The infection is not direct, that is, the tick does not feed on one host then pass to another carrying the disease germs with it. Unlike many other ticks the Texas fever tick does not leave its host until it is fully developed. When the female is full grown and gorged she drops to the ground and lays from 2,000 to 4,000 eggs which soon hatch into the minute "seed-ticks" which make their way to the nearest blade of grass or weed or shrub and patiently wait for the cattle to come along.
If the mother tick had been feeding on an animal that was infected with the Texas fever parasite, her body was filled with the minute organisms of which some found their way into the eggs so that the young ticks hatching from them were already infected and ready to carry the infection to the first animal they fed upon.
It took many years of hard patient work to learn all this, but the knowledge thus obtained cleared up much of the mystery in connection with the occurrence of the fever in the north and, as we shall see, suggested the possibility of other diseases being communicated in the same way.
It was found that the southern cattle in the region where the ticks occur normally, usually have a mild attack of the disease when they are young and although they may be infected with the parasite all the rest of their lives it does not affect them seriously. These cattle are almost always infected with ticks, and when taken north where the ticks do not occur naturally and where the cattle are therefore non-immune, some of the mature ticks drop to the ground and lay their eggs which in a few weeks hatch out and are ready to infect any animal that passes by. The northern cattle not being used to the disease soon sicken and die.
It is estimated that the annual loss due to this disease and the ravages of the tick in the United States is over $100,000,000, so of course most determined efforts are being made to stamp it out. Formerly various devices for dipping the tick-infested cattle into some solution that would kill the ticks were resorted to, but it was always expensive and never very satisfactory. The immunizing of the cattle by inoculating them when they were young with infected blood has been practised. Very recent investigations have shown that it is possible and practicable to rid pastures of ticks by a system of feed-lots and pasture rotation. The aim is to have as many of the ticks as possible drop to the ground on land where they may be destroyed and to so regulate the use of the pasture that the ticks in all of them may eventually be left to starve.
Several similar diseases of cattle, many of them probably identical with Texas fever, occur in other parts of the world where the losses are sometimes appalling. Horses, sheep, dogs, and other animals are also affected with diseases caused by the same group of Protozoan parasites. Most of them have been shown to be transmitted by various species of ticks ([Fig. 17]) so that from an economical standpoint these little pests are becoming of prime importance. Not only do they transmit the disease germs that infect domestic animals but they are known to be responsible for at least two diseases of men, Rocky Mountain spotted fever and the relapsing fevers.
Spotted Fever. The first of these is a disease that for some years has been puzzling the physicians in Idaho and Montana and other mountainous states. A few years ago certain observers recorded finding Protozoan parasites in the blood of those suffering from the disease, and although more recent investigations have failed to confirm these particular observations it is now quite generally believed that the disease is caused by some such parasite and that the organism is transferred from one host to another by certain species of ticks that live on wild mammals of the region where the disease exists. Dr. H.T. Ricketts, who has made a special study of the disease, has shown:
"1. That the period of activity of the disease is limited to the season during which the adult female and male ticks attack man.