Abstract
Background : The conjunctiva of the eye is a mucosal surface
that is colonized by various bacteria. Following surgeries, trauma, or
other underlying conditions the normal flora of these bacteria may cause
some eye infections such as conjunctival, corneal infections, or
endophthalmitis.
Objectives : This study aimed to investigate the bacterial
population and antibiotic resistance of the conjunctiva in healthy
horses.
Study design : Random sampling of ocular samples from 20 horse
during the winter (2019).
Methods : Swab samples were collected from the eyes of 20
healthy horses of different breeds, ages, and sexes in Tehran province,
Iran. The swabs were cultured on blood agar and MacConkey agar and
incubated at 37°C for 24-48 hours. The isolated bacteria were
characterized using cellular morphology, gram-staining, and biochemical
tests. Sensitivity and resistance to 10 different antibiotics were
measured using an antibiogram test.
Results : Results showed that the bacterial population consisted
of various species, with Bacillus cereus being the most frequent. There
were no significant differences in the frequency of isolated bacteria
between sexes and age groups. There were no significant differences in
the sensitivity of bacterial flora of the eyes to Colistin, ceftiofur,
florfenicol, amoxicillin, and ampicillin between neither different age
groups nor different sexes of the horses (P>0.05). However,
sensitivity to enrofloxacin and ciprofloxacin was significantly
correlated with the age groups of the horses (P>0.05).
Significant difference in sensitivity to trimethoprim-sulfamethoxazole
between various sexes of horses was observed.
Main limitations : The non-uniformity of the follow-up
examination reliance owners or caregiver of the horse for follow-up
information and Lack of adequate financial resources to study more
horses.
Conclusions : The study suggests that the frequency of bacterial
flora in horses’ eyes is influenced by housing and management conditions
rather than age and sex.
Keywords: Microbial flora, Conjunctiva, Eyes, Horse, Mucosal
surface.
Introduction
Eye surfaces are continually exposed to the environment, providing a
habitat for different bacterial and fungal microorganisms. This
microbiota contributes to the ocular defense mechanism by producing
antibacterial substances that prevent colonization by pathogenic
microorganisms (Chiang and Chern, 2022, LiYi and Wei, 2020). In
physiological conditions, the surface of the eye is protected by several
innate and adaptive immune systems, such as mucin and epithelium barrier
functions, tear film that enables flushing of the ocular surface,
antimicrobial tear components (lysozyme, beta-lysin, lactoferrin, blood
cells, IgA), antigen-presenting cells and special antigen-recognition
receptors called Toll-like receptors(Bolaños-Jiménez et al., 2015, Zak
et al., 2018, SkopińskiKrawczyk and Ambroziak, 2013, Santibáñez et al.,
2022). Eye infections are prevalent in horses, despite the protective
mechanisms of the ocular surface. These infections are often caused by
members of the conjunctival commensal microbiota that act
as opportunistic pathogens after damage to the integrity of the ocular
surface or surrounding areas(Santibáñez et al., 2022). The injured
equine cornea may be more susceptible to infection due to the ubiquitous
nature of bacteria and fungi in the outdoor or barn environment and the
poor efficacy of host immune responses to prevent infection (Plummer,
2017). In horses, gram-positive bacteria are the predominant normal
conjunctival flora, whereas gram-negative bacteria are isolated with
greater frequency in eyes with extraocular disease(Zak et al., 2018).
The most common gram-positive bacteria isolated from normal equine
eyes are Staphylococcus, Streptococcus, Corynebacterium, and Bacillus
spp. On the other hand, gram-negative bacteria such as Escherichia coli,
Pseudomonas, Moraxella, Acinetobacter, and Neisseria spp are less
frequently isolated from normal equine eyes. Fungal organisms such as
Cladosporium, Alternaria, Fusarium, Aspergillus, and Penicillium spp
have also been isolated from normal equine eyes (Hampson et al., 2019,
Awosile et al., 2018). Several studies have shown the type of bacterial
flora in the conjunctival sac may vary depending on the geographical
location, the climate zone, age, and breed of the horses. Identifying
the physiological commensal flora in the conjunctival sac is crucial for
diagnosing inflammatory ocular disorders and selecting the most
effective antibacterial treatment before obtaining antibiogram
results(Johns et al., 2011). Risk factors for equine infectious
keratitis include seasonal influences like weather, temperature,
humidity as well as housing conditions, and use of topical
corticosteroids and antimicrobials(MustikkaGrönthal and Pietilä, 2020,
Tahoun et al., 2020). Bacterial resistance to certain antimicrobials was
found to be higher in isolates obtained after antimicrobial treatment
compared to those obtained before treatment(Hashemi et al., 2020,
Hashemizadeh et al., 2022). This study aimed to investigate the
population of bacterial flora in the eyes of healthy adult horses in
Iran and to assess their antibiotic resistance to prevent the emergence
of further antibiotic resistance in horses This study was also performed
to identify any sex and age-related differences in bacterial populations
and antibiotic resistance, which could be useful for developing targeted
antibiotic therapies in the future.
Materials and methods