﻿84 BULLETllsr 1369, TJ. S. DEPARTMENT OF AGRICtJLTtTRE 



more localized. In the latter case there is a tendency for the skin 

 to close the grub opening and cut off its air supply. In certain 

 animals there is also a strong tendency to form pus in and about 

 the cyst, and in such cases the grubs usually die. The writers' obser- 

 vations show that the mortality in the subcutaneous tissues of the 

 back is very high. Their figures indicate that 60.5 per cent of those 

 which have cut holes through the skin die before reaching maturity. 

 Of this percentage approximately 16 per cent succumb in the third 

 stage, 46 per cent in the fourth stage, and 38 per cent in the fifth 

 stage. It appears also that occasionally third-stage larvse which 

 reach the subdermal tissues die before they succeed in cutting 

 through the skin. 



There is a marked difference in the degree of infestation of 

 animals of different ages. Young stock, notably animals from 1 to 

 3 years old, almost always show a higher percentage of infestation 

 than mature animals. There is also some reason to believe that as 

 cattle become very old the degree of infestation is again increased. 

 Seymour- Jones {91) advances the theory that in young stock the 

 tenderness of the muscle {panniculus carnosus) immediately under 

 the skin enables the grubs to penetrate them, whereas in older cattle 

 they do not all get through. 



Had wen (55, I1.O) is of the opinion that the difference in de- 

 gree of infestation between young and matured animals may be 

 explained by a gradual development of immunity following the 

 repeated attacks of parasites. He believes that a first invasion of 

 parasites in an unresisting host stimulates the production of both 

 antisubstances and eosinophiles to neutralize their cast-off products, 

 and in addition to this, that there must be a third substance which is 

 antagonistic to the parasites themselves; he believes that this sub- 

 stance is secreted by the eosinophiles, and that it paralyzes or kills 

 the parasites which are surrounded by the eosinophiles in the tissues. 



The writers believe that the difference in the number of grubs 

 found in young and old cattle may be explained in part by this 

 reaction, but that other factors enter into the question. In general, 

 young animals are more exposed to the attack of the flies, both be- 

 cause they are not housed so long in the spring and because they are 

 seldom placed in barns during the time when the flies are oviposit- 

 ing. It is also true that the young stock are frequently not as well 

 fed as the older animals. The statement is made frequently by 

 dairymen and stockmen that poor animals are more heavily in- 

 fested than fat ones. The writers have observed many instances in 

 which very fat animals were fairly well infested with grubs, but 

 their observations indicate that the poor cattle are usually more 

 heavily infested. It appears logical to believe that well fed, healthy 

 animals have more natural resistance to grubs than those in poor 

 flesh and with lower vitality. It is also certain that cattle in an 

 extremely impoverished condition will fall more ready prey to 

 the flies at the time of oviposition. It seems evident, however, that 

 the presence of a large number of grubs, especially in cattle not re- 

 ceiving an optimum amount of feed, will reduce flesh condition. 



No marked difference in susceptibility seems to exist in different 

 breeds. When water or shade is readily available it serves as a 

 means of partially protecting from infestation the stock which have 



