They found no significant association between antimicrobial resistance and isolation rate for any year under study, but they found a significant reduction in methicillin resistance among S. aureus isolates, ranging from a maximum of 13.4% to a minimum of 1.3%.
Although more samples were infected with viruses than with bacteria, it is hard to distinguish them, so “it is not unreasonable to prescribe broad-spectrum antimicrobials prior to obtaining culture results,” the researchers wrote.
Compared with many other topical antibacterial agents, chloramphenicol penetrates well following topical application, they pointed out.
However, they warned against using chloramphenicol in contact lens related cases of conjunctivitis, or when Pseudomonas aeruginosa is suspected.
The prevalence of S. aureus was lower in these samples than in studies in Japan and the United States, even though it was the most common organism identified. Researchers looking at other organisms have also documented a reduction of resistance to methicillin in S. aureus over time. They have attributed it to enhanced screening and decolonisation.
Silvester et al. advised caution in interpreting their results because BSAC breakpoints are derived for systemically rather than topically delivered antimicrobials. Breakpoints are becoming available for keratitis, they noted, but are not yet available for conjunctivitis.
They also acknowledged a low-positive isolation rate as a limitation of this study. Such rates have varied in other studies, they said. They explained this variation by the inclusion of all bacteria isolated in some studies rather than pathogenic bacteria only.
They also pointed out that bacterial swabs in this study were taken from all patients with suspected conjunctivitis, including those with viral conjunctivitis. Many patients have already started topical antibiotic treatment by the time the patients present at clinics, which may have reduced the positive isolation rate.
“Despite the ready availability of chloramphenicol over the counter during the last seven years, there has been no increase in chloramphenicol resistance nor a change in the bacterial spectrum in this cohort of patients,” the researchers concluded.