FILARIAL DISEASES 303 



but it now becomes active again and migrates into the connective 

 tissue of the anterior parts of the body of its host, and ultimately 

 into the proboscis (Fig. 126). Here the worms lie in pairs, or 

 several pairs together, awaiting an opportunity to re-enter a 

 human host. 



The length of time required for the metamorphosis and de- 

 velopment in the mosquito varies from about two weeks under 

 ideal conditions to several weeks under less favorable circum- 

 stances. When the infected mosquito bites a human being, the 

 worms emerge from the proboscis and bore through the skin 

 in the immediate vicinity of the wound, though not directly 

 through the puncture. Experiments have shown that the larvae 

 can not be deceived into entering vegetable tissue, such as a 

 banana, even though for many days they have been at the tip 

 of the proboscis, ready to emerge when the mosquito bites into 

 warm-blooded flesh. 



It is possible that these parasites may occasionally find entrance 

 to the human body by other paths than the mosquito's bite but 

 this has not yet been proved. The popular belief that bad water 

 is the cause of filarial infection is probably due to the effect of 

 stagnant water on the abundance of mosquitoes, and not to the 

 emergence of the larvae from the bodies of mosquitoes into water. 

 Bahr has shown that the larvae will live in water only seven 

 hours. 



Once back in a human body from this period of " purgatory " 

 in the body of a mosquito the larvae migrate to the lymphatic 

 vessels, there to attain sexual maturity, copulate and reproduce. 

 The larvae of the next generation escape again to the blood as 

 microfilariae, and the cycle is complete. The adult worms may 

 live for many years and even the microfilariae are able to live for 

 a considerable time, as shown by their continued presence after 

 the death of the parents. 



Filarial Diseases. The disease symptoms which are asso- 

 ciated with Filaria bancrofti can all be traced to interference 

 with the lymphatic system. In many cases there are no ill 

 effects of the infection felt for many years, or perhaps never, 

 though sooner or later there is usually produced anemia, en- 

 largement of the spleen and fever. More serious are the effects 

 produced by obstruction of the lymphatics. This causes great 

 enlargement of the lymph vessels and the diversion of the lymph 



