Long-term efficacy of single-dose mass treatment with diethylcarbamazine citrate against diurnally subperiodic Wuchereria bancrofti: eight years' experience in Samoa.

A single-dose of diethylcarbamazine citrate (DEC, 6 mg per kg body weight) was administered in three mass treatment campaigns to > 80% of the estimated total Samoan population (160,000) in 1982, 1983, and 1986. The effect of the drug was evaluated before and after each campaign by conducting four blood surveys covering 9600 to 13,700 people from 26-34 villages on each occasion. The drug reduced the prevalence of Wuchereria bancrofti microfilariae from 5.6% to 2.5% (a 55% reduction), while the transmission potential (the estimated mosquito infection rate if everyone is equally exposed to mosquito bites) dropped from 2.18 to 0.67 (a 70% reduction). The total number of microfilariae in the Samoan population is estimated to have been reduced by 80%. A spaced, single-dose treatment with DEC at a 1-2-year interval therefore seems to be an effective control measure against diurnally subperiodic W. bancrofti.

Distribution of vectors, transmission indices and microfilaria rates of subperiodic Wuchereria bancrofti in relation to village ecotypes in Samoa.

Aedes polynesiensis and Ae. samoanus biting densities and Wuchereria bancrofti infection and infective rates were studied in 47 villages throughout the islands of Samoa Upolu, Manono and Savaii during 1978-79, and microfilaria (mf) rates were surveyed in 28 of the villages. The mf rate was correlated with both infection and infective rates of Ae. polynesiensis in Upolu, but not of Ae. samoanus. In Upolu, Ae. polynesiensis was apparently the major vector. It was relatively more abundant in more cultivated and populated areas, along the northern coast of Upolu, except Apia town area. In Savaii, Ae. samoanus predominated over Ae. polynesiensis except in "plantation" villages. Relatively high biting densities and rates of infection and infectivity indicated that Ae. samoanus was not less important than Ae. polynesiensis as a vector in Savaii. Ae. samoanus preferred natural vegetation, in contrast to Ae. polynesiensis which was found near human habitations in cultivated land. There was no difference between the biting densities of Ae. polynesiensis in "coastal" and "inland" villages, indicating that crab holes (numerous only in some coastal villages) may not influence the density of Ae. polynesiensis. Higher mf rates were associated with villages where Ae. polynesiensis, rather than Ae. samoanus, was dominant, indicating that Ae. polynesiensis was generally a more efficient vector. In the former villages, the difference in mf rates between males and females was smaller than in the latter, probably reflecting a difference in biting habits of the vectors. Ae. polynesiensis infections were recorded in plantations over 2 km from any village, suggesting that both habitats were foci of transmission.

Epidemiology of subperiodic bancroftian filariasis in Samoa 8 years after control by mass treatment with diethylcarbamazine.

In 1979, a microfilarial prevalence study was conducted in a population of 8385 persons inhabiting 28 villages in Samoa using both the nuclepore filtration (NP) method (with 1 ml blood) and the fingerprick (FP) method (with 60 mm(3) blood). The overall prevalence rate was 4.5% by the NP method and 3.8% by the FP method. The average microfilarial prevalence in males was 2.3 times higher than in females, and the rate among males aged 30 years and over was as high as 20%. The positive cases were found to be concentrated in certain households.The median microfilarial density (MfD-50) for the whole of Samoa was 18.6 using 60-mm(3) blood samples (males, 21.4; females 14.2). While the MfD-50 of any village has a positive association with the microfilarial prevalence rate of that village, a relatively high MfD-50 was noticed among young people under 20 years of age together with low prevalence rates.The negative binomial distribution was fitted to the data on the distribution of microfilarial counts in Samoa and gave a better fit than the log-normal distribution. The data having been fitted to the negative binomial, the number of false negatives could be determined as 9% of the estimated number of positives in the survey population when the NP method was employed and about 25% with the FP method.Further studies revealed that 15.1% of the microfilaria carriers presented some clinical manifestation, the most common being an attack of filarial fever (13.1%). The average duration of a fever attack was 3.5 days and the total period with fever/person/year averaged 27.1 days.

Low-density microfilaraemia in subperiodic bancroftian filariasis in Samoa.

Among microfilaria (mf) carriers of subperiodic bancroftian filariasis in Samoa, the low-density level of microfilaraemia was defined as 1-20 mf/ml, and the occurrence of low-density carriers (90 in the present study) was analysed by age, sex, and village in relation to the microfilarial prevalence rate. The low-density carriers were more numerous among those under 20 years and over 60 years old than in other age groups. The ratio of low-density carriers to the total of mf-positive subjects in a village decreased as the prevalence rate of Wuchereria bancrofti in the village increased.The epidemiological significance of low-density carriers was assessed in connection with the infectivity of vector mosquitos (Aedes polynesiensis) produced by them, the possible change of these carriers to carriers of a higher density, and the production of new low-density carriers by diethylcarbamazine citrate (DEC-C) treatment. The mosquito infectