The tonsils of eleven pigs and the mammary glands of a sow were used to investigate actinomycotic lesions due to Actinomyces sp. infection. At necropsy, there was no abnormality on these tonsils, on the other hand, numerous abscesses containing sulfur granules were found in the mammary. Histopathologically, the Actinomyces sp. lesions were noted as crypt abscesses in the tonsils and as pus-forming granulomas in the mammary glands. The microorganisms in both lesions were composed of bead-like cocci, bacillary cells and short, branching filaments, those cells being positive by the Gram's and Grocott's methods. Clubs were formed around the microbial clumps in these lesions. Immunohistochemically, there were cross-reactions between antibodies of Actinomyces sp. Chiba 101 (101) and swine actinomycetes of 7 species: A. bovis, A. hyovaginalis, A. israeli, A. naeslundii, A. pyogenes, A. suis) formerly Eubacterium suits) and A. viscosus. However it was possible to differentiate Actinomyces sp. 101 from them by absorption and dilution of the antiserum, then the microorganisms in the tonsillar crypt abscesses and the granulomatous mastitis were labelled with an immunoperoxidase technique using the absorbed Actinomyces sp. 101 antiserum. Thus, these immunolabelling properties are suggestive of the presence of 'A. suis' (Grässer) Franke 1973.

Summary The tonsils of ten fattening pigs were compared with those of ten breeding sows to determine the relative incidence of Tonsillophilus suis and Actinomyces sp lesions. Macroscopic observation revealed mildly focal upheavals on the tonsillar mucosal surfaces of two of the breeding sows. In tonsillar slices from both pig groups, tiny nodules containing sulfur granules were found. Histopathologically, the T. suis and Actinomyces sp lesions were noted as tonsillar crypt abscesses in both pig groups, and granulomas due to T. suis were seen in the breeding sows. T. suis showed larger ‘club‐shaped’ structures than Actinomyces sp, characterized as clubs showing a broom‐like structure with degenerated central thalli. Breeding sows are known to have a higher incidence of tonsillar T. suis lesions than fattening pigs, and in this study, those fattening pigs showing greater propensity to infection by Actinomyces sp also showed a mixed infection with T. suis that produced the tonsillar crypt abscesses. In addition, it is known that in breeding sows, the T. suis lesion develops from a crypt abscess into a granuloma.

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The present experiment was performed to test the pathogenicity of Actinomyces‐like bacteria in experimental animals and swine. Two rough (R) strains of Actinomyces‐like bacteria isolated from a site of arthritis and from the tonsil in pigs were used as inocula. To investigate their susceptibility to Actinomyces‐like bacteria, BALB/c, SS and ddY mice and guinea‐pigs were inoculated intraperitoneally with the strains of Actinomyces‐like bacteria. The ddY mice were used for the long‐term observation of Actinomyces‐like bacteria lesions and the mammary tissue of a sow was inoculated with Actinomyces‐like bacteria isolated from swine tonsil. Macroscopic observation revealed many abscesses on the surfaces of the abdominal and/or thoracic organs in the mice, but not on those of the guinea‐pigs. The sow developed firm nodules at the inoculation site in the mammary glands. Histopathologically, the lesions in the mice were characterized as actinomycotic abscesses in the early stage and as pus‐forming granuloma (PFG) in the advanced stage; the lesions were accompanied by crystalloid particles. Actinomyces‐like bacteria induced granulomatous mastitis in the sow, and the lesion was characterized as PFG. The characteristic actinomycotic lesions in swine mammary glands were reproduced by experimental infection.