316 PARASITES OF THE DOMESTIC ANIMALS 



malignant parasitism is reached when the spirochetes acquire the habit 

 of living in the blood. In this case it is evident that, except under cer- 

 tain conditions of contact, the transfer from host to host cannot be direct, 

 but that the intervention of an intermediate host is necessary. This must 

 be a blood-sucking invertebrate, and, in certain known cases of spiroche- 

 tosis of domestic animals, has been found to be a tick, as the tick Argas 

 miniatus, the carrier of Spirocheta gallinarum which causes a spiroche- 

 tosis, in fowls, and the cattle tick Boophilus decoloratus, the inter- 

 mediate host of Spirocheta theileri, the cause of a disease among South 

 American cattle. 



The malaria parasites afford study in the evolution of pathogenicity 

 of other Protozoa. These organisms indicate in their morphology and 

 development that they are closely allied to the Coccidia, which are 

 protozoan cell parasites attacking and entering tissue cells, especially 

 epithelium, of arthropods and vertebrates. There is little doubt that 

 the malaria parasites were originally Coccidia of insects that, with 

 change of habitat, developed increased pathogenicity toward the new 

 host. 



Granting this, we have, then, in the malaria parasites an example 

 of the evolution of disease in the past, while disease in the making is 

 evidenced to-day more especially in the case of certain parasitic flag- 

 ellates of the genus Herpetomonas. 



The introduction of herpetomads into vertebrates by the latter 

 swallowing infected insects, or by the way of wounds of the skin, has 

 been shown to result in pathogenic effects in the vertebrate host. A 

 series of experiments extending over some six years (Fantham and 

 Porter, Journal of Parasitology, June, 1916) have shown that certain 

 herpetomads normally parasitic in insects, when introduced into ver- 

 tebrates will produce a condition resembling kala-azar, an infectious 

 disease of man common in certain regions of India, China, and countries 

 bordering on the Mediterranean, caused by the herpetomad Leishmania 

 (Herpetomonas) donovani. The symptoms developed and the mor- 

 phology of the parasite found in the vertebrate host show that here at 

 least are examples of kala-azar in process of evolution. 



Plate III. — EvoLurioN of the Parasite of Kala-Azar. Figs. 1 to 6. Parasites of 

 kala-azar. 1. Isslated parasites of different forms in tiie spleen and liver. 2. Division 

 forms from liver and bone marrow. 3. Mononuclear spleen cells containing the parasites. 

 4. Groups of parasites. 5. Phagocytosis of a parasite by a polynuclear leucocyte. Figs. 

 6 to 15. Parasites from cultures. 6. First changes in the parasites. The protoplasm has 

 increased in bulk and the nucleus has become larger. 7. Further increase in size. Vacuoli- 

 zation of the protoplasm. 8. Division of the enlarged parasite. 9. Evolution of the 

 flagella. 10. Small piriform parasite showing flagellura. 11. Further development and 

 division of the parasite. 12. Flagellated trypanosome-like form. 13, 14. Flagellated 

 forms dividing by a splitting-off of a portion of the protoplasm. 15. Narrow flagellated 

 parasites which have arisen by the type of division shown in Figs. 13 and 14. (After. Craw- 

 ley, from Mense's "Handbuch," after Leishman, Cir. No. 194, Bu. An. Ind., U. S. Dept. 

 Agr.). 



