This potentially fatal disease is caused by excessive burdens of encysted small redworm, from which neglected and abandoned horses are often found to be suffering.
Encysted small redworm (ESRW) are the hibernating larval stages of the small redworm that bury themselves in the horse’s gut wall. While every horse is at risk of encysted small redworm and associated diseases such as cyathostominosis, animals less than five years old or those with previously poorly managed burdens are the most susceptible.
During the autumn and winter, when the environmental conditions are less favourable for small redworm larvae to develop on the pasture, the proportion of encysted larval stages within the horse’s gut wall gradually increases. Instead of continuing their normal development within cysts in the gut wall, larvae stop developing and enter a hibernating state where they are known as encysted small redworm. In horses where pasture management has been poor and worm control has been inadequate, large encysted burdens can accumulate.
ESRW larvae typically emerge from their cysts in early spring when the environmental conditions start to improve. Larvae re-emerge into the lumen of the gut where they finish their development into egg laying adults. Mass emergence of ESRW can cause severe damage and inflammation to the gut lining resulting in the disease syndrome ‘larval cyathostomonisis’, Once clinical signs of the disease are evident, there is a 50% mortality rate, despite treatment. Incidence is higher in horses less than five years old and in the spring.
Horses that develop larval cyathostominosis may have a history of being treated inadequately during the previous grazing season or been kept on over-stocked and highly infected pastures. These circumstances can cause them to accumulate large burdens of encysted larvae.
“Typically horses of around two to five years of age appear to be at the greatest risk,” explains Dr Wendy Talbot, National Equine Veterinary Manager at Zoetis. “We still don’t know exactly why this is the case and both a reduced or increased immune response to the dangerous larval stages of the redworm have been suggested as a cause’.
Larval cyathostominosis can also occur following treatment with a non-larvicidal wormer ie; one that is not effective for the treatment of the larval stages of the small redworm. While such a wormer will kill the more mature stages of ESRW it will subsequently encourage the larval stages to progress with their emergence, potentially causing cyathostominosis.
Wendy has put together a checklist to help horse owners keep their horses safe from encysted small redworm:
· Remember encysted small redworm won’t show up in a faecal worm egg count: Horses can harbour several million larvae yet show negative or low faecal egg counts.
· Treat every horse for encysted small redworm once a year: Ideally treat in the late autumn or early winter each year but certainly before the spring.
· Use the right wormer: A single dose of moxidectin or a five-day course of fenbendazole
· Remember youngsters are at the highest risk: Be extra vigilant with horses of less than five years old.
· Beware of resistance: There is widespread evidence of resistance in small redworm to fenbendazole, including the five-day dose so a resistance test is recommended before using it.
· Keep redworm under control in summer: Regular faecal worm egg counts from early March until October and treating according to the results will help keep redworm under control and reduce the risk of large hidden encysted burdens forming.
· Use faecal egg count reduction tests during the grazing season: The best way to ensure that your wormers are working properly is to ask your vet to perform a faecal egg count reduction test. This involves taking a FWEC immediately before and two weeks after worming to assess the level of worm eggs being shed.
· Be rigorous with pasture management: Daily poo-picking, regular rotation and resting of fields and cross grazing with sheep or cattle will help keep pasture worm burdens under control.
· Seek veterinary advice: If you have a vulnerable young horse showing any clinical signs it is important to speak to your vet before using a wormer.