G. H. F. Nuttall and G. S. Graham-Smith 227 
died, but no parasites could be found in their blood at death. This 
result suggests that a partial immunity may have been obtained by the 
treatment. 
2. Regarding attempts at immunization by means of inoculations 
with blood containing dead parasites. Of the five dogs which we 
attempted to immunize in the manner we have described, three died of 
acute piroplasmosis; one died without parasites in its blood although 
parasites had been previously found ; one dog died on the 36th day from 
chronic piroplasmosis. The absence of parasites in the blood of one dog 
at autopsy and the occurrence of chronic piroplasmosis in another dog 
may or may not indicate a partial acquisition of immunity. In any case 
the experiments afford no evidence that practical results are likely to 
follow further investigation of this character. 
3. The duration of immunity following recovery is undetermined 
since there are no experiments to prove that animals are immune after 
the parasites have completely disappeared from their blood. In the so- 
called “ immune,” “ recovered,” or “ salted ” dogs, the animals harbour 
parasites in small numbers for weeks, months or years, consequently 
such dogs are suffering from a mild chronic form of piroplasmosis. Whilst 
subject to this mild form of the disease dogs usually escape acute 
infection when reinoculated with the parasite. 
4. The parasites of piroplasmosis canis may persist in the blood of 
apparently recovered dogs for a considerable length of time, 6 months 
to 2 years, and so long as they are present in the blood the latter 
remains fully virulent for clean dogs. Consequently there is no evidence 
that the African Piroplasma canis becomes modified in its virulence 
during the course of chronic piroplasmosis. According to Nocard and 
Motas the European P. canis does become modified in its virulence in 
dogs suffering from chronic piroplasmosis. It should be noted that the 
European disease is a milder affection than the African. 
5. The passage of the African P. canis through two series of dogs, 
that is through upwards of 90 animals in the course of two and a half 
years, has shown that the parasite may be communicated by inoculation 
from dog to dog for an indefinite period. No evidence was obtained 
that P. canis is in any way modified in its virulence by passage through 
dogs. In a number of dogs which were inoculated with post-mortem 
blood the disease developed more slowly than usual but nevertheless led 
to a fatal issue. 
6. The onset of fever after inoculation with virulent blood may 
precede or succeed the appearance of P. canis in the peripheral circula- 
15—2 
