worms dio tlu'y Iiecoino forciK" bodies. Inflamiiiation follows 

 their death and degeiieiation. s"'i"K rise to lymphangitis, 

 lYnii)liadenitis, intlammatovy varicose j^roin glands, abscess and 

 fever. The obstructed and dilated lymphatics sometimes rup- 

 ture, with escape of chyle into the bladder, and less frequently 

 into the intestine and the abdominal cavity, and thus (j'^'c ''se 

 to chyluria, chylous diarrhea and cliylous ascites. Without 

 ru|)ture of the vessel, superficial or deeji lymph varices may 

 develop, such as varicose (flands of the );roin or axilla, h.ydrocelo 

 and lymph scrotum. 



All active changes are associated with the defeneration and 

 absorption of dead parasites. The end result is always fibro 

 sis with complete occlusion of the parasitized vessel. Elephan- 

 tiasis, one of the commonest lesions, is the result of long and 

 widespread lymphatic obstruction. 



Pyogenic Ijactcria, streptococci and stajihylococci have been 

 isolated fairly frequently from the region of the lymphedema. 

 Their responsibility in the disease syndrome, however, is not 

 clear. Drinker, Field and Homans (1934) and Drinker (1936) 

 have shown experimentally that loss of lymph circulation pre- 

 disposes to streptococcic infection, that these liacteria cause 

 attacks of severe chill and high fever, and usually can be iso- 

 lated from the tissue fluids only in the early stages of the 

 seizures. 



The diagnosis of infections with W. bancrofti is made by 

 finding the characteristic microfilariae in the blood. Many 

 cases, however, having clinical symiitoms show no microfilariae 

 in the blood nor in the contents of the dilated vessels. In such 

 instances the infection is usually of long standing and either 

 the adult worms have died or the lymphatics draining the af- 

 fected area have become obstructed by the worms and their 

 products to such an extent that the microfilariae cannot pass 

 along the vessels to enter the circulating blood. 



EPIDEMIOLOGY 



Bancroftian filariasis characteristically occurs in low lying 

 coastal areas and along the shores of lakes and rivers. In- 

 digenous infections are seldom to be found in the foothills or 

 beyond coastal ranges. The incidence of infection and clini- 

 cal manifestations in endemic regions vary greatly in adjoin- 

 ing areas. The incidence of infection is always in direct rela- 

 tion to the prevalence of the mosquito concerned; and, in turn, 

 the prevalence of these mosquitoes in an area is in direct rela- 

 tion to the favorableness of that area for moscpiito breeding. 

 The parasite is naturally limited to the range of its insect 

 vectors, and local physical factors, such as temperature, hu- 

 midity, porosity of the soil, prevailing winds, and character 

 of the vegetation which may influence the development and 

 presence of the vector, will also indirectly influence the inci- 

 dence and intensity of filariasis in the human population. 



O'Connor (1923) found a very low incidence of filariasis 

 and but a very few mosquitoes on some atolls of the Ellice 

 group having narrow, broken land strips and lacking depth of 

 bush favorable to Aerlr.t rarirgaUi.i. On other atolls with larger 

 land areas, the central lagoon reduced to a swamp, and cov- 

 ered with d?nse, dark bush, Aedrx raruiiatiis was obs?rved in 

 swarms throughout the day, and over 70 percent of the in- 

 habitants over 16 years of age showed some sign of filarial in- 

 fection. Again, a favorable high temperature with a suitable 

 amount of moisture is absolutely necessary for the development 

 of the parasite in the mosquito and for its transfer from the 

 mosquito to the human skin. The low incidence of infection 

 or absence of infection in many places, particularly in the in- 

 terior of China where proved mosquito carriers are present, is 

 attributed to cold or to dryness and high temperature (Feng, 

 1931). Thus, local conditions may have a marked influence on 

 the distribution of the infection within an endemic area. 



While the mosquito is the sole vector of W iichrreria bancrofti, 

 as in malaria, the transmission is accomplished with much less 

 certainty and promptness in the case of filariasis. There is no 

 .multiplication of filaria larvae in the intermediate host. There 

 develops but one infective larva from each microfilaria sucked 

 in by the mosquito, whereas the malaria parasite multiplies 

 enormously and the chance of the infection being returned to 

 man is by thousands of times more likely. The actual number 

 of microfilariae sucked up by the mosquito is also relatively 

 small in comparison with the number of malarial organisms in 

 the blood which may be taken up in a similar manner. A high 

 mortality occurs among the microfilariae which actually reach 

 the .stomach of the mosquito. 



O'Connor and Beatty (193S) estimated that about 3.5 per- 

 cent of the microfilariae ingested by mosquitoes die within 20 

 hours after an infective meal in the stomach blood clot, that a 

 very heavy mortality of larvae may occur after their arrival 

 in the thorax, and that many infected mosquitoes may die dur- 

 ing the first f^ew days after such feeding. Of ,").nOO wild Culex 

 fatiganx collected over a period of 12 consecutive months in St. 



