By Vet Freya Wood
As we start to approach the end of the winter and look forward to the spring, now is a good time to start thinking about a parasite control plan for your horse.
Historically, parasite treatment was given regularly throughout the year as a preventative measure, but sadly this has led to widespread resistance in the parasite population to the anthelmintic (worming) drugs that we have available. With no new treatments on the horizon, we must be proactive in protecting what we have to ensure it remains effective.
While we must be careful with all antiparasitic drugs, moxidectin is a drug we must be particularly conscious of protecting. It is the only effective treatment for encysted redworm, whose mass emergence can cause fatal colitis and colic. Use of moxidectin outside of the winter treatment, unless specifically advised by your vet, is inappropriate. A 5-day course of fenbendazole was previously advocated for treatment of encysted redworm in the winter, but due to huge levels of resistance, it is no longer considered to be appropriate based on the most up to date research.
The aim of parasite control in horses is not to eliminate every parasite in the gastrointestinal tract – a low level is art of the normal gut flora of the horse. The aim is to control the parasite burden in the gastrointestinal tract to reduce clinical disease due to excessive numbers, and to reduce pasture contamination that allow burdens to increase by the parasite life cycle continuing by ingestion by the horse.
80% of the UK horse population are considered “low shedders” for redworm and therefore do not need regular anthelmintic treatment beyond an appropriate winter treatment. A large proportion of UK horses also do not require treatment for tapeworm. A targeted, diagnostic-led approach is required. While there is some variation between horses, a general approach for most adult horses is as follows:
Spring, summer and early autumn – faecal worm egg counts should be performed at approximately 3-month intervals to detect "high shedders” of redworm who are contaminating pasture. High shedders can then be treated appropriately with an ivermectin based worming product.
Winter – a winter treatment should be selected. In some circumstances, a small redworm blood test can be used to detect exposure and should be seriously considered to restrict moxidectin use. The CANTER group has some excellent resources for owners regarding risk profiling horses to determine if this may be appropriate. This is not appropriate for all horses and all management systems, in which case a moxidectin treatment should be given.
Tapeworm testing should be performed via a blood test or saliva swab at least once a year. Testing should be performed no more than 6 monthly if a test is positive and treatment required as the test detects exposure and levels take up to 6 months to reduce after treatment, so testing earlier can lead to false positives.
The mainstay of parasite control should be management, not reliance on drugs. Regular removal of droppings goes a long way to reducing pasture contamination. This is also important after treatment with an anthelmintic drug as residues will be present in the droppings for a number of days after treatment. These residues are harmful to the environment as well as being toxic to many dog breeds if ingested.
Syringes for any worming treatments should be returned to your veterinary practice to be disposed of in clinical waste and should not go into household waste.
For more information please visit https://canterforhorses.org.uk/horse-owners/ and utilise their resources. Your vet is also an excellent point of contact to discuss worming plans.