward that it may be because colts eat more poop than fillies. Think about that the next time one of them wants to give you a cute little baby kiss. Foals start shedding eggs pretty quickly, and peak at around 6-8 months of age. Foals were also susceptible to Anoplocephala sp (tapeworms), but it seems to take them 4-6 months to even get them. I felt like foals just like to spread their parasites around. It does make sense since their immune systems aren’t quite fully on-line yet, but jeesh, do they have to go contaminating everything? 

What About Pregnant Mares?

There’s work in cows and sheep to show an increase in parasite egg shedding around the time of calving and lambing. Based on the work on these horses, that doesn’t seem to hold true for horses. The very smart human (I mean, she is a Dr. Dr.) did caution this was a small group of horses. I say this is even more reason to do a fecal egg count before deworming! Why spend the money, and the time, and the aggravation deworming your mama mare if you don’t have to? Science says see if she’s even shedding parasites at all.

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Cat Conclusions

What’s this all mean? Never deworm again? That can’t be true. Tony, you’ve gone crazy, you say. This is a study that looked at a very specific herd of horses and asked a very specific question about shedding of parasites. This study didn’t discuss the risk level of the parasite load in the horses in the study. It does give us great insight into parasite egg shedding patterns when there is no influence of dewormers. For this wise cat, it also makes me an even stronger believer in fecal egg counts before deworming any more than once yearly. 

P.S. My Docs recommend once yearly because it minimizes the risk of a deadly type of colic caused by Strongylus vulgaris (large strongyles), and the less deadly, more annoying gas colics that Anoplocephala sp (tapeworms) cause. This study did not look at risk factors for complications when not deworming for 40 years, it simply looked at what worms were there and how many of them. In general though, it seems horses tolerate their parasites fairly well so don’t deworm them every month!! 

P.P.S. My Docs are excellent at helping you formulate a parasite plan for your property, and your horse’s lifestyle. Don’t go all willy nilly. The drugs available now all have resistance. With a little bit of planning, you can help them last as long as possible. Give my minions a call at (352) 472-1620. 

Until next week,


Tuesdays with Tony is the official blog of Tony the Clinic Cat at Springhill Equine Veterinary Clinic in Newberry, Florida. If you liked this blog, please subscribe below, and share it with your friends on social media! For more information, please call us at (352) 472-1620, visit our website at SpringhillEquine.com, or follow us on Facebook!

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