116 



necessity to fall back on such an hypothesis as this. It seems 

 possible that S. calcitrant infested with larvae of H. micro- 

 stoma may inoculate these larvae' into the skin of a horse with 

 the production of a granuloma. It has been shown experi- 

 mentally that larvae of H . microstoma are capable of produc- 

 ing a typical granuloma in the subcutaneous tissues. The 

 occurrence of such a granuloma under natural conditions, 

 however, has not been definitely observed in Southern 

 Australia by the present writer, although the lesion taken 

 from the metacarpus may possibly have been due to these 

 larvae. 



As far as can be ascertained the granulomata observed 

 on the sheath and limbs have not resulted from infection of a 

 wound. In no case has there been a history of a previous 

 wound. It is possible that small wounds may have been over- 

 looked, but it must be conceded that the sheath is a very 

 uncommon site for wounds. 



These granulomata appear about the sheath and limbs, 

 sites commonly attacked by Stomoxys calcitrans. When one 

 or more of these flies bite they often produce some swelling 

 in the skin, and an exudation of blood or serum occurs through 

 the puncture wound. It seems possible that Musca domestica 

 when coming to feed upon this exudate may contaminate the 

 site with larvae of Habronema. The larvae would find 

 sufficient moisture to prevent their desiccation, and would 

 probably be able to make their way through the puncture 

 wounds into the skin and subcutaneous tissues. 



Wounds would often present ideal conditions for contam- 

 ination by larvae and their subsequent penetration into the 

 deeper tissues. It is probable that this method of infection 

 does occur, but there seems to be no doubt that it is not the 

 only method of infection. 



Habronemiasis is so common in horses that it is rare to 

 find a stomach free from one or other of the three species. 

 This being so, it is remarkable that habronemic granulomata 

 should be of such infrequent occurrence in Southern Australia 

 and other temperate countries. There are several possible 

 reasons for this : — (1) H. megastoma is not as common as the 

 other species. It has been suggested that larvae of H . mega- 

 stoma are probably the commonest cause of habronemic 

 granulomata. In the experience of the present writer H . 

 megastoma is the least common of the three species to be 

 found in the stomach of the horse. This fact would tend to 

 lessen the frequency of the occurrence of the granulomata if 

 larvae of H. megastoma are the causal organisms. (2) The 

 escape of larvae from the proboscis of flies is not a common 

 occurrence. It would appear that the larvae may escape from 



