Up to 70% of equine properties in New Zealand now have some degree of drench resistance. This growth in resistance, which has been seen worldwide, has led to changes in advice around equine parasite management practices in recent years.  

Most equine parasites are transmitted by horses ingesting the larvae from infected faeces while grazing. If present in large numbers, these parasites have the potential to severely affect equine health and can result in poor body condition, colic, and general poor health. A heavy worm burden can also damage the horse's intestines and some other internal organs.  

To combat these parasites, horse owners were previously advised to regularly drench their horses, however, VetSouth’s equine team now recommends first testing for worms and then developing risk-based deworming programmes based on the results and grazing management on the property.  

Blanket drenching horses is no longer advised because this can cause drench resistance.

This develops because there will always be some random genetic mutations in parasites, meaning that some of them will naturally be resistant to certain drenches.  

That does not become a problem until we inadvertently apply selection pressure by deworming the animal. When we drench the horse, we kill all the susceptible worms, but the resistant worms survive, and they are the ones that go on to reproduce. So, the more we drench, the more we select for resistance.  

This issue is made worse when we underdose by giving the worms too small an amount of the drug. They are more likely to survive the underdose, develop resistance, and then go on to reproduce.  

Something we need to remember is that all drugs within a drench family have the same overall mechanism. So, if you have a parasite that becomes resistant to one specific drug, it can also have the genes to be resistant to other drugs in that family.  

The scary thing about this is that we do not have any new drench families in development. We have to think about what we will do when the drenches stop working!   

As such, regular drenching based on calendar intervals, or blanket rotational drenching, is no longer recommended by the VetSouth equine team.

Alternatively, using faecal egg count testing to first check for the presence of adult worms in horse faeces helps to establish which horses are contributing the most to the parasite contamination of the pasture.   

This is because it is normal for all horses to have some worms, all of the time. However, 20% of the horses grazing a pasture will produce 80% of the worm eggs that are contaminating that pasture, so it is important to follow a risk-based deworming programme that only treats the high-shedding adult horses.   

In a risk-based deworming programme, the no-shedding or low-shedding adult horses may not need drenching, because they are not contributing to pasture contamination, and are also maintaining populations of susceptible worms.    

We want to keep these populations of susceptible worms because it will minimise the increase of drench resistance in overall worm populations.   

VetSouth advises working together with your equine vet to create an appropriate risk-based deworming programme that is suitable for your property and takes into account the number of horses you have grazing, what age they are, and what paddock management strategies you follow.    

Initially, this may seem like an increase in cost for the horse owner, with the extra cost of getting the faecal egg count testing done.    

However, the overall cost of parasite management may go down, as the horse owner will no longer be drenching all of their horses all of the time. Instead, they will just be drenching the horses that need drenching, and only at the exact times they need to be drenched.   

Changes in paddock management and grazing strategies may also help to keep pasture contamination levels down, further reducing the need to drench all horses all the time.