ON THE SUBJECT OF PLAGUE 
365 
find, then, that where B. pestis kills, the B. pseudotuberculosis 
rodentium is harmless to these animals. 
Vitality of the Plague Bacillus outside the Body. — I have 
described to you now the form of the bacillus and how it 
is grown. Let us turn to the important point as to the degree 
of resistance to destruction which it manifests when outside 
the living body. In general the resistance is slight ; but pure 
cultures, kept in the dark and prevented from drying, can live 
for months or even years, depending a good deal on the 
temperature at which they are kept. Otto was able to demon- 
strate that the plague bacillus taken from putrid cadavers of 
plague rats were alive up to sixty-one days, provided they were 
kept at as low a temperature as 6° C. In bubonic pus it has 
been found to survive for twenty days, and also for that length 
of time in fresh water, and in sea water for forty-seven days. 
In cow-dung it has been found to live for months when kept 
moist. You may perhaps be aware that the common custom 
in India is to plaster the sides of the walls and the floors of 
the houses with moist cow-dung, which is then allowed to dry. 
On such floors plague bacilli were found to remain infective 
for forty-eight hours. 
The resistance of B. pestis to drying is not great. Thus, 
plague matter dried on wool, silk, linen, glass, wood, &c., died 
within six days. In dust it rapidly dies out ; and in Hankin’s 
experiments it was found to die out within thirteen days in 
grain and meal. Exposure to light causes rapid sterilisation of 
plague cultures — three to four hours sufficing. Dry heat at 
160° C. kills in a minute. Cold, however, has very little effect 
on plague bacillus. It was found alive after forty days, after 
the severe treatment of freezing and thawing it daily. 
Chemical disinfectants rapidly and easily kill it. Thus, 1 per 
cent, carbolic acid or lysol will kill in ten minutes. 
Pathogenic Action of B. pestis on Animals. — What happens 
when this bacillus gets inoculated into animals ? And which 
are susceptible to its action ? The most susceptible are all 
kinds of rats, also guinea-pigs, mice, monkeys, mongooses, 
squirrels, bats, jerboas, and marmots. Dogs, jackals, and 
hyenas are mostly unsusceptible. Cats are not highly so, and 
the bird family is immune. 
