Study on the current resistance of the ocular microflora of the horse – Suitable locally applicable antibiotics for the initial treatment of conjunctivitis and keratitis in the horse

Studie zur aktuellen Resistenzlage der okulären Mikroflora des Pferdes – Geeignete Antibiotika zur Erstversorgung bei Konjunktivitis und Keratitis des Pferdes

Schieder A-K, Müller E, Heusinger A, Eule J. C.

DOI: 10.21836/PEM20210311
Year: 2021
Volume: 37
Issue: 3
Pages: 292–301

Horses are frequently affected by ocular inflammatory diseases. In most cases, bacteria are secondary pathogens. However, bacterial infections can complicate the disease progression seriously. Bacteria can infiltrate the eye if it is damaged in any way making targeted local antibiotic treatment necessary to avoid secondary infections. The aim of this study was to determine the aerobic bacteria found in diseased equine eyes and the in vitro effectiveness of locally applicable antibiotics against the isolated bacteria. 844 swabs from the ocular surface of horses were cultured for aerobic bacteria in 2018. All swabs were submitted for routine veterinary diagnostics. Bacterial isolates were identified based on culture morphology, biochemical methods, and matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF). Antibiotic susceptibility testing was performed by microdilution using the breakpoint method with the following antibiotics: oxacillin, gentamicin, neomycin, kanamycin, enrofloxacin, tetracycline, florfenicol and polymyxin. In result no bacterial growth was observed in 8 % of the equine eye swabs (n = 65/844). A mixed culture of Gram-positive and Gram-negative bacteria was obtained from 36 % of the swabs (n = 308/844). Only Gram-positive bacteria were isolated from 31 % of the swabs (n = 264/844), while only Gram-negative bacteria were isolated from 25 % (n = 207/844). There were significant differences between the seasons: The majority of mixed cultures of Gram-negative and -positive bacteria and the lowest number of negative cultures were obtained in the autumn. Most of the negative samples were submitted in the winter. A total of 1510 bacterial isolates were obtained during the study. Most of the isolates belonged to the families Enterobacteriaceae, Staphylococcaceae, and Bacillaceae. Streptococcaceae, Moraxellaceae, and Pseudomonadaceae were also common. Antibiotic susceptibility testing was carried out with 1421 of the isolated bacteria. Enrofloxacin was the most effective antibiotic against the Gram-negative isolates (n = 654/625 susceptible to enrofloxacin), closely followed by gentamicin (n = 620/654) and neomycin (n = 618/654). Most of the Gram-positive isolates (n = 767) were sensitive to florfenicol (n = 713/767), enrofloxacin (n = 710/767) and tetracycline (n = 679/767). In conclusion a wide range of Gram-positive and -negative bacteria can be found on the ocular surface in horses. The great differences found in antibiotic resistance pattern between these bacterial strains in some cases highlights the need for susceptibility testing. Looking at the resistance of single bacterial species or bacterial families, the differences become even clearer. Streptococci, for example, have a natural resistance against aminoglycosides while methicillin-resistant staphylococci have a different resistance pattern than that of other Gram-positive bacteria. It is therefore more important to determine the resistance pattern of individual isolates than that of larger groups, e.g. Gram-negative or -positive bacteria. There are only three approved antibiotic compounds for local eye treatment for horses available in Germany. They contain either tetracycline or cloxacillin. A comparison of these two antibiotics showed that tetracycline has a broader spectrum of activity, while cloxacillin is mainly effective against Gram-positive bacteria. Tetracycline therefore appears to be a better choice for initiating treatment. Because of the lack of antibiotic compounds approved for use in horses, off-label use of drugs approved for use in other species or in humans is often necessary. According to the “Verordnung über tierärztliche Hausapotheken (TÄHAV)”, veterinarians are required to perform antibiotic susceptibility testing for any off-label use of an antibiotic.