LIPE-HISTOET OF FILARIA BAN^CBOFTI. 367 



without obvious injury to the bearer. This immunity, in reality, 

 proves nothing. If, for example, we take the parallel case of 

 Trichina we find that several millions of entozoa may exist in the 

 human or, at all events, in the animal bearer without the pro- 

 duction of any palpable symptom of discomfort. In such cases 

 it is not possible to determine the strict limits of health and 

 disease ; nevertheless, were we to double the amount of infection, 

 an imaginary line of demarcation is at once bridged over, and the 

 parasites become acknowledged as directly responsible for grave 

 symptoms which may even prove fatal to the bearer. Again, the 

 relative strength and size of the infested host constitute factors 

 that will materially modify or limit the power of the parasite for 

 injury. Where the entozoa are of minute size, and where their 

 injurious action is primarily due to the mechanical obstructions 

 they set up, it is clear that the virulence of the helminthiases, or 

 resulting diseased conditions, will mainly depend upon the number 

 of intruders. 



Another consideration of the highest value in relation to epi- 

 demiology generally, and more especially in regard to the prac- 

 tical question as to the best methods of stamping out parasitic 

 plagues, is that which refers to the life-history of the entozoon 

 itself. It must be obvious that in all cases where the interme- 

 diate host can be captured and destroyed, the life-cycle of the 

 parasite can be broken or interrupted ; and if thus broken there is 

 an end to the further propagation of the species. The knowledge 

 that we have acquired by experimental research in this con- 

 nexion has already enabled us to set a limit upon the prevalence of 

 certain well-known disorders, such as trichinosis, cestode tuber- 

 culosis, and so forth. In the case of epizootics, however, which 

 are not merely dependent upon minute entozoa, but which are 

 also, in the way that we have seen, indirectly due to the action of 

 intermediary hosts that cannot be readily captured or destroyed, 

 our power of arresting the disease is comparatively limited. In 

 the case of Filaria hancrofti it is probably not necessary either 

 that a dead or living mosquito should be swallowed to ensure in- 

 fection ; but it is necessary that the parasitic larvse should have 

 dwelt within the mosquito in order to arrive at the highest stage 

 of larval growth prior to their re-entrance within the human ter- 

 ritory. Undoubtedly the larvse of Filaria hancrofti are swal- 

 lowed with potable waters. The perfect filtration of these waters 

 before use would certainly check and, in course of time, would pro- 



