In the mild type of the disease, which occurs in October and November, symptoms of disease are well-nigh absent. There is little if any fever, and if it were not for loss of flesh and more or less dullness the disease may pass unnoticed, as it undoubtedly does in a majority of cases. If, however, the blood corpuscles are counted from time to time a gradually diminishing number will be found, and after several weeks only about one-fifth or one-sixth of the normal number are present. It is indeed surprising how little impression upon the animal this very impoverished condition of the blood appears to make. It is probable, however, that if two animals kept under the same conditions, one healthy and the other at the end of one of these mild attacks, are weighed, the difference would be plainly shown.
Pathological changes observable after death.—In the preceding pages some of these have already been referred to in describing the nature of the disease. It is very important at times to determine whether a certain disease is Texas fever or some other disease, like anthrax, for example. This fact can, as a rule, be determined at once by a thorough microscopic examination of the blood. The necessary apparatus and the requisite qualifications for this task leave this method entirely in the hands of experts. There are, however, a considerable number of changes caused by this disease which may be detected by the naked eye when the body has been opened. Put together they make a mistake quite impossible. The presence of small ticks on the skin of the escutcheon, the thighs, and the udder is a very important sign in herds north of the Texas-fever line, as it indicates that they have been brought in some manner from the South and have carried the disease with them, as will be explained later. Another very important sign is the thin, watery condition of the blood, either just before death or when the fever has been present for four or five days. A little incision into the skin will enable any one to determine this point. Frequently the skin is so poor in blood that it may require several incisions to draw a drop or more.
The changes in the internal organs, as found on post-mortem examinations, are briefly as follows: The spleen, or milt, is much larger than in healthy animals. It may weigh three or four times as much. When it is incised the contents or pulp is blackish (see [Pl. XLIV], fig. 1), and may even well out as a disintegrated mass. The markings of the healthy spleen (fig. 2) are all effaced by the enormous number of blood corpuscles which have collected in it, and to which the enlargement is attributable. Next to the spleen the liver will arouse our attention. (See [ Pl. XLV], fig. 2.) It is larger than in the healthy state, has lost its natural brownish color (fig. 1), and now has on the surface a paler, yellowish hue. When it is incised this yellowish tinge, or mahogany color, as it has been called by some, is still more prominent. This is owing to the large quantity of bile in the finest bile capillaries, and as these are not uniformly filled with it the cut surface has a more or less mottled appearance. This bile injection causes in many cases a fatty degeneration of the liver cells, which makes the organ appear still lighter in color.
In all cases the gall bladder should be examined. This is distended with bile, which holds in suspension a large number of yellow flakes, so that when it is poured into a tall bottle to settle fully one-half or more of the column of fluid will be occupied by a layer of flakes. If mucus is present at the same time, the bile may become so viscid that when it is poured from one glass to another it forms long bands. The bile in health is a limpid fluid, containing no solid particles.
If the animal during life has not been observed to pass urine colored with blood or red water, the bladder should be opened. This quite invariably, in acute cases, contains urine which varies in color from a deep port wine to a light claret. In many cases the color is so dense that light will not pass through even a thin layer. ([Pl. XLV], fig. 3.) The kidneys are always found congested in the acute attack. The disease exerts but little effect on the stomach and intestines beyond more or less reddening of the mucous membrane; hence an examination of them may be safely omitted. The lungs are, as a rule, not diseased. The heart usually shows patches of blood extravasation on the inside (left ventricle) and less markedly on the outer surface.
We have observed jaundice of the various tissues but very rarely. It has been observed by some quite regularly, however.
During the hot season about 90 per cent of the susceptible mature animals from a noninfected district die, but later, in the cool weather, the disease assumes a milder type, with a consequent decrease in the number of deaths.
The cattle tick, Margaropus annulatus, as the carrier of Texas fever. ([Pls. XLVI], [XLVII], and [XLVIII].) —The cattle tick is, as its name indicates, a parasite of cattle in the southern part of the United States. It belongs to the group of Arthropoda and to the genus Margaropus (or Boophilus), which is included in the order Acarina. Its life history is quite simple and easily traced from one generation to another. It is essentially a parasite, attaching itself to the skin ([Pl. XLVIII]) and drawing the blood of its host. It is unable to come to maturity and reproduce its kind unless it becomes attached to the skin of cattle, whence it may obtain its food.
The eggs laid on the ground after the female has dropped from the host begin to develop at once. When the embryo is fully formed within the shell it ruptures this and gains its freedom. The time required from the laying of the eggs to their hatching varies considerably, according to the temperature. In the laboratory in the heat of midsummer this was accomplished in about 13 days. In the late fall, under the same conditions, it required from four to six weeks. The larva after emerging from the egg is very minute, six-legged, and is just visible to the naked eye. ([Pl. XLVI], fig. 3.) If these larvæ are kept on a layer of moist sand or earth in a covered dish, they may remain alive for months, but there is no appreciable increase in size. So soon, however, as they are placed upon cattle growth begins.
On pastures these little creatures soon find their way on to cattle. They attach themselves by preference to the tender skin on the escutcheon, the inside of the thighs, and on the base of the udder. Yet when they are very numerous they may be found in small numbers on various parts of the body, such as the neck, the chest, and the ears. ([Pl. XLVIII] and [Pl. XLIX], fig. 1.)