238 MR. E. G. BOULEXGER : REPORT OX 



raised by ai'tiftcial metliods in the labovatoiy and a innaber of 

 Hat Viruses were placed on the maiket. 



The manufacturers of these claimed that the bacillus was so 

 virulent that it would kill rodents in about ten days, and that 

 within a, month the disease would spread with fatal results to the 

 whole of the rat — or mouse — population of the area under treat- 

 ment. The results of recent investigations on the efficiency of 

 some of the viruses on the market have been disappointing. 

 Bainbridge* has experimented on a very large number of luts in 

 captivity, and the results of his experiments showed that the 

 destructive power of all the viruses he tested was inconstant, 

 the death-iate in the different experiments varjnng from 20 to 

 50 per cent. Further, according to this expei'imentator the 

 presence of agglutinins in the serum of the rats which survived 

 after being fed on virus gave reason to suppose that a certain 

 proportion acquired immunity and were therefore unlikely to 

 succumb to a second infection. 



Experiments with the rat-viruses conducted some years ago 

 during the outbreak of plague in San Francisco also gave poor 

 results. 



It must be recognized that, if geneially successful, this method 

 of exterminating rats and mice by spreading among them a 

 disease, not afiecting Man and domestic animals, could not be 

 improved upon, and I was therefore anxioiis to give a thorough 

 test to all the viruses on the market in the hoj^e that the resiilts 

 of the experiments under natural conditions would difiVr from 

 those obtained in the Inboratory. 



We made in all 7 tests with different virus prepai-ations, and 

 of these one only was an unqualified success; three were partial 

 successes, a. veiy slight reduction in the number of rnts being 

 recorded, and three were absolute failures. A number of reliable 

 witnesses who visited the Exhibition in the course of the summer 

 informed us of their experiences, and we iiscertained the successes 

 in these cases to amount to about 33 per cent. 



Traqyping. 



Although we found that trapping alone will not always rid us 

 of the rats where they are present in very large numbers, never- 

 theless if the best traps ai'e used skilfully and persistently the 

 vermin can be greatly reduced. Owing to the abundance of food 

 in our Gardens, the locality is not one where much success woidd 

 be expected. Nevertheless, in the four months leferred to in 

 this report, we caught with traps alone 736 rats. Of the many 

 different ti-aps laid down, three types accounted for 85 per cent, 

 of the total catch. Some of the traps that proved to be failures 

 were those we found most frequently in use in the rat-infested 

 localities which Ave visited, while the most successful type of 

 trap, several of which had been laid down in our Gardens for 



* Journal of PatlioloKV, vol. xiii. 1909. 



