Granulomatous lesions in lymph nodes of slaughter pigs bacteriologically negative for Mycobacterium avium subsp. avium and positive for Rhodococcus equi

The prevalence of granulomatous lesions in lymph nodes of pigs was studied. From January till August 2004 in two slaughterhouses in The Netherlands 2,116,536 pigs were examined for the presence of granulomatous lesions in the sub-maxillary lymph nodes. In 15,900 (0.75%) of these pigs, lesions could be detected. Nine farms with the highest incidence of lesions were selected for a more detailed pathological and bacteriological examination. On these farms, the prevalence of lesions in sub-maxillary lymph nodes ranged from 2.3 to 5.7% with a mean of 3.0%. From 1276 pigs that were sampled, 98 (7.7%) displayed granulomatous lesions in the sub-maxillary lymph nodes and one (0.1%) pig showed lesions in its mesenteric lymph node. Mycobacterium avium subsp. avium (MAA) could not be isolated from the lymph nodes of the 99 pigs with lesions and from a selection of lymph nodes (n = 61) of pigs without lesions. Rhodococcus equi was isolated from 44 out of 98 (44.9%) of the sub-maxillary lymph nodes with granulomatous lesions and from two mesenteric lymph nodes without lesions. A comparison of former studies and the current results indicate that the prevalence of MAA infections in slaughter pigs has strongly decreased over the last decade, whereas R. equi is highly prevalent. The high incidence of granulomatous lesions associated with the bacteriological presence of R. equi could be considered as a serious cause of misdiagnosis of MAA infections in cases where meat inspection is carried out by inspection for granulomatous changes of lymph nodes only.