61
Although the latent period of rat-surra was so prolonged in the first instance when
inoculated from rat to horse (average 40.7 days), it immediately returned to the normal
period (average 7 to 8 days), when a second horse was inoculated with the blood of the first
affected. The latent period, however, becomes still more reduced (5 days) when a third animal
is inoculated with the blood of the second.
Horses unsuccessfully inoculated and re-inoculated subcutaneously with the rat
hæmatozoon, when subsequently inoculated with horse-surra blood, show no protection from
the former, but acquire the disease after an incubation period of 6 to 10 days.
Animals other than horses which contracted the disease were bovines, monkeys
and field rats; while the rabbit, guinea-pig, fowl, cat, dog and donkey proved refractory
to the rat hæmatozoon; but all the above animals, with the exception of the fowl, were very
susceptible to the rat-hæmatozoon, when it had first been passed through the horse.
Experiments on the gastric ingestion of rat's excrement.
The above experiments raised the question as to how rats could cause the disease in
horses; for the food would rarely, if ever, be soiled with the blood of these animals. It
therefore only remained to be tried whether the excreta of rats, in whose blood the infu-
sorian was present at the time of its collection, could cause the disease when mixed with
grain.
In 1892 experiments were undertaken at Poona, on the Deccan plateau, where the
disease is not enzootic, and where the difficulty of producing it was fully recognized owing to
the dryness of the situation at that period of the year (December to February). These
experiments, as expected, proved negative. At a later date, during the latter half of the hot
season and the rains of 1893—further experiments with the same end in view were carried
out in Bombay, where the climate is favourable from its humidity, and the disease is enzootic.
From these, positive results were obtained: incubation period occupying four months, about
June, July, August and September. But the incubation period of the disease will probably
be found to vary with the amount of moisture present in the atmosphere and amount of the
materies morbi ingested by the animal. This points to the probability of a number of the
surra cases in Bombay being contracted through the ingestion of rat's or bandicoot's excreta
mixed with the corn. Although not yet definitely proved, it is more than likely that the
excreta of young and half-grown bandicoots have the same power of producing the disease,
as the hæmatozoon is also-frequently present in their circulation.
351. Experiments have also been conducted with bovines and dogs.
Dr. Lingard concludes a most instructive report, with the following re-
marks on the etiology of surra as far as it is known at present:—
Our knowledge of the etiology of surra is, up to the present, as follows:—
1. The disease occurs spontaneously in elephants, camels, solipeds, bovines, and the
canine species (dog, jackal, hyæna).
2. In most of these animals the disease appears between June and December, i.e.,
the rainy season; while in hounds it occurs during the hunting months, Jan-
uary to end of March.
3. The causes of the disease in animals other than canines are—
(a) Drinking concentrated water contaminated with the infusorian towards the end
of the hot season.
(b) Eating soiled herbage obtained from localities liable to inundation.
(c) Ingestion with corn of excrement of rats and bandicoots affected with the
hæmatozoon.
(d) In hounds, by eating carrion or the soiled blood of the animals hunted; in other
dogs, by destroying affected rats and bandicoots.
In the case of horses contracting the disease there can be little doubt that many of the
outbreaks are due to the ingestion of rat's or bandicoot's excrement with the corn. In the
hot season the hæmatozoon or its resting form in the excrement would naturally be destroyed
by the continued drought; while in the rains the moisture would be found sufficient to sustain
the life of the infusorian and enable the resting form to germinate, and possibly mature in situ.
Hence the dung of rats and bandicoots is a source of danger, and protective
measures should therefore be taken at all times to remove rat and bandicoot excrement from
the corn given to equines.
As, however, the complete removal of the excrement would be a matter of
some difficulty, owing to its breaking up into fragments, artificial means to