Another variety of trypanosomiasis is mal de caderas, seen in South America, particularly in Argentina and Brazil. It affects horses. According to Koch, the parasites of mal de caderas exactly resemble the tsetse and surra parasites. Other observers, however, declare that the mal de caderas parasites are distinguished from those before mentioned by their particularly small centrosome. Mal de caderas affects not only horses, but all the other animals which suffer from tsetse.
Another variety of trypanosoma, the trypanosoma Theileri, is especially striking on account of its size. It is only found in oxen, and exhibits a very slight degree of virulence.
Koch divides trypanosomata into two great groups.
The grouping is based on three important peculiarities: firstly, the morphology of the parasite; secondly, its virulence; and, thirdly, its relation to the host.
The first group only exists in one species of animal. They have become so completely accustomed to this method of life that they cannot exist under other circumstances. Their virulence is slight but constant. This group comprises the trypanosoma of rats and the trypanosoma Theileri.
The second group (to which all other trypanosomata belong) shows great variation in virulence and in form. These trypanosomata are not peculiar to any one species, but may affect dogs, rats, horses, etc. Their morphological peculiarities also vary according to the animals in which they are found. Thus, the tsetse parasites when cultivated in the bodies of dogs and rats become much smaller than usual, and the centrosome appears near the end; when cultivated in horses the end appears pointed, and the centrosome lies near the centre; in the pig the parasites lose their peculiar short flagellum. Their virulence also varies within wide limits.
It has been found possible, as in the case of bacteria, to modify the virulence of trypanosomata by successive passages through different animals. By inoculating dogs with comparatively innocuous trypanosomata and conveying the disease from dog to dog the virulence is markedly increased. On the other hand, parasites which prove very virulent for oxen become much less active for these animals after passages through rats and dogs. This apparently trifling discovery laid the foundation for protective inoculation experiments. Parasites of the second group can also exist in the bodies of almost all mammals.
Koch is of opinion that the parasites of surra in India and of tsetse disease in Africa are absolutely identical. Laveran, on the other hand, states that he has protected animals against tsetse, and that they have nevertheless suffered from surra.
That this in no way disproves the identity of the two parasites is shown by other experiments.
Koch, whilst in Dar-es-Salaam, made some interesting experiments for the purpose of discovering a method of protective inoculation. He had found that the virulence of the ox parasites could be modified. He therefore inoculated oxen first with these weakened parasites and afterwards with others of high virulence. All the control animals died while those treated as above remained alive.