Worming horses

Parasite control can become a burden on both horses and their owners, so here are a few basic concepts to consider when thinking about drenching your horse(s).

In the past, we have relied on faecal egg counts (FEC) to monitor worm burden in horses. Unfortunately this method is now known to be unreliable in horses - FEC results do not properly reflect worm burden. However they do still give useful information and should be used to:

  • Identify horses which are contaminating the pastures (high shedders)
  • Identify anthelmintic resistance and assess drench efficacy
  • Monitor the types of eggs foals are excreting for accurate treatment

FEC results should be interpreted on an individual basis and discussed with your vet, as a suitable cut-off point for drenching can vary. We know that 20% of the adult horse population will excrete 80% of the total egg output. Therefore, by selecting an appropriate FEC level we can effectively decrease the pasture contamination. Instead of removing all the eggs from the pasture, we want to decrease the level so the parasites still stimulate and improve the horses' immune response to them.

Young stock, including two year olds, are high shedders as they are yet to build an effective immune response to the parasites, so they often require more regular drenching to control parasite contamination. In comparison adult horses should be able to build an immune response to parasites, however sometimes their immune response wanes, or it cannot control a very high parasite burden, in which case drenching will be required. In the spring and autumn ALL ages of horses should be drenched with a product containing the active moxidectin (for example Ultra-moxTM) to control cyathostomes which inhibit over winter, hiding from the immune system, and can cause colitis/colic.

Monitoring drench efficacy is also very important and is easy to do. By collecting a fresh faecal sample 10 to 14 days post-drenching and doing a FEC we can identify if resistant parasites are present. Resistance to most drench families has been identified so early warning of resistance emerging on your property is critical.

Other ways to help control parasites include:

  • Picking up faeces once or twice weekly is an excellent way to reduce the pasture contamination and increase the grazing area by around 50%. This is because most larvae migrate less than 15cm from the faeces and few horses graze close to faeces unless the stocking rate is high.
  • Other helpful management practices include cross grazing with other livestock species and decreasing the stocking rate.
  • Harrowing only controls worms if the weather is hot and dry for extended periods of time (conditions which are not common in the Manawatu!) so unlikely to be of use.

If you are concerned about the health or parasite burden of your horse please contact your vet to discuss an individualised parasite management plan best suited to your horse(s).