Drs. Martin Nielsen, Christian Sauermann and Dave Leathwick have used computer modeling to simulate small strongyle resistance to ivermectin over the next 40 years. Small strongyles are very common and infect almost every horse that grazes on pasture. The worms can trigger a disease called larval cyathostominosis, which occurs when encysted small strongyle larva emerge in large quantities from the intestinal wall and cause swelling and damage the mucosa.
Nielson, who works out of the Maxwell H. Gluck Equine Research Center at the University of Kentucky, said that drug resistance is slow to develop in worms; it takes decades to show resistance, so tracking the effects in real-time is extremely difficult. The model was created to better understand drug resistance and find ways to counteract resistance. Horse owners have been urged to use fecal egg counts to determine if and when horses need deworming medications, but some owners still use dewormers based on a calendar cycle.
The computer model Nielson and his colleagues developed replicates the life stages of small strongyles both in the pasture and in the horse. The pasture-based model took into account precipitation and temperature for four different climates; the horse-based model took into account every life stage of the worms and factors that affected development. The model also took into account the genetics related to dewormer resistance.
The researchers found that the increasing the frequency of deworming increased worm resistance in all climates, but moving to only four treatments a year resulted in only a minor reduction in resistance development.
They reported that to significantly slow resistance development, deworming treatments may need to be reduced to as few as two a year if all horses are treated at the same time. The month in which the medication is administered becomes important if horses are being dewormed only twice a year; resistance was slower to develop when deworming medications were given in winter or early spring. However, the timing of deworming administration did not affect the subtropical climate as conditions are always favorable for small strongyle development.
The researchers concluded that selective deworming based on climate should be done in the spring, while treatments of all horses on a pasture are most beneficial in the fall.
Read more at HorseTalk.
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