"GROUSE DISEASE" STRONG YLOSIS 233 



of stock on a large moor may seem very low proportionately to the whole 

 area, but when estimated in proportion to the food area it may prove 

 exceptionally high, and this means a high potential capacity for the pro- 

 duction of Trichostrongylosis, whilst the entrance of a few bacteria or protozoan 

 parasites into the body may suffice to cause serious diseases owing to the rapid 

 multiplication of the original germs. In helminthic infections, as we have 

 shown, the parasite cannot multiply inside the bird. Birds with few worms 

 remain healthy. The progress of the disease is correlated with the actual 

 number of parasites entering and surviving in the body. The more heavily 

 infected the food, the more heavily infected does the bird become. 



The following facts connected with the growth of the parasite outside the 

 body of the Grouse emerged from our Inquiry, viz. : (1) that moisture is 

 necessary for the development of the egg ; (2) that a minimal tempera- 

 ture of several degrees above freezing point is essential not only for facts 



.. , ,' - established. 



the development ol the egg, but also for the metamorphosis of the 

 embryo ; (3) that the embryo ascends the heather only after metamorphosis. 

 These facts afford us some explanation of the disease being a fatal one in the 

 spring months. During the summer months many of the csecal droppings must 

 be dried by the sun and wind shortly after they are passed, and the eggs 

 thereby killed. The same agencies must also desiccate beyond revival a large 

 number of the encysted larvse upon the heather. During the winter months, 

 however, this loss does not occur. Owing to the low temperature and continual 

 wet the eggs remain in a living but quiescent condition. Even if an occasional 

 spell of warmer weather occurs, an,d the eggs develop into embryos, it would 

 be necessary that such period of high temperature should continue for at least 

 a fortnight to enable these embryos to become converted into active migrating 

 larvae. The result probably is that there accumulates upon the moors during 

 the whole winter vast numbers of undeveloped eggs and unmetamorphosed 

 embryos. The low temperature does not kill them, but merely suspends 

 their growth for the time being. At the spring-time the minimal temperature 

 rises gradually to such a point as to allow the continuous development of 

 the eggs and embryos to and through metamorphosis, with the result that 

 at this period there presumably ascends the heather the accumulated result 

 of feecal contamination during the winter months. The frequent rains and mists 

 give the larvse ample opportunity to reach the topmost tips of the heather 

 at this time. 



