Evolution of the Prevalence of Antibiotic Resistance to Staphylococcus spp. Isolated from Horses in Florida over a 10-Year Period

SIMPLE SUMMARY: The purpose of this study was to determine how common antibiotic-resistant infections are in horses, particularly Staphylococcus species. These are bacteria that are normally found on the skin of horses. Overgrowth of these bacteria can lead to infection. In recent years, the emergence of resistance to antibiotics to treat these infections has been shown with these bacteria in humans and dogs. Determining how widespread Staphylococcal resistant bacteria are in horses helps to educate the veterinary profession on potential changes to horse resistance. This can help guide appropriate antibiotic usage as well as prove the need for innovative treatment options for both veterinary and human medicine. This study found increasing resistance in a class of antibiotics in the population observed at our institution. In addition, the species of Staphylococcal bacteria affects the resistance. Larger studies with more horses are needed to evaluate the clinical usefulness of these results. ABSTRACT: Previous studies documented antibiotic resistance in horses but did not focus on skin specifically. We investigated antibiotic resistance and correlations between resistance patterns in skin infections. Records from 2009 to 2019 were searched for Staphylococcal infection and susceptibility results. Seventy-seven cases were included. Organisms identified were S. aureus (48/77), S. pseudintermedius (7/77), non-hemolytic Staphylococcus (8/77), beta-hemolytic Staphylococcus (6/77), and other species (8/77). Samples included pyoderma (36/77), wounds (10/77), abscesses (15/77), incision sites (5/77), nose (8/77), and foot (3/77). A trend analysis using non-parametric Spearman’s test showed significant upward trend of resistance (p < 0.05) for 3/15 antibiotics (ampicillin, cefazolin, penicillin). Susceptibility was significantly different by Staphylococcal species for 8/15 antibiotics. Gentamicin showed significant susceptibility differences based on source (all abscesses were susceptible to gentamicin). Steel-Dwass test showed statistically significant (p = 0.003) difference between incision sites and abscesses. A non-parametric Kendall’s T-test found significantly negative correlation between cefazolin and amikacin sensitivity (p = 0.0108) and multiple positive correlations of resistance (p < 0.05). This study confirms increasing resistance in dermatologic samples. It is unlikely that the sample source affects resistance, but Staphylococcus species may affect it. Study limitations include lack of information about previous antibiotic use and small sample size.