50 THE BROWN EAT IN THE UNITED STATES. 



is authority for the statement that in May, 1901, after the steam- 

 ship Minnehaha of the Atlantic Transport Company had been disin- 

 fected at the London dock by the use of sulphur dioxid over 1,300 

 dead rats were found in the holds. The vessel had been in commis- 

 sion only about nine months, carrying flour, grain, and provisions 

 between New York and London, and the finding of so many rats 

 was a complete surprise to the ship's officers. 



MICRO-ORGANISMS. 



At irregular intervals when rodents become overabundant, con- 

 tagious diseases break out and destroy large numbers. Such dis- 

 eases have been known to occur among ground squirrels, prairie dogs, 

 rabbits, lemmings, house mice, field mice, and rats. They occur 

 also, occasionally, where numbers of the animals are brought to- 

 gether in confined quarters. Observations of such epidemics have 

 encouraged bacteriologists to experiment in inducing diseases arti- 

 ficially for the purpose of destroying rodent pests. 



The problem of the bacteriologist is to find an organism that will 

 destroy a given species by contagious disease, and yet be harmless to 

 all other animals, whether wild or domesticated. From a scientific 

 standpoint some progress has been made toward the solution of such 

 problems, but practical results are still lacking. The rat has been 

 the subject of more experiments than any other animal, but attempts 

 to destroy it by epidemics have not yet advanced beyond the ex- 

 perimental stage. 



Several micro-organisms, or bacteria, have been exploited in Europe 

 and America for destroying rats. A number are on the market in 

 the United States. The Biological Survey has made laboratory and 

 field experiments with some of them, and has also received many 

 reports from others who have tried the cultures in a practical way. 

 The results are by no means uniform, although the majority are nega- 

 tive. The cultures tested by the Survey have given poor results. 



The chief defects to be overcome before the cultures can be recom- 

 mended for general use are — 



1. The virulence is not great enough to kill a sufficiently high 

 percentage of rats that eat food containing the micro-organisms. 



2. The virulence decreases with the age of the cultures. They 

 deteriorate in warm weather and in bright sunlight. 



3. The diseases resulting from the micro-organisms are not conta- 

 gious and do not spread by contact of diseased with healthy animals. 



4. The comparative cost of the cultures is too great for general 

 use. Since they have no advantages over the common poisons, 

 except that they are harmless to man and other animals, they should 

 be equally cheap; their actual cost is much greater. Moreover, con- 



