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Attention has already been drawn to the fact that the 

 tissue reaction and necrosis found in habronemic granulomata 

 are essentially the same as those found quite commonly in 

 the internal organs of herbiverous animals following the death 

 in the tissues of migrating parasites. Experience leads one to 

 believe that a granuloma containing necrotic foci, showing a 

 marked eosinophilic invasion and the formation of multi- 

 nucleated cells, is due to the reaction of the tissues against an 

 invasion by a larval or adult metazoan parasite, usually a 

 Nematode. There seems to be no reason to suggest that 

 "swamp cancer" offers an exception. 



It is now known that these lesions in the skin and external 

 mucous membranes of the horse are most commonly due to a 

 larval Habronema, and there is strong presumptive evidence 

 that "swamp cancer" is due to a similar larva. 



It has already been shown that in older lesions larval 

 Habronema cannot be demonstrated. It is difficult in a 

 country like the Northern Territory of Australia to obtain 

 early lesions, for the animals are not under constant super- 

 vision. Lesions that have been sent to the present writer and 

 described as early lesions have been found on examination to 

 be small lesions, but of some standing. It seems almost certain 

 that many of these small lesions would never develop into the 

 large, chronic lesions. Their small size appears to depend 

 upon the few necrotic areas present and a resistance on the 

 part of the animal which is apparently absent in those animals 

 which develop large, chronic lesions. 



Experimentally it has been shown that larvae belonging 

 to all three species of Habronema are capable of penetrating 

 the tissues for some distance from the point of entry. At least 

 two of these can set up a chronic irritation which leads to the 

 formation of a granuloma containing necrotic foci. Experiment- 

 ally it has also been shown that apparently the tissues of some 

 animals offer a strong resistance to the presence of these larvae, 

 and are capable of quickly destroying them before they are 

 able to produce much reaction. It will be seen, therefore, that 

 the possibility is that "swamp cancer" may be due to any one 

 of the three species of Habronema. Evidence is not in favour, 

 however, of the probability of "swamp cancer" being due to 

 larvae of either H. muscae or H. megastoma. These two forms 

 pass through their larval stages in Musca domestica, and as 

 this fly is not usually to be found far afield, it seems probable 

 that it is in no way associated with the occurrence of "swamp 

 cancer." It seems more probable that H. microstoma may be 

 the species responsible for the lesion. As this species passes 

 through its larval stage in Stomoxys calcitrans there is more 

 chance of horses in the field becoming inoculated with these 



