Association between antimicrobial treatment of subclinical pneumonia in foals and selection of macrolide- and rifampicin-resistant Rhodococcus equi strains at horse-breeding farms in central Kentucky

Abstract

OBJECTIVE To compare soil concentrations of macrolide- and rifampicin-resistant Rhodococcus equi strains (MRRE) on horse-breeding farms that used thoracic ultrasonographic screening (TUS) to identify foals with subclinical pneumonia combined with subsequent administration of macrolides and rifampin to affected foals (TUS farms) versus soil concentrations on farms that did not (non-TUS farms), determine whether the combined use of TUS and antimicrobial treatment of subclinically affected foals was associated with soil concentration of MRRE, and assess whether there were temporal effects on soil concentrations of MRRE during the foaling season. SAMPLES 720 soil samples and 20 completed questionnaires from 20 horse-breeding farms (10 TUS farms and 10 non-TUS farms) in central Kentucky. PROCEDURES A questionnaire was used to gather information from participating farms about their 2019 foaling season. Soil samples were collected during January, March, May, and July 2019 for bacterial culture and antimicrobial susceptibility testing to identify any isolates of MRRE. Results were compared for TUS farms versus non-TUS farms. Linear mixed-effects modeling was used to evaluate for potential associations between the soil concentration of MRRE and the use of TUS. RESULTS Overall, the sum of the mean soil concentrations of MRRE was significantly higher for TUS farms (8.85 log10-transformed CFUs/g) versus non-TUS farms (7.37 log10-transformed CFUs/g). CONCLUSIONS AND CLINICAL RELEVANCE Our findings indicated that farms that use TUS to identify foals with subclinical pneumonia for antimicrobial treatment select for antimicrobial-resistant R equi strains. Because prognosis is worse for foals infected with resistant versus nonresistant strains of R equi, prudent use of antimicrobials to treat foals with subclinical pulmonary lesions attributed to R equi is recommended.

Genomic Characteristics and Pan-Genome Analysis of Rhodococcus equi

“…The mortality of AIDS patients with combined R. equi infection was as high as 20%~58% ( Ahumada et al., 2020 ). With the extensive application of antibiotics, reports of multidrug-resistance in R. equi are increasing, including resistance to trimethoprim, sulfamethoxazole, ampicillin sulbactam, clindamycin, erythromycin and rifampicin ( Petry et al., 2020 ; Alvarez-Narvaez et al., 2021a ; Huber et al., 2021 ).…”

Rhodococcus equifoal pneumonia: Update on epidemiology, immunity, treatment and prevention

“…In that study, the number of foals treated with macrolides and rifampin was significantly associated with the prevalence of MDR‐ R. equi in the environment 27 . Subsequently, the use of thoracic ultrasonography was associated both with greater use of antimicrobials to treat subclinical R. equi pneumonia among and with greater environmental concentrations of MDR‐ R. equi at horse‐breeding farms 26 . Genetic sequencing of 144 isolates collected from the soil of 100 horse‐breeding farms in the previously cited study 27 identified MDR‐ R. equi carrying both the known pRErm46 plasmid and a novel MLS B resistance gene, erm (51) 49 .…”

“…R. equi invades and replicates within macrophages due to the presence of virulence‐associated protein A (VapA), a surface‐expressed protein necessary to cause disease in foals 21 . Infection with R. equi occurs by inhalation of aerosolised virulent R. equi which are widely distributed in the environment at horse‐breeding farms 10,16,22–28 . Factors contributing to the aerosolisation of R. equi , such as a dry climate and density of mares and foals in paddocks, are associated with higher airborne concentration of R. equi and to higher infection rates 10 .…”

Fecal concentration of Rhodococcus equi determined by quantitative polymerase chain reaction of rectal swab samples to differentiate foals with pneumonia from healthy foals

“…An accurate minimally invasive test for diagnosis of foals presumed to have pneumonia caused by R. equi would be useful for equine veterinary medicine because TBAs are not routinely performed by veterinarians at farms where R. equi is endemic and because overuse of antimicrobials to treat foals with subclinical pneumonia has contributed to the emergence of an alarming level of antimicrobial resistance in R. equi isolated from foals and their environment in the United States. [10][11][12][13][14][15][16][17] For this reason, we investigated the ability of qPCR of fecal swabs (which can be collected conveniently from foals) to differentiate between cases of pneumonia attributed to R. equi and healthy controls at 2 horse breeding farms in New York. Fecal concentrations of vapA from rectal swab samples were significantly greater in samples from foals with pneumonia attributed to R. equi than agematched controls and the typical magnitude of difference was nearly 100-fold (Figure 2).…”