Chloramphenicol remains effective against Gram-positive bacteria, the most common agent in bacterial conjunctivitis, researchers say.
Making the antibiotic available over the counter has not increased bacterial resistance to it, wrote Alexander Silvester, Royal Liverpool University Hospital, Liverpool, United Kingdom, and colleagues. They published the finding in BMJ Open Ophthalmology.
The incidence of acute infective conjunctivitis is about 135 per 10,000 in the United States. In severe cases it can cause conjunctival scarring and damage to the ocular surface.
Antimicrobials have the positive effect of improving remission rates, but many cases of conjunctivitis are self-limiting, and inappropriate use of antimicrobials can lead to increased bacterial resistance.
A global movement is underway to reduce prescribing of antimicrobials, and general practitioners in the United Kingdom have been writing fewer chloramphenicol prescriptions for conjunctivitis. However, the use of the drug has increased several-fold since 2005 when it became the first antibiotic available over the counter.
Research into trends in prescription patterns and antimicrobial resistance can help guide prescription decisions. Researchers in the United States have undertaken such investigations, though chloramphenicol is not widely used there because of concerns about aplastic anaemia – a concern dismissed by reviewers studying its use in the United Kingdom, Silvester and colleagues reported.
To examine whether resistance to chloramphenicol was developing in the United Kingdom, Silvester and colleagues analysed laboratory records for all bacterial swabs taken from adult patients with suspected bacterial conjunctivitis presenting to the Royal Liverpool University Hospital from 2001 to 2012.
Samples were incubated for the presence of pathogens, and starting in 2006 were analysed for viruses using real time polymerase chain reaction (PCR). Bacteria were tested for susceptibility to chloramphenicol, ciprofloxacin, and gentamicin.
The researchers used British Society for Antimicrobial Chemotherapy (BSAC) guidelines and templates to interpret zone inhibition diameters for chloramphenicol, ciprofloxacin, and gentamicin.
Out of 8209 conjunctival swabs, 1300 (15.9%) were considered positive for bacteria. Of these, 977 (74.2%) were Gram-positive and 323 (24.8%) were Gram-negative. The most prevalent organism identified was Staphylococcus aureus.
The researchers counted 3293 viral samples, of with 707 were adenovirus. They counted 21 (2.97%) in which both viruses and bacteria were identified.
Resistance to chloramphenicol varied from 3.0% to 16.4% in bacterial culture-positive samples. Most of the resistant organisms were Gram-negative. The rate of resistance to chloramphenicol did not appear to change after 2005 when it became available over the counter.
The researchers calculated that 16.4% of samples were resistant to ciprofloxacin, of which 80.1% were Gram-positive and 14.0% were resistant to gentamicin. Of those, 69.2% were Gram-positive.