All horses have worms. It is well known that infestation with internal parasites, or worms, can have a severe effect on the health and performance of all classes of horses, especially foals, working and aged horses. The side-effects of worm burdens are related to which species are present, their relative numbers and the age or use of the horse.

Over the last 3 decades, horse owners have been led to believe that complete control of worms of all types was possible with worming alone by the exclusive use of drugs at regular intervals. This has resulted in less

emphasis being  placed on pasture and other hygiene related measures to minimise re-infestation, allowing natural immunity to suppress worm activity and lack of strategic worming to help control seasonal worm burdens.

The risk of development of resistance by Small Strongyle worms to even the most recent worming compounds, the ‘mectin’ group, has highlighted the need for a more complete and co-ordinated approach to worm control, rather than relying on chemical or anthelmintic use alone to keep horses relatively worm free.

Dr. Craig Reinemeyer DVM, PhD of Tennessee, USA presented a review of methods to control Strongyle parasites in horses, entitled “A Mandate for Change” at the American Assoc. Equine Practitioners Convention in Dec 2009.

The take home message was that we must act now to ensure greater sustainability of worm control, as there are no new compounds available in the near future. He advocates a less frequent, more targeted and strategic seasonal approach  to worm control based on a combination of using drugs, improved hygiene and environmental control and worming

Increasing Burdens of Small Strongyles (Cyathostomes)

Over the last 30 years, many parasitologists have been concerned about the targeted control of Large Strongyle worms (predominately Strongylus vulgaris – the ‘Bloodworm’) by worming at regular 6 – 8 week intervals. 30 years ago, 90% of colic cases in horses were due to migratory Bloodworm larvae damaging the arteries (forming ‘aneurysms’) which supply blood to the gastro-intestinal tract. This targeted control has been very effective to prevent migratory Bloodworm larvae, which return to the large bowel and develop to egg laying mature forms in 6 – 8 weeks, spreading eggs into a horse’s environment. However, although targeted control has reduced the incidence of colic related to Bloodworm larvae to less than 5% and eradicated Bloodworms on many studs and horse properties, it has facilitated increased populations of Small Strongyles, or Small Redworms (Cyathostomes). The common types of Cyathostomes of which about 12 varieties out of 50 or more affect horses, cause internal damage, ill-thrift and impaired digestion and a risk of colic (as well as an increased risk of sand colic) in grazing and stabled horses.

Early studies in the 1980’s indicated that 99% of potential Strongyle-type worm populations existed on pasture as ‘infective larvae’, with follow-up surveys in the mid 1990’s suggesting that 95- 100% of these are Small Redworm larvae.       read the full article>>PDF