INFLUENCE OF TEMPERATURE ON INFECTION 739. 



The parasite remains passive in the egg until it is hatched, and deve- 

 lops at a later stage, so that the second generation becomes infective ; 

 infection of the third from the second generation may 

 occur, and so on indefinitely. It is often a matter of 

 considerable difficulty to obtain individuals free from 

 infection for the purposes of experiments. As examples of hereditary 

 infection Piroplasma canis and bigeminum, and Spirochaeta duttoni, 

 all transmitted by ticks as already stated; Crithidia melophaga, a 

 natural flagellate of Melophagus ovinus; Crithidia haemaphysalidis 

 from Haemaphysalis bispinosa, may be mentioned. 



Hereditary transmission is not of course final, as the parasite has 

 still to reach its vertebrate host. In the case of those arthropods which 

 are blood-sucking and parasitic in all the instars, as are the ticks, this 

 introduces still further complications ; any one or more of the instars 

 may be the stage at which the parasite is taken up from the vertebrate, 

 and any of the succeeding stages of that or the next generation may 

 pass it back again. In Piroplasma bigeminum only the adult female 

 Margaropus annulatus can serve to take up the parasite, which is 

 returned to a new host by the young larva ; the nymphs may ingest 

 the parasite transmitted by the larvae and if dislodged may infect another 

 animal. One may imagine still more complex conditions in connection 

 with those ticks which feed on different species of vertebrates in their 

 several instars. 



The occurrence of any one of these methods of transmission does not 

 of course exclude the others, though it is probable that in each case one 

 method is usual, others being adopted more or less by accident. In the 

 early experiments on the transmission of Trypanosoma gambiense, for 

 instance, mechanical infection was found to occur, though very irreg- 

 ularly, and was thought to be the usual method. Three methods of 

 transmission of Trypanosoma lewisi have been demonstrated, by biting 

 ('accidental' infection), by the deposition of infected faeces on the 

 skin of the rat, and by the eating of the invertebrate host by the rat. 



In experimental observations with known transmitters and susceptible 

 animals it is not usual to find that all become infected, even though 



the experiments are carried out under apparently 



. - . . _, r , , , Conditions determin 



natural conditions. The reasons for these apparent jn g jnfect i on 



failures are in many respects obscure. The tempera- 

 ture at which the experiments are carried out has an undoubted influence 

 in most if not all cases, either affecting the length of the latent 

 period, or even totally inhibiting the cycle of development. The parasite 



