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Historical events and suitable environmental conditions in the southwestern coastal areas of Sri Lanka have led to the establishment of a zone of endemic filariasis caused by Wuchereria bancrofti and transmitted by Culex pipiens fatigans. The previous Brugia malayi foci, scattered over widely dispersed areas of the island, were apparently completely eliminated as a result of control of the Mansonia vectors by the destruction of the larval host plants in their swamp habitats. Control measures by the Anti-Filariasis Campaign against W. bancrofti and C. p. fatigans have greatly reduced the human infection rates in the endemic coastal belt and have kept the rate in the dense population to less than 1% over the last several years. This paper assesses the entomological aspects of the control programme during the years 1970-72.

The in vitro diffusion of the organophosphate famphur from polydimethylsiloxane (PDS) capsules into plasma and their effects on Litomosoides carinii and Dipetalonema witei in jirds was studied. The in vitro rate of diffusion per 24 hours was constant and was directly proportional to the capsule lumen surface area and inversely proportional to the capsule wall thickness. One capsule implanted subcutaneously into each jird almost completely eliminated the microfilaremia of L. carinii while in situ (5 weeks) but had no effect on the levels of microfilaremia of D. witei. The adults of both species were unaffected. The capsules were well tolerated by the jirds and little tissue response to them was noted. The possibilities of using PDS-incapsulated drugs for prophylactic chemotherapy of canine and human filariasis is discussed.

An effective filariasis research and control programme has been recently established in Tahiti. This article describes the methods used for preliminary investigation of the clinical manifestations of the disease, the prevalence of the parasite and the role of the vector mosquitos. Control was effected by eliminating mosquito breeding-places and administering diethylcarbamazine to the population. An estimate is made of the costs and comparative value of the various procedures which were tried.

The ways in which repellents and attractants could be used to interrupt the transmission of bancroftian filariasis, especially by Culex pipiens fatigans, are assessed. Dimethyl phthalate (DMP) is an effective repellent against C. p. fatigans. It is also very toxic to all larval stages of Wuchereria bancrofti. The median lethal time to microfilariae of a solution of about 0.2% in normal saline is about 10 minutes. Other stages are similarly sensitive. Contact with DMP in very thin films is almost immediately lethal. It is highly unlikely that infection could take place under ordinary conditions through skin treated with DMP for at least many hours afterwards. This toxicity made it impossible to evaluate repellency in vitro. An in vivo test of the effect of skin applications of DMP on microfilaria counts gave anomalous results. A schedule for filariasis control, involving education, garbage disposal, rodent control, sewage disposal, and mosquito control, is proposed, and the research needed to allow repellents and attractants to play a full role in such a schedule is delineated.

District filariasis surveys in Tahiti in the years after the Second World War yielded the highest microfilaria rates in the South Pacific area, ranging from 25% to 44%; the mean elephantiasis rate was 5% and microfilaria densities (MfD(50)) ranged from 18 to 31. A co-operative filariasis research programme was begun and a control programme was inaugurated in 1953, using diethylcarbamazine therapy supported by mosquito larvae control measures. By 1959 fifteen districts had received mass treatment. Re-examination after 12 months showed that the mean microfilaria rate had dropped from 31% to 3% and the MfD(50) from 23 to 4. Subsequently, only positives were re-treated and by 1964 the microfilaria rate had risen to 6.8%. The increase caused concern and the significance of this reservoir of infection was investigated.Administration of diethylcarbamazine in periodic mass treatments only in American Samoa began in 1963 and was accompanied by a drop in the microfilaria rate to less than 1%, leading to interruption of transmission in some areas.

Bancroftian filariasis became a problem in Ceylon after the Second World War. The only vector is Culex pipiens fatigans, which is widely distributed throughout the country and is prevalent throughout the year. It is a domestic mosquito found resting mainly on hangings and household articles. Its biting hours are from dusk to dawn, with peaks around midnight and in the early hours of the morning. So far this mosquito has not been incriminated as either the natural or the laboratory vector of animal filariasis in Ceylon. C. p. fatigans collected from northern, eastern and north-central parts of the country are capable of transmitting bancroftian infection. This mosquito was able to survive after ingesting high numbers of microfilariae. The main larval habitat consists of collections of polluted water. Husk pits, underground drains and receptables around houses constitute special problems. The control results so far obtained are encouraging.

Since larvicidal treatment to reduce the numbers of Culex fatigans, the tropical house mosquito, offers a promising means of preventing the spread of filariasis, an assessment was made of the liability of C. fatigans to develop resistance to certain important insecticides. Strains of this mosquito from Burma, Cameroon, Sierra Leone and the USA (California) were submitted to larval selection for periods of seven to 23 generations. It was found that resistance to DDT or deutero-DDT rapidly developed to high levels and that to dieldrin or CP-47412 to moderate levels; dieldrin did not select for DDT-resistance.The organophosphorus compounds chosen developed only a modest tolerance-fivefold to sixfold with malathion or parathion, twofold to fourfold with fenthion or diazinon; the carbamate insecticide Hercules 5727 induced a threefold to fivefold tolerance. There was a general cross-tolerance from the selecting agent to all of the organophosphorus compounds tested and to the carbamate. A Rangoon strain that had already been made DDT-resistant subsequently developed a ninefold resistance to fenthion after 23 generations of selection with this organophosphorus compound, but this is still insufficient to annual the larvicidal effectiveness of fenthion.

In small-scale pilot trials, filarial infection can usually be reduced to low levels by oral administration of diethylcarbamazine to all the persons concerned; but in mass campaigns it is often difficult to persuade large numbers of people to swallow the tablets. In order to overcome this difficulty the authors propose that the compound be incorporated into cooking salt, as has been done with chloroquine to control malaria. There are many reasons why this method of medication should be more effective against filariasis than it has often been against malaria.Laboratory trials showed that cooking the compound in food did not make it toxic for rats or diminish its antifilarial activity. A pilot trial was carried out at Recife, Brazil, in which 1000 adults received salt containing 0.4% diethylcarbamazine (corresponding to a daily intake of 100 mg/day) for 40 days, and then salt containing 0.1% compound for a year. This medication was simple to administer; it was quite acceptable to the subjects; it caused no untoward effects; and it removed almost all the microfilariae from the blood. Administration of medicated salt (0.3%) for 18 days to another group of 1300 adults was well tolerated and produced a considerable reduction of the microfilarial load; but this short period was insufficient to remove all the microfilariae.The authors recommend that this method of administering diethylcarbamazine to large numbers of people should be investigated further to see if it could be used for mass campaigns to control filariasis.