Youngstock parasite control: Understanding the new CANTER recommendations
Claire Shand - Director at Westgate Labs, AMTRA SQP, CANTER Core Steering Group Member
Sustainable parasite control centres around evidence based, diagnostic led prescribing to slow the development of anthelmintic resistance. Young horses, however, present a different challenge. Their developing immune systems, changing parasite populations and increased susceptibility to disease require a different balance between protecting the immediate health of the individual and preserving wormer efficacy for the future.
The recently published Chapter 11 of the CANTER Guidelines for Sustainable Equine Parasite Control sets out age specific protocols for foals, weanlings, yearlings and youngstock from birth to five years of age. Understanding the principles behind those recommendations, rather than simply following a protocol, will help SQPs apply them confidently in practice.
Youngstock are automatically high risk
One of the key messages within the CANTER Risk Profile is that all horses under five years of age are automatically considered High Risk of parasite infection. This reflects their developing immunity, changing parasite burdens and increased susceptibility to parasite associated disease.
The Guidelines also recognise that not all young horses face the same level of challenge. Those living on stud farms with high stocking densities and sub optimal pasture management may fall into a critical risk category, whereas youngsters kept in smaller groups with good grazing hygiene represent a lower level of risk despite remaining within the High-Risk category because of their age.
This distinction reinforces the need for individual risk assessment. Prescribing decisions should always take account of the horse's age, management, previous parasite control measures, recent test results (where available), history of parasite associated disease and likely infection pressure on the premises before treatment is recommended. On large, high risk breeding operations, or where there is a recent history of any clinical disease veterinary involvement is advised.
While some young horses will inevitably require more intervention than others, blanket treatment should still be avoided wherever possible for foals over six months old. Reducing paddock contamination through appropriate pasture management remains fundamental, with frequent poo picking, avoiding overstocking, resting paddocks, using clean grazing and co grazing with ruminants all helping to reduce reliance on anthelmintics.
Keeping pace with changing parasite populations
The lifecycle of some parasites is short; infection happens fast. Foals are no longer recommended a preventative treatment for threadworm (Strongyloides westeri). If needed a worm egg count, combined with vet assessment at 2-5 weeks can identify those that may need support. Begin proactive worming treatments for ascarids in foals from 8 weeks old, treating strategically until they are around six months old. Autumn/winter treatments for encysted small redworm are recommended for most horses under five.
Alongside strategic treatment, diagnostic testing is central to decision making. As young horses mature, the predominant parasite species change. Testing helps identify these transitions, informs treatment choice and increasingly provides evidence of whether treatments continue to be effective as resistance profiles evolve as well as providing valuable information on the effectiveness of management measures.
Figure 1 illustrates how the predominant parasite challenge changes as young horses mature. Threadworm may be encountered during the first few weeks of life, followed by a period where ascarids become the principal parasite of concern. As immunity to these parasites develops, cyathostomins become increasingly important, with tapeworm establishing from around six months onwards. The Guidelines also recognise that a second wave of ascarid infection can occur in some weanlings, reinforcing the need for continued diagnostic monitoring.
The table below shows the available tests and how to use them effectively to monitor different parasite types and efficacy:
Choosing and evaluating treatment
Parasite infection should always be considered as a potential cause of poor growth, weight loss or failure to thrive in young horses. Encouraging owners to monitor weight and development not only helps identify disease early, but also ensures treatments are accurately dosed. Underdosing reduces efficacy and accelerates the development of resistance. Horses showing signs of clinical disease should be referred to their vet.
is central to decision making.
Choosing the right wormer starts with identifying the parasite(s) involved. The horse's age, the predominant parasite type, the time of year and, importantly, the known resistance profile on the premises should all influence prescribing decisions. Ascarids remain most susceptible to fenbendazole and pyrantel on most premises, while ivermectin and moxidectin (and, in some cases, pyrantel) continue to be the principal treatments for adult cyathostomins. Where mixed infections occur, some youngsters may require two different drug classes, although previous worm egg count reduction tests may demonstrate that a single product remains effective against both parasite groups.
Resistance or reduced efficacy has now been reported for both licensed tapeworm actives, pyrantel and praziquantel, making post treatment monitoring increasingly important. This is where the Proudman faecal flotation technique may add value. Compared to commonly used egg count method, it requires a bigger sample to account for the uneven distributions of eggs in dung; however false negatives can occur due to the known intermittent egg shedding patterns of tapeworm or when there’s the presence of high proportions of immature or sterile tapeworms. It allows tapeworm eggs to be detected before treatment and again 14 days later, providing an opportunity to assess treatment efficacy and monitor for emerging resistance.
The emphasis throughout the Youngstock chapter is that every treatment should become an opportunity to learn something. Reduction testing helps build an understanding of which products continue to work effectively on an individual premises, allowing future prescribing decisions to be based on evidence rather than assumption.
Management remains the foundation
No parasite control programme can rely on wormers alone. Young horses often produce higher egg outputs than mature horses, increasing contamination of grazing and the infection challenge for future groups of foals.
Thinking of parasites as a transmissible disease, rather than simply a problem affecting an individual horse, can help frame conversations with owners. Every egg shed onto pasture increases infection pressure for the next group of susceptible youngsters.
As SQPs we are urged to reinforce the importance of pasture hygiene alongside every prescribing decision. Regular poo picking, avoiding overstocking, rotating grazing, resting paddocks, co grazing with cattle or sheep, and avoiding the repeated use of the same paddocks for successive groups of foals all help reduce reliance on anthelmintics.
Supporting better decisions
The Youngstock chapter is intended to support consistent, evidence-based prescribing. By combining thoughtful risk assessment, appropriate diagnostics, targeted treatment and good management, SQPs have a key role to play in protecting both the health of today's young horses and the long-term effectiveness of our anthelmintics.
The full CANTER Guidelines for Sustainable Equine Parasite Control, including the new Youngstock chapter, are available free of charge at www.canterforhorses.org.uk.
