A review of the filariasis control programme in Tahiti from November 1967 to January 1968.

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.

Treatment failure in filariasis mass treatment programmes.

The data from the first 3 years of the Samoan pilot filariasis control programme were reanalysed using incidence instead of prevalence statistics. With these statistics, failures of diethylcarbamazine treatment can be roughly divided into three groups: primary treatment failures, manifested by persistent microfilaraemia; secondary treatment failures, manifested by microfilaraemia recurring within the prepatent period after apparently successful treatment; and new infections. When cases of persistent and recurrent microfilaraemia are excluded by appropriate statistical manipulations, the incidence of new infections is seen to be many times higher in persons who originally showed microfilaraemia. This suggests that susceptibility or exposure, or both, are not homogeneously distributed in the population, and indicates that proposed mathematical models of filarial epidemiology must be further refined. It also indicates that filariasis control programmes should devote more attention to studies and control methods aimed at this high-risk subgroup of the population.

Periodic mass treatment with diethylcarbamazine for the control of filariasis in American Samoa.

Filariasis surveys made during 1962-63 in 5 villages in American Samoa among persons over 5 years of age gave an elephantiasis rate of 3.4%, a microfilarial rate of 26% and a median microfilarial rate (MfD(50)) of 29. These rates were somewhat higher than those found in surveys made in the same villages among villagers of the same ages some 20 years previously. A mass treatment programme with diethylcarbamazine was then decided on.Preliminary treatment with a single total regimen of 72 mg of the drug per kg of body weight, given in 12 doses of 6 mg/kg, was administered in several villages over a period of 6 months. Among villagers of all ages at the end of 3 years, the microfilarial rate was 7.3% and the MfD(50) value was 2. These results appeared to be inadequate for a successful control programme and a periodic mass treatment project of 2 or more regimens of 72 mg/kg to be administered every 2 years was instituted on the Island of Tutuila, to be continued until considered no longer necessary.Surveys made 2 years after the second mass treatment in the villages of Amouli, Amanave, Onenoa, Tula, Alao, Utumea and Alofau, in which 1407 persons were examined, showed that 3 villages were negative for microfilariae, that the average microfilarial rate in all 7 villages was 0.4% and that the MfD(50) was 1. These villages were also negative in intensive mosquito surveys for infective-stage larvae of Wuchereria bancrofti. It is suggested that interruption of transmission has been observed in the 3 villages negative for both microfilariae and infective-stage larvae and that, if favourable surveys continue, eradication programmes should be established.