22 Oestrus Ovis. 



Occurrence. The disease is found all over the world, ex- 

 cepting- Australia, especially in young animals (yearlings) ; it 

 occurs" enzootically, more rarely sporadically. It is exception- 



In fat-tailed sheep the larvae of GRstnis purpureus likewise appear to occur. 

 Ill the nose of buffaloes and camels the larvae of G-]strus iiiaculatus have been found, 

 and in the pharynx of deer the larvaa of Pharyngoniyia picta and Cephaloniyia 

 rufibarbis; in roes G. stimulator; in reindeer C. trompe and in moose C. Ulrichii. 



M Etiology. The larvae of (Estrus (cephalo- 



myia) ovis, sheep fly, are 2 to 30 mm. long, 

 according to their varying stage of develop- 

 ment. Their bodies are elongated and oval, 

 flat on the ventral surface, convex on the dor- 

 sal and show ten or eleven rings. The young- 

 Fig. 1. Larva of est individuals are white, and transparent, 

 (Estrus oris (seen ^j (^^^^y^ yellowish white. The mature ones 



frnm thf» linstprior <- ^ ^ J .. PIT • J.1 



show a transverse striation or the rings; the 



from the posterior 

 surface at the left 



seen from the ven- autcrior end is more elongated than the pos- 

 terior one. The head ring carries two mouth 

 hooks (Fig. 1). 



tral surface to the 

 right) 



The larvae are developed from the ova of the sheep fly. They are 

 10 to 12 inni. long, yellowish-gray with transparent wings. The larvae 

 Avhich leave the ova already in the oviduct are deposited by the female 

 flies in the neighborhood of the anterior nares of the sheep, whence they 

 wander into the nasal cavities, frontal sinuses, ethmoidal cells, and even 

 higher up. They become adherent to the mucosa and progress in 

 development. They mature after about ten months, i. e., in spring, 

 migrate out of the middle nasal duct and change in the soil vdthin 

 twenty-four hours into pupae. The fly leaves the latter after four to six 

 weeks; after fertilization the females again deposit their ova in the 

 neighborhood of the anterior nares of sheep. 



Sheep flies swarm from the middle of May to October, for 

 a shorter time in colder clhnates, especially during the noon 

 hours. They rest preferably in the holes and clefts of sheep 

 stables and "among the brushes. From here the fertilized fe- 

 males hunt up herds of sheep to deposit the larva) in the neigh- 

 borhood of the anterior nares. On the approach of the flies; the 

 sheep run together, put their heads together or place them on 

 the ground, or hold their noses between tlieir legs. If the lar- 

 vae are deposited in spite of this, and after they have begatn 

 to creep upward into the nasal cavities, the sheep become very 

 restless, rub their heads on the soil or lietween their feet, shake 

 themselves, sneeze frequently, and run around restlessly. The 

 restlessness, however, soon disappears and the animals remain 

 quiet for about ten months. 



Anatomical Changes. The mature larva^ are, as a rule, 

 found in hirgc numbers (according to Ziirn, GO to 80) in the 



