BACTERIAL INVASION 15 



the buffalo, that appeared to be immune to the disease, 

 to other species intensely susceptible to the action of this 

 organism, the horse and the dog. The importance of 

 these lower mammals as reservoirs or harbourers of the 

 parasite, and of insects as vectors, or carriers, was thus 

 made clear. Long before this, William Williams of Edin- 

 burgh had maintained most stoutly that ticks were in 

 some way or other associated with the transmission of the 

 infective material of Redwater fever from one bovine to 

 another, an opinion he later translated into partial demon- 

 stration. This opinion has now been confirmed beyond 

 doubt by numerous observers. Bruce, applying his reser- 

 voir theory to Malta fever, found that goats, apparently 

 healthy, provided a continuous supply of the Micrococcus 

 melitensis, which, distributed through milk, infects sus- 

 ceptible human beings. Similarly, the rat forms a ' reser- 

 voir ' for the plague bacillus, this organism, moreover, 

 being carried from rat to rat by the rat flea, and thence 

 even to the human subject by the same vector. Following 

 these observations, there have been described during the 

 last ten years a regular swarm of protozoal, blood and other 

 parasites, which are carried principally by arthropods 

 insects, ticks, crustaceans from one animal to another. 

 In certain instances the carrier seems to act as an inter- 

 mediate host, some phase of alternation of generation, 

 usually the sexual phase, being here passed through. In 

 connection with human diseases we have mosquitoes 

 (Stegomyia fasciata) harbouring and carrying the in- 

 fective material of yellow fever. Malaria we have already 

 mentioned. Sleeping sickness appears to be the result of 

 infection by an organism very like that of Nagana, but 

 carried to the human subject by another tsetse-fly, the 

 Olossina palpalis. Nuttall points out that many of the 

 hsematozooal parasites belonging to the genus Piroplasma 

 are transmitted by ticks ; as are also certain relapsing fevers 

 of the fowl and the human subject. Amongst these he 

 mentions the Redwater in cattle, transmitted by certain 

 species of Boophilus, of Ixodes, of Rhipicephalus ; Rho- 

 desian fever in cattle, Redwater in sheep, malignant jaundice 

 in dogs, biliary fever in horses, spirochaetoses in cattle, 



