Your Guide to Horse Worming

Worming is a critical part of keeping your horse healthy and thriving. Understanding the basics of worm control, from choosing the right treatment to creating an effective schedule, ensures your horse stays protected year-round.

Worms will compete with horses for their food and, in some cases, suck the blood from the horse’s digestive tract, causing damage that will be permanent and can affect the horse secreting the chemicals necessary for it to digest its food properly.

So here's the essential steps to take in your worming strategy, and keep your horse healthy.

Conduct regular manure egg counts

The only way to truly determine whether horse deworming products are working is to conduct fecal egg count reduction tests.

This strategy for horse worm control strongly recommends the use of fecal egg counts (worm egg manure counts) to monitor the efficiency following worming and to identify individual horses in a group which have higher egg counts due to low natural resistance, close grazing habits which increases the ‘pick-up 'of infective larvae from pasture, as well as resistance build-up against a specific worming compound due to over-use and underdosing. Fecal Egg Count Reduction Testing (FECRT) can be used to monitor the seasonal prevalence of worms and response to a worming compound or combination of compounds.

Monitor worming populations

It is helpful to identify the individual horses which have a naturally high burden of Small Strongyles despite regular worming by collecting droppings and performing Faecal Egg Counts. This is best carried out 6 weeks after the last worming with an AZOLES (active ingredients such as oxfendazole and oxibendazole), and 10-12 weeks for a MECTIN (active ingredients such as moxidectin), as these wormers can suppress egg reappearance for a longer period.

Horses with Strongyle egg counts above 200 eggs per gram at 6 weeks or 12 weeks after worming, depending on the wormer used, should be retreated. After 10-14 days following worming, allowing time for female worms not killed to start laying eggs again, collect droppings to check if the wormer has been effective and calculate the Faecal Egg Count Reduction percentage.

FECR% = Initial EPG - After Treatment EPG x 100

If the wormer used results in a 95-100% reduction in egg count after 10-14 days, it is not likely to have a build-up of worm resistance.

Ideally, an annual check should be carried out to monitor the effectiveness of the wormer against Small strongyles. Any wormer which fails to reduce FEC by at least 90% should not be used.

Rotate the worming compounds

It is commonly recommended to change to a different class of worming product every 12 months, in order to minimise the risk of the development of resistant worms. This is called SLOW ROTATION and forms the basis for best practice rotational worming advice. This means that you should use one class of active ingredient for an entire year, and then switch to a different class of active ingredient the next year.

It’s important to know that the majority of horse worming products currently on the market have one of two major classes of active ingredient – they are either Avermectin (Mectin) based or Benzimidazole (Azole) based. If you are currently using any wormer with an active ingredient ending in ‘...ectin’ then your rotational choices are limited to wormers that do not contain any actives ending in ‘..ectin’. This is regardless of the other active ingredients contained within the wormer. To make the correct choice you must change to a wormer with an active ending in ‘...azole’. There are other classes of the active ingredient, but they are less commonly used and are generally used in combination with other actives to target specific parasites.

What are the types of worms that affect horses?

Small strongyles (cyathostomes): These are common and cause direct damage to the gut wall. The immature stages can encyst in the wall of the intestine for 1-2 months, and when they are released in spring, they can cause colic and diarrhoea. Resistance to anthelmintics (worming pastes) in these worms is becoming a serious problem.

Large strongyles: Some members of this group of worms invade the blood vessels supplying the intestines and can cause serious damage leading to colic, ill-thrift, diarrhoea and even death. Resistance is also increasing in this group.

Roundworms: These worms are generally only of significance in horses younger than 2 years. They can cause intestinal blockage, ill-thrift, diarrhoea, and even respiratory problems.

Pinworms: The most common sign of pinworm infection is tail-rubbing, due to the adult worms laying their eggs around the horse’s anus and causing irritation.

Tapeworms: These can cause colic, weight loss, diarrhoea, gut rupture and death. Their life cycle involves the oribatid mite that lives on pasture.

Bots: The adult bot is actually a fly that lays its eggs on the horse’s coat. These hatch into larvae and are ingested into the horse’s stomach, where they can cause ulceration and may even penetrate the wall of the stomach. Bots are extremely common in temperate climates.

To find out more about getting Faecal Worm Egg Counts done, visit http://www.dpi.nsw.gov.au/Worm-control-in-horses.pdf

Please use this article as an information guide only. Please consult your vet for more specific advice for your situation or if you are concerned that your wormer is not as effective as it should be.