Global spread of new antibiotic-multiresistant bacteria

New study reports that an antibiotic-multiresistant strain of Rhodococcus equi bacteria that emerged in the United States and was later found in Ireland, has spread to Japan and China, highlighting its potential for global establishment.

There has been a rise in resistance to some critical antibiotics – called macrolides – in Rhodococcus equi bacterium. Rhodococcus equi can cause life-threatening pneumonia in young foals, posing a serious problem for horse welfare and the equine industry. The infection can also affect other animals and humans, where it is always severe, making the spread of antibiotic resistance a major concern.

Researchers from the Vázquez-Boland group, who are internationally recognised for their work on the genetics of this bacterium, investigated why resistance to macrolides has been rising in Rhodococcus equi. By analysing samples from sick foals, they discovered that the resistance came from a small piece of DNA that allows the bacteria to survive antibiotic treatment.

One particular bacterial strain carrying this resistance was able to outcompete others on horse farms where antibiotics were used, allowing it to spread widely. Worryingly, this strain is resistant to multiple antibiotics and has been circulating on US horse farms since the early 2000s. It has since been detected internationally, including in Ireland, highlighting the global threat of antibiotic resistance.

World map of the northern hemisphere with red arrows showing direction of infection — Credit northern hemisphere chart: Sean Baker (Marvin01 | talk) - https://commons.wikimedia.org/w/index.php?curid=387292

Working with international partners, the Vázquez-Boland group set up a global surveillance network of 19 laboratories across 13 countries in Europe, North and South America, Africa, the Middle East, Asia and Australia. Using this network, they previously identified the same multidrug-resistant (MDR) strain of Rhodococcus equi in Ireland in samples from 2016 and again in 2021, suggesting the strain became established there after being introduced from the United States.

In the current study, the researchers analysed two samples collected in Japan in 2021 and 2023 that matched their screening criteria. Genetic testing confirmed that both belonged to the same highly resistant strain. By examining international genetic databases, they also found evidence that this strain is circulating in China.

The discovery of the strain in East Asia shows that multidrug-resistant Rhodococcus equi has now spread globally, likely through the international movement of horses. The study also found signs of continued importation of this strain into Ireland.

Rhodococcus pneumonia is notoriously difficult to treat, and the global establishment of the bacterial variant with resistance to the only antibiotics with proven efficacy against the disease in foals is a worrying development for the equine industry.
Since Rhodococcus equi infections can be transmitted from animals to humans, the global spread of the multidrug-resistant variant also represents a public health threat.

International monitoring of this resistant strain must continue and be strengthened. Horses are frequently transported for breeding, racing, and other sporting events, creating opportunities for the bacteria to spread. Testing horses for carriage of the multidrug-resistant variant of Rhodococcus equi before international transport is essential, as the bacterium can be present without obvious symptoms.

Research into new treatments and the development of a vaccine should also be prioritised, since no commercial vaccine currently exists to protect horses against this dangerous infection.

This work was funded by the Horserace Betting Levy Board.

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