[Rhodococcus equi infections in humans: an emerging zoonotic pathogen].

Rhodococcus equi is a facultative intracellular gram-positive coccobacillus which is a well-known cause of foal pneumonia and/or enteritis in equine veterinary medicine. More than 300 cases of R. equi infection have been reported since the first description of human disease in 1968. Most patients who become infected with R equi are immunocompromised, such as those infected with human immunodeficiency virus (HIV), recipients of organ transplantation, and patients receiving cancer treatment. However, there are increasing reports of the immunocompetent hosts. The pathogenicity of R. equi has been attributed to the presence of plasmid-encoded virulence-associated proteins (Vap). To date, three host-associated virulence plasmid types of R. equi have been identified as follows: the circular pVAPA and pVAPB, related, respectively, to equine and porcine isolates in 1991 and 1995, and a recently described linear pVAPN plasmid associated with bovine and caprine strains in 2015. More recently, these three plasmid types have been re-found in the human isolates which were isolated during 1980s to 1990s. Not only horses, but also pigs, goats, cattle and their environment should be considered as a potential source of R. equi for humans. In this review, we shed light on the current understanding of R. equi as an emerging zoonotic pathogen.

Isolation of vapB-positive Rhodococcus equi from submaxillary lymph nodes with or without granulomatous lesions in growing-finishing pigs in Japan.

To investigate the etiological role of vapB-positive Rhodococcus equi in pigs, R. equi was isolated from the submaxillary lymph nodes with or without macroscopically detectable lesions of apparently healthy growing-finishing pigs at a slaughterhouse in Toyama Prefecture, Japan. R. equi was isolated from 57 (24.6%) of 232 pigs with macroscopically detectable lymph node lesions, and 56 (98.2%) of the 57 isolates were vapB-positive. R. equi was isolated from 10 (2.4%) of 420 pigs without lymph node lesions, and six (60%) of the 10 isolates were vapB-positive. Plasmid DNA was isolated from the 62 vapB-positive isolates and digested with EcoRI and NsiI to obtain the plasmid profile. Fifty-two (83.9%), three (4.8%), and four (6.5%) isolates contained pVAPB subtypes 1, 2, and 3, respectively, while the remaining three isolates were of pVAPB subtypes 9, 13, and 14, respectively. Twelve specimens from lymph nodes with macroscopically detectable lesions were randomly selected for histopathological staining. Granulomatous lesions resembling tuberculosis were found in 11 of the 12 specimens, and the remaining specimen showed typical foci of malakoplakia in the lymph node. The isolation rates of R. equi and vapB-positive R. equi from lymph nodes with macroscopically detectable lesions were significantly higher (P<0.05) than those of lymph nodes without lesions, suggesting an etiologic association between vapB-positive R. equi and macroscopically detectable granulomatous lesions in porcine submaxillary lymph nodes. Previous reports on the prevalence of vapB-positive R. equi in pigs are reviewed and discussed.

Experimental infection of goats with pVAPN-harboring Rhodococcus equi causes latent infection in the lymph nodes.

Rhodococcus equi has recently been identified in various animals, including ruminants. Several studies have highlighted the emergence of pVAPN-harboring strains, isolated from multiple abscesses, in the liver and lungs of ruminants. Epidemiological evidence strongly suggests that pVAPN-harboring strains are pathogenic in ruminants. This study aims to replicate the disease in goats through experimental infection. Intravenous administration of the pVAPN-harboring strain (Yokkaichi), pVAPA-harboring strain (ATCC33701), and pVAPN-cured strain (Yokkaichi_P-), each at 1.0 × 107 CFU/head, was conducted in 24-month-old goats (n = 1 per group). During the observation period, goats treated with Yokkaichi or ATCC33701 exhibited transient increases in body temperature and white blood cell count, alongside a decrease in body weight from the administration day. Conversely, goats treated with Yokkaichi_P- displayed no significant changes in these values. The Yokkaichi-treated goat demonstrated a >10-fold increase in anti-VapN antibody titers from 11 to 14 days postadministration, whereas the other two goats exhibited no variation in anti-VapA and VapN antibody titers. Pathological autopsy analysis of organs harvested 28 days postadministration revealed no characteristic lesions on gross examination. However, the inoculated strain (vapN-positive R. equi) was exclusively recovered from the tracheobronchial lymph node in the Yokkaichi-treated goat. Immunohistochemistry detected a VapN-positive reaction in the tracheobronchial lymph node, confirming latent infection despite the absence of dramatic suppurative lesions seen in ruminants. Overall, this study highlights the latent infection in lymph nodes induced by the pVAPN-harboring strain, despite the absence of overt pathological manifestations.