06/11/2024
LETS TALK WORMS!
All about Parasites- Part 1
In 2023, the UK reported the first cases of moxidectin resistance in equine parasite populations. This is a scary finding, particularly as it is likely NZ is following a similar pattern to the UK. It is imperative we begin to challenge old fashioned worming practices and change the way we worm our horses to protect the efficacy of our wormers as well as our equines.
It is common practice in NZ for owners to worm horses more frequently than they likely need. Many are still worming 3 monthly (or more), and most are blanket deworming all horses in Autumn in fear of red worms. Instead of doing what we have always done, we need to stop and ask ourselves- why are we doing this? Is this still the best way to protect our horses? Is this protecting the efficacy of our wormers? In the last two years there have been updates published around the world on this topic and new management guidelines recommended by varying organisations. To make sense of these new recommendations, and to help you understand the science behind worming practices, Forme Equine alongside veterinary professionals, has created this short informative series.
Before we can get into what to do and not to do, we need to know what we are fighting!
Broadly these days we are concerned primarily about the following worms:
Tapeworm
Ascarids (roundworms)
Cyathostomes (redworms)
This is not a comprehensive list. There are other worms that affect horses, but these are the ‘big 3’ we shall discuss in this article. Owners often cite pinworm and bots as concerns as well so we will touch on those briefly. Strongylus vulgaris (aka the blood worm) will be mentioned in later articles
Ascarids:
These are the roundworms and they can measure up to 50cm long! Generally we think of them as worms that affect foals or youngstock. They can affect adult horses if there is a high burden on the pasture, and particularly if there are comorbidities.
Ascarid larvae are hardy- their larvae live inside a protective egg that can survive months-years even after a prolonged frost. Once ingested, they migrate via the bloodstream, through the liver and lungs and wriggle to the throat where they are swallowed, reaching the small intestine 3 weeks post-infection. It takes them many weeks to mature, producing eggs within 10-16 weeks of infection.
There can be no clinical signs but some foals present as ‘ill thrifty’ or colicky. Large
burdens of ascarids can cause impactions that require surgery to be resolved, so they are not to be trifled with.
Ascarids are widely resistant to the class of wormers called macrocylic lactones (MLs) which includes ivermectin, moxidectin, and abamectin. They are, however, generally sensitive to benzimidazoles (BZs) such as fenbendazole. Unfortunately, resistance is reported in some areas of the world.
Tapeworm:
Equine tapeworms are found towards the junction of the small intestine and the caecum, and large burdens have an association with specific forms of colic in adult horses. They can be hard to pick up on egg counts, as eggs are found within segments of the worms itself (called proglottids) that are shed intermittently into the faeces.
Praziquantel is our main treatment for tapeworm in NZ, and is often included in wormers as a combination. As of writing, there are no studies looking into resistance of tapeworms but it seems likely that it is developing as praziquantel has been used frequently for a long time. Generally, once a year treatment is effective at controlling tapeworm burdens in horses.
Pinworms and bots:
The main problem with pinworm infestation is the itchiness it causes around the perineum and tail. Washing this area is the most effective way to address this, to mechanically remove the sticky eggs that cause the itching. The use of wormers topically is not recommended as it does not get to the adult population of worms, and washing alone is sufficient for the eggs.
Bots are often a reason people want to worm their horses, but bots are rarely of clinical concern in the stomach and their presence alone does not justify a worming treatment. Bots can be annoying to horses and owners, so exercising fly control is recommended rather than deworming treatments.
Cyathostomes:
These are the most prevalent worms we see in adult horses and are also known as red worms or small strongyles. They are ubiquitous in all grazing horses with a prevalence of 90-100%. More than 40 species of cyathostome exist, and we consider these to be more relevant in adults than foals, but can affect both.
Cyathostomes have a stage in their lifecycle where the larvae, once ingested by the horse, can encyst into the lining of the gut. This tends to happen over the Autumnal period, and emergence of the larvae occurs in Spring. Mass emergence can potentially lead to significant damage to the gut, resulting in diarrhoea, colic signs, and potentially very severe disease. This is known as larval cyathostominosis. It is the main reason why worming recommendations in the last several years have focused on blanket deworming in Autumn with moxidectin.
The caveat- fortunately, larval cyathostominosis as a condition is pretty uncommon these days. However, as moxidectin has been leant heavily for control of cyathostomes, resistance is now starting to emerge with the first reported case in the UK in 2023. This is concerning because if larval cyathostominosis (though rare) does occur we need our wormers to be effective and it is clear that efficacy is being lost. Rather than blanket deworming, good management of horses and pasture,risk assessments, and well timed efficacious worming is key. Additionally, when we do faecal egg counts, we measure the adult burden not the larval burden of cyathostomes. Currently in NZ there is no way to assess a larval burden in a live horse. Thus, knowing what wormers are effective against what stage of the lifecycle, and the epidemiology of the worm is crucial in guiding the recommendations your vets are making.
Take home points:
- Worms have variations in their lifecycles and affect different age groups in different proportions.
- This means there is no one wormer that has a perfect strike rate against all worms, at all stages, in all horses.
- Therefore, we need to know what we are targeting, when we want to target it, and why
- We need to have a multimodal approach to protect our wormers, and our horses.
In the next part of the series we are going to look at the wormers available and how their efficacy has changed over time. We will also highlight current concerns with parasite management, and common pitfalls.
Diagram credit: https://www.worldhorsewelfare.org/advice/worms-how-to-control-them-in-horses
This diagram, sourced from World Horse Welfare takes us through the basic lifecycle of equine worms. Different parasites have some variation on this, and the time it takes each worm species to go through this lifecyle varies however this gives you a good basic understanding.
