When the infection is disseminated beyond the permanently infected district, the roads, pastures, pens, and other inclosures are dangerous for susceptible animals until freezing weather. The infection then disappears, and cattle may be driven over the grounds or kept in the inclosures the succeeding summer and the disease will not reappear. There are some exceptions to this rule in the section just north of the boundary line of the infected district. In this locality the infection sometimes resists the winters, especially if they are mild.
In regard to the manner in which the disease is communicated, experience shows that this does not occur by animals coming near or in contact with one another. It is an indirect infection. The cattle from the infected district first infect the pastures, roads, pens, cars, etc., whence the susceptible cattle obtain the virus secondhand. Usually animals do not contract the disease when separated from infected pastures by a fence. If, however, there is any drainage or washing by rains across the line of fence this rule does not hold good.
The investigations made by the Bureau of Animal Industry demonstrate that the ticks which adhere to cattle from the infected district are the only known means of conveying the infection to susceptible cattle. The infection is not spread by the saliva, the urine, or the manure of cattle from the infected district. In studying the causation and prevention of this disease, attention must therefore be largely given to the tick, and it now seems apparent that if cattle could be freed from this parasite when leaving the infected district they would not be able to spread the malady. The discovery of the connection of the ticks with the production of the disease has played a very important part in determining the methods that should be adopted in preventing its spread. It established an essential point and indicated many lines of investigation which have yielded and are still likely to yield very important results.
Nature of the disease.—Texas fever is caused by an organism which lives within the red blood corpuscles and breaks them up. It is therefore simply a blood disease. The organism does not belong to the bacteria but to the protozoa. It is not, in other words, a microscopic plant, but it belongs to the lowest forms of the animal kingdom. This very minute organism multiplies very rapidly in the body of the infected animal, and in acute cases causes an enormous destruction of red corpuscles in a few days. How it gets into the red corpuscle it is not possible to state, but it appears that it enters as an exceedingly minute body, probably endowed with motion, and only after it has succeeded in entering the corpuscle does it begin to enlarge. Plate XLV, figure 4, illustrates an early stage of this blood parasite. The red corpuscle contains a very minute, roundish body which is stained blue to bring it into view. The body is, as a rule, situated near the edge of the corpuscle. Figure 5 illustrates an older stage in the growth of the parasite, in fact the largest which has thus far been detected. It will be noticed that there are usually two bodies in a corpuscle. These bodies are in general pear-shaped. The narrow ends are always toward each other when two are present in the same corpuscle. If we bear in mind that the average diameter of the red blood corpuscles of cattle is from 1/4000 to 1/5000 inch, the size of the contained parasite may be at once appreciated by a glance at the figures referred to.
The various disease processes which go on in Texas fever, and which we may observe by examining the organs after death, all result from the destruction of the red corpuscles; this destruction may be extremely rapid or slow. When it is rapid we have the acute, usually fatal, type of Texas fever, which is always witnessed in the height of the Texas-fever season, that is, during the latter weeks of August and the early weeks of September. When the destruction of corpuscles is slower, a mild, usually nonfatal, type of the disease is called forth, which is only witnessed late in autumn or more rarely in July and the early part of August. Cases of the mild type occurring thus early usually become acute later and terminate fatally.
The acute disease is fatal in most cases, and the fatality is due not so much to the loss of blood corpuscles as to the difficulty which the organs have in getting rid of the waste products arising from this wholesale destruction. How great this may be a simple calculation will serve to illustrate. In a steer weighing 1,000 pounds, the blood in its body weighs about 50 pounds, if we assume that the blood represents one-twentieth of the weight of the body, which is a rather low estimate. According to experimental determination at the bureau station, which consists in counting the number of blood corpuscles in a given quantity of blood from day to day in such an animal, the corpuscles contained in from 5 to 10 pounds of blood may be destroyed within 24 hours. The remains of these corpuscles and the coloring matter in them must be either converted into bile or excreted unchanged. The result of this effort on the part of the liver causes extensive disease of this organ. The bile secreted by the liver cells contains so much solid material that it stagnates in the finest bile canals and chokes these up completely. This in turn interferes with the nutrition of the liver cells and they undergo fatty degeneration and perish. The functions of the liver are thereby completely suspended and death is the result. This enormous destruction of corpuscles takes place to a large extent in the kidneys, where a great number of corpuscles containing the parasites are always found in acute cases. This accounts largely for the blood-colored urine, or red water, which is such a characteristic feature of Texas fever. The corpuscles themselves are not found in the urine; it is the red coloring matter, or hemoglobin, which leaves them when they break up and pass into the urine.
Symptoms.—After a period of exposure to infected soil, which may vary from 13 to 90 days, and which will be more fully discussed under the subject of cattle ticks as bearers of the Texas-fever parasite, the disease first shows itself in dullness, loss of appetite, and a tendency to leave the herd and stand or lie down alone. A few days before these symptoms appear the presence of a high fever may be detected by the clinical thermometer. The temperature rises from a normal of 101° to 103° F. to 106° and 107° F. There seems to be little or no change in temperature until recovery or death ensues. The period of high temperature or fever varies considerably. As it indicates the intensity of the disease process going on within, the higher it is the more rapid the fatal end. When it does not rise above 104° F. the disease is milder and more prolonged.
The bowels are mostly constipated during the fever; toward the end the feces may become softer and rather deeply tinged with bile. The urine shows nothing abnormal during the course of the disease until near the fatal termination, when it may be deeply stained with the coloring matter of the blood. (Hemoglobinuria; see [Pl. XLV], fig. 3.) Although this symptom is occasionally observed in animals which recover, yet it may generally be regarded as an indication of approaching death. The pulse and respiration are usually much more rapid than during health.
Other symptoms in addition to those mentioned have been described by observers, but they do not seem to be constant, and only those described above are nearly always present. As the end approaches emaciation becomes very marked, the blood is very thin and watery, and the closing of any wound of the skin by clots is retarded. The animal manifests increasing stupor and may lie down much of the time. Signs of delirium have been observed in some cases. Death occurs most frequently in the night.
The duration of the disease is very variable. Death may ensue in from three days to several weeks after the beginning of the fever. Those that recover ultimately do so very slowly, owing to the great poverty of the blood in red corpuscles. The flesh is regained but very gradually, and the animal may be subjected to a second, though mild, attack later on in the autumn, which pushes the full recovery onward to the beginning of winter.