Worm control. Postdoctoral research fellow at the University of Queensland, Dr Anne Beasley, explains the importance of sustainable worm control and why we must no longer rely on the old-school interval treatment regime.

This article provides all the basics on parasite biology and the drugs available for treatment. This knowledge will empower you to do better and contribute to slowing down the slide towards resistance. 

Most people would agree that worm control in horses can be confusing and is further complicated by a plethora of available worming products containing different ingredients and conflicting advice on how to best use them. Unfortunately, there is no ‘one-size fits all’ approach to worm control – not even on a regional basis. You and your neighbour may well require different worming strategies to best manage worms on your property.

The good news is that researchers have moved in leaps and bounds towards understanding the many factors that contribute to sustainable worm control. The bad news is that worming practices of the past, which were developed under very different circumstances, are still being used today.

The slippery slope to resistance

The approach of treating all horses at regular, and sometimes short, intervals leads us down a slippery slope towards the emergence of drug-resistant worms. This article introduces a variety of information that can be used to create an individualised worm control program that aims to prevent heavy pasture contamination and, perhaps more importantly, slows down the development of drug-resistant worms.

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What worms are important?

Horses are host to a large number of internal parasites, some more of a health risk than others. Rather than review them all in depth, here I will focus on those that should be the main target of your worming program in young and mature horses.

Foals are born with very little resistance to worms and so they are particularly vulnerable to infection. The most important worm infecting foals is Parascaris equorum, the intestinal roundworm. The pre-patent period (time taken from ingestion to eggs being passed in faeces) is between 72 and 110 days, so the foal will have adult worms in the intestine from about three months and heavy infestations can quickly build up.

Signs of infection in foals will vary depending on the stage of the worm life cycle and the number of worms present. Small numbers usually cause no harm, but large numbers of migrating larvae can cause symptoms of respiratory disease, such as fever, coughing and nasal discharge. Due to their exceptional size (about the dimensions of a pencil), large numbers of mature Parascaris in the intestine can cause blockage, severe colic and, in the worst instance, intestinal rupture.

Within the foal’s first year, strong immunity develops to Parascaris, so that by 12-15 months of age most will have cured themselves of infection.

As the foal ages and Parascaris becomes less of a problem, the cyathostomins, or small strongyles, step into the spotlight. Horses of all ages are susceptible, but particularly weanlings and yearlings while their immunity is still under development.

Cyathostomins are by far the most prevalent group of worms (there are 54 species!) and they cause disease in the horse when large numbers of larvae simultaneously emerge after a period of arrested development. The disease, known as larval cyathostomiasis, is characterised by a sudden onset of diarrhoea followed by severe weight loss.

Tapeworms (Anoplocephala perfoliata), in large numbers, can induce poor growth, ill–thrift, spasmodic colic and ileal impaction.

The threadworm (Strongyloides westeri), pinworms (Oxyuris equi), stomach hair worms (Trichostrongylus axei), the spirurid worms (Habronema spp and Draschia megastoma) and bots (which are not worms, but larvae of the bot fly – Gasterophillus spp), are less likely to be associated with clinical symptoms and take a back seat to parascaris and cyathostomins as targets of treatment.

Once considered a major threat to horse health (and rightly so), were the large strongyles (Strongylus vulgaris and others), however, with the introduction of interval treatments using highly effective anthelmintics, these species are now much less prevalent.

Horses usually harbour a mixed infestation of many of these worms and otherwise healthy horses can carry small or moderate burdens and continue to thrive with very little, if any, measurable impact on health and performance.

Clinical disease is only a risk in the case of heavy infestations or in horses that are otherwise immunocompromised (e.g. nutritionally deficient).

Some of the worms you might recognise in your horse’s manure following worming are: cyathostomins (small stongyles) which are small, approximately 1cm in length, white or pink/red in colour; Parascaris (roundworms) which are large, approximately 20cm in length, white in colur; pinworm, which are large, approximately 10cm in length, white with characteristic tapering tail, and tapeworm, which are pale, about 1-2cm in length, slightly triangular in shape, wide and flattened.

Anthelmintics (worming products)

Current horse wormers contain active ingredients which belong to one of three separate anthelmintic classes, and sometimes a combination of two. Each class has a different mode of action against gastrointestinal parasites and slightly different efficacy against each, so careful consideration needs to be given when choosing an appropriate product. The anti-cestodal drug, ‘praziquantel’, is often used in combination with anthelmintics for the control of tapeworm.

A useful summary of horse worming products and their effectiveness against various horse worms has been prepared by the NSW Department of Primary Industries in Primefact 976, Worm control in horses.

An important feature of each drug class is its ‘egg reappearance period’ (ERP), which is the time taken for eggs to reappear in the faeces after treatment. A summary is given in Table 1 below:

Table 1. Classes of drugs available for treatment of horse worms

  • BENZIMIDAZOLES (BZ’s) Oxibendazole, Oxfendazole, Fenbendazole – The ERP is four weeks
  • TETRAHYDROPYRIMIDINES (THP’s) Morantel, Pyrantel – The ERP is four weeks
  • MACROCYCLIC LACTONES (ML’s) Abamectin – The ERP is six to eight weeks
  • MACROCYCLIC LACTONES (ML’s) Ivermectin, Moxidectin – The ERP is 12 weeks

Modern worm control

Out with the old-school interval treatment approach to worm control

This system was originally recommended many years ago when the very harmful large strongyles (particularly Strongylus vulgaris) were highly prevalent and the predominant target for worm control. The aim was to keep egg-shedding as low as possible and, as such, anthelmintics were administered to all horses in the mob at short, regular intervals.

Indeed, the reign of large strongyles as the major parasite threat to horses is over and other worms (Parascaris and cyathostomins) have taken their place as primary targets. Although now outdated, the interval approach is still widely used. Most often, a ML product (e.g. Ivermectin) is administered every six-eight weeks (the observed ERP) – that’s up to eight treatments every year.
We now know that frequent treatment of all horses in this way selects strongly for drug-resistant worms – and once resistant, always resistant. It’s a one way street with no u-turns! Therefore, it is absolutely urgent that we adopt more sustainable approaches to worm control.

How do we move away from the interval treatment approach?

There are a number of factors that we can use to our advantage when designing a worm control program, primarily targeting cyathostomins (small strongyles) in mature horses, that relies on fewer worming treatments – remembering that our aim is now two-fold; firstly, to minimise pasture contamination with infective larvae and, secondly, to slow down the development of drug-resistant worms.

Work with nature

Nature herself has a major impact on the availability of infective larvae on pasture. For eggs in faeces to develop to hatching stage, they need warm conditions (within the broad range of 6oC – 38oC, the optimum being around 25oC). The cooler it is, the slower the development. Once hatching has taken place, however, the infective larvae are affected by temperature in the opposite way. Warmer temperatures (especially above 32oC) greatly reduce their survival, whereas cooler temperatures are more favourable. In fact, larvae can even tolerate freezing.In the context of a south-east Queensland (sub-tropical) property, for example, the bad news is that climatic conditions allow egg development and larval survival year-round, making paddock spelling an unreliable method of preparing ‘safe’ pasture. Some handy knowledge though is that peak pasture infectivity usually occurs during Spring/early Summer and again in Autumn/early Winter.

Treating horses immediately prior to these high-risk periods (i.e. in late Summer and late Winter) helps to reduce those peaks. Winter is also an effective time to treat for bots as the bot larvae ‘overwinter’ in the stomach and wait for warmer conditions to pass out of the horse and pupate into flies.

Unfortunately, nature never gets the upper hand on parascaris, whose eggs are so resilient that they are found wherever there are young horses world-wide.

Not all horses are equal

Pasture contamination with cyathostomin eggs is not generated equally by all horses in a herd. It is now known that the majority of parasites (80%) are harboured by a minority (20%) of individual horses, making these few the source of most of the contamination.

If these individuals can be identified (via a faecal egg count a couple of weeks after the ERP of the previous worming product), an opportunity for targeted treatment exists, whereby more treatments can be administered to these horses and fewer to those that shed fewer eggs.

The commonly used thresholds for ranking horses on faecal egg counts are: <200 epg = low egg-shedder; 200-500 epg = moderate egg shedder, and >500 epg = high egg-shedder.

Pasture management 

Pasture hygiene can have a big impact on pasture contamination. Weekly (minimum) removal of manure from yards/small areas greatly reduces the number of larvae available to infect your horses.

