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Winter WormsWe all expect modern wormers to kill off all worms immediately. However if we delve into the mysteries of pharmacology and parasitology to find out what it’s really all about; we come out better educated but still confused.Fact – no wormer is 100% effective – i.e. after you’ve wormed your horse / pony it still has some worms! Sorry but it’s the best we can do and an awful lot better than 2 or 3 decades ago. Fact – there isn’t one wormer group that kills all worm types/ stages. But now there is a combination that gets close to this ideal. Fact – if you use the same wormer / pharmacological group continually , you are selecting for worm resistance. This is a serious problem in its own right but beyond the scope of this piece. This article is about a type / stage of roundworms called cyathostomes. These are the small redworms and over the last 20 years they have been the ‘new challenge’. Before this time they were considered not to be very important. This was because other worm species caused more disease and also we had few ways of effectively diagnosing or treating these worms. ‘Larval Cyathostomyasis’ is an infrequent seasonal disease which can vary from mild to severe and even cause fatalities. It usually occurs in the spring and affects the young or elderly. These small redworms have adapted to the changing seasons to improve their survival by hibernating / encysting within the bowel lining.  To explain the reasons why this happens we need to understand the basic life cycle of the worm. Starting from the egg stage within the faeces the egg hatches into a 1st stage larvae and then into a 2nd and 3rd. This 3rd stage larva is the infective stage on the pasture and climbs the grass sward settling into a water droplet waiting to be eaten. Once eaten it burrows into the bowel wall and either continues development into an adult worm and re-enters the bowel lumen and continues the life cycle of egg production or encysts within the bowel wall. The reasons behind this are to do with the ambient environmental temperature. Larval development is restricted at low temperatures and death rates increase so by arresting development until spring it allows a greater percentage of larvae to survive. It is thought that a period of frosts initiates this process and young or old horses are more susceptible due to a lower overall immunity. When the encysted larvae do continue their development and hatch out into the bowel it can occur en-masse causing hundreds of thousands to hatch at once, each one causing a microscopic wound which leaks blood and protein. This means the bowel can leak significant volumes of blood and protein very quickly causing diarrhoea, anaemia, weight loss and even death. Normal worming doses have little to no effect upon this condition. Even Moxidectin which can kill encysted stages can be caught out by poor timing. E.g. this year we had a dry summer and when the rains came there was a mass pasture hatch. Whilst a Moxidectin dose would kill what was present, it doesn’t last forever and when the frosts arrived early this year larvae still on the pasture were primed to encyst. Once eaten and encysted the weather warmed up and although we don’t understand the trigger factors for hatching it is likely that temperature is important. I don’t suppose they thought ‘ That was a short winter!’- but I certainly had to deal with the unseasonal Cyathostome disease that followed. PreventionTargeted worm doses to kill the encysted larvae before they hatch. Either a 5 day course of Fenbendazole (Panacur Equine Guard) or a single dose of Moxidectin (Equest). These are best given in late November/early December and again repeated in late March/early April.There are no management regimes that will alter or reduce the risk of larval cyathostomyasis. |
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