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

Abstract

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.

Keywords: Rhodococcus equi; vapB; lymph node; pig; plasmid.

Fig. 1.

(A) Macroscopic observation of pig submaxillary lymph nodes with lesions. Arrows indicate caseous foci. (B) Histology of the lesion with Hematoxylin–eosin (HE) staining. Focal granulomatous inflammation was bounded by thin fibrous capsules. Bar, 0.5 mm. (C) Histology of the lesion with Gram staining. Arrows shows Gram-positive coccobacillary bacteria in the cytoplasm of some epithelioid cell and multinucleated giant cells. Bar, 20 μm. (D) Immunohistochemical examinations with a rabbit hyperimmune serum against R. equi strain A5 expressing VapB antigen. Coccobacillary bacteria (arrow) were observed in the cytoplasm of some epithelioid cells and multinucleated giant cells. Bar, 20 μm. (E) Histology of the lesion with HE staining. Arrows indicate “owl-eye” shaped intracytoplasmic inclusions (Michaelis—Gutmann bodies). Bar, 20 μm. (F) Histology of the lesion with Von Kossa, Periodic acid-Schiff, and immunohistochemical stainings. Arrows indicate “owl-eye” shaped intracytoplasmic inclusions (Michaelis—Gutmann bodies). Bar, 20 μm.