Among the pathogenic species are to be found the causative organisms of some of the most serious diseases of domestic animals and even of man. It is probable that these pathogenic species secrete a specific poison. The majority of them are tropical in distribution.
Though we are concerned especially with the species which infest man, we shall first consider two of the trypanosomes of lower animals, known long before any of those of man had been found.
Fleas and Lice as Carriers of Trypanosoma lewisi.—Trypanosoma lewisi, the first mammalian trypanosome known, is to be found in the blood of wild rats. Like its host, it appears to be cosmopolitan in distribution, having been reported from several localities in the United States, Brazil, Argentine, England, Germany, France, Italy, Russia, Asia and Africa.
This species is usually regarded as non-pathogenic, but in experimental work, especially with white rats, heavy infestations often result fatally, and naturally infested specimens sometimes show evidence of injury. Rats which have been infested exhibit at least temporary immunity against new infection.
Trypanosoma lewisi is transmitted from rat to rat by fleas and by lice. Rabinowitsch and Kempner (1899) first found that healthy rats which were kept with infested rats, showed trypanosomes in their blood after about two weeks. They found the trypanosomes in the alimentary canal of fleas which had fed on the diseased rats. On teasing such fleas in physiological salt solution and inoculating them into fresh rats they were able to produce the infection. Finally, they showed that the fleas which had fed upon infested rats were able to carry the parasites to healthy rats. Corresponding experiments with lice were not successful. Prowazek (1905) found in the rat louse (Hæmatopinus spinulosus) organisms which he regarded as developmental stages of the Trypanosoma lewisi. He believed that the sexual cycle was undergone in this insect.
Nuttall (1908) readily transmitted the trypanosomes through the agency of fleas, (Ceratophyllus fasciatus and Ctenopthalmus agyrtes). He believes that these insects are probably the chief transmitters of the parasite. He was also able to transmit it from diseased to healthy rats through the agency of the rat louse. He was unable to trace any developmental stages in the louse and inclined to the opinion that Prowazek was deceived by the presence of extraneous flagellates such as are known to exist in a number of blood-sucking arthropods.
Nuttall concludes that since three distinct kinds of blood-sucking insects are capable of transmitting Trypanosoma lewisi it appears doubtful that this flagellate is a parasite of the invertebrate "host" in the sense claimed by Prowazek and other investigators.
Tsetse-flies and Nagana—One of the greatest factors in retarding the development of certain regions of Africa has been the presence of a small fly, little larger than the common house-fly. This is the tsetse-fly, Glossina morsitans ([fig. 165]) renowned on account of the supposed virulence of its bite for cattle, horses and other domestic mammals.
The technical characteristics of the tsetse-flies, or Glossinas, and their several species, will be found in a later chapter. We need emphasize only that they are blood-sucking Muscidæ and that, unlike the mosquitoes, the sexes resemble each other closely in structure of the mouth-parts, and in feeding habits.
In 1894, Colonel David Bruce discovered that the fly was not in itself poisonous but that the deadly effect of its bite was due to the fact that it transmitted a highly pathogenic blood parasite, Trypanosoma brucei. This trypanosome Bruce had discovered in the blood of South African cattle suffering from a highly fatal disease known as "nagana". On inoculating the blood of infected cattle into horses and dogs he produced the disease and found the blood teeming with the causative organism. In the course of his work he established beyond question that the "nagana" and the tsetse-fly disease were identical.