Worming science

HOW DO YOU PLAN YOUR HORSES’ WORMING? DO YOU ADMINISTER SEASONAL DOSES ON AN AD HOC BASIS, OR TAKE A MORE SCIENTIFIC APPROACH WITH A TARGETED PROGRAMME? BARBARA YOUNG REPORTS ON THE LATEST VETERINARY THINKING.

With an array of wormers on offer, gaining a full understanding of worming your horse effectively can seem a daunting task and it’s easy to get confused. hile horse owners recognise that worms can seriously affect the health of their horse, the importance of informed worming should never be underestimated. xperts agree that to maintain effective control and prevent resistance, it’s important to target worming with a customised individual approach, rather than relying on interval dosing or blanket treatments. r sther awlinson, technical services manager at oehringer Ingelheim Animal ealth UK td, explains that wormers anthelmintics are licensed veterinary medicines that have been approved by the regulatory authorities as being safe and effective, when used as directed, in killing different types and lifestages of worm.

“Traditionally, horses were routinely wormed at set intervals. It is now recognised that resistance to wormers is becoming a significant problem, and we must take a sustainable approach to worming in order to preserve wormers for when they are really needed.

“The aim of any worming programme is to prevent horses becoming ill due to worm burdens and to reduce pasture contamination with worm eggs, not to eliminate all worms.

“It has been shown that just of horses carry of the worm burden . y only treating the horses that need it, we are leaving a population of worms untreated. These are in refugia’ and dilute the effect of any resistant worms that have survived treatment in the wormed horses. “Key in our worming programme is to identify the one in five horses that need worming so that we can target treatment and don’t worm any horses unnecessarily.”

Dr Rawlinson says that in order to maintain horses’ health, it’s vital to ensure that “we administer the right wormer to the right horse at the right time” .

“Although there are many brands, there are only five active ingredients available

ABOVE: WITH RESISTANCE TO WORMERS BECOMING A SIGNIFICANT PROBLEM, IT’S ESSENTIAL TO ESTABLISH WHETHER YOUR HORSE REALLY NEEDS TREATING. INSET RIGHT: STRONGYLES AND ASCARID EGGS VIEWED UNDER THE MICROSCOPE FROM A

ABOVE: WHEN USING A WORM COUNT SERVICE, SAMPLES ARE TAKEN FROM A FRESH DROPPING (ABOVE) AND SENT TO THE LAB FOR TESTING (RIGHT INSET). BELOW: FOLLOWING A PROGRAMME OF WORM EGG COUNTING, PLUS SIX-MONTHLY SALIVA TESTS AND EITHER TESTING OR TREATING FOR ENCYCSTED REDWORM, CAN HELP ASSESS A HORSE’S RISK AND ESTABLISH A TARGETED PARASITE CONTROL PROGRAMME.

Worm egg count for redworm and ascarids.

Blood test with your vet and/or treat for possible encysted redworm, depending on risk. “MINIMISING EXPOSURE TO THE WORMING CHEMICALS (BY TESTING) TREATS THE INDIVIDUALS THAT NEED IT, WHILE DRAMATICALLY SLOWING DOWN THE RATE THAT RESISTANCE IS DEVELOPING TO THESE CHEMICALS.”

– fenbendazole, ivermectin, moxidectin, praziquantel and pyrantel – and which to use will depend on which worm(s) need to be treated and whether resistance is present on the premises.” Dr Rawlinson points out that for treatment of encysted redworm (which is done in winter), only moxidectin and fenbendazole fi ve day course are licensed. “Due to resistance to fenbendazole, in many cases moxidectin is the only eff ective product. It is therefore recommended to reserve moxidectin for only the treatment of encysted redworm, to preserve its e cacy for when it is really needed”

Studies have shown that parasites can become resistant to certain worming treatments which would have previously been able to kill them at the normal dosage. Therefore, there is increasing concern that if a wormer is used when not required, we do more harm than good and in so doing help the resistant worm population to thrive, which is why it is so important to carry out regular FECs (faecal egg counts).

