205
in human cases of plague with encouraging results. With the diminution
of the incidence of plague in India and the difficulty of obtaining sufficient
numbers of cases under hospital conditions it has not been possible to
obtain trials on the scale necessary to obtain full statistical evidence of the
value of the serum.
(c) Epidemiological studies.—It had been observed by the Plague Re-
search Commission that rats in areas where plague had been severely epi-
demic showed a high resistance to experimental infection while on the
other hand in areas, such as Madras City, which had been free from plague,
the rats were highly susceptible. An investigation has been in progress
at the Haffkine Institute on the relative immunity of rats from places all
over India showing varying degrees of incidence of the disease. The work
has confirmed that the immunity of the rats is roughly proportionate to the
degree of incidence of plague in the areas from which they are collected,
and has also shown that the resistance to infection persists over a prolonged
period of years. These observations suggest the existence of a factor which
is probably responsible for the decline in the incidence of plague. It is
possible that the occurrence of the immunity in the rodent herd may ex-
plain the epidemic cycles of plague and be responsible for its eventual dis-
appearance in certain areas.. The basis for this immunity is being studied.
9. Cumbum Valley Enquiry.—As a sequel to a series of rat-flea surveys
of the Madras Presidency the Cumbum Valley was selected as an endemic
area for the study of the "carry over" of plague, and a field unit has work-
ed there since 1930. The most important findings are that climatic condi-
tions in rat burrows are much more suitable for flea survival than ordinary
meteorological observations would suggest, and that Plague-infected starved
fleas may be responsible for the recrudescence of plague after periods of as
long as a month. A development of this enquiry was a field trial of hydro-
gen cyanide gas as a lethal agent for rats and fleas. This method of com-
batting plague is now well established in India. The value of rat-proof
grain godowns and the disinfestation of grain has also been studied. This
investigation still continues on a small scale.
KALA-AZAR.
As pointed out by Rogers (1897) there is little doubt that the epidemic
fever in Burdwan between the years 1850 and 1875 was kala-azax. The
accounts of it and its method of spread, by various contemporary
observers, such as Elliot and French, could have been applied, word for
word, to later epidemics after the true causation of kala-azar was known.
2. The disease was investigated in Assam in the 1890s by Rogers and
Ross and, later, by Bentley. By these early workers it was considered,
variously, to be a form of malaria, malaria complicated with other infec-
tions, or a disease allied to Malta fever.
3. In 1903 Leishman recorded the discovery of the parasite, now
known as Leishmania donovani, Ross 1903, in the spleen of a soldier. who
Contributed by Lt-Col. H. E. Shortt, M.D., I.M.S