DISEASES DUE TO INTERNAL PARASITES. 209 



fly is fully matured — it darts off with remarkable swiftness to 

 find a flock of sheep and single out its victim. 



Symptoms. 



On the fly touching the nostrils of the sheep the latter im- 

 mediately shakes its head, at the same time striking the ground 

 with its fore-feet. The sheep commences to run, holding its nose 

 close to the ground, endeavoring to escape its pursuer. The ani- 

 mal runs into the middle of the flock. The whole flock becomes 

 alarmed even if only one fly is in evidence. They place' their 

 noses close to the ground and huddle together as closely as possi- 

 ble to make it difficult for the fly to attain its object. The 

 young larvae deposited on the mucous membranes create an in- 

 tense irritation from the insertion of their hooks and spines, 

 drawing themselves np the nasal chamber by means of these pro- 

 jections, leaving dark points of hemorrhage on the mucous sur- 

 face wherever the points have been inserted. On proceeding up- 

 ward they reach the turbinated bones, entering the frontal 

 and even superior maxillary sinuses. Those that reach the 

 superior maxillary sinuses grow in size, but never escape 

 from their location, finally dying and undergoing degeneration. 

 Those which reach the frontal sinuses, however, on reaching ma- 

 turity find their way backward to the nasal chambers, and are 

 voided to the ground. As the larvae grow in size in the sheep's 

 head the animal becomes affected with a catarrhal discharge. 

 This is at first clear, but later becomes thick with mucus. The 

 sheep frequently sneezes, these efforts resulting in the expulsion 

 of mucus and oftentimes matured larvae. The animal will lie 

 down and hold the nose pointed in the air, gradually turning its 

 head roimd and backwards. The eyes become red and watery, 

 the sheep moves with the nose close to the ground, stepping high 

 with the hind legs, frequently raising the head to the wind and 

 bending it sharply backwards. They may be seized with giddi- 



