Over winter, the management of parasites in horses should focus more on the external population of worms, rather than the worms inside your horse. Winter across most of Australia is cool or mild, and down south, wet. Damp and cool conditions are optimal for worm larvae to survive on pasture. Worm egg/larvae development will be slowed by the cooler weather, but the larvae will not die. They will remain infective for about 6 months in mild and damp conditions. It is important to note that of all the worms that live on your property, at any one time, only 10% live inside a horse – the other 90% of the population exist as free-living larvae on the grass, waiting to be ingested by the horse.
Therefore, the key to parasite management over winter is to prevent a build-up of larvae on the pasture – via removal of manure (remove the manure, stop the eggs from hatching into infective larvae!). The best way to do this is to collect manure, at least twice a week. If you harrow, remember that harrowing spreads larvae across the paddock – do not harrow paddocks unless you can cross-graze with another animal species (e.g. cows/sheep) to clean up the infective larvae that has been spread across the entire grazing area, or can rest the paddock for more than 6 months.
When it comes to managing the worms that do live inside the horse, only deworm them if it is indicated as required via a FEC. If your horses were wormed in the autumn for an annual clean out and to remove any bot larvae, chances are they will not require another deworming. To check if your horse does need treatment over winter, be sure to wait 10-14 weeks after deworming to check via FEC as any sooner can give results that are influenced by the lasting effects of the autumn treatment. If you did not treat your horses for bot-flies in autumn, it is good practise to do so in early winter. By late winter bot-larvae may already have passed out, and begun their development in the soil ready to pupate for summer.
If your horse had a low autumn egg count, or has a low winter egg count, they can be left until spring before checking again. Remember: worming an adult horse is about preventing egg shedding and pasture contamination, rather than removing all worms. If horses are checked in winter, only ~20% of horses will require worming.