Croix, V. I., fr(un within and near dwellings occupied by per 

 sons with filariasis, only 2.3 percent were found to contain 

 fully "infective" larvae. These authors believe that the ])er- 

 centage of C. fatiflaii.s which actually transmit the infection to 

 man is much smaller, due to death of mosquitoes from various 

 causes such as strong winds, torrential rains, spiders, bats, liz- 

 ards and chickens. Chickens wandering into the laboratory 

 were observed to search and eagerly devour mosquitoes resting 

 in the darker corners of the room. It was also observed that 

 r. fali(/a)i.s readily feeds upon domestic fowl. It is probable 

 that many parasites are deposited on the skin or feathers of 

 birds and thus liecome lost. 



Bancroftian filariasis char.'icteristically occurs in small, dense 

 ly populated and poorly .sanitated villages. It is particularly 

 common in the overcrowded dwellings of poor people, and the 

 incidence and morbidit.y in a given family ma.v be striking. 

 Infection is usually commoner in males than in females, which 

 difference in China is attributed to the custom among women 

 of wearing more clothes while sleeping at night, and thus ex- 

 posing less body surface to the attack of mosquitoes (Lee, 

 1926). The severity of the disease characteristically increases 

 with advancing age, thus indicating absence of any develop- 

 ment of immunit.Y in filarial infection. All races of mankind 

 appear to be equall.v susceptible. Differences noted, particu- 

 larly absence of infection or lighter infection among North 

 Americans and Europeans residing in the area are due to their 

 better sanitary conditions, better protection against mosquitoes, 

 and homes removed from the over crowded dwellings of the 

 native population. Better housing is always essential in control. 



In view of the fact that the parasite is transmitted solely 

 through the bites of mosquitoes, its prevention is primarily 

 one of mosquito control, and measures taken against these 

 insects in the control of malaria and yellow fever are eciually 

 effective against infections with TTuchereria bancrofti. Culex 

 fatifiann is a domestic mosquito which breeds near dwellings 

 in cisterns, rain liarrels, discarded tin cans, sullage drains, 

 ditches, etc. Tight screens and gauze coverings will prevent 

 mosquito breeding in cisterns, vats and rain barrels. Discarded 

 pots, tins and other utensils should be buried or destroyed, and 

 drains arid ponds kept clear of vegetation in order to effect 

 proper mosquito control. All breeding places should receive 

 weekly treatment with larvicides. In the Oceanic Islands, 

 where Ai^des variegatus is the most important vector, attention 

 must be centered on discarded cocoanut husks and shells, natu- 

 ral and artificial cavities in trees, tin cans, and other possible 

 containers of clean water. O'Connor (1923) observed that the 

 Pacific rat makes breeding places for A. variegatus in trees by 

 gnawing and cutting young cocoa-pods. The pods then die, 

 become dry, and form hanging breeding places for the mos- 

 quito. 



With modifications to meet local conditions, the methods ad- 

 vocated by O'Connor and Beatty (193S) to reduce Bancroftian 

 filariasis in Christiansted, St. Croix, might be effectively ap- 

 plied elsewhere. They include the following recommendations: 



-4. The general measures adopted should be as follows: 



1. The incidence of persons with microfilariae should be de- 

 termined at the same time for the whole population. 



2. The percentage of infective mosquitoes should be deter- 

 mined in tlie same houses and outhouses, etc. The mosquito "in- 

 fective" incidence may be more valuable than the mierofilarial 

 incidence, partly because while some natives do not readily sub- 

 mit to having blood taken from them, yet when the reasons 

 are explained to them they rarely ob.iect to their mosquitoes 

 lieing collected. Furthermore, a person with microfilariae hav- 

 ing been infected in another locality may be in a place where 

 there are few or no mosquitoes and so will not be a serious 

 menace. On the other hand, the repe.'.ted finding of infected 

 mosquitoes is proof positive that one or more persons with 

 microfilariae is near l)y. 



3. These studies might well be repeated aliout every 3 years. 



B. In houses of high human and mosquito infectivity inci- 

 dence, the following local measures should be carried out: 



1. The nature of filariasis, its transmission and prevention 

 should be completely and simply explained to the occupants of 

 the house where control measures are in.stituted. 



2. Suitable containers for potable and other water supplies 

 should be adequately .screened with wire netting. Where con- 

 tainers are not suitable they should be replaced. 



3. The use of the mosquito net should be demonstrated. (If 

 the occupants cannot afford them these should be provided from 

 public funds.) 



4. The proper maintenance and us? of all screening should be 

 supervised at intervals by the existing sanitary officers. 



5. When possible occupants should be encouraged to keep 

 fowls in their yards near the house. 



6. The number of mosc|uitoes in the houses and the percentage 

 of these which are infective should be recorded from time to 



319 



