56

Till we are quite certain of the method of infection
preventive measures are likely to prove somewhat ineffectual.

ALTERATION IN VIRULENCE OF THE BACILLUS.

Growing the bacillus at higher temperatures than 37°C.
greatly alters its virulence for animals, but growing it at low
temperatures does not alter its virulence in the slightest.
Grown for long periods in vitro at 37°C. it loses its virulence
sooner than when grown for the same length of time at
30°C. This was shown by cultivating the bacillus from a
rabbit which died in 12 hours, and from one tube two sub-
cultures were made.

One subculture was kept at 30°C. and the other at
37°C. for six weeks, each being subcultured every few days.

At the end of this period three rabbits were inoculated
from the 30°C. culture, and three with similar doses from
the 37°C. culture, with the result that the former died in
23, 14 and 10 hours respectively, whereas two of the latter
recovered and the 3rd died in 36 hours, i.e., a distinct
decrease in virulence in the cultures kept at the higher
temperatures.

Other conditions being favourable, mere length of time
cultivated outside the living animal body decreases the
virulence.

A few hours' drying greatly alters the virulence. Bacilli
minus their toxins have only feeble pathogenic powers.

Passage through certain animals with a natural resist-
ance to the disease greatly decreases the virulence.

In Italy it is said that passage of the bacillus through
pigeons so alters the virulence that the blood will protect
buffaloes against the disease.

Here in India, I tried to alter the virulence for rabbits
by passage of the organism from pigeon to pigeon, and then
cultivating on agar and inoculating into rabbits, but without
success.

Passage from sheep to sheep greatly alters the virulence
for rabbits, one rabbit dying after 10 days. Passage through
the camel does the same. In the case of Camel No. 7
previously mentioned, the bacillus of normal virulence for
the rabbit took 31 days to kill the camel, and afterwards