408 QUEENSLAND AGRICULTURAL JOURNAL. [1 Nov., 1897. 
The first question that anyone proposing to inoculate his cattle usually 
asks this :—‘‘ How am I to know what blood to use? “What is the essential 
point in regard to the past history or present condition of any given animal that 
makes its blood efficient as a vaccine?” And it has to be confessed that our 
knowledge, in respect to this very matter, is by no means as definite as could 
be desired. In the first place, it is well known that tick-infested cattle harbour 
in their blood those peculiar, free, amceboid bodies known as the extra- 
corpuscular forms of the tick-fever organism ; and, so far as our present 
knowledge goes, tick infestment and this form of blood infection are constantly 
associated. The bodies in question are also known to be intimately associated 
with acute tick fever. From these facts it might seem not unreasonable to 
suppose that their presence is the one thing needful to render blood effective 
for inoculation purposes. But, in opposition to such an idea, it must be borne 
in mind that cattle frequently suffer from gross tick infestment (and almost 
certainly from the blood infection accompanying it) for a long time before 
they show any outward sign of disease. In other words, though tick infested, 
they are still susceptible. No inoculation experiments with the blood of such 
cattle, however, have, so far, been recorded; and, in the absence of any such 
positive evidence, it is certainly hard to understand how clean cattle can be 
rendered immune by the injection of blood from these tick-infested ones, 
since the latter are, as regards susceptibility, in practically the same condition 
as the clean ones. . 
The experimental evidence, so far to hand, would seem to indicate that 
recovery from acute tick fever, rather than past or present tick infestment, is 
the essential condition that renders the blood effective for inoculation purposes. 
Tn one experiment, at any rate, immunity has been established in a number of 
cattle by the injection of blood from a steer that had never been in contact 
with ticks, but had recovered from an attack of the acute fever, artificially 
induced by the injection of virulent blood. 
The prolonged exposure of an animal to virulent tick infestment, subse- 
quent to recovery, might be assumed to accentuate the protective power of its 
blood. And it is certain that the blood of recovered animals that have been 
so exposed is generally found to contain a much larger number of the 
characteristic free organisms than that of such as have recovered from 
artificially induced attacks of the specific fever apart from ticks. Whether 
thore is any corresponding difference in the protective efficacy of blood from 
these two classes of animals, is not yet determined. But there is little doubt 
that the blood of the former class is much more apt to set up fever in 
susceptible animals than that of the latter. For the present it is, therefore, 
probably the wisest course to use, for ordinary inoculations, the blood of 
recovered animals which have since been, for some time, exposed to virulent 
tick infestment. In the case of very valuable animals, more particularly such 
as are known to be specially liable to succumb to acute tick tever—as, for 
example, stud bulls—it would probably be a wise precaution to prepare the 
way for this inoculation by a preliminary injection of blood from an animal 
that had recovered from the artificially induced disease.* 
Now as to the actual state of cattle “infested with ticks but as yet 
showing no sign of disease”: Such animals may truly be said to be in an 
interesting condition. We know, as already stated, that they harbour one 
form of the microparasite in their blood. We have some evidence to show 
that, though they are to all appearance perfectly healthy, their internal organs 
are not quite in the normal condition,t and for these reasons it seems necessary 
* Tt should be clearly understood that these are merely individual opinions, advanced for 
what they may be worth. ° The evidence on this particular point is still somewhat conflicting, 
and is certainly not such as to warrant any positive statements. The very heavy mortality that 
has occurred amongst some bulls, recently inoculated at Rosedale in the south, shows the necessity 
of proceeding with the utmost caution in the case of such highly susceptible animals. 
+ The average spleen weight of six such tick-infested animals was found to be *45 per cent. of 
total body weight as against ‘29 per cent. in the case of six normal clean cattle. Itis of interest 
to note also that the average absolute spleen weight of immune cattle in permanently infested 
areas in the States has been found to be considerably greater than in the case of normal cattle. 
