A stockowner who, independently of his fellows, adopts one of the above expedients in an infested district, is however confronted by the risk of the infection of his herd, by the accidental or careless contact of his cattle with outside ones, and especially with the places where they have been. A broken fence and the entrance of tick infested cattle, or the escape of his tick-free cattle into infested lands, will be the death warrant of all that have not been previously exposed to the disease. Another consideration is that this rigid seclusion of the protected herd must be continued indefinitely so long as ticks are maintained anywhere in the district. The protected animals cannot be driven over a highway without exposing them to almost certain death. Even if a group of adjacent stockowners agree to purify their respective farms, they cannot debar their less careful neighbors from using the highways for tick-infected stock, nor from turning such out on adjoining fields. The veriest scrubs, admitted to the highways, woods and unfenced grounds, keep up the general diffusion of the fever germ and its tick bearer and undo the best directed efforts of any combination of owners of high class and valuable stock.
Well directed legislation, excluding cattle for one or two years, from all woods and unenclosed lands, and enforcing some one of the available methods for the clearing of fenced and stocked lands (cultivation, pasturage by cattle on alternate years, frequent dipping or smearing, passing the stock through a succession of pens), could be made to put an end for all time to the obnoxious tick. If even some other than bovine animals should be discovered to harbor the boöphilus and pyroplasma it could be included in the prohibition and the work made complete. The results would far more than compensate for any necessary outlay. Illinois, with 55,414 square miles of area has over 3,000,000 cattle. The coast states from Virginia to Texas, with Arkansas, Indian Territory and Oklahoma, amount to 767,215 square miles, and in the same ratio should sustain 43,340,040 head. A stock of 25,000,000 at $20 per head would amount to a capital of $500,000,000. Immune from the pyroplasma these cattle would draw freely on the best blood of the north and under the milder skies would compete with the northern cattle on more than equal terms. Living in the open air, they would in the main escape tuberculosis and the other stable-propagated diseases of the north, and their dairy and beef products would enter the market free from suspicion, and command a readier sale, if not a higher price. The stock themselves could be moved to northern markets at all seasons without restriction, and escape the serious losses that now come from a sudden transfer, while pyroplasma-infected and susceptible, to the violent excitement of travel, and the frost-bound destination. Their owners could watch the markets and sell in the best, in place of being compelled, as at present, to hurry them in during November and December, and to sell often at a ruinous sacrifice. With the extinction of the boöphilus the present unrestricted pasturage and all other privileges now enjoyed would return, freighted with a value never borne before, and the few Southern cattle and cattle products need fear no competition in the markets of the world, and could no longer be justly subjected to any restriction.
There would remain the constant danger of the introduction of the boöphilus anew from Mexico, the West Indian Islands and the Central and South American States, where in the absence of frosts the boöphilus cannot be extirpated in the same way, and here accordingly all importation must be forbidden. Cattle from Southern Florida and from islands on our Southern Coast may demand a similar exclusion. There is too much at stake to permit any laxity, and no infected area should be allowed to send out its cattle until it has been abundantly well proved that such district or State is absolutely tick-free.
2d. Immunization: Encreasing the Resistance to the Piroplasma Bigeminum. That cattle can be fortified to resist the attacks of the piroplasma is shown in the immunity possessed by the indigenous herds generally, in the regions infested with this parasite and the boöphilus. To begin with, there may be a survival of the fittest, the more susceptible strains of blood having been long ago cut off. But the immune southern cattle if kept for years outside of the infested area and then returned to it, suffer a mortality about as great as that of northern cattle in the same circumstances. Their earlier immunity, therefore, is not merely a racial difference, but must be due in greatest part to an acquired resistance, and further, this resistance is not permanent but must be renewed at short intervals. The immunity may be in part acquired in the womb of the infested dam, in the last months of gestation, but it is chiefly post-natal through the attacks of the ticks.
a. Infection of sucking calf. The indigenous cattle acquire immunity mainly through the attacks of the boöphilus, in the first month or two when they are still on an exclusively milk diet, that renders the piroplasma practically harmless. Following a parallel method, calves, living on milk alone, can be taken into the infected regions and exposed to the attacks of the ticks with safety, and with the result of protection for the future. Escaping the first invasion, they continue to harbor as many ticks summer after summer, as will reinforce yearly their acquired power of resistance, so that they continue measurably safe though spending the life in the area of the infection.
