Treatment of (rrass L< did for Klint'inafion of l>'}sease. 85 



to the aci-e. We have no knowledge of the special eflt'ect of 

 anj- chemical manure on the organisms which pollute pastiires, 

 but when it is realised .that gi-ass land may be contaminated by 

 organisms discharged in the excreta of infested animals, it will 

 1)6 evident that most of the chemically prepared manures are 

 far less likely to convey the seeds of disease than animal 

 manure. It will be realised that top-dressing of grass land 

 with farmyard manure made by animals discharging disease 

 germs may prove a facile means of rendering it dangerous to 

 grazing animals. 



The treatment of grass lands most likely to prove eflFective 

 for the elimination of parasitic worm disease is ploughing ; 

 few, if any, of the eggs or embryos, we think, long survive 

 their burial in this way, while the absence of animals would 

 ensure it against fresh pollution. It is, however, questionable 

 whether the effects of ploughing are sufficiently enduring to 

 warrant its adoption in case of grass land which, except for the 

 existence of disease caused by parasitic worms, is more valuable 

 as pasture than as cultivated land. There must always be the 

 risk of pollution by deposit of ova or embryos as soon as such 

 land is again laid down to grass, and stocked. 



Contamination of grass land is not, however, limited to 

 disease induced by parasitic worms. Grazing stock are liable 

 to acquire l^acterial and other maladies from polluted herbage, 

 water, &c., and these are so numerous that but very cursory 

 reference can be made to some more salient features of a 

 few, and this mainly with a view of indicating that attempts 

 at elimination must be governed by the special features 

 of each form of contamination. As in the case of parasitic 

 worms, the great majority of l)acteria must spend some 

 part of their existence in animals, and do not retain their 

 disease-producing powers long apart from them. They live 

 in the outer world for varying periods, but most of them 

 succumb to natural conditions soon after being discharged 

 from the affected animal, sunlight being a potent factor 

 in their destruction. The temperature usually obtaining 

 in our climate does not favour their multiplication outside 

 the animal body. It is within the infested animal that 

 their increase takes place after varying periods of incubation, 

 and though to some extent the effect will depend on the 

 number entering an animal, a very small dose is usually 

 sufficient to produce the full effect. Some are discharged 

 during illness of affected animals and scattered over pas- 

 tures ; others are mainly confined to the animal, and, for 

 pollution of grass land, derived from affected carcasses. The 

 effects of some bacteria are also limited to animals of certain 

 species. 



