344: QUEENSLAND AGRICULTURAL JOURNAL. [1 Supr., 1901. 
When the experimental infection has produced the disease in the mild 
form, without hemoglobinurea, but infecting some of the red corpuscles, the 
consequent immunity is very strong if the trial inoculation has not been delayed 
too long. Thus, the injection of 10 ¢.c. of virulent blood into the jugular of a 
bullock attacked two or three weeks previously by the mild disease, remains 
quite negative ; hematozoa are not again found in the blood. 
Occasionally, however, after three or four months these animals again have 
a slight sensibility to inoculation which is shown by a passing fever and the 
appearance of the Piroplasma in the blood. 
The virulent form of bovine malaria furnishes an absolute immunity much 
more durable, since I have found it impossible to re-infect, even slightly, animals 
which had been sick eight months before. 
The bovine malaria, then, confers immunity as the result of a first attack. 
This attack may have been light or severe, yet the trial inoculation, be it as 
severe as one might wish, has no effect, and contrarily to what has been 
advanced by Messrs. Nicolle and Adil-Bey, there is not, from the mere fact of a 
fresh inoculation, any re-appearance of the Péiroplasma in the blood. If, 
immediately after a cure of the disease, whatever may have been its form, the 
immunity is identical, we see that it diminishes with time, slower when it is a 
question of the virulent form; at the same time, the animal may benefit for a 
long time from the immunity resulting from a mild attack. 
Immunity seems to keep step with infection. A strong and durable 
immunity will correspond with a virulent attack; a fleeting immunity, with a 
mild attack. It is between these two extremes that we find the medium. 
Is immunity, acquired naturally or artificially, available against infection 
by tick ? 
Observation has often shown me, in an indisputable manner, the 
resistance of cattle naturally infected, when they are subjected to another 
attack of the ticks. I have mentioned an epidemic in which, of 1,000 head, 
650 cattle perished from malaria; now, about fifty very fine cows—cross-bred 
Durhams—coming from a neighbouring estate, had accidentally got mixed up 
with the diseased beasts; their perfect state of health during the whole course 
of the epidemic was in singular contrast to the miserable appearance of the 
contaminated animals. These cows, taken off the property in the preceding 
year, had paid their tribute to the malaria (400 dead out of 600), so that they 
were spared in spite of the numerous ticks which they harboured. 
When immunity has been acquired by means of sub-cutaneous or intra- 
venous injection of virulent products, the resistance to infection by ticks is evident. 
Animals showing severe disease have completely resisted infection by ticks, 
whilst under the same conditions, the controls contracted the disease. 
Tf the original infection has been mild, contamination by ticks was all the 
more easy, as the second trial was undertaken some time after the first attack. 
According to the opinion I expressed as to the particular réle of the ticks 
in the transmission of malaria, it appears that re-infection by these parasites is 
easier than by artificial inoculation, since they would inject, at the same time as 
the malarial virus, a substance eminently adapted to the development of 
hematozoa, and, as a fact, amongst many experiments in this direction, some 
have fully confirmed this view. Ticks overcome immunity better than experi- 
mental inoculations; at the same time, the animal which has been rendered 
immune against the latter, and then has been infected by ticks, still manifests 
its immunity by taking, at the worst, only the mild form of the malaria. 
My own researches prove then, in the most categoric manner, that a first 
attack confers an immunity all the more: durable against artificial infection ; 
that this original attack had been more severe. According to my researches, 
immunity consequent on a severe attack lasts, at the very least, for one year. 
Since the power of resistance resulting from a light attack is considerable 
during the first few weeks, it is clear that the animals thus rendered immune 
should be placed as soon as possible in the infested zones; by so doing, not 
