Localisations in the udder and peritoneum are commonest in pigs, and it is believed that inoculation occurs either through the galactophorous sinuses or through the abdominal wounds made for purposes of castration.
Causation. The cause of actinomycosis is to be sought in the development of Actinomyces bovis within living tissues. It seems problematical whether the germs to be found in the pus or saliva of affected animals ever directly infect new hosts, and it is difficult to carry out infection in this manner even in very sensitive experimental animals. Nevertheless, the persistence of the disease in certain byres would seem to support the view of direct infection.
On the other hand, it is proved that the actinomyces is a parasite affecting vegetables, principally the graminaceæ, and that domestic animals are most commonly infected through injuries caused by vegetable substances. This is suggested by the discovery of the débris of grain at the point where the lesions have originated.
Fig. 273.—Actinomycosis of mammary gland (cow), showing growth invading lobules. (Figs. 272 and 273 are from blocks kindly supplied by Mr. Gilruth, F.R.C.V.S.).
Inoculation is commonest in the mouth and on the surface of the tongue, parts which are, so to speak, permanently excoriated. The shedding of the temporary molars favours such accidents, and this is why actinomycosis of the jaw is, relatively, so common. The incisor region may also be inoculated during the shedding of the milk teeth, but as the infected food comes more closely and for much longer periods in contact with the molars, it is easy to understand why actinomycosis is rarer in the incisor region.
The conditions are less favourable for inoculation of the pharynx, because food does not remain in position there for more than a second or two, but when the epithelium has been shed as a consequence of laryngitis or pharyngitis, infection may occur.
As regards cutaneous inoculation, the parasite only seems dangerous when the skin is excoriated or injured either accidentally or as the result of surgical interference.
Fig. 274.—Old-standing bone lesions in a case of actinomycosis of the jaw.