TEXAS FEVER. 
337 
may differ in virulence ; and that it may vary in the same 
animal from time to time. In work now in progress on cattle 
that have not yet been sent south, two supply animals have 
been used, one a Texan cow that has been north since the sum¬ 
mer of 1896, but has been kept on infected grounds the past 
two summers ; the other a Northern cow that was inoculated 
during the past winter and suffered from an acute attack of the 
fever, but recovered and has carried ticks during the summer 
and fall without apparent illness. An equal number of cattle 
have been inoculated from these two supply animals. More 
severe symptoms have been caused by the blood of the recovered 
native. This was probably due to the fact that this animal was 
more grossly invested with ticks during the latter part of the 
summer and fall than the Texan, and was well infested at the 
time the blood was used. Blood from the same supply animal 
was used in midsummer, in doses of 2 to 6 cc. on a few cheap 
experiment animals without showing as severe symptoms as 
appeared in others inoculated in the fall with doses of 1 to 3 cc. 
after the supply animal had become grossly infested with 
ticks. It is probable that the safest supply animal is one that 
is thoroughly immune, and in which great variations in viru¬ 
lence of the blood is avoided by keeping free from ticks. The 
supply animal should be at the place where the inoculations 
are made and the blood should be used when perfectly fresh. 
Size of Dose .—The size of the doze will depend largely 
upon the degree of virulence of the blood used. If the blood of 
a tick-infested animal is used, doses of from 1 to 2.5 cc. appear 
to be sufficiently large. Severe attacks of fever have been pro¬ 
duced by the use of 1 cc. of blood from an animal grossly in¬ 
fested with ticks. As the susceptibility of animals varies 
greatly, it is prudent to give a small initial dose and repeat if 
necessary. In the Bullock and Rhea inoculations, pages 19, 2i> 
large quantities were given, because the blood could not be 
used for a day or so after being drawn ; and in these cases an 
antiseptic was added to prevent decomposition ; and no doubt 
many of the Texas fever germs were destroyed. 
