In the rainy season the Indian rats swarm with the Trypanosoma Lewisi, an entirely different species, and though they can be successfully inoculated with the T. Evansi of Surra, the T. Equiperdum of Dourine, the T. Brucii of Nagana, and it is alleged the T. Equinum of Mal de Caderas, yet these are not their common parasite. The presumption is that the rat affected with Trypanosoma Evansi could transmit the disease to the horse through one of the many possible insect channels or otherwise.
Neither condition, sex nor age appears to affect receptivity. Open sores especially open the way for infection.
The position of stables, yards, pastures or picket grounds near stagnant water, manure or rubbish heaps, abattoirs or other places that breed or attract flies, is a much more important consideration. Foul stables, or those having light from both sides are more exposed to flies.
Pathology and Lesions. The pathology of the disease consists in the rapid destruction of the red blood globules by the trypanosomata. It is a form of rapidly advancing pernicious anæmia due to the great and active voracity of the trypanosoma. The swarming of the trypanosoma in the blood at the period of the relapse and the absence of the mature form in the intervals is remarkable. The trembling movement in the blood at the period of swarming results from their prodigiously active movements. The red globules may assume various forms, crenate, echinated, (Ranking), and more or less broken up or disintegrated, their numbers steadily decrease, leaving the blood thin and watery with rusty serum and yellow (icteric) staining of the white tissues, even the bones. Encrease of white globules, actual or relative, has been a marked feature (always present in typical cases, Burke).
Petechiæ are especially common on the conjunctiva, vaginal mucosa, endocardium, and less marked in the nose, mouth, and serosæ.
Œdemas are common, yellowish, gelatinoid exudate at the base of the heart, subcutem, between the muscles of the limbs or elsewhere, and as effusions into the pleuræ, pericardium or peritoneum.
Distinct blood extravasations have been noted beneath the endocardium.
The spleen is often enlarged, excessively so if death occurred during a paroxysm. The lymph glands are swollen and appear dropsical.
The whole body is emaciated and shrunken, the visible (unpetechiated) mucosæ are pale and bloodless, often yellow, yet rigor mortis is well marked.
Gastric ulcers are common (Steel, Burke, G. W. Evans) apparently preceded in many cases by capillary embolism, congestion and degeneration. Intestinal congestions are frequent (Steel, Burke), but ulcers are rare (Geo. H. Evans).