Francis and Connoway applied this to Jersey calves of two to six weeks old applying to each 25 to 50 ticks. It led to slight hyperthermia, some dulness and inappetence, but on recovery they all gained flesh and condition. Two died from exposure but necropsy showed no sign of Texas fever. The following summer all were infested with 200 to 500 ticks apiece but not one sickened in consequence. These were Jersey calves (the least susceptible breed) and the experiments were made in cool weather in autumn. The limitation of the practice to the cool fall or winter months renders the operation much more safe.
b. Infecting Older Animals by a limited number of Ticks. Yearling Jerseys, Holsteins and Shorthorn were subjected to 25 to 50 ticks in July, they showed only slight rise of temperature, and later resisted the free exposure to tick infestation. It must be recognized that these were still young animals, with presumably greater resisting power than the mature, but on the other hand they were of the susceptible northern herds, they were first infested in the hottest season, and the acquired resistance appears to have been perfect throughout the succeeding summer. The added precaution of subjecting them, in late autumn or winter only, to the ticks raised in a warm room or thermostat, would add greatly to the safety of the operation. After recovery from the effects of the first crop of ticks, a second crop of 50 to 100 should be placed on the skin so that the system may be thoroughly habituated to them and the measure of resistance correspondingly strengthened.
This measure may be advantageously applied to valuable cattle that are to be moved into the infecting territory, but it has serious drawbacks. The relative strength of the poison introduced by the ticks to the susceptibility of the animals on which they are placed, can never be perfectly gauged, and a certain small but appreciable number of deaths result from this first infesting. This has been observed in North and South America, Australia, Roumania and Turkey. Again, the plan entails the necessity for clean, non infected premises (lots or buildings) for each fresh lot of cattle, as the places previously used are left in a tick-infested condition, and are likely to furnish a dangerous excess of ticks to any susceptible animal. The buildings could, of course, be disinfected and purified, but this entails considerable expense.
c. Infection by Graduated Injections of Blood Containing the Pyroplasma. Up to the present this is the most promising method of securing resistance to the pyroplasma. It is advised to take the blood from an immunized northern animal or from one indigenous to the infected district. Such an animal is not, however, strictly speaking immunized. It has acquired a tolerance so that it is no longer in much danger of succumbing to the pyroplasma, but it does not exclude the pyroplasma from its system. The micro-parasite is still found in the blood, though mainly in the coccus-like form in the interior of the red globules. The animal to be fortified against the disease is therefore inoculated with the germ of the disease itself, though it may be, at the time, in a somewhat inactive form. If, however, the inoculated animal is specially susceptible, or if the dose is excessive, the disease is produced in deadly form. The virulence is less in the case of blood drawn from a northern animal just recovered from the disease, than from an animal indigenous to the infected district, and which harbors the pyroplasma, it may be in spore form (Lignieres), without showing obvious disease. The former source of the blood is therefore the more desirable, while the latter is the more easily obtained. The precaution, however, should be adopted of reducing the dose when taken from an indigenous animal. Another important precaution is to select the winter or cooler season for the operation rather than the summer.
The animal which is to furnish the blood may be fixed in stocks, or held with a bull ring, or it may be cast so that it can be kept still. The hair is clipped or shaved from over the jugular vein in the upper third of the neck and the surface is washed with soap and water and with a five per cent. aqueous solution of carbolic acid. A thick cord inch (¼ inch) is tied tightly around the back part of the neck so as to compress and raise the jugulars. With a sharp pointed bistuory sterilized by boiling, a small incision is made through the skin, directly over the centre of the jugular and a cannula and trochar ⅒th inch in diameter and sterilized by boiling, is passed obliquely upward through the coats of the vein and the trochar withdrawn. The blood flows through the cannula and is received in a sterilized (scalded) glass beaker. The blood is stirred slowly with a sterilized glass rod until all the fibrine has coagulated when the latter is lifted out and the remaining liquid blood is ready for use. The blood is injected with a hypodermic syringe which, with its nozzle, has been thoroughly sterilized by boiling. The point selected for injection is usually back of the scapula on the middle of the chest. The skin is clipped or shaved, washed with soap, soaked in a five per cent. carbolic acid solution, then pinched up, perforated with the point of the bistuory, and with the nozzle of the syringe passed through this wound the blood is injected into the subcutaneous connective tissue. The slight wound may then be covered with tar or collodion or merely left undressed. The mass of blood in the connective tissue may be diffused through its meshes by rubbing so as to favor absorption.