A gene that enables bacteria to be highly resistant to linezolid, an antibiotic that is used as a last resort for treating infections in humans, has been found in bacterial samples  from cats and a dog at a small-animal hospital in the UK for the first time. The new research is being presented at this year's European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Amsterdam, Netherlands (13-16 April).
Linezolid is licensed for the treatment of certain bacterial infections in humans including streptococci and methicillin-resistant staphylococci (MRSA), but it is not used in animals in the UK. However, samples from companion animals in a small-animal hospital in the UK indicated that pets could carry bacteria which are resistant to linezolid.
The new research suggests that there is potential for the gene (optrA) that plays a key role in bacterial resistance to linezolid to spread between different bacterial populations in animals and humans.
"We believe this is the first report of optrA-positive enterococci isolated from companion animals in the UK", says Dr Katie Hopkins from Public Health England who led the research. "This is concerning as transmission of this organism to owners carries the potential for spread to other bacteria, including Staphylococcus aureus. This may lead to difficult-to-treat infections. In order to minimise transmission of resistant bacteria between companion animals and people veterinary surgeries need to ensure adequate cleaning takes place and pet owners should wash their hands after handling pets."
"Whilst linezolid is not licensed for veterinary use in the UK, optrA also is involved in resistance to florfenicol, which is used in animals. However, standard protocols for the management of colonised or infected animals should prevent transmission to veterinary staff, and therapeutic options (eg, ampicillin or glycopeptides) are available should an infection occur."
Linezolid resistance is still rare in enterococci (<1% of bacterial isolates), but has been detected in isolates from both humans and animals, and is most commonly within chromosomal genes making the resistance mechanism stable and incapable of spreading to other bacteria. However, in recent years genes causing resistance to linezolid, such as optrA, have been identified on mobile bits of DNA caused plasmids, meaning that these gene can spread to other bacterial populations.
During routine testing for antibiotic resistance, an Enterococcus faecalis isolate from a cat wound swab was referred by the veterinary diagnostic laboratory to Public Health England's Antimicrobial Resistance and Healthcare Associated Infections Reference Unit. Subsequently, three further E. faecalis isolates from other two cats and one dog from the same small-animal hospital (but different households) were analysed.
Four isolates from three wound swabs (two cats, one dog), and a third cat rectal swab were confirmed to be resistant to linezolid and gentamicin, but susceptible to the antibiotics teicoplanin, vancomycin, and daptomycin.
Concerningly, all four isolates were positive for optrA and there was evidence that there was transmission between animals.
"Our findings further the 'One-Health' view that antibiotic-resistant bacteria can be shared by animals and humans, although the direction of transfer is often difficult to prove. We currently do not know the prevalence of linezolid-resistant enterococci in companion animals and therefore a joint approach to monitoring emergence and dissemination of resistance mechanisms of public health importance is needed", says Dr Hopkins. "In this instance, further transmission was stopped by cleaning and decontamination and we have no evidence that any people acquired an infection from these animals."