16 SCIENCE BULLETIN, No. 21. 



along the needle track. There is no unusual odour in the incised affected 

 muscles and no haemorrhages. No other gross lesions have been observed 

 save moderate congestion of the mucosa of the small intestines. The bacilli 

 are recovered in ourity from the inoculation area, but elsewhere is bacteria 

 ' free. 



The Pigeon. 



This bird is as susceptible to artificial infection as the guinea-pig. The 

 intramuscular injection of (H c.c. of virulent culture is usually followed by 

 death in about eighteen to twenty hours. On autopsy no cutaneous . changes 

 are evident. The subcutaneous tissues of the breast and abdomen are infil- 

 trated with the usual exudate. There is no odour or gas formation. A 

 distinctly necrotic line follows the needle track and extends laterally for a 

 short distance into the muscles, the lateral extension being irregular in width. 

 At the edges of this necrotic zone the muscles show evidence of intense 

 inflammatory congestion, but there are no distinct haemorrhages and no gas. 

 On incision, the odour of the affected muscles is rather unpleasant but not 

 distinctly putrefactive. No other gross lesions have been observed. The 

 bacilli are confined to the lesions from which they have been recovered in 

 purity. 



Up to the present, no experimental inoculations have been made into the 

 !horse, ox or pig. 



The Natural Method of Infection. 



Before entering upon a discussion as to the natural method of infection in 

 black disease, it may first be advisable to note some points that have been 

 .observed clinically and experimentally in connection with this condition, 

 most of which are already recorded in this and my previous article. 



Clinical. The disease is seasonal, the vast majority of cases occurring 

 from late summer to early winter. In mild seasons occasional deaths will 

 iake place throughout the winter. On infected properties, sheep grazing in 

 paddocks well watered by springs but in which the ground is not well 

 drained or which contain swampy areas, not necessarily of very large 

 -extent, are subject to a heavy annual mortality from black disease, except 

 in seasons of unusual dryness when the moist places have dried up, whereas 

 sheep kept in paddocks having water from the same sources running 

 through them, but where the ground is well drained naturally, show few 

 losses or none at all. The division between the infected and non-infected 

 paddocks may be merely a wire fence or a stone wall. In infected paddocks 

 that have been artificially drained the mortality has been considerably 

 diminished. In a very dry season, such as that experienced in 1918-19, the 

 death-rate in paddocks known to be heavily infected was unusually light. 



There is no evidence of any kind to show that the disease can be com- 

 municated directly from one animal to another. It is stated that after the 

 transfer of the whole of an affected flock to a clean paddock a few cases 

 occur for a short time, and then the mortality ceases. These are apparently 

 Ceases of infection, or rather (as will be shown) of potential infection 



