, that the parasite may
enter the body only through a wound of some kind. Apparently by far the
most common method is through wounds produced by biting flies whose
mouth parts are moist with the infected blood of some animal bitten by
the same flies immediately before biting the healthy animal. Crows may
also transmit the infection by pecking at sores on a diseased animal,
soiling their beaks with blood, and transferring this infected blood to
a healthy animal. Likewise, if a scratch is made on a horse and then
infected blood is rubbed on the scratch, the horse will become diseased.
If, in experiment, infected blood is fed to a healthy animal, the latter
may contract surra in case it has an abraded or wounded spot in the
mouth; but if no part of the lining of the alimentary canal is wounded,
infection does not take place. Thus dogs and cats may contract the
disease by wounding the lining of the mouth (as with splinters of bone)
while feeding on the carcasses of surra subjects. All available evidence
indicates that under normal conditions of pregnancy the disease is not
transmitted from mother to fetus.

There is a popular view that surra may be contracted by drinking
stagnant water and by eating grass and other vegetation grown upon land
subject to inundation, but there is no good experimental evidence to
support this view: Probably the correct interpretation of the facts
cited in support of this theory is that biting flies are numerous around
stagnant water and in inundated pastures; hence, that a great number of
possible transmitters of the disease are present in these places.

_Symptoms._[7]--The invasion of this disease when contracted naturally
is usually marked by symptoms of a trivial character; the skin feels
hot, and there may be more or less fever; there is also slight loss of
appetite, and the animal appears dull and stumbles during action; early
a symptom sometimes appears which may be the first intimation of the
animal's indisposition, and which, as a guide to diagnosis, is of great
importance; it is the presence of a general or localized urticarial
eruption. If the blood is examined microscopically, it may be found to
present a normal appearance; but in the majority of cases a few small,
rapidly moving organisms will be observed, giving to the blood, as it
passes among the corpuscles, a peculiar, vibrating movement, which if
once observed will not easily be forgotten. If the parasite has not been
discover