1 Noy., 1897.] QUEENSLAND AGRICULTURAL JOURNAL. 405 
may be due, not to any fresh tick infection at all, but to the direct effect of 
such adverse influences as exertion, privation, and exposure, in rekindling, as it 
were, the fires of fever which have been lying dormant in their blood, or, as 
drovers say, “in bringing out the disease.” But we know that persons who 
have suffered from malarial fever are very apt to get fresh attacks, apart from 
fresh infection, under the like unfavourable conditions.* No one, however, on 
this account, doubts that a certain degree of immunity is acquired by persons 
residing in malarious districts. And if it can be definitely shown that droving, 
in conjunction with, or apart from, fresh tick infestment, brings out the 
disease in animals that are immune on their own runs, then we shail have to 
recognise two grades or degrees of immunity—station immunity and road 
immunity. 
The broad fact that this increased resistance—“ immunity” in its various 
degrees—is, actually, brought about in Nature, is by far the most important 
ground for anticipating satisfactory results from inoculation. If no “immu- 
nity” were established in Nature, then, assuredly, it would be vain to anticipate 
that such a condition could be established by any artificial means, such as 
inoculation. The second reason for sanguine anticipations is that the disease, 
which is followed by ‘‘immunity,’ when brought about in Nature by ticks, can 
be equally well produced, artificially, with a syringe. These are the two 
cardinal points. On them depends the whole hope and the whole theory of 
protective inoculation. The practical value, however, of these facts lies very 
largely in the application thereof; and the many inoculation experiments that 
have been undertaken may be regarded as, essentially, efforts to apply these 
cardinal facts, in their various details and circumstances, to meet the exigencies 
of the situation. 
If blood be taken from an animal suffering from acute tick fever, and 
injected into the vein or beneath the skin of a susceptible one, an acute attack 
of tick fever results. And the disease may be thus passed on from animal to 
animal without any apparent diminution in virulence. Cattle that have sur- 
vived the disease, thus intentionally brought about, are found to be immune to 
subsequent attacks from further injections of virulent blood; and there is 
evidence that they are also, at any rate, highly resistant when exposed to 
prolonged and virulent tick infestment. Unfortunately, the mortality which 
has attended the disease thus artificially set up has been almost, if not quite, 
as great as that attending the most virulent tick infestment. Hence it is 
obvious that this method of direct inoculation with blood of an acutely diseased 
beast cannot, without some modification, be employed as a means of securing 
protection. A milder expedient must be found. ‘he blood used for inocula- 
tion must be in some way weakened or attenuated. Inasmuch, however, as 
the micro-organism of tick fevert has not been found susceptible of artificial 
cultivation after the manner of the microbes of many bacterial diseases, the 
methods of attenuation employed in such cases are not applicable. And as 
tick fever is apparently an exclusively bovine disease, it is not practicable to 
seek to attenuate its virulence by passing it through the organism of some 
more resistant animal, as can be done, for instance, in the case of anthrax, by 
passing it through the organism of a dog, which is known to be very highly 
resistant to that disease. 
A way out of these difficulties, however, seemed to be indicated by the 
remarkable observation, made at the Experimental Station at Washington, 
that the blood of a certain cow, that had been brought up from one of the per- 
manently infested Southern States, was, still, after a number of years’ sojourn in 
clean country, capable of setting up fever when injected into susceptible cattle. 
* Some striking instances in illustration of this fact have recently come under the notice of 
the writer, in the case of persons returned from New Guinea after severe attacks of malarial fever. 
‘A few days’ hard work on the Winton Railway extension works has, in several instances, sufficed 
to bring back the fever, with an abundant, development of the intra-corpuscular form of the: 
malarial plasmodium. 
Now generally regarded as a hwmatozoon, an order of things belonging to the lowest order 
of animal life. 
