icts no unfavorable or
unhygienic surroundings produce the affection. As both native and
imported horses are equally susceptible, there is no indication of an
acquired immunity to be observed.

Theiler has recently stated that his experiments in transfusing blood
from diseased to normal horses were negative, and has suggested that the
causative agent may be transmitted by an intermediate host only, as in
the case of Texas fever. He draws attention to this method of spreading
East African coast fever, although blood inoculations, as in
osteoporosis, are always without result. We know that coast fever is
infectious, and that it can not be transmitted by blood inoculations,
but is conveyed with remarkable ease by ticks from diseased cattle. That
the cause has not been observed may be accounted for by its being
invisible even to the high magnification of the microscope.

On some farms and in some stables bighead is quite prevalent, a number
of cases following one after another. On one farm of Thoroughbreds in
Pennsylvania all the yearling colts and some of the aged horses were
affected during one year, and on a similar farm in Virginia a large
proportion of the horses for several years were diseased, although the
cows and sheep of this farm remained unaffected.

_Symptoms._--The commencement of the disease is usually unobserved by
the owner, and these symptoms which do develop are generally not well
marked or are misleading unless other cases have been noted in the
vicinity. Until the bones become enlarged the symptoms remain so vague
as not to be diagnosed readily. The disease may be present itself under
a variety of symptoms. If the bones of the hock become affected, the
animal will first show a hock lameness. If the long bones are involved,
symptoms of rheumatism will be the first observed, while if the dorsal
or lumbar vertebrae are affected indications of a strain of the lumbar
region are in evidence. Probably the first symptom to be noticed is a
loss of vitality combined with an irregular appetite or other digestive
disturbance and with a tendency to stumble while in action. These
earlier symptoms, however, may pass unobserved, and the appearance of an
intermittent or migratory lameness without any visible cause may be the
first sign to attract attention. This shifting and indefinite lameness,
involving first one leg and then the other, is very suggestive, and is
even more important when it is associated with a tend