Choosing an Antibiotic for Use in Foals

Treating bacterial diseases in horses—or really, in any species—is much easier when the causative agents are sensitive to available antibiotics. But unfortunately, this isn't always the case anymore: antibiotic-resistant bacteria are popping up all over the world.

"While the development of new antimicrobials initially outpaced the development of bacterial resistance, the trend has reversed over the last 10 years," explained Steeve Giguère, DVM, PhD, Dipl. ACVIM, a professor at the University of Georgia College of Veterinary Medicine and the Marguerite Thomas Hodgson Chair in Equine Studies. "This has resulted in the emergence of resistant bacteria as a major worldwide problem.

"Judicious use of antimicrobials is essential to maximize treatment efficacy while minimizing development of bacterial resistance, occurrence of adverse reactions, and relapses," Giguère said.

So when a veterinarian is faced with a sick equine neonate, how can he or she ensure they're treating the problem effectively without adding to the resistant bacteria problem? Giguère offered some tips at the 2013 Society for Theriogenology Conference, held Aug. 7-10 in Louisville, Ky.

Factors to Consider

Before reaching for any old antimicrobial, "there are a variety of factors you need to think of when you're presented with a foal," Giguère told the veterinarians in attendance. He reviewed these considerations in depth.

  • What's the likelihood of identifying a bacteriologic cause of disease? Or in other words, does my patient really need antibiotics? If the answer is no, Giguère said, it might be better not to use them. But many of the conditions found in newborn foals—including pneumonia, septic arthritis/osteomyelitis, umbilical infections, diarrhea, and meningitis—should all be treated with antibiotics. For this reason, Giguère said that ailments should be considered septic until proven otherwise. In one study researchers found that 20-50% of sick neonatal foals had positive blood culture (i.e., bacteria in their bloodstream). 
  • What organism is likely causing my patient's problems? To answer this question, veterinarians must be familiar with the common pathogenic bacteria in their regions and what illness they cause in foals. Giguère said some commonly identified pathogens in sick neonatal foals include enteric Gram-negative bacteria (including Escherichia coli and Klebsiella spp.), nonenteric Gram-negative bacteria (including Pasturella spp.), and Gram-positive bacteria (including Streptococcus equi spp. zooepidemicus and Enterococcus spp.). Giguère said Gram-negative bacteria account for 70-90% of the microorganisms isolated from sick neonatal foals' blood, while Gram-positive bacteria account for about 25% of the isolates. 
  • What's the suspected pathogen's likely susceptibility profile? If you think you know what's causing your patient's problems, you should have a good idea of what that pathogen is commonly susceptible to so initial treatment can begin quickly. But remember, "In all situations, therapy should be adjusted based on initial clinical response and, when available, results of culture and susceptibility testing," Giguère said. 
  • Do I need to run a culture or in vitro susceptibility profile test? If a veterinarian is unsure what pathogen is causing a foal's illness or what drug a pathogen is susceptible to, he or she should run a culture or an in vitro (in the laboratory) susceptibility profile test. The former test will help identify the causative bacteria, while the latter will provide information regarding whether the bacteria is susceptible or resistant to certain drugs. This test will help ensure the foal receives the most effective treatment against the bacteria causing him problems. 
  • What is the site of infection? Determining where within the foal's body the infection is located is important, Giguère said, as some drugs can't penetrate certain body parts. Veterinarians should ensure they use an appropriate drug to reach the infection's origin. 
  • How will the drug be administered? Both systemic and local therapy can be effective in treating bacterial infections, Giguère said. The key is for the veterinarian to determine which method will be most effective in each individual circumstance. 
  • Are there any potential adverse effects to consider? Depending on the drug, potential side effects can include renal (kidney) failure, a decrease in liver function or complete liver failure, arthropathy (joint disease), and enterocolitis (inflammation of the small intestine and colon). Veterinarians should use caution when selecting an antimicrobial as these side effects can be exaggerated in foals that are already ill. 
  • Are there any drug interactions to be aware of? Some drugs don't interact well together, while others can essentially nullify any effect other drugs might have on a foal's infection if used in conjunction with one another. 
  • Is the drug cost-effective or prohibitive for the owner? Medications aren't inexpensive, and treating a foal—many of whom are already hospitalized—with a pricey medication might not appeal to owners, Giguère said. Consider other options that might be less expensive, but none less effective. 
  • What pharmacokinetics are at play? Giguère said that understanding the pharmacokinetics (the factors affecting the disposition of the drug in the body, which in turn affects the onset, duration, and magnitude of effect) of certain drugs in the foal's body is also important. "Foals absorb drugs better than adults, so they can consume some drugs orally that wouldn't usually work in adult horses," he explained. Additionally, he said, foals frequently require higher doses due to their higher proportion of extracellular fluid. Other factors that could impact a drug's pharmacokinetics include the foal's lower plasma protein concentrations, lower body fat content, less developed blood/brain barrier, and immature metabolism (which typically matures within the first two weeks of life). Another challenge veterinarians face regarding pharmacokinetics is that researchers still aren't sure if there are differences in the way healthy and sick foals' bodies handle medications.

Common Drugs

Keeping that in mind, Giguère reviewed some common antimicrobials used to treat equine neonates. He noted three antimicrobial combinations that are good choices for treating neonates with bacterial illness:

  • Ampicillin (a penicillin drug) plus amikacin (an aminoglycoside drug);
  • Ceftiofur (a cephalosporin drug) plus amikacin; or
  • Penicillin plus amikacin.

Of course, veterinarians have many other drug choices at their disposal should other options be required.

Take-Home Message

Giguère concluded that "judicious use of antimicrobials" involves careful selection and administration of medications that maximizes treatment efficacy while minimizing adverse effects and treatment failure.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.