PATHOGENICITY OF RHODOCOCCUS-EQUI STRAINS POSSESSING VIRULENCE-ASSOCIATED 15-KDA TO 17-KDA AND 20-KDA ANTIGENS - EXPERIMENTAL AND NATURAL CASES IN PIGS

The pathogenic role of Rhodococcus equi in pigs remains controversial. Small numbers of pigs were inoculated intravenously (i.v.), or intram uscular-ly (i.m.) around the mouth, with a Virulent, an intermediately virulent, or an avirulent strain of R. equi and killed 14 days later. None showed clinical signs other than transient fever and weight loss . The virulent and intermediately virulent strains were recovered in c ulture from various organs and lymph nodes of pigs inoculated i.v., bu t only from the mandibular lymph nodes of pig inoculated i.m. The avir ulent strain was not recovered from any site. None of the pigs develop ed macroscopically visible lesions, bur they. show cd reactive hyperpl asia of the mandibular lymph nodes. The latter contained scattered pha gocytic cells, which were labelled immunohistochemically for virulence -associated antigens (15- to 17-kDa antigens or 20-kDa antigen). Inter mediately virulent and virulent strains of R. equi were isolated from mandibular lymph nodes of 5.5% of apparently healthy abattoir pigs (n = 1615). Virulence associated antigens were detected in phagocytic cel ls of culture-positive nodes, but the latter showed no lesions other t han reactive lymphoid hyperplasia. The results would seem to question the pathogenic role of R. equi in pigs, and it is speculated that the organism survives in the lymph nodes without causing pathognomonic les ions. (C) 1998 W.B. Saunders Company Limited.