1921.] 



Kesearch (Hat) Laboratory. 



71B 



After many experiments he succeeded in isolating i\m\ cultivating 

 tlie germ artificially, and raising its virulence until it was strong 

 enough to kill not only mice but also rats. It had, however, no- 

 effect on other animals. The virus method depends on feeding 

 the rats vdth baits containing cultures of living germs; there 

 infect the animals, which die in 10 or 12 days, and the disease 

 spreads among the survivors. Several t}^es of virus or germ 

 rat poisons have been placed on the market. Unfortunately 

 there are objections to this method which, in many respects, 

 offers an ideal means of dewstruction. These objecaons are : — (1) 

 the varying susceptibility of the rats to the disease, as some do 

 not die when they take the germs; (2) sublethal doses tend to 

 immunise the rats to the disease, so that a relatively immune 

 race of rats would be evolved : and ^8) there is the risk of the 

 organism used being either initially pathogenic to other animals 

 or developing in its passage through the rat a higher virulence 

 v.'hich might affect other animals, and even human beings. Cases 

 of illness in human beings have been attributed to rat virus. 

 (4^1 Lastly, there is some doubt as to the extent to which the 

 disease is transmitted from rat to rat, for as it is not a con-" 

 lagious disease it can only be transmitted by ingestion of the 

 f^erms. For these reasons the Ministry of Agiiculture and the 

 Ministry of Health do not favour the wus method of rat destruc- 

 tion. Bacteriological science has not, however, said its last word 

 on this subject,- and the objections may be overcome in the 

 future. 



Poisoning is by far the most important method. With proper 

 precautions it can be used anwhere ; when suitably applied it 

 is remarkably effective ; and it therefore offers the best solution 

 of the problem at present. Poisons are substances which, when 

 introduced into the living organism, cause death or injury. 

 (Strictly they include disease germs, but for the present 

 purpose these are excluded.^ Some substances, such as strych- 

 nine and morphine, are dangerous poisons, but if given in small 

 quantities are valuable drugs. Other substances, such as 

 bismuth or iron salts, and strong vegetable purgatives, are not 

 oi'dinarily regarded as poisons, but if taken in excessive amounts 

 may come within the above definition. There is a large field 

 from which to choose a rat poison. 



It should be noted that all methods of rat destruction should 

 be carried out in conjunction with rat proofing, for in the end 

 it is largely the amount of shelter and food available which 

 determines the r?t pop'ilRtion. 



