Correlation between frequency of isolated bacteria and sex
Significant differences in the frequency of isolated bacteria between various sexes of the horses were not indicated (p>0.05). Obviously, due to the isolation of each of Listeria monocytogenes , Actinomyces spp., Mannheimia haemolytica,and Corynebacterium bovis from only one individual horse, statistical analysis is impossible.
Discussion
The Conjunctiva of the eye is a mucosal surface which is colonized by different bacteria. Following surgeries, trauma, or other underlying conditions the normal flora of the conjunctiva may cause some kinds of infections in the eye including conjunctivitis, keratitis, or endophthalmitis (Sthapit and Tuladhar, 2014). Staphylococcus, Streptococcus, Corynebacterium, and Bacillus spp. are the most common gram-positive bacteria isolated from normal equine eyes(Hampson et al., 2019). Conjunctiva and cornea have a very strong unique immune system against infections which consists of tear, phagocytosis, mechanical barriers, and bacteriostatic functions of lysozyme (Sousa et al., 2011). Any agents including stress, dysfunctions of the immune system, and systemic as well as traumatic diseases that cause an imbalance of the mentioned immune system can put the eye at risk of probable infections with internal and external origins(Rosa et al., 2003). The primary objective of this study was to determine the frequency of bacterial isolates from the eyes of healthy horses and to investigate any potential correlation between this frequency and the age or sex of the horses. Additionally, the study aimed to assess the sensitivity of the bacterial isolates. This study found that Bacillus cereus was the most frequently isolated bacteria from the conjunctiva of healthy horses, followed by Micrococcus luteus , Bacillus licheniformis , Pasteurella multocida , Bacillus mutans ,Staphylococcus spp., and others. Gram-positive bacteria were the most commonly found. In a study conducted by Zak et al, the majority of the isolated bacteria consisted of gram-positive bacteria besides the lack of significant effect of neither age nor sex on the frequency of the isolated bacteria resulted the same as the current study(Zak et al., 2018). Several previous studies have indicated that most frequent bacteria species are gram-positive(Johns et al., 2011, Gemensky-Metzler et al., 2005). Despite the similarities of the mentioned studies to the current one, the major isolated species were different from each other. As an assumption, this difference may be resulted by various climate and housing conditions, including the ingredient of the feed. However, in a study conducted by Johns et al, the effects of geographic location and type of housing (either housing in the field or stable) on bacteria isolation were not significant(Johns et al., 2011). It was also resulted from this study that sex and age have no significant effect on the frequency of bacteria isolation which is similar to the results of the current study (Johns et al., 2011). One of the reasons for the high frequency of bacterial flora in horse eyes is the mechanical transmission of microorganisms by insects contacting eyes which can be considered an important factor in determining the type of bacterial population of the conjunctiva. In a study conducted by Butler et al, isolated Staphylococcus sciuri from horse eyes was isolated from Musca domestica fly as well. This result proves the correctness of the aforementioned claim(Butler et al., 2010). Health condition is another reason for the variety in the bacterial flora of eyes as indicated in the current study that sensitivity to trimethoprim-sulfamethoxazole was significantly higher in neutered horses compared with intact mares which can be due to the better health conditions in housing neutered horses resulting in the lower bacterial population of neutered horses (total of 22 isolated bacteria) compared with intact ones (total of 26 isolated bacteria). Therefore, the lower the frequency of the isolated bacteria, the lower the amount of sensitivity to antibiotics. Araghi-Sooreh et al. (2013) in 2013 indicated 100% sensitivity of isolated bacteria to ciprofloxacin and florfenicol as well as 100% sensitivity of gram-negative bacteria to gentamycin. In the current study, 100% of isolated bacteria were sensitive to gentamycin and neomycin. In another study in Brazil, the bacterial flora of 100 healthy horses with no clinical disorders was assessed. Like the current study, the majority of isolated bacteria were gram-positive (65.5%). Sensitivity to antibiotics was also examined and 94%, 85%, 81%, and 68% of isolated bacteria were sensitive to ciprofloxacin, gentamycin, chloramphenicol, and tobramycin, respectively(Ferreira et al., 2017). In a study conducted by Hampson et al, bacteria and fungi of the normal flora of eyes in 95 horses were evaluated and the most frequent isolated bacteria were detected as gram-positive. Furthermore, most of the isolated bacteria were sensitive to neomycin and florfenicol(Hampson et al., 2019). In a study in Iran, Tamarzadeh et al studied bacterial flora of the eyes in 50 healthy mules with 5-12 years old of age using swab sample collection from conjunctiva as well as the sensitivity of the isolated bacteria to different 12 antibiotics. 85% of the isolated bacteria were gram-positive and the bacteria were mostly sensitive to florfenicol (90.82%) followed by flumequine (70.64%) and gentamycin (67.88%) (Tamarzadeh and Araghi-Sooreh, 2014).
In conclusion, this study aimed to evaluate the frequency of the bacterial flora in the conjunctiva of horses with different sex and age groups as well as their sensitivity to various antibiotics in order to prevent any further bacterial resistance to antibiotics by administration of antibiotics according to sex and age groups of the horses in future. Results of the current study indicated that the frequency of bacterial flora mainly depends on housing and management conditions while it was not significantly correlated with age and sex. Among 10 examined antibiotics, only sensitivity to ciprofloxacin and enrofloxacin was correlated with age which was only significantly different between age groups of 1-5 and 5-10 as well as 1-5 years old and older than 10. Sensitivity to ciprofloxacin and enrofloxacin was increased by age increment as no resistance to antibiotics was shown in horses older than 10 years of old which may be related to more administration of these drugs in young ages, and consequently altering bacterial population which results in resistance of bacteria to the antibiotics in younger horses. In addition to previous results, the sensitivity of the isolated bacteria to trimethoprim-sulfamethoxazole in intact mares was significantly more than in neutered horses which may be due to the lower number of total isolated bacteria from neutered horses’ eyes (total number of 22) compared with intact mares (total number of 26) probably because of better care and health conditions in neutered horses. It should be beard in mind that this result may be also related to the lower number of studied neutered horses (table 1).