Horse Parasites and Gut Microbiota

Some horses are prone to intestinal parasites; others, less so. But why? French researchers believe it might have something to do with the microbiota—the community of microorganisms—living in horses’ guts.

The numbers of certain bacteria and protozoa in the digestive tracts of ponies that are susceptible to parasite infection differ slightly compared to microbiota from ponies that resist parasite infection, said to Guillaume Sallé, DVM, PhD, of the French National Agricultural Institute (INRA), in Nouzilly.

Those findings could lead to a better understanding of how parasites “choose” their hosts within a herd, Sallé said. And equally importantly, it could lead to the discovery of certain biomarkers that would help scientists detect which equids are more likely to be “wormy.”

“Our study was part of a project dedicated to the identification of biomarkers of infestation,” said Sallé. “We chose to work with animals that varied in their potential of resistance (to parasites) in order to bring out the differences observed while working with a small number of equids.”

Sallé and colleagues followed a group of 20 Welsh ponies, belonging to INRA, for five months. They selected these animals from more than 100 INRA ponies because they had known tendencies to either resist parasites or show particular susceptibility to them. (The ponies had been part of experiments on parasite infestations for the previous two years, so there was sufficient data to reveal the extremes.) Half the ponies in the study group were the 10 most resistant, and the other half were the 10 most susceptible.

The scientists studied the ponies’ gut microbiota five times throughout the study to follow the evolution through different seasons and infestation periods. They also monitored ponies’ body weight, fecal egg counts, and blood analyses.

They found slight but distinct differences in immune system biochemistry profiles and in basic gut microbiota between the two groups when the ponies were worm-free, Sallé said. Once the ponies had become naturally infested with parasites at pasture, the scientists noted that the gut microbiota was mostly similar between the two groups, with a few exceptions.

The slight differences—with or without parasite infestation—mainly included an increase in circulating eosinophils and monocytes and lower levels of lymphocytes in resistant ponies, said Sallé. Eosinophils and monocytes are disease-fighting white blood cells, and eosinophils are particularly common in fighting parasitic infections. Lymphocytes are small white blood cells circulating in the lymphatic system.

Meanwhile, during infestation, susceptible ponies tended to have more protozoa—which could mean both the worms and the protozoa benefit from similar gut conditions, he said. Protozoa are single-celled organisms that can live like parasites in the gut, but their effects—harmful and/or beneficial—in the equine digestive system aren’t yet fully understood.

However, it’s too soon to make conclusions about these potential biomarkers, Sallé cautioned. The study group was small—only 20 ponies—and the climactic conditions during the study period could have affected the results.

“The study was conducted at pasture during an especially hot and dry year,” he said. “We were not able to detect strong infestations of parasites nor strong effects on the fecal flora. Therefore, at the current time, we still do not have robust predictive markers.”

In the absence of reliable markers, handlers should continue to monitor intestinal parasite infestations through fecal egg counts and treat them appropriately, he added.

“Coproscopy (microscopic analysis of stool specimens) remains the best way to evaluate the ‘potential’ of an animal,” he said. “This doesn’t necessarily reveal its entire potential of resistance (average number of worms) but, critically, its potential as a contaminator (to the environment and other animals).”

The study, “Strongyle Infection and Gut Microbiota: Profiling of Resistant and Susceptible Horses Over a Grazing Season,” was published in Frontiers in Physiology.