Britain’s horse owners have been warned that the country’s mild and wet winter may increase the early-spring risk to horses from larval cyathostominosis, a potentially fatal syndrome caused by the mass emergence of small redworm from their dormant, encysted state.
Encysted small redworm are one of the most harmful parasites to affect horses in Britain.
They are larval stages of the small redworm that hibernate in the lining of the gut and do not show up in a standard faecal worm egg count.
They usually “wake up” in the early spring and their mass emergence can lead to larval cyathostominosis, causing diarrhoea and colic, with up to a 50 percent mortality rate.
Normally, during winter months, lower temperatures prevent worm eggs and larvae from developing on the pasture, meaning that re-infection of horses does not occur to a significant extent until the following spring.
However, during unusually mild, wet winters such as this year’s, worm eggs and larvae can develop on the pasture and grazing horses can become re-infected.
Even horses that have been treated for encysted small redworms in late autumn/early winter may still be at risk of re-infection, particularly if they have been turned out on heavily used pasture.
“It is recommended that all horses receive a treatment for encysted small redworms during the late autumn/winter, regardless of their faecal worm egg count,” said Wendy Talbot, a veterinarian with animal health company Zoetas.
“In some circumstances, such as if the preceding winter has been especially mild, then it is advisable to consider a second encysted small redworm dose in the spring for those horses most at risk.”
All horses can develop larval cyathostominosis, but those at particular risk are youngsters, old or immune-compromised horses (such as those with Cushing’s disease), those with an unknown or sub-optimal worming history and those that were not dosed correctly in late autumn/early winter.
Talbot said it was important for owners to contact their veterinarian as soon as possible if they had a horse showing signs possibly related to a worm burden such as loss of condition, sudden weight loss or diarrhoea.