8
Some Aspects of Anthrax in Bengal
[VOL. XVIII, PART I,
It may be concluded from this that while anthrax may break out in any part of Bengal at any
time of the year, the majority of the outbreaks occur during the summer months. Thus, of the 998
outbreaks which occurred during 1937-1946 more than 65 per cent were during the four months of
April, May, June and July. Analysis of the seizure and mortality figures for this period has also
revealed a very much higher incidence of the disease in these four months than in the other months.
It is generally believed that in warm countries the maximum incidence of anthrax is in the months
of June, July, August and September [Topley and Wilson, 1936]. The present study, however,
shows that the period of maximum incidence in Bengal slightly precedes this, it being the summer
and the first half of the rainy season.
On comparing the incidence curve of anthrax outbreaks in Bengal with the prevailing weather
conditions the two factors which appear to mainly coincide with this incidence are the atmospheric
temperature and the rainfall, probably as they affect the soil and other environmental conditions.
Thus, in December and January (Tables I and II), when the weather is cool and dry and the rainfall
is at its minimum, the incidence of anthrax outbreaks is very low. In February and March, when
both maximum and minimum temperatures rise, rainfall increases a little (though the humidity is
still low), the incidence, though still rather low, shows a tendency to increase. In April, when the
temperature rises to very near the most optimum temperature for vegetation and multiplication of
B. anthracis, rainfall increases a little more, and the humidity, though still low, shows a tendency to
rise, the incidence increases considerably. In May, when the mean temperature is about the same as
in April, the rainfall increases considerably and the humidity also rises, the incidence is at its highest.
In June, when the maximum temperature comes down and the minimum temperature rises, both by
a couple of degrees, the rainfall is heavy and the humidity high, the incidence is almost as high as
in May. In July, when the temperature is still favourable, humidity very high and the rainfall heavy,
the incidence is moderately high but on the decline. In August, when the temperature is almost
the same as in July, humidity a little higher still and the rainfall continues to be heavy, the incidence
decreases considerably. In September, when the temperature and humidity remain about the same
as in July and August, and the rainfall decreases a little, the incidence is almost at its lowest point in
the year. In October and November, when the temperature, though gradually coming down, is still
favourable, humidity about the same as in June, cloudiness and rainfall further decreased and the
soil comparatively less soaked with water than before, there is a small increase in the incidence before
the final decrease in December.
To sum up, the incidence of anthrax outbreaks in Bengal seems primarily to vary with the at-
mospheric temperature, being low in winter months, gradually rising with the approach of summer
and reaching the peak when the mean temperature approaches the most optimum temperature for the
growth and multiplication of B. anthracis. Moderately high humidity and moderately heavy rainfall
seem to favour high incidence, very heavy rainfall and very high humidity keeping it low. The
great decrease in the incidence from July to September may be due chiefly to a cumulative effect of
the heavy and continued monsoon showers which can be expected to wash out most of the existing
soil contamination, leach out the available food elements, dilute the remaining contamination, and
by raising the atmospheric humidity and the moisture content and acidity of the soil, interfere with
aerobiosis and multiplication of the organisms.
The finding, that anthrax in Bengal shows a well-defined seasonal incidence can be profitably
utilized in case it is decided to adopt prophylactic vaccination in any locality. Such operations can
be so timed that the most active period of the immunity conferred covers the major anthrax season.
Thus, the best time to undertake prophylactic vaccination against the disease in Bengal, if
done only once a year, would be the period soon after the beginning of February. As it is advised
to use the vaccine as fresh as possible, its manufacture and stocking in bulk may also be adjusted
according to the seasonal prevalence of the disease.