AS we think more and more of housing our horses before the winter, we should think of what this transition time means to them. ‘Dr Green’ generally provides for our equines, in particular on limestone-rich land. But housing, even part-time, means a change in diet and management. It is important that we make the transition as gradual as possible and minimise the potential for unintended consequences, such as colic. And our horses may take particular health issues with them from one environment into the next, obesity for example.

Thinking parasitic disease in particular, despite the name, not all individual parasites are truly harmful to all hosts at all times. Some, like an uninvited dinner guest might just eat your food but then go quietly on their way to tomorrow’s free lunch! Increasingly we understand that balance is everything, we shouldn’t aim to exterminate all parasites with drugs at all times! Because attempting to do so, we now understand, promotes the survival of drug-resistant organisms.

Nature abhors a vacuum, and space created by the killing of sensitive parasites is soon filled by resistant ones. ‘Refugia’ is the new watchword: it seems counter-intuitive but we must encourage manageable levels of (drug-sensitive) parasites to survive. More specifically, promote the survival of drug-sensitive strains of parasite at low levels in adult animals that can well tolerate them.

Antiparasitic drugs, like antibiotics, do a great job when used in the right place at the right time, in animals carrying significant disease-causing parasite burdens, timed for maximum benefit.

But sadly, drug resistance (among parasitic as well as bacterial organisms) is a very real, scientifically proven and ever escalating emergency. The scientific reality is that every use of an anti-parasitic drug (as for antibiotic use) selects for any resistant organisms encountered. In the animal’s gut, on the animal’s skin, in the environment, wherever we use chemical agents and administer drugs.

Published studies related to drug resistance of gut worms in grazing ruminants in Ireland include one recently by Kelleher and others. We must not think that grazing equines are in any way immune to this problem. We have to recognise the limitations of even ultra-modern medicines; the emergence and survival of resistant strains of parasite; and the growing need to safeguard these drugs for future use.

Modern thinking on parasite control is not to treat blindly, but first to diagnose the need and then to treat with a product containing the appropriate drug, at the correct dose and in a targeted manner. Many of these products are currently labelled LM, supplied via licensed merchants, equestrian stores and pharmacies as well as veterinary practices; they can be sold over the counter by Responsible Persons.

New rules

From January next, antiparasitic products (when licensed for use in food-producing animals, which horses legally are) will become prescription only medicines (POMs). In Ireland only vets can legally write prescriptions for animal medicines. Vet nurses, pharmacists and Responsible Persons currently dispense certain POM medicines; they never prescribe. The declared position of DAFM is that the dispensing outlets for antiparasitic drugs will remain unaltered next year; the new requirement is that there be a veterinary prescription to match the supply. This would then be similar to the current regime regarding the supply of mastitis control tubes, intramammaries for dairy cows, often dispensed on foot of a veterinary prescription via agricultural merchants.

This change in relation to antiparasitics has proved controversial and excited much comment. It came on foot of recommendations made by a panel of experts set up by the Advisory Committee on Veterinary Medicines, a part of the HPRA (Health Products Regulatory Authority). This is a committee I now chair, but had no involvement in their deliberations at that time. I also organise and teach on a DAFM-approved course for Responsible Persons – who go on to dispense a wide range of animal remedies.

We should bear in mind that the status of a type of drug is largely motivated by a concern to protect animal and/or human health: agents we use to anaesthetise animals, for example, that have the potential to do harm, are generally classified as VPO – Veterinary Practitioner Only.

Similarly, a change in status of drugs such as antiparasitics to POM is motivated by a need to protect animal health and welfare by preserving future efficacy of these drugs. Just like reserving certain antibiotics for use in humans only and not animals, is motivated by the need to have ‘antibiotics of last resort’ to treat serious infections in people.

Manufacturers, wholesalers, retailers and end-users understandably may not favour further regulation or restriction on supply. But we have to use less of these drugs in the future and what we do use, do so in an increasingly targeted fashion. The major and very real worry is that if we continue as we have done over the past 30 years since I graduated, we won’t have any effective anti-parasitic products in another 30 years. For anyone to prescribe or dispense, for owners to administer or for animals to benefit from.

So look for evidence that your horses harbour parasites – lice, small and large red worms (stronglyles), liver fluke, lung worm, bots, tapeworm – in numbers that cause harm, before reaching for the medicine cabinet. We can’t assume that every ‘wormy-looking’ equid truly has worms. Faecal (dung) worm egg counts, skin scrapings and blood tests all form part of the armoury.

Be suspicious of drug resistance among the gut worms your horse herd harbours if treatment doesn’t yield the desired results. We then need to perform faecal egg count reduction tests, taking samples before and at set intervals after treating, to determine how effective the drug treatment has been.

Additionally, pay attention to the environment your horses occupy. Management of parasite levels on horse pasture is best achieved by removing dung (by hand, sweeper or vacuum), resting for a season, change of use (e.g., tilling) and grazing by cattle or sheep.

These strategies have the added benefit of improving both the nutritional quality and appearance of your land. Not all are practical in all situations, but horse-folk could consider which work best for them. Better this than an environment populated by drug-resistant parasites causing disease in horses that we can’t then effectively treat.

We need to strike a balance with nature, not work against her. Resilient, otherwise healthy adult horses can well cope with moderate burdens of bots in their stomach, small stronglyles in their small intestine and tapeworm at the next junction lower down (ileo-caeco-colic if you must!).

Treating blindly with a de-wormer kills off all drug-sensitive worms leaving only drug-resistance worms in situ; to divide, multiply and conquer!

To read more on a study related to drug resistance of gut worms in grazing ruminants in Ireland: irishvetjournal.biomedcentral.com/articles/10.1186/s13620-020-00167-x