There are numerous types of gastrointestinal parasites or “worms” as they are more frequently referred as. Worms are a burden on the horse’s gastrointestinal tract (GIT). This burden can range from being completely sub-clinical (the horse shows no ill effects from the infestation) to reducing a horse’s performance in events, causing colic, diarrhoea, weight loss and in severe cases horses can die as a result of parasitic infestation.
The most common and pathogenic worms that infest horses are, cyathostomins (small worms or small redworms), large strongyles (large red worms) and tapeworms. Large red worms were historically a problem but are now far less prevalent and cause less clinical disease. Small red worms are very common affecting all ages but mainly young horses (1-4 years old). Most infections are subclinical.
The small red worm life cycle: the larvae are eaten by the horse off the pasture and move through the GIT. They then burrow into the gut wall and can stay there for weeks, months or even years. They stay there until conditions are just right then they emerge from the gut wall and mature into adults where the adults produce eggs that are shed in the faeces. The eggs in the faeces then develop on the pasture into the larvae that are eaten and so the cycle continues.
The larvae in the gut wall are called “encysted” and these are very difficult to treat. Only two drugs can target these encysted larvae: a 5 day course of Fenbendazole or Moxidectin. If a large number of worms erupt from the gut wall at the same time then horses can develop severe problems. Encysted larvae don't produce eggs and so it is important to remember that a horse can have a low faecal egg count (FEC) but still have a high worm burden.
The tapeworm life cycle is different as they don't burrow into the wall but they shed their eggs intermittently so a negative WEC does not mean no tapeworms. A high tapeworm burden can cause colic. A blood test is available to detect antibodies against tapeworms, but this only gives an indication that they have been exposed at some point in the last 6 months.
|Adults||Encysted Larvae||Annual/6 month dose|
|1 d Fenbendazole||√||χ||χ|
|5 d Fendendazole||√||√||χ|
Anthelmintic “wormers” have different effects and so target different worms at different stages of their life cycle. This table shows which worms are killed by the different active ingredients in wormers.
80% of worms produced in a field come from 20% of the horses. This means that the majority of horses on a pasture have few worms and so produce only a very small number of eggs. These horses are ‘low egg shedders’ and they do not need worming as they won’t be suffering adverse effects of having worms and won’t be significantly contributing to pasture contamination. The small group of horses that make up the 20% will be ‘high egg shedders’. These horses will have a FEC>200 eggs per gram. For an unknown reason these horses will be persistent egg shedders despite the same or similar pasture management as their herd mates. It is these horses we need to target with wormers to reduce pasture contamination and infectivity.
Each time you worm your horse a small number of the worms present will be resistant or “immune” to the effects of the wormers you use and so will survive worming. If you repeatedly give the same type of wormer to all horses on the pasture, over time the number of resistant worms in your horses' GIT will increase until all the worms are resistant and your wormer will no longer work. The oldest wormers (Fenbendazoles) have the highest resistance and the newest wormers have the least resistance (Moxidectin). It is important to understand that there are no new wormers being generated. Donkeys are already resistant to Moxidectin.
If you are worried about resistance developing on your yard then your vet can perform a faecal egg count reduction test (FECRT). This test measures the efficacy of the wormer you have used.
To reduce the development of resistance we can do four things:
1. Allow a population of sensitive worms “refugia” to survive the worming process. These refugee worms will dilute and compete with the resistant worms for resources. This competition will stop the development of an entirely resistant population. To do this we have to use a targeted worming protocol.
2. Reduce the use of wormers by only worming those horses with a FEC above 200 epg.
3. Manage the environment to minimise our reliance of wormers. We can do this by reducing the number of horses per acre, regular poo picking (everyone’s favourite summer pastime), mixed grazing with sheep and cattle and finally harrowing fields on hot dry days so the sun destroys the eggs in the soil.
4. Quarantine new horses so they don't bring resistant worms onto the yard. New horses should also be wormed for tapeworm and encysted larvae before being turned out.
Firstly, no one protocol will work for every yard and if you have any concerns please consult your vet about your specific yard requirements so we can tailor a programme for you.
Regular worming - worming every 4-6 weeks will reduce disease BUT will lead to resistance and a shift in the type of worms you will see, so this is NOT a sustainable management protocol. Worming at specific times of the year can be successful in disrupting the worms' life cycle, however, changes in weather pattern or the introduction of heavily contaminated individuals will reduce your success and won’t help heavily burdened horses.
Targeted worming - treating each horse as an individual is the BEST strategy. You must use a FEC to select those horses who are shedding >200 epg and only worm those horses, the “high egg shedders”. By doing this you will reduce pasture contamination and reduce the development of resistance. It is also a CHEAPER worming protocol. You must have a FEC from every horse because 80% of the worms are produced by 20% of the horses so in a herd of 30 horses only 6 will be significantly shedding eggs but you won’t know which six unless you sample all 30.
We recommend that as part of any worming strategy, tapeworm should be targeted twice a year in autumn and spring, and that encysted worms are targeted in winter.
Chris Baldwin, BVetMed, MRCVS