6 CONDITIONS AFFECTING TRANSMISSION [CH. 



the parasite and its host has become estabUshed and the former 

 does not increase beyond a certain hmit, with the result that 

 the health of the animal from which it is deriving nourishment 

 is not very seriously affected. This is the case with numerous 

 parasites, such as the majority of trypanosomes in vertebrates, 

 Hcemoproteus in birds, Haemogregarines in reptiles, etc. It is 

 only in rare cases that the parasites increase to such an extent 

 that they markedly affect the health of their hosts, but since 

 our attention is only called to diseased animals, the cases in 

 which disease is caused by the parasites are more generally 

 noticed. 



A pathogenic blood parasite cannot persist indefinitely 

 unless one or more of the following conditions are fulfilled : 



(i) The disease which it produces may be of long duration 

 so that the invertebrate host has numerous opportunities of 

 becoming infected with the parasite and spreading the infec- 

 tion to other hosts. Of course the parasite must be present 

 in the blood in sufficient numbers and at a suitable stage of 

 development to ensure the infection of the biting host which 

 transmits the disease. If these conditions were fulfilled an 

 excessively fatal, though chronic disease, possessed of efficient 

 means of distribution would be able to persist until the exter- 

 mination of its host had been accomplished, after which it would 

 also cease to exist, unless it became adapted to some other host. 



(2) The infection may be hereditarily transmitted in the 

 invertebrate host, so that the offspring of an infected parent 

 also carries the infection. Fortunately, with the doubtful 

 exception of yellow fever in Stegomyia, no biting-fly is known 

 to transmit any disease agent to its offspring. One of the 

 best examples of this mode of hereditary infection in other 

 groups is that of SpirochcBta duttoni in its invertebrate host 

 the tick, Ornithodorus fnoubata. 



(3) The parasite may infest more than one species of 

 vertebrate host, in one or more of which the equilibrium be- 

 tween parasite and host has become established. This is by 

 far the most important condition from an epidemiological 

 point of view. If a parasite is able to live in several verte- 

 brate hosts and is non-pathogenic towards any one of them, 



