Prevalence and Antibiotic Susceptibility of Bacterial Isolates From Dogs With Ulcerative Keratitis in Midwestern United States

Abstract

The objective of this study was to describe bacterial culture and antibiotic susceptibility results in 476 dogs presenting with suspected bacterial keratitis in Iowa and surrounding Midwestern states, further detailing trends in patient characteristics, seasonality, and antimicrobial resistance. Corneal swabs yielded 465 bacterial isolates and 220 cultures (46.2%) with no apparent growth (0–5 isolates per culture). The most frequent bacterial genera were Staphylococcus (32.3%), Streptococcus (19.1%), and Pseudomonas (12.5%), while the most common bacterial species were Staphylococcus pseudintermedius (26.7%), Streptococcus canis (12%), and Pseudomonas aeruginosa (7.5%). Compared to mixed-breed dogs, canine breeds most likely to be examined for ulcerative keratitis included Boston terrier, Cavalier King Charles spaniel, miniature pinscher, pug, rat terrier, Saint Bernard, shih tzu, and silky terriers. In summer, the likelihood to yield a negative culture was reduced while the likelihood to culture Pseudomonas species was increased. Bacteria considered multidrug resistant (MDR, resistant to ≥ 3 antibiotic classes) represented 20% of all canine isolates and were most prevalent for Staphylococcus species (33%). An alarming, escalating trend of MDR prevalence was noted between 2016 (5%) and 2020 (34%). Individual ophthalmic preparations (i.e., single antibiotics or commercially available antibiotic combinations) with highest efficacy against all bacterial isolates included chloramphenicol (83%), ceftiofur (79%), amikacin (77%), neomycin-polymyxin B-bacitracin (77%), and gentamicin (74%). Efficacy of systemic antibiotics and combinations of ophthalmic preparations was also evaluated. Based on the present findings, triple antibiotic (Neo-Poly-Bac) is recommended as empirical monotherapy for prophylactic antibiotic therapy in dogs with simple corneal ulcers, while a chloramphenicol-ciprofloxacin combination is empirically recommended for therapeutic management of infected corneal ulcers. Pending culture and susceptibility results, appropriate selection of empiric antibiotic therapy is important to enhance therapeutic outcome and reduce antibacterial resistance in dogs with corneal ulceration.

Albumin in Tears Modulates Bacterial Susceptibility to Topical Antibiotics in Ophthalmology

“…Bacterial keratitis is a vision- and globe-threatening condition that requires prompt diagnosis and treatment to prevent unfavorable outcomes. Timely antimicrobial therapy must be started on the basis of clinical and laboratory evaluation; however, clinical success can be hindered by several challenges such as the rise of antimicrobial resistance ( 6 , 20 – 22 ) or progression of keratomalacia despite the appropriate use of antibiotics ( 23 – 25 ). Here, we describe an important challenge not previously recognized in the scientific literature, that is, the negative impact of protein binding in tear fluid on the efficacy of antibiotics commonly used to treat ocular infections.…”

“…Albumin concentrations had a significant impact on the MIC of several antibiotics commonly used in ophthalmology, a finding confirmed for 3 bacterial species ( Staphylococcus sp., Streptococcus sp., Pseudomonas sp.) that are commonly identified in ocular infections of canine ( 6 , 28 ) and human patients ( 23 , 27 ). The impact of protein binding may be overlooked in cases of highly susceptible bacteria ( 34 ), therefore 10 different isolates were tested for each bacterial species making the present findings more broadly applicable.…”

“…The study focused on the 3 most common bacterial species identified in dogs with ulcerative keratitis, that is, Staphylococcus pseudintermedius, Streptococcus canis , and Pseudomonas aeruginosa ( 6 , 14 ). Ten bacterial isolates were selected for each bacterial species, providing a total of 30 bacterial isolates for the study.…”

Bacterial Cross-Contamination in a Veterinary Ophthalmology Setting

“…Last, multiple swabs were lumped into “environment” and “equipment” groups in the prospective surveillance sessions due to cost constraints, limiting our understanding of specific sites of contamination for targeted disinfection. Of note, enrichment cultures were not performed in the present study as the culture media used (blood agar, MacConkey agar) were deemed sufficient to grow the majority of isolates considered pathogenic in canine eyes (e.g., S. pseudintermedius, Streptococcus canis, Pseudomonas aeruginosa ) ( 9 ). Nonetheless, the use of enriched media could be considered in future studies to identify a greater variety of microbials involved in cross-contamination.…”

“…In clinical patients, corneal cultures were collected by anesthetizing the ocular surface (0.5% propracaine ophthalmic solution) followed by gentle rub of the pre-moistened swab tip against the edges of the corneal defect, with care not to touch the conjunctiva or eyelids to avoid contamination. As previously described ( 9 ), corneal samples were processed for aerobic microbiologic assessment via inoculation of a non-selective medium [tryptic soy agar with 5% sheep blood (Blood Agar, Hardy Diagnostics, catalog #A10)] and a Gram-negative selective medium (MacConkey Agar, Hardy Diagnostics, catalog #G35). The blood agar was incubated at 35°C ± 2°C with 5–10% CO 2 for a total length of 4 days while the MacConkey agar was incubated 35°C ± 2°C without CO 2 for a total length of 2 days.…”