189 



aud seem rather to be the contracted, hardened remains of the cheesy 

 mass. There are sometimes found in certain lungs raised patches of a 

 rather dry, emphysematous tissue, which seems to be due to the deeper 

 lying parasites. In other lungs the patches whicli once showed the 

 pneumonia have becomed thicliened, firmer, denser, and a cut across 

 them shows the thiciiening to extend to a considerable depth. 



The abundance of the nodules and patches of j)neumonia is very vari- 

 able. There may be a dozen nodules of different sizes aud two or three 

 patches, or the nodules may be diffusely scattered over the whole pos- 

 terior surfaces of the lung, or there may be associated with them numbers 

 of patches due in part to the close proximity of the nodules and in part 

 to the extension of the disease. In other cases there may be a few of 

 the nodules with a series of patches ranged along the dorsum of the 

 lung. Each lung seems to present a slightly different phase, dependent 

 on the degree and the time of infection and possible reinfection. 



Source of infection. — That verminous pneumonia is caused by a worm 

 [Strongylus ovis-pulnionalis) and that sheep become infested while feed- 

 ing or drinking has already been enlarged upon. It is obvious, there- 

 fore, that the best way to keep the sheep well is in some way to pre- 

 vent them from becoming infected with the parasite while feeding. 



Preventive treatment. — In giving rules for prevention the value of 

 knowing the complete life history is fully illustrated. The unexplained 

 gaps in this history are two, viz : there has been no complete demonstra- 

 tion of the manner and place in which the parasite spends its life be- 

 tween the time of its escape from the lung of one animal and its recep- 

 tion into that of another, nor has it been demonstrated that the worm 

 must escape from the lung before it may complete its development. 

 This latter item is an important one, for if the worms can continue 

 multiplying indefinitely in the lung then there is little hope of freeing 

 a sheep after it is once infected. On the other hand, if an infected 

 sheep is to bp regarded as incapable of continuing the infection within 

 itself then the case is more hopeful. If the parasite must become para- 

 sitic on a second host while external to the sheep, as some claim, this 

 is an important factor in its life history, for its continuance, then depends 

 on the presence, abundance, and seasonal appearance of this second 

 host, and influences adverse to the life of the second host would be un- 

 favorable to the parasite. 



As the parasites are present in the lungs of sheep throughout the 

 year in all stages, this theory does not seem to hold good. The infec- 

 tion of lambs is proof enough that the parasites are continually passing 

 from one sheep to another, and whatever be the mode of living there 

 are certain precautions which may be taken to keep the sheep less in- 

 fected if not to entirely exclude the worms. The older sheep, which 

 seem to be more infected and which are the source of infection for young 

 ones, should.be marketed. Lambs should be weaned as early as they 

 safely can be, separated from the older sheep, pastured in fields whef§ 



