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Contagious Abortion of Cattle, Sheep and Goats
[VOL. XVIII, PART III
After drying, the vacuum was released and some unused papers were stored over CaCl 2 at 42°C ;
from time to time they were pulped and the suspensions counted. Despite the irregularities already
mentioned, the counts indicated that the death rate was dramatically swift (Fig. 1).
In a second experiment, using the same technique the artificial drying in vacuum was omitted
and the samples were placed directly in the 42°C incubator, some over CaCl 2 and others over H 2 O
and H 2 SO 2 variously mixed to produce relative atmospheric humidities of 40 per cent, 80 per cent
and 90 per cent. This experiment failed because the death-rate was so rapid that it quite outstripped
the dilutions selected for counts, all that could be learnt was that the samples over CaCl 2 showed
a count of nil in the 1/100 and 1/10 dilution at 24 and 38 hours respectively (lower plates were not
made). In the 90 per cent humidity, on the other hand, one count of 1.4 millions per cc. was obtain-
ed at the 56 hour.
It is to be noted that in the CaCl 2 atmosphere the papers were nearly dry at 20 hours, when a
count of 800 million was obtained. Whereas, at 32 hours the paper was perfectly dry and in the
interim the count had fallen almost to nothing. Conversely, the paper in the 90 per cent humidity
was not dry at the 92 hour.
These preliminaries suggest that in the quite ordinary summer plains temperature of 42°C
Brucella die within 36 hours, in dry air but in moist, remain alive for three or four days.
The foregoing technique was now abandoned and instead, 0.5 cc. amounts of suspension were
pipetted into several covered, empty, 1½ in. petri dishes, which in turn were placed in the various
atmospheres. A suspension was reconstructed by adding 5.0 cc. of saline to a dry plate, whilst an
estimated quantity between 4.5 and 5.0 cc. was added to one partially dry. (The error necessarily
occurring here is appreciated, but, as such large differences are concerned it is probably not important.)
Again, as 42°C is rather higher than the usual day maximum in the humid parts of India, the storage
temperature was reduced to 37°C which is commonplace in winter and summer in the endemic areas.
Using this method, it was found that 0.5 cc. of suspension over CaCl 2 dried completely between the
26 and 30 hour and that during the period 18 to 42 hours the count dropped from 990 millions
to 0.066 million. In 80 per cent humidity on the other hand, the count dropped from 5,800 millions
to 220 millions in the same period, while drying was incomplete at 90 hours (Fig. 2).
The above experiment was, nevertheless, unsatisfactory, mainly owing to incorrect estimates
of which dilutions to plate at the later counts. Again it was thought that the salt in the saline
suspension might be playing an inimical part as it became concentrated during drying. So the
experiment was repeated but bacteria suspended in distilled water, instead of saline were used. The
results appear in Fig. 3 and, while both death-rates are considerably slower in pure water than in
saline, the mortality in dry atmosphere is again more rapid than in the humid. It is to be noted
moreover, that natural drying of Brucella is unlikely to occur often in a substrate entirely free from
salts.
The foregoing experiments contribute towards a better knowledge of the roles played by humidity
and dryness in the dissemination of Brucella in Asia and reviewing them in conjunction with the
actual findings of the Indian field, it might not be too rash to suggest that a cornerstone has been
laid in the construction of a hitherto neglected aspect of Brucella epidemiology.
The role of seasonal breeding
When the buffaloes' habit of breeding seasonally causes them to be advanced in pregnancy dur-
ing the monsoon, Brucellosis, once established, will tend to spread. This tendency might possibly
be corrected by inducing the animals to breed at a more favourable season.
Reservoirs of infection
The principal reservoirs of Brucellosis in India are to be found where the environment favours
dissemination viz., (i) in congested city stables, as found in Calcutta and Bombay, whence the disease