8 SOME MINUTE ANIMAL PARASITES 



There are some remarkably interesting points to 

 note in connexion with the transmission of certain 

 parasites from host to host. In a few cases the 

 parasite has become so well accustomed to its host 

 that the reproductive organs of that host are invaded 

 by the parasites, and the young are born infected in 

 consequence. One of the best-known cases of the 

 occurrence of hereditary infection is that found in 

 silkworms, occasioning the disastrous disease known 

 as " p^brine," which devastated the silk industry of 

 France, before Pasteur discovered the cause and 

 devised preventive measures. Either of the parent 

 moths might be infected, and in that case every egg 

 was infected with the parasite called Nosema bombycis, 

 and every such egg gave an infected silkworm that 

 died before arriving at the silk-producing stage. 



Redwater of cattle is due to a minute blood 

 parasite, and is carried from cow to cow by ticks. 

 The infected parent tick can pass on the infection to 

 the offspring, and the latter are the great agents in 

 spreading the disease among cattle. 



A more human interest attaches to the ticks that 

 infect man with African relapsing or tick fever by 

 means of their excretions. The organisms causing 

 these maladies are spirochsetes, and the ticks that 

 usually transmit them are the very young ticks or 

 nymphs. The nymphs are so small that they are 

 easily overlooked. Each individual, about the 

 size of a very small pin's head, has the appear- 

 ance of a small fragment of sawdust, which it re- 

 sembles in colour and spikiness. Both the nymphs 

 and their bite are very minute and easily overlooked, 



