FILAEIA EMBEYOS IN BLOOD 309 



body of the mosquito. The actual mode of transmission from the mosquito to 

 man remained unknown for a long time and gave rise to a variety of conjectures. 

 It has been determined only within the last few years, as we shall see farther on. 

 The relation of the human parasites of this genus to mosquitoes has been es- 

 tablished for several species, although there seems to be some confusion from the 

 difficulty of always identifying with certainty the larval filarige. The species 

 concerned are: Filaria hancrofti, F. perstans, F. demarquayi and F. philip- 

 pinensis. With other species of Filaria investigations and experiments with 

 mosquitoes have been entirely negative and these are supposed to have some 

 other blood-sucking arthropod for an intermediate host. In addition to certain 

 human filarige the transmission by the mosquito has also been proved for Filaria 

 immitis of the dog. 



THE FILARIAL WORMS. 



The adult filarige are long, thread-like, transparent worms. The female is con- 

 siderably larger than the male. In Filaria bancrofti the female measures from 

 85-90 mm. (3f-4y^e- inches) in length by 0.24-0.25 mm. in diameter; the male 

 measures about 40 mm. (1^ in.) in length and 0.1 mm. in diameter. 



The adults of Filaria hancrofti, as already stated, live in the lymphatics or in 

 the connective tissues of different parts of the body of human beings ; the two 

 sexes are often side by side, or a number of individuals are associated, and they 

 are capable of living there for a long time. When present in the lymphatic 

 vessels they may produce serious pathologic conditions, caused, according to 

 Manson, by obstructing at certain points the flow of the lymph. This accumu- 

 lates and dilates the vessels and Ijonphatic spaces, and this mechanical dis- 

 tension is accompanied by an irritation of the vessels and of the surrounding 

 connective tissue. It causes frequently very great swelling, and sometimes 

 results in the extraordinary deformities of different parts of the body known as 

 elephantiasis arabum. It seems, however, that these effects do not appear until 

 long after the infection and it is well known that in elephantiasis the filarise 

 are no longer present. Dr. Lebredo particularly points out the abundance of 

 the parasites in children and the large number of cases in which marked symp- 

 toms of the disease are absent. 



The female Filaria, enclosed in the lymphatic vessels, is viviparous and gives 

 birth to an immense number of microscopic embryos. These embryos are eel- 

 shaped, enveloped in a loose sheath, and are very active. They spread in the 

 lymph and then with it into the blood, and it is in the blood that they are com- 

 monly observed 



PERIODICITY OF FILARI/E. 



The embryos of Filaria hancrofti are usually in abundance in the blood of the 

 peripheral circulation only during the night, or during sleep ; during the day, 

 or when the patient is awake, they can not be found. There seems to be no good 

 explanation of this periodicity, nor is it absolutely exact, but it appears to con- 



