Pet dogs may spread resistance to last-resort antibiotic to owners

The study raises concerns that pets can act as reservoirs of the gene and so aid the spread of resistance to precious last-line antibiotics, said researchers from the University of Lisbon.

They found two cases, in which both dog and owner were harbouring the gene.

The mcr-1 gene, first reported in China in 2015, confers resistance to colistin -- an antibiotic of last resort used to treat infections from some bacteria resistant to all other antibiotics. If mcr-1 combines with already drug-resistant bacteria, it can create a truly untreatable infection, the researchers said.

"Colistin is used when all other antibiotics have failed -- it is a crucial treatment of last resort. If bacteria resistant to all drugs acquire this resistance gene, they would become untreatable, and that's a scenario we must avoid at all costs," said Juliana Menezes from the varsity's Centre of Interdisciplinary Research in Animal Health, Faculty of Veterinary Medicine.

"We know that the overuse of antibiotics drives resistance and it is vital that they are used responsibly, not just in medicine but also in veterinary medicine and in farming," Menezes added.

For the study, the team looked for resistance to colistin in bacteria in faecal samples from people and pets.

Samples were taken from 126 healthy people living with 102 cats and dogs in 80 households in Lisbon between February 2018 and February 2020. All of the humans and 61 of the pets were healthy. A total of 23 pets had skin and soft tissue infections (SSTI) and 18 had urinary tract infections (UTI).

Eight dogs out of the 102 pets (7.8 per cent) and four humans out of 126 (3.2 per cent) harboured bacteria with the mcr-1 gene. Three of the dogs were healthy, four had SSTIs and one had a UTI. None of the cats were carrying the gene. Further analysis showed that the bacteria isolated from all 12 samples that were mcr-1 positive were resistant to multiple antibiotics.

In two households with dogs with SSTIs, the mcr-1 gene was found in both dog and owner. Genetic analysis of the samples suggested that in one of these two cases, the gene had been transmitted between pet and owner.

While transmission in both directions is possible, it is thought that in this case, the gene passed from dog to human, Menezes said.

The owners did not have infections and so did not need treatment. The sick dogs were successfully treated.

The study was presented at the ongoing European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) taking place online between July 9 and 12.