The way forward is to think about worm control in an entirely different way. All too often we find ourselves with the question - what should I worm with now? This may or not be coupled with a worm leg count result from a faecal sample, but the question remains the same. Is interval worming wrong? Should we all be worm egg counting every horse and avoiding drugs altogether? May be it's time we asked a different question.
What are we trying to achieve?The actual aim should be effective worm burden control. This isn't a strive for a zero eggs per gram result every time we send of a poo sample; it's about preventing worm related disease in our horses.
There is an increasing weight of evidence to suggest that there are potential benefits to exposure to a low parasite burden. There is little to no progress being made into the development of new drug classes and ever increasing levels of resistance to the drugs we already have access to; thus, if we continue managing our horses but answering the origin question "what do I worm with now?", very soon we will run out of effective answers.
So what should we be doing?Unfortunately, there is no one size fits all solution for deworming strategies. What we do need to do is shift our mindset from a reliance on medication to a broader, more holistic approach focussing on three areas:
Effective pasture management.Removal of droppings from the pasture at least once a fortnight, will be effective in minimising pasture contamination with infective worm larvae. Droppings must be removed completely and only after storage for 6 months in an effective muck heap where the high temperatures will kill the eggs can the droppings be considered safe to spread on the pasture. Adequate grazing densities will promote the natural formation of grazed ‘lawns’ and un-grazed ‘roughs’ where most horses will choose to pass droppings.
Identifying and monitoring horses with significant parasite burdens.Faecal worm egg counts: Faecal worm egg counts (WEC) form the basis of both identifying those horses with significant intestinal parasite burdens and monitoring them. WECs also play a role in monitoring the effectiveness of the anthelmintics and identifying resistance.
- WEC results need to establish, more importantly than a specific numerical value those horses with significant intestinal parasite burdens as a separate population from those with no or acceptable levels of burden. What represents an acceptable burden may change throughout a horse's life. WECs should be first performed in the spring (March - May). A fresh dropping sample should be collected in an airtight container or bag and kept cool and delivered the same day to your veterinary practice.
- Horses with persistent low or zero WECs can have the interval between testing increased. Horses with high or significant WECs should be treated and re-tested in 2 - 3 months time; if they are high again, then interval worming should be used. Horses with very high burdens should be retested following worming to check for resistance to the wormer used, this is called a egg count reduction test.
- Monitoring for Tapeworm: Traditional faecal WECs are not a reliable method for identifying tapeworm burden in the horse. However, an ELISA assay can be performed on blood or saliva samples for tapeworm antibodies which will identify horses with significant intestinal tapeworm burdens.
- The modern approach to use of this assay is to test horses once in the spring. If this result reflects a low burden the horse is treated once yearly for tapeworm in the autumn. Those horses with a high result are treated twice yearly, once in Spring and once in Autumn. There is no requirement to repeatedly test these horses, but just base their on-going tapeworm treatment on the initial test results. Single dose praziquantel or double dose pyrantel are both effective in treating tapeworm.
Utilizing our available chemical treatments in the most effective manner.When treating horses with high faecal worm egg counts, the use of either ivermectin or pyrantel is recommended, using manufacturer's interval recommendations (4 - 6 weeks for pyrantel, 8 - 10 weeks for ivermectin). It is suggested that moxidectin is too efficient and as such, encourages the worm population to produce resistance. Once resistance is formed to moxidectin, cross resistance to ivermectin is inevitable. Moxidectin should ideally be reserved for annual encysted cyathostome (small redworm) treatments. Widespread resistance to benzimidazoles (Panacur) at single dose means that benzimidazoles should only be used as a five day course, again for encysted cyathostomes.