Let them hide

Perhaps one of the biggest advances in the understanding of the development of drug resistance is the concept of ‘refugia’.

Put simply, the portion of the parasite population that escapes exposure to the drug is considered ‘in refuge’ – for cyathostomins this comprises encysted larvae in the wall of the large intestine (most products do not kill these), larvae already on the pasture and worms in any untreated animals.

If there is little or no refugia, resistant worms surviving a drug treatment go on to make up the majority of the breeding population for the next generation. If there is a substantial pool of worms in refugia, then resistant worms are in effect ‘diluted’ among the susceptible worms, thereby slowing the progression of resistance. The historical approach of treating all animals on a property at the same time, and at excessively frequent intervals, removes a large amount of refugia and, therefore, encourages the development of resistance.

Never under-dose

Always administer your worming treatments based on an accurate measurement of horse weight. Most people underestimate their horses’ weight by at least 10%, so if you can’t weigh your animals on an equine specific weighbridge, use a weight estimation formula or a weight tape and add 10% to be sure you are not under-dosing. Resistance develops rapidly when worms are exposed to a sub-lethal concentration of drug.

Recommendations and advice

Evidence-based, Targeted, Strategic worming (E.T.S.)

Eradication of worms from horses is not achievable and nor is it sensible. Rather, a balance should be sought that allows adequate exposure (for the development of immunity) and provides sufficient refugia (to slow down resistance), while maintaining clinical health of our horses.

The best way to achieve this is by using an ‘E.T.S.’ treatment regime; one that is evidence-based, targeted and strategic.

E. is for the ‘evidence-based’ part of this regime, which is based on periodically monitoring faecal egg counts of horses and only worming when necessary. It also means only using drug classes you know are still effective on your property. There are various diagnostic laboratories nationwide that offer faecal egg counting services, and you can carry out a faecal egg count reduction test by collecting and submitting samples from a group of horses on the day of treatment and again 10-14 days after treatment. Monitoring is often cheaper than worming a whole mob of horses unnecessarily or with a product that doesn’t even work!

T. is for the ‘targeted’ part, which means identifying those horses on the property that require more frequent treatments and giving fewer treatments to those that are low egg-shedders.

S. is the ‘strategic’ component, which comes from applying knowledge about the worm life-cycle and ecology. As discussed earlier, timing treatments to pre-empt high-risk seasons and using pasture hygiene (if practical) to break the life-cycle are very effective. Another very effective weapon (where available) is to graze horses in rotation with cattle/sheep (who harbour different worm species not infective to horses). The ruminants will ‘vacuum’ up much of the equine worms.

These approaches should help to reduce the number of treatments required every year for mature, grazing horses. This includes broodmares which, on such a monitored and controlled program, should not need an additional treatment at foaling to protect against Strongyloides westeri infection – leading parasitologists regard this worm as a non-issue.

There is less room for improvement in treating foals and young horses as they are more susceptible and their egg counts quickly rise to high levels following the ERP. These horses (weanlings, yearlings) should be grazed separately to older stock and it is still imperative to test the efficacy of your worming product (especially the ML class) against Parascaris, the main target for treatment in young horses. Despite the common recommendation that foals be treated as early as four-six weeks of age, there is no real value in treating earlier than eight weeks.

Remember, the primary target should be mature Parascaris worms that are not yet passing eggs (this is the easiest stage to kill). Regular treatment will then usually be required within a couple of weeks of the ERP until the horse starts to regulate its worm burden through acquired immunity.

Horses housed in stables or dirt yards that are not exposed to contaminated pastures should require less frequent treatments, provided manure is regularly removed. If the horse is wormed into the stable/yard, treatment every three-four months should suffice (maybe even less for low egg-shedders) – check that treatment is required first by monitoring faecal egg count.

It’s time we stopped relying solely on the convenience of the old-school interval treatment regime and put the effort into getting to know the ins and outs of sustainable worm control. It begins with learning the basics of parasite biology and a little about the products available for treatment – and when we know better, we can do better. The threat of resistance should be a motivating factor in changing the way we manage worms on our properties.

To download a handy 6-Step Guide to Worm Management click here. (Sign up required)