Intestinal worms have been associated with causing serious problems such as weight loss, colic and diarrhea in horses and as a result horse owners spend significant amounts of money every year on internal parasite control. Download a PDF of this article to print.
Traditionally the so called blood worms or large strongyles were the target of strategic worming programs and given their life cycle generally they are well controlled with 6 monthly worm treatments. However, small stronglyes or Cyathostomes are becoming increasingly recognized as a problem in horse populations. Their life cycle is more complex and generally it has been recommended that horses be wormed every 6-8 weeks, often with different worming products on each occasion.
It was thought that rotating worm products and using a different product each time may help prevent drug resistance, however more recently the effectiveness of this approach has been questioned. Unfortunately it would seem that in Europe at least, this approach is encouraging resistance to develop in these parasites to some of the products used. The small strongyles have encysted larval stages that lie within the gut wall and are often not killed by commonly used wormers. To account for this veterinarians often recommend using wormers that contain moxidectin or fenbendazole. For the past few decades horse owners and veterinarians alike have been following these guidelines, however new research suggests we should be taking another approach. While resistance to some of the older worming products has been suspected for a long time and in some cases proven to exist, the concern now is that resistance may be developing to some of the newer products available to horse owners.
Studies in Europe have found that not all horses carry the same numbers of worms and in fact approximately 20% of horses shed 80% of worm eggs. This has led researchers to recommend performing faecal egg counts on all horses on property and then only worming the horse that are shedding the highest number of worm eggs as an effective way to reduce the expense associated with worming and reduce the risk of developing drug resistance.
Management strategies to prevent worm burdens:
- Use a program of regular drenching. For example, use Equest Plus Tape every 14-16 weeks.
- Drench all horses in a group at the same time.
- Drench all new horses before the enter the property and wait 48 hours before introducing them to pasture.
- Remove manure regularly.
- Feed horses in bins or buckets to keep their feed off the ground.
- Keep stocking rates as low as possible.
- Keep horses in age groups with the youngest horses on the least contaminated pasture.
- Ask your veterinarian to perform faecal egg counts to see which horses are shedding the most worm eggs.