In a world of increasing pathogen resistance to available antibiotics, many veterinarians, owners, and caretakers worry when faced with a sick horse, especially a delicate neonate. Will the medications that once worked to save a sick foal still be effective?

There’s good news: Researchers at the University of California, Davis, School of Veterinary Medicine have confirmed that the “go-to” antibiotics frequently used to treat septic neonatal foals are, indeed, still effective.

“Sepsis, defined as the presence of bacteria in the bloodstream, is a rapidly progressive disease that requires immediate antimicrobial therapy in foals,” said David Wilson, BVMS, MS, Hon Dipl. ACVIM, who presented the group’s study results on behalf of Mathijs Theelen, DVM, Dipl. ECEIM, at the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7 in Orlando, Florida.

In most cases, the exact bacteria causing infection aren’t known when treatment begins. The veterinarian administers antibiotics “empirically,” selecting one or more to begin treatment based on the most likely bacterial cause of infection until bacterial culture results become available—a process that usually takes several days.

Common antimicrobials used empirically either alone or in various combinations in septic foals include: amikacin, penicillin, ampicillin, gentamicin, ceftiofur and related drugs, chloramphenicol, and trimethoprim/sulfamethoxazole. To determine if these medications were still good choices for this use, the researchers isolated bacteria from 213 septic foals brought to the hospital and tested their drug sensitivity.

Theelen and colleagues found that a combination of amikacin and ampicillin was one of the most effective for empirical treatment of septic foals less than 30 days old. In their study, bacteria in 91.5% of the foals were susceptible to this antimicrobial combo (i.e., it was effective).

Most of the other combinations were also effective, with susceptibilities between 80 and 90%. Trimethoprim/sulfamethoxazole and gentamicin alone were the least effective of the tested antimicrobials, with susceptibilities of only 59.6% and 62%, respectively.

The researchers also tested imipenem, which proved highly effective for treating septic foals; however, this antibiotic should not be routinely used in foals as it is considered a critically important antimicrobial in human medicine, and excessive use in veterinary situations could fuel microbial resistance and impair the drug’s efficacy long term.

“The combination of amikacin and ampicillin remains an excellent choice for empirical treatment of septic foals, assuming normal renal (kidney) function and while awaiting bacteriological culture and susceptibility results,” said Wilson.