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(i) Horse Surra is enzootic in Bombay with an average rainfall of 72.7 inches, and mean yearly
humidity of 81 per cent. of saturation; but the reverse is the case in Poona, on the Deccan plateau at
an elevation of 1,857 feet, with an average rainfall of 27.6 inches, and mean yearly humidity of 53 per
cent. of saturation.
(ii) It is a well ascertained fact, and the statistics in this and Volume I, furnish the proof, that in
Bombay the deaths from Surra as a rule commence in August or September, and new cases practically
cease at the end of December. Consequently, this experiment was undertaken at too late a period of the
year, and any future experiment in this line should be undertaken in Bombay, or at all events below
the ghâts, towards the end of the hot weather, and continued through the rain, if necessary.
(iii) Another point which has been noticed in animals submitted to inoculation with rat's blood is
that the hæmatozoon in the blood of rats (kept for any time in captivity, and fed on clean corn, &c.)
appears to lose in virulence.
The positive cases which have been recorded in horses in this report were inoculated with blood
taken from rats which had been caught only a few hours previous to their blood being used for the pur-
pose of inoculation. Some species of rats are very clean feeders, viz., Nesokia providens, but the
Mus decumanus of this country is not so, and eats any food which is procurable; when no grain is
obtainable, it having recourse to offal and fæcal matter.
For the above reasons, it was deemed advisable to make further experiments with the same end in
view—in Bombay.
Two horses (Nos. LXXXIX and XCI) were therefore transported in May 1893 to Bombay, and
through the courtesy of Veterinary-Captain J. Mills, were located at the Veterinary College Hospital
at Parel. A regular supply of rats was maintained principally from the kennels at Bandora, and their
blood was examined by Mr. Dhakmarvala. Morning and evening the excrement was collected, weighed
and a known quantity mixed with the ration of corn, previous to its being given to the animals under
experiment. The experiments commenced on the morning of May 6th, and were terminated on
September 26th, 1893, on which latter date the two horses appeared to be in perfect health and pre-
sented a good condition. Later the animals were taken several miles to the railway station and were
sent back to the Laboratory Hospital at Poona. A period, therefore, of 145 days, commencing during
the hottest and driest part of the year (May) and carried on during the whole of the rains, was devoted
to this experiment. The temperature according to the charts, kindly kept by Mr. N. D. Dhakmarvala,
during the animal's sojourn in Bombay, never exceeded the limits of health, in fact, during prolonged
periods the diurnal variation never differed more than 2/5 ths of a degree. It was not until 24 hours
after the animals arrived in Poona on the Deccau plateau that an urticarial eruption made its appear-
ance on horse No. XCI and the Surra hæmatozoon was discovered on microscopical examination of
its blood. Of course it may be said that the animal simply contracted the disease in the ordinary
way, from the fodder, &c., during the rains. We have, however, the fact before us that the control
animal horse No. LXXXIX standing by it, and all other horses in the same stable remained healthy,
which certainly points to the extreme probability of the animal under experiment having acquired Surra
through the agency of rat excrement. The period (incubation) occupied in this case was from first to
last 146 days.
The latent period in the case of experimental inoculation by gastric ingestion of small quantities
of blood known to contain the hæmatozoon, and mixed with water (vide horse No. XX, Vol. I) occu-
pied 130 days from the first, and 75 days from the final exhibition of the soiled blood, until the appear-
ance of the hæmatozoon in the general circulation. Consequently, if we consider it probable that the
cause of the outbreak at the Bombay Tramway Company's stables was due to the horses consuming
rat-excrement in their rations of corn, and we take as an instance the outbreak of 1892, when three
horses were attacked with Surra on the 12th and 14th September, these animals must have commenced
the ingestion of the grain soiled with the damp excrement of the rat, about July 1st, 1892, according
to the shorter, or on the 6th May 1892, according to the longer computation. The grain we must
suppose is contaminated during the whole year, but certain changes in the rat excrement take place at
different seasons, the most important being dryness after the rains in October, but which is still more
accentuated during the hot weather, at the commencement of the rains the dung becomes soft, and
remains so, thus allowing the lower forms of life present in it to be developed.
The above cause, namely rat excrement, may be means of some animals, especially in towns and
those fed on corn, acquiring Surra, but it must be remembered that enormous numbers of equines,
especially in the Punjab, North-West Province, and Burmah, contract the disease during certain years,
and these animals are turned out in the open and have to find their own fodder. One point, however,
is quite certain, whatever be the means by which an animal contracts Surra, when once a case of the
disease exists, flies may be and are in many instances the means of spreading it to healthy animals, just
as in the case of anthrax and rinderpest, the smallest trace of virulent blood, placed in contact with a
small scratch of the skin of an animal will be sufficient to reproduce the disease in a few days.
Animals other than horses, which contracted the disease, were bovines, monkeys and field rats
(Nesokia providens) ; while the rabbit, guinea pig, cat, dog and donkey proved refractory to the rat
hæmatozoon. But all the above animals were very susceptible to the rat hæmatozoon, when it had
been passed through the horse. Fowls proved refractory to the rat hæmatozoon, as also after its
passage through the horse.
B 2204—30