Key Worming

By Ben Sturgeoan, BSc, BVM&S, Cert EP, MRCVSPolVam

Currently there are no new types or classes of wormer on the immediate horizon and the major conclusion of recent research was that “worm control programmes should be designed to be sustainable using presently available drugs.”

Historically, a blanket zero tolerance approach was undertaken and potentially still has its place in some yards where horses come and go frequently and in a relatively uncontrolled fashion. However, the consensus now is to undertake a strategic or targeted approach utilising diagnostic tests such as faecal egg counts (FECs) and ELISA blood tests (for tapeworm diagnosis). This has not only proven to be cost effective, when compared to a blanket repeat worming protocol, but also reduces resistance, promotes horse health and provides information on herd and farm/yard field health.

Previous blanket worming control, over frequent dosing and under-dosing have made wormer resistance a common finding. A fundamental and pivotal fact is that 80% of worms are only carried in 20% of horses. In other words, the vast majority of horses have low and in fact clinically insignificant numbers of worms. Additionally and importantly low numbers of worms in horses are not inherently unhealthy and faecal egg counts of up to 400 eggs per gram (200 in young or old) are not considered significant, so a zero tolerance attitude is wrong. Secondly, allowing horses to maintain this or a low level of worms, that are not exposed to treatment, reduces the incidence of resistance. This is called “refugia” and is paramount to avoiding future resistance by increasing the effectiveness of wormers.

Much confusion lies at the heart of resistance with many owners basing their choice of wormer on brand name or colour of the box. Four major classes of wormer chemical exist:

And many products combine the various classes:

This need to become a professor of pharmacology leaves most with a headache.
The important points are that:
1. Overuse of one group will result in resistance to that group
2. Overuse of similar compoundswithin the macocyclic lactones (i.e. avermectin and milbemycin) will result in emerging cross-resistance

Importantly, any wormer chosen or implicated by a FEC should be based on the worm expected or indeed proven to be present. Hence, in the pasture season worms tend to be mature adult round worms, both and redworms rather than larval stages and use of an adultacide pyrantel, fenbendazole or less commonly recommended ivermectin (because of possible cross resistance). Compare this with winter where encysted small redworms are the major culprit so use of moxidectin usually with a tapeworm praziquantel (especially if ELISA bloods have not been performed) is recommended.

Adopting this approach means that any wormer is used infrequently, that when used the wormer is appropriate for the targeted worm so achieving the balance between a healthy worm burden, worm challenge and refugia.

Importantly this approach allows the identification of high egg shedding horses which may be due to a resistance problem, previous poor worm management common in young stock or even areas of the farm that are worm sick or have poor management (i.e. no poo picking policy or are in need of pasture management.) This somewhat holistic consideration means seasonality, yard history and yard management are encompassed giving a much greater level of information.

• Accurate dosing is essential and use of a weigh bridge or weigh tape should be adopted where possible. You can calculate approximate weights using girth (at the withers) and length (shoulder to buttock) measurements: weight (kg) = (girth x girth x length (cm))/11877.
• Moxidectin is dangerous if eaten by dogs.
• Ivermectins are dangerous to cats, dogs and aquatic life
• Foals should be wormed from 6 weeks but moxidectin should be avoided

The aim of a modern worming programme is to:

Author: Ben Sturgeon

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