Fighting Infections in Horses, Foals by Boosting Immunity

Years ago, veterinarians were rarely called until a horse was in a health crisis, usually after the handler’s roster of home remedies had already failed. Considering the tinctures and potions used by many horse owners back then, veterinarians would sometimes cringe, knowing the so-called cure might be just as likely to cause death as whatever was ailing the horse.

The advent of the antimicrobial age early in the 20th century brought about a magical turn of events, whereby a veterinarian’s advice was sought earlier in the course of disease and success rates improved markedly. Overuse of antimicrobials, however, has brought us full circle, as horses are again succumbing to treatable disease due to lack of effective medications.

Antimicrobial resistance occurs among animals, just as it does in humans. To help demonstrate that antimicrobial resistance is a threat to the equine industry, a group of American veterinary researchers provided a real-life example of antimicrobial resistance involving foal pneumonia.^*

Foals with subclinical infection caused by Rhodococcus equi often receive prophylactic treatment using a combination of a macrolide antimicrobial (such as erythromycin) and rifampin. This practice occurs despite the fact that many foals with subclinical infections do not ever develop pneumonia and may, in fact, not require treatment at all.

Key findings from the study found that foals treated with a macrolide and rifampin:

• Resulted in dysbiosis, which is the reduction in the abundance and diversity of the intestinal microbiome;
• Increased the abundance and diversity of resistance genes (genes that provide the ability for bacteria to resist the effects of antimicrobial drugs) in their fecal bacteria; and
• Promoted the survival of macrolide-resistant equi in the soil, where the bacteria thrive and serve as a source of infection for foals.

“There is no doubt that R. equi infection has major health and economic consequences; however, the prophylactic administration of antimicrobials to subclinically infected foals leads to widespread antimicrobial resistance. Further, those resistance genes can be passed on to other bacteria, leaving foals and horses susceptible to other diseases,” explained Catherine Whitehouse, M.S., a Kentucky Equine Research advisor.

The benefits of omega-3 fatty acids in boosting immunity are widely reported, particularly when fed to broodmares. Foals are born without any antibodies and must derive all their infection-fighting abilities from the mare’s colostrum until their own immune systems mature.

^*Álvarez-Narváez, S., L.J. Berghaus, E.R.A. Morris, et al. 2020. A common practice of widespread antimicrobial use in horse production promotes multi-drug resistance. Scientific Reports 10:911.