Same Paddock, Different Count
Why do horses in the same paddock have different worm egg counts??
One question that I am asked a lot, and many clients struggle to understand, is how horses in the same paddock under the same management conditions can have such variation in the FEC results.
So, I am going to do my best to explain why in this post.
Imagine this… We have a herd of 10 horses, with no new horses coming or going. They all live in the same paddock, and we are committed to worm control. We regularly poo pick the paddock, there are no other horses nearby and the poo from the poo picking is taken far away from the paddock.
The horses regularly have worm egg counts every 2-3 months throughout the year. Here is their typical results:
Sunny 0, George 50, Sam 150, Max 0, Rudi 650, Colin 50, Red 0, Coco 0, Stanley 900, Milo 0.
The numbers mean eggs per gram (epg). Anything below 200epg is considered low, 200-500epg is medium, and over 500epg would be high.
We want to know why Rudi and Stanley have high worm egg counts when the rest of the field don't, despite the hard work of poo picking 2-3 nights a week.
The answer is in the 80:20 rule! We know that only 20% of the horses in a well-managed herd will contribute to 80% of the worm burden in the paddock. But what does that mean?
Looking at the diagram/picture… there are 10 horses in the paddock, and the orange things inside their belly are worms. As you can see, only 2 out of the 10 horses have a high level of worms (circled in red). These 2 horses will be pooping the highest amount of worm eggs onto the paddock. But why only these 2 horses?
It just comes down to individual susceptibility and immunity to the worms. Assuming we don't have young or old horses or horses with any kind of immune compromising disease (such as Cushing's) there will always be some horses that are just more likely to have worms than others.
We want to identify these high shedding horses before they contaminate the paddocks too much and this is where the worm egg counting becomes really valuable - it helps us to identify the high shedders in the paddock so we can treat only those horses with wormers.
This means on well managed herd only 20% of the horses will need to be wormed.
Despite the best management these high shedding horses regularly come back with high counts - that is just unfortunate and comes down to individual susceptibility. Other times we manage to reduce those worm burdens and keep them down long term. But the only way to know for sure is to check - every 2-3 months.
In summary, worm egg counts are the best way to manage worming in your horse. Only 1 in 5 horses will likely need to be wormed.
Reference & inspiration: This explanation is based on a fantastic example shared by Ridings Equine Vets (www.ridingsequinevets.co.uk), who do a wonderful job of educating horse owners about strategic worming.
