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Infectious Diseases
Macrolide/Rifampin Prophylaxis for R. equi Leading to Multidrug Resistance
By Adam Marcus
The overuse of macrolide/rifampin for prophylactic therapy to treat foals infected with Rhodococcus equi has led to the emergence of antibiotic-resistant forms of the bacteria that could pose a threat to other farm animals and even people, researchers have found.
The growing spread of macrolide/rifampin-resistant R. equi, first detected in 2002 in Kentucky, has been linked to a clone of the microbe, named R. equi 2287, which relies on a chromosomal mutation to evade rifampin and a set of resistance genes that protects bacteria against macrolides, lincosamides, streptogramins, tetracycline and sulfamethoxazole. A new resistant clone, labeled G2016—for the year it was first discovered—suggests that the problem may be getting worse.
“The use of prophylactic chemotherapy and the treatment of subclinically pneumonic foals with antibiotics represents a clear example of antimicrobial overuse that is driving the selection of multidrug resistant bacteria, including R. equi, in the animals and the environment,” said Sonsiray Álvarez-Narváez, DVM, of the Athens Veterinary Diagnostic Laboratory at the University of Georgia College of Veterinary Medicine, who led the new study. “An increase of resistant bacteria in the farm environment directly translates into an increase in the chances of a foal being exposed and potentially ill by resistant bacteria, whose infections are more challenging to treat and have a worse prognosis.”
For the study, Dr. Álvarez-Narváez and her colleagues performed genetic analyses of samples of R. equi from foals being raised in Florida, Kentucky, Louisiana, New York and Texas between 2002 and 2017. Of the samples, 38 were resistant to a macrolide and rifampin, 8 were resistant to macrolides only and 24 were susceptible to the drugs.
In addition to finding samples of the 2287 clone of the bacteria, which has the same core genome as the typical MaR resistant strain, they also found the new resistant clone, G2016. As with the 2287 clone, this one has a chromosomal mutation to evade rifampin and gained its multidrug resistance to the other antimicrobials by acquiring a plasmid, or ring of DNA,called pRErm46.
According to Dr. Álvarez- Narváez, that meant 2 things: “Up to 2015, pRErm46 was not transferred to other genotypes, and dissemination of macrolide and rifampin resistance in R. equi across the U.S. was caused by the same bacteria [clone 2287] that passed from 1 animal to another.”
How clone 2287 emerged is still a mystery, she said. “But once it did, the action of the 2 antibiotics made it easy for these resistant bacteria to take over, because most competition was killed by the action of the dual antibiotic combination,” Dr. Álvarez-Narváez told Modern Equine Vet.
“This way, dual therapy makes it more difficult for resistant bacteria to emerge but once they do, clonal populations carrying the key resistance elements are favored.”
Dr. Álvarez-Narváez said the findings, and similar studies, offer a clear message to equine veterinarians: “They should stop using antibiotics prophylactically or to treat foals with subclinical R. equi pneumonia with antimicrobials because there are several studies showing that, in many cases, foals presenting subclinical lung lesions recover spontaneously without the help of chemotherapy.
“I understand that in many cases clinicians are under a lot of pressure from breeders and owners to treat the animal, but they should minimize the use of antimicrobials to strictly necessary cases because by trying to assistthe foals with subclinical pneumonia, they may be creating a larger problem of antimicrobial resistance.”
For more information:
Álvarez-Narváez S, Giuère S, Cohen N, et al. Spread of multidrug-resistant Rhodococcus equi, United States. Emerg Infect Dis. 2021;27(2):529-537. https://doi.org/10.3201/eid2702.203030 (https://wwwnc.cdc.gov/eid/article/27/2/20-3030_article)
