The pathogeny of Texas fever may be shortly summed up as follows:—Animals suffering from the disease carry in their blood a protozoan organism called the Piroplasma bigeminum, analogous to the parasite of human malaria; once introduced into the blood, this organism remains there in an active condition throughout the animal’s life; it is transferred to susceptible cattle either within or without the infected district by the Southern (U.S.) cattle tick Boophilus annulatus; Southern cattle, although carrying the protozoa, are harmless unless infested by this particular tick: the mature ticks and their eggs contain the protozoa, and the mystery of certain grounds over which infected animals have passed being first dangerous, then harmless, and again dangerous depends on—(a) the infestation of the ground with mature infected ticks; (b) the destruction or death of the mature ticks; and (c) the hatching out of new (infected) ticks from the eggs laid on the ground by the mature female ticks.
Dr. Salmon states that in Texas a successful method of protection is in practice based on the observations that young cattle do not suffer so severely as adults, and that the disease always assumes a milder form in winter. Young animals introduced during the winter are inoculated with virulent blood. They contract a mild form of disease, and afterwards resist. In this way the losses, which previously amounted to 90 per cent. of all freshly introduced stock, have been reduced to about 10 per cent.
A remarkable and very interesting observation (if absolutely reliable) deserves to be mentioned, viz., that the ticks develop regularly in the natural prairie, but do not develop in parts artificially sown with grass such as lucern, and that when contaminated or diseased animals are transferred to artificial meadows they do not convey the disease to other animals already there; the latter are proof against it.
Diagnosis. The disease is so typical that it cannot be mistaken for anthrax. In anthrax the urine is never hæmoglobinuric and very rarely hæmaturic, and the fæces are sometimes blood-stained, a symptom never present in piroplasmosis. Anthrax can be transmitted to experimental animals, but piroplasmosis cannot.
Prognosis. The prognosis is generally grave.
Treatment. Van Hellens recommends the use of quinine in large doses. He give 5 drachms in one dose, and repeats it for the next two, three, or four days.
Lignières says that he has never obtained the slightest success with quinine, though it is true he has never given higher doses than 2½ drachms.
Attempts have been made to confer immunity by injecting animals with serum from others which have recovered. Vaccination with the blood of patients arrived at the period of convalescence has also been tried. The results, however, have not been very satisfactory.
Lignières has formulated an efficient method of vaccination, of which he has not yet published the full details, but which appeared by reason of its simplicity likely to render great service. Nevertheless, his most recent reports seem to show that vaccination is not always efficacious, and that in the Argentine Republic alone several varieties of the disease exist, two being caused by allied but different parasites. The vaccine used against one variety is powerless against the other. The problem of vaccination would therefore appear to be much more complex than in the case where one form only occurs in any particular country.
The immunity arising from attacks of piroplasmosis is in direct ratio to the gravity of the disease, and according to Lignières’ views this acquired immunity is due to the secretion by the piroplasma of a substance which is toxic for the red blood corpuscles. This toxic substance provokes, as in other diseases, an organic antitoxic reaction.