406 QUEENSLAND AGRICULTURAL JOURNAL. [1 Nov., 1897. 
Here, apparently, was the very attenuation desired—brought about, by a natural 
process, in the blood of immune animals. For it seemed only reasonable to 
suppose that, though the microparasite was thus shown to persist for a very 
long time in the blood of an immune animal, it must exist there in a com- 
paratively mild or non-pathogenic form, since it was absolutely innocuous to its 
host. 
This, then, was the basis of the idea, which has recently been developed in 
Queensland, of using the blood of a recovered (immune) animal for inoculating 
purposes. 
The first point in pursuance of this idea was to obtain, by independent 
experiment, confirmation of the Washington observation, that the blood of a 
‘recovered animal was actually capable of setting up fever in a susceptible one. 
The second was to ascertain whether the fever (if any) was of a mild or 
attenuated kind; for it seemed by no means irrational to conjecture that, 
though the micro-organism was, apparently, as it were, held in check, and 
rendered harmless in the blood of an animal in which immunity had been 
established, it might still spring again into virulent activity when introduced 
into that of another, in which this condition did not exist. 
To carry out these objects it was, obviously, essential that experiments 
should be made with the blood of an animal that was known to be “immune” 
on the one hand, and with cattle that were free from all suspicion of past or 
present tick infestment on the other. Accordingly, two animals, that had 
survived attacks of acute fever in a virulently tick-infested spot in the North, 
were cleansed and shipped south of the infested area, and varying quantities 
of their blood injected into a number of susceptible cattle.* ‘The result was 
a sharp attack of tick fever in the majority of the animals operated on—so 
sharp indeed that as many as 10 per cent. of them succumbed. It is worthy 
of note that the microparasite of tick was, after microscopical examination, 
reported to be present in greater or less extent in the blood of all these 
animals; and it was subsequently found that the blood of these in turn was 
capable of setting up the like fever in other susceptible animals, and the 
blood of these again in yet others; and that, as in the case of virulent blood 
before referred to, no obvious diminution in the severity of the disease resulted 
from this transmission through a series of animals. - 
A similar experiment carried out amongst (presumably) clean Northern 
cattlet showed that, amongst these also, the injection of ‘recovered blood” 
caused, in many cases, a sharp rise of temperature. The positive results 
obtained in both these experiments substantially confirmed the Washington 
observation as to the pathogenic power of the blood of immune animals from 
infested place. 
The premises being so far established, it remained to ascertain if the 
practical inferences that had been drawn from them could be verified—i.e., if 
cattle in which the fever had been produced with “recovered blood” were 
subsequently immune. ' 
The answer to this, if not equally definite, was, at least, very encouraging :— 
Of the animals inoculated by this method in the South, only 14:8 per cent, had 
fever when they were subsequently sent Northt and exposed to virulent tick 
infestment, and 3°4 per cent. of the number died; whilst of other animals of 
the same class, that had not been inoculated at all, or had been inoculated 
otherwise than with “recovered blood,’’ all suffered from fever, and 23'2 per 
cent. died. 
In the case of the Northern animals experimented on, none of those 
inoculated showed any fever when subsequently exposed to tick infestment, 
and also injected with virulent blood ; whilst of the controls similarly tested 
no less than 75 per cent. had fever, though none died. 
* At Mundoolun, on the Logan. ‘+ At Glendower, on the Flinders. 
+ To Inkerman. 
