The parasite can only be cultivated in defibrinated blood from a hæmoglobinuric subject, and the cultures do not always yield more than reproductions of the round form, the pear-shaped form only being produced with red blood corpuscles in the living animal body.
Regarding the method of growth of the piroplasma in the body and in cultures, Lignières believes that the parasite may produce two forms of spores differing in their nature. One, the active spore, has little resisting power. It soon degenerates outside the animal body, forms rapidly at the expense of one of the pear-shaped parasites, and may immediately reinfect another red blood corpuscle. The other, called the passive spore, is very resistant, and retains its vitality for a long time outside the body, being produced at the expense of spherical parasites already withdrawn and incapable of producing the disease.
The form of piroplasmosis at present under consideration is peculiar to the ox, and none of the other domestic animals or experimental subjects can be inoculated with it.
Subcutaneous or intravenous inoculation of the ox with 5 to 10 cubic centimètres always gives positive results when made with blood or active products, such as the pear-shaped parasites or active spores, but is ineffectual when the parasites have already begun to retract in order to form passive spores.
Calves seldom contract more than the benign form of the disease, and do not die.
In the grave form following experimental infection the temperature begins to rise between the third and sixth day, and corresponds with a marked increase in the number of parasites to be found within the red blood corpuscles. The urine at first becomes albuminous, then hæmoglobinuric, whilst the red blood corpuscles diminish in number to a very marked extent, falling from about six or seven millions to one million, or even a few hundred thousands, in the course of a few days. The temperature, which may previously have risen to above 105° Fahr., suddenly falls, indicating the approach of death.
If an immediate autopsy is made, the spleen is always found to be enlarged, the intestinal mucous membrane reddish in tint or blood-stained, and the serous membranes, particularly the endocardium, covered with petechiæ.
Few or no parasites can be discovered except in the blood from the cardiac muscle and the kidneys.
The grave form may end in recovery. This end is indicated by the temperature remaining normal after defervescence, the appearance of hæmaphæic icterus of an obstinate character, and the progressive return of appetite.
The disease is usually transmitted by adult and larval ticks carrying the parasite from infected animals. Lignières has proved that this transmission occurs through the medium of passive spores, which, though themselves incapable of producing the disease, become active and infective in consequence of the local irritation produced by the poisonous saliva of the ticks.