Another equine health company, Eggscope, were in attendance with the Jenquine team at the recent Horse Care Expo in Samford QLD, Australia. With Dr Stewart’s experience and history regarding equine worms this sparked exciting conversation and with research always on the move we thought it would make a great edition to the blog topics. We hope you enjoy!


Let’s blog worms – horse worms! It’s nearly 40 years since I took a research sabbatical from my veterinary degree and spent a year collecting manure from horses all over NSW, Queensland and Victoria to harvest and hatch worm eggs and identify resistance to worm pastes.

Most of us are familiar with the frightening development of antibiotic resistance but may not be so aware that anthelmintic resistance is an ever-present danger. Because wormers were effective, we became a little complacent – but now that biological reality is reducing their effectiveness, we must develop smarter ways for controlling worms. In sheep and cattle, a combination of 4 or more drugs is often required to reduce mortalities from worms. And we are now facing the same prospect with our horses.

For my research, after incubating and hatching the eggs in a test tube, I could look at the larvae through a microscope and identify the worm species. The studies found that, as in sheep and cattle, BZ resistance (a decrease in the efficacy of a compound against a population of worms that were previously susceptible) was indeed occurring. Since my work in parasitology, nearly 400 generations of worms have passed through our horses and paddocks and ongoing, current international research is finding resistance is worsening, with some worms resistant to 3 or 4 different types of worm pastes – and what’s even more of a call to arms, is that resistant worms never again become sensitive to the worm pastes.

After the publication of the work on resistance, rotation of wormer types was widely adopted. But resistance to anthelmintics has diminished the effectiveness of rotating wormers and routinely worming every 2 months. All wormers have a finite lifespan because the worms adapt genetically to the drugs and worms today are not susceptible to the drugs that would have killed their ancestors. Also our understanding of the biology and ecology of horse-parasite relationships has expanded so new control strategies are possible – even so, many of us still think worm control is a simple recipe of bimonthly treatment.

The movement of horses across Australia and between countries promotes the virtually global spread of worm resistance. In Europe, resistance to one or more wormers has been reported in a range of countries. Of pressing urgency is the resistance of round worm (Parascaris equorum) – a big problem for foals, especially on studs. For adult horses the major threats are the small strongyles (cyathostomes) – a cause of colitis; and tapeworm (Anoplocephala perfoliata) a cause of ileocaecal colic. Both these parasites can and do kill horses. We must prolong, for as long as we can and until new drugs are available, the effectiveness of available wormers.

The idea of monitoring eggs in manure through a faecal egg count (FEC) has been embraced by livestock industries to reduce both the financial cost of worm control and the development of resistance in the worms. Resistance is defined as a less than 90-95% reduction in FEC, 14-17 days after treatment. A reduction in the time between treatment and the reappearance of eggs in the manure (ERP – egg reappearance time) is an early indicator of a shift towards resistance.

We need to have better, more responsible management of equine worms, improving the way we use anthelmintics to avoid unnecessary or ineffective treatments. Limiting the frequency of dosing, ensuring correct dose rates and treating on the basis of FEC is highly effective in reducing worming frequency and pasture contamination. Combining with manure collection from pasture, is more effective than worming alone to reduce pasture larval levels and slow development of resistance. There are no new equine anthelmintics under development, so it is imperative that the efficacy of any currently‐effective drug classes be maintained for as long as possible. In many countries it is now routine to monitor anthelmintic effectiveness using FEC before and after treatment.

Most of us have little knowledge about the true prevalence of anthelmintic resistance or the resistance status in our horses and on our pastures – you could be religiously worming every 6-8 weeks with a drug that is totally ineffective. The recommendation that horses be wormed bimonthly was developed in the 1960’s primarily to control large strongyles (bloodworms), and the occurrence of verminous colic is now unusual. At that time we considered small strongyles just a ‘bit of a nuisance’. Now we find ourselves in a situation where they have developed high rates of resistance to all commonly used wormers except avermectin/milbemycins and are considered the principal parasitic pathogens of horses.

The FEC and ERP are the gold standards for drug resistance and your veterinarian will use them to determine the presence of drug-resistant worms in a horse and on a farm. When done on a regular basis, FEC help measure the effectiveness of a worm control program and identify those horses that do not need frequent treatments. Frequent use of anthelmintics in an attempt to keep FEC near zero is not a sustainable approach and may actually increase the risk of disease by inhibiting the development of immunity. Rotating drugs with each treatment does not slow resistance, and can in fact increase it and the risk of multi-drug resistance.

When we ask why we worm our horses, the obvious answer is…’to kill worms!’ But, killing worms is not the objective - especially for encysted small strongyles (cyathostomes) that do most damage before they are susceptible to wormers. The only practical way to decrease infection is to decrease the number of eggs on the pasture – by killing female worms before they produce eggs. So the answer to our question really is ‘…to prevent pasture contamination with eggs’. The pasture survival of the eggs and hatching larvae is dependent on climate and weather – so worming schedule depends on geographical location, the FEC and the ERP. For the best advice, discuss your program, drug selection and frequency and how to FEC with your veterinarian.

Dr Jennifer Stewart