New test could be a life-saver for foals developing pneumonia

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The test not only identifies the presence Rhodococcus equi, but key elements of its resistance profile.
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Researchers have developed a potentially life-saving molecular test to quickly identify the presence and resistance profile of a common pneumonia-causing pathogen in foals.

Multidrug-resistant Rhodococcus equi is rapidly spreading across the United States in horse breeding farms, causing devastating untreatable disease in foals. Indeed, the bacterium is a leading cause of severe bronchopneumonia in foals, Sonsiray Álvarez Narváez and her fellow researchers wrote in the journal Animals.

The emergence and spread of multidrug-resistant R. equi represents a significant threat to the horse industry because of its economic impact. Infections can result in long-term treatment costs, veterinary and nursing care costs, and lost income from animal losses.

Multidrug resistance of this pathogen is also a concern to public health as the antibiotic therapy of choice to treat infected horses – a macrolide in combination with rifampin – is also routinely used to treat bacterial infections in humans.

“There is a critical need for new diagnostic tools that can identify R. equi and its antibiotic resistance profile accurately and fast.”

The authors noted that the current diagnosis for R. equi pneumonia in foals generally involves a cytology report, a preliminary qPCR detecting R. equi directly from the respiratory sample, and subsequent bacteria isolation and antimicrobial susceptibility profiling.

Cytology and PCR results are usually available to the veterinarian in the first 12 hours. However, R. equi isolation and corresponding antimicrobial susceptibility testing can take up to 72 hours.

“Due to the insidious nature of this disease, fast action is crucial, and clinicians generally start treatment with a macrolide (clarithromycin) and rifampin before the susceptibility profiles are available.”

This strategy, they said, has been effective so far as resistance to macrolides and rifampin was below 5% in all R. equi cases recorded in some studies.

However, over the last 15 years, the number of resistant isolates has increased markedly, to the point that resistant R. equi to all macrolides and rifampin is being cultured from up to 40% of foals at a farm in Kentucky.

The increasing prevalence of multidrug-resistant R. equi in the US has been recently evidenced by research, they noted.

Such findings require the development of new diagnostic tools that can quickly and accurately identify not only the pathogen, but relevant resistance information.

University of Georgia researchers set out to develop and standardize a new multiplex quantitative real-time PCR (qPCR) test to detect R. equi and its most clinically relevant antimicrobial resistance genes directly from equine respiratory samples.

The resulting test not only identifies the presence of R. equi, but the genes that point to the resistance profile of each isolate. The qPCR test showed high sensitivity and specificity in testing, they said.

They found the test was not reactive to normal bacterial communities present in the respiratory tract of horses, including genetically similar organisms.

They believe the test can be used for a fast-preliminary diagnosis of R. equi and the simultaneous prediction of its most critical resistance profile.

“The new molecular diagnostic tool presented here will shorten the waiting time from the moment the practitioner sees the equid patient until it is diagnosed and appropriately treated.”

The study team comprised Narváez, Ingrid Fernández, Nikita Patel and Susan Sánchez, all with the College of Veterinary Medicine at the University of Georgia. Their work was supported by a grant from the US Department of Agriculture.

Narváez, S.Á.; Fernández, I.; Patel, N.V.; Sánchez, S. Novel Quantitative PCR for Rhodococcus equi and Macrolide Resistance Detection in Equine Respiratory Samples. Animals 2022, 12, 1172. https://doi.org/10.3390/ani12091172

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

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