Experts agree that managing worm burdens requires an individual approach. While some

horses will be high egg shedders that need worming frequently, others are low shedders that can be wormed less frequently.

It is also crucial to always worm in accordance with your horse’s weight, as under-dosing can lead to resistance in surviving worms, while over-dosing provides no added benefi t.

“Minimising exposure to the worming chemicals this way treats the individuals that need it, while dramatically slowing down the rate that resistance is developing to these chemicals,” explains Claire Shand, qualifi ed A A egistered Animal edicines Advisor from Westgate Laboratories. “Small redworm, one of the most numerous and dangerous of horse parasites, is now showing resistance to every one of the licenced wormers in the UK. A very worrying state of play that threatens our ability to keep horses healthy from parasite damage.”

Claire explains that when using a worm count service, the customer will be provided with a sample collection kit and a glove and asked to take four to fi ve pinches from across a fresh dropping, fi lling and labelling a pot for each horse. This is then sent to the lab in the post-paid envelope for testing.

According to laire, a single worm egg count provides a useful snapshot but it’s a series across a season that forms the cornerstone of a targeted parasite control programme.

“They enable us to pinpoint what treatment is needed where and identify the ‘wormy’ horses that are more susceptible to parasites. In a herd all kept the same, 80% of the parasites will be in 20% of the horses. They can also be a barometer of health. or a horse that has always had a low susceptibility to parasites, rising worm egg counts can be due to a new worm challenge in the environment or underlying illness developing, which weaken the horse’s overall immune response.” Claire explains that most healthy adult horses can follow a simple programme of a worm

MOST HEALTHY egg count every eight to 12 weeks (which monitors ADULT HORSES CAN adult, egg laying stages of redworm and ascarid and FOLLOW A SIMPLE an EquiSal tapeworm test to target tapeworm every PROGRAMME OF A six months. “A worm egg count WORM EGG COUNT cannot detect the presence of any encysted redworm EVERY EIGHT TO 12 – these are larval stages WEEKS of the small redworm parasite buried into the gut wall. Blood test by a vet or treating proactively with an appropriate product in the late autumn/ winter will help to guard against this possibility. “Testing this way allows us to be much more selective about our treatment choices. As well as slowing resistance, using fewer chemicals is better for our horses, better for the environment and often works out cheaper too.” References: * Rendle, D. et al. Equine de-worming: a consensus on current best practice. UK-Vet Equine (2019) 3 (1) 3-14

The growing resistance to dewormers came under the spotlight at this year’s 30th National Equine Forum (NEF), where David Rendle, President Elect at the British Equine Veterinary Association, gave a veterinary perspective.

David Rendle made the point that 90% of vets surveyed believed restricting availability of de-wormers would help to improve horse welfare, yet he feared the wider industry “don’t appreciate the importance of the situation.”

He maintained that education alone didn’t work as it was preaching to the converted and the science of human behaviour indicated that suffi cient change will not result unless there is some restriction on access to de-wormers.

Mr Rendle presented alarming fi gures that 120,000 faecal worm egg counts were used per year compared to 1.13 million doses of de-wormer being sold, equating to 11 doses for each test (rather than an average of approximately four tests per wormer if a targeted approach was being followed).

Twenty years of research and education had failed to change horse owner behaviour suffi ciently and he suggested that if an equine welfare crisis was to be averted, the supply of wormers would have to be limited to use within a diagnostic led plan that was revised every year.

“This information will hopefully encourage horse owners to change to a diagnostic test-led, or at least a more strategic approach with routine drug-resistance testing. In so doing, we can help avert the potential equine welfare crisis that will inevitably ensue if the equine industry continues with the indiscriminate use of anthelmintics. I would urge anyone who has not discussed worming with their vets recently to do so.”

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