Resistant form of bacterium that causes foal pneumonia spreading in the US, study finds

A slide culture showing Rhodococcus group bacteria that has been cultured on tap-water agar. Photo: David Berd, public domain, via Wikimedia Commons

Scientists report the growing prevalence of multidrug-resistant Rhodococcus equi in the United States, which they say is likely to spread internationally through horse movements.

R. equi is a soilborne bacteria capable of infecting multiple species, including humans.

It is a common cause of life-threatening pneumonia in young foals and people with compromised immunity. The bacterium is able to colonize horses, pigs, and ruminants through three different host-specific types. Human cases originate from animals.

R. equi is common in horse-breeding farms worldwide. For decades, the standard treatment for R. equi pneumonia in foals has been an antibiotic combination of macrolide and rifampin.

Many horse-breeding farms rely on early ultrasonographic detection of infected foals and start treatment with the two antibiotics before clinical signs of the disease.

In the United States, where foal rhodococcosis is often endemic, this practice has been linked to the emergence of R. equi that is resistant to both macrolide and rifampin, Dr Sonsiray Álvarez-Narváez and her fellow researchers noted in the journal, Emerging Infectious Diseases.

Such resistance was first noticed in the late 1990s and is becoming more prevalent, posing a substantial problem because no clinically proven therapeutic alternative is available for affected foals. This form also represents a potential hazard to humans because of the risk of cross-species transmission, they said.

For their study, Álvarez-Narváez and her colleagues delved into the spread of multi-drug resistant R. equi in the US.

They sequenced the genome of 30 macrolide-resistant and 18 macrolide-susceptible R. equi strains recovered from foals with pneumonia in five states — Florida, Kentucky, Louisiana, New York, and Texas — between 2012 and 2017. They also undertook antimicrobial susceptibility testing.

The study team said the evidence indicated increasing prevalence of multi-drug resistant R. equi since it was first formally documented in 2002.

It is caused by a clone, dubbed R. equi 2287, attributable to changes driven by macrolide/rifampin therapy.

The clone likely emerged after large-scale use of the antibiotic combination in the US to prevent the onset of pneumonia in foals.

The authors said their analyses showed that R. equi 2287 has diversified since its first documented isolation. They found resistance to macrolides, lincosamides, streptogramins, and, in a substantial proportion, also to tetracycline.

“All of these antibiotic drugs are listed as critically or highly important for human medicine by the World Health Organization.”

They continued: “Our findings illustrate that overuse of antimicrobial prophylaxis in animals can generate multi-drug resistant pathogens with zoonotic potential.

Resistant forms of the bacterium are currently disseminating in the US and are likely to spread internationally through horse movements, indicating the need for interventions to control its spread, they said.

They noted that around 9% of human R. equi infections are caused by equine-derived strains, and about half of human cases are caused by pig-derived isolates.

“Therefore, in addition to compromising the therapeutic management of equine R. equi infection, these isolates represent a potential hazard to human health because of the risk of zoonotic transmission.”

They said their study probably underestimates the extent of the spread of the multi-drug resistant form.

The evidence points to a pattern of multi-drug resistant R. equi spread and evolution directly determined by antibiotic pressure in equine farms, they said.

The study team comprised Álvarez-Narváez and Steeve Giguère, with the University of Georgia; Noah Cohen, with Texas A&M University; Nathan Slovis, with the Hagyard Equine Medical Institute in Kentucky; and José Vázquez-Boland, with the University of Edinburgh in Scotland.

Álvarez-Narváez S, Giguère S, Cohen N, et al. Spread of Multidrug-Resistant Rhodococcus equi, United States. Emerging Infectious Diseases. 2021;27(2):529-537. doi:10.3201/eid2702.203030.

The study, published under a Creative Commons License, can be read here

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