FOUBTH ANNUAL REPORT. 41 



fatty degeneration of the liver, and general congestion of the intestines and 

 the peritoneum. Cohabitation suffices to communicate the infection. A 

 hypodermic injection of 0.1 cc. of a decoction of cultures of 24 hours' stand- 

 ing generally kills the mice in 12 to 24 hours. 



Before putting virus at the disposal of the farmers, experiments were 

 made with rodents in the fields belonging to two agricultural schools. In 

 the first few days of March, 1893, pieces of bread impregnated with the 

 virus were distributed in the infested fields. From the fourth day after- 

 ward, dead mice were found in the field and in adjoining fields lying on the 

 surface of the ground. In April, the field was partially plowed, and sub- 

 terranean passages were brought to light filled with the bodies of mice all 

 more or less eaten. The rapid propagation of the disease is partially due to 

 the fact that the mice eat their dead. In immediately adjacent fields, as in 

 the ones experimented upon, the mice were exterminated. No injurious 

 effects were noted in the farm animals that eat of the dead mice. 



After the efficacy of this virus had been shown by actual field experi- 

 ments, the farmers demanded its application to their own fields. Trials on 

 a large scale in different parts of France were made with complete success. 

 The expense amounted to 3 fr., 15 cents, per Hectare. 



In a storehouse at Amiens, all the mice (Mus musculus) disappeared in 

 20 days after the distribution of the saturated bread. As in a neighboring 

 storehouse they kept on increasing, their disappearance must have been due 

 to the virus. 



In the instructions furnished the farmers, it is recommended that two or 

 three distributions of the saturated bread be made in order to completely 

 eradicate the pest. The mice are most numerous in August and September, 

 and the virus acts best at that time on account of their density. 



The first distribution of impregnated bread should begin in August, 

 right after the harvesting of cereals, and continued in September and Oc- 

 tober in fields of beets, potatoes, etc. This first application generally de- 

 stroys 90 to 95 per cent, of the mice. The epidemic thus started continues 

 of itself for at least six weeks. One month or six weeks after the first dis- 

 tribution, the impregnated bread should be placed at all the new holes in 

 the field. The total cost of three distributions should not exceed more than 

 5 francs per hectare (about 50 cents per acre). If the whole farm cannot 

 be treated at once, portions of it can be infected at any time, as the rodents 

 are not migratory. 



The virus preserves all of its properties in the laboratory for several 

 months. A maximum virulence is obtained from 5 to 20 days after the 

 preparation of the tube. 



For the destruction of the large gray rats (Mtis decumanus), which cause 

 annually numerous losses to crops in many countries, a stronger virus must 

 be used. The same microbe, rendered more active by a series of inocula- 

 tions on these rodents of larger size, is found to act as successfully as does 

 the weaker virus in the case of the field mice. 



