Does hierachy influence worm burden?

There are a number of advantages to having a high status in the horse herd. These horses have first access to food or drinking water and are less often disturbed during rest periods. According to new research, there is something else as well: they also seem to have lower worm burdens. 


“While looking for ways to prevent dewormer resistance, we should also be able to identify and treat individuals who are more at risk of worm infestation and therefore need a dewormer,” said Léa Briard, PhD, from the University of Strasbourg in France. “Our study is the first to show that the hierarchy is related to worm burden and that lower-ranked horses appear to have more worms in their intestines.” Briard presented her work at the 42nd French Equine Research Day held in Paris on March 17. 


For this study, Briard and her colleagues looked at a herd of 13 horses and ponies ages 4 to 27. None of these horses had been dewormed before. The researchers observed the horses six hours a day, six days a week for three months to determine the herd hierarchy. They also regularly tested the horses for worms by collecting manure twice a week for manure testing. Briard said the team saw a clear link between the hierarchy and the amount of worms in the horse. “The more dominant an individual was, the lower the worm burden.” 


It is possible that a lower rank causes stress, which compromises the immune system and therefore makes it easier for worms to infest the horse. However, this is unlikely as other recent studies have shown that lower-ranked horses do not necessarily experience more stress, Briard said. Another hypothesis is that lower-ranked horses eat poorer quality food more often, as they generally eat after a higher-ranked horse. In the case of a shared food source (such as a hay bale or hay rack), they are more likely to be left with the ‘leftovers’. The only problem with this hypothesis is that all horses in the study appeared to be in good shape, regardless of their rank in the herd. 


A more likely theory is that the lower-ranked horses mainly use the less favoured parts of a paddock or pasture, where the ground quality is poorer and there is more manure. “Previous studies showed that horses naturally avoid manure-contaminated areas,” said Briard. So the lower-ranked horses may expose themselves more to worms by grazing in these risky zones. More studies on the grazing behaviour of low-ranked horses are needed to confirm this, Briard indicates. 


What is interesting is that the researchers saw that horses with more social contacts were less affected by worms than isolated horses. This is a surprising result, the team said, expecting close contact with other horses, especially grooming around the hindquarters and anus, would increase the risk. It is possible that the physiological benefits of grooming (lower heart rate, increased production of endorphins and stimulating the immune system) may lead to this result, but there are no concrete explanations yet. 


“Our research is certainly limited because it focuses on only one herd of horses,” said Briard. “But we hope to open a door to get better horses for research and to encourage more research in this area.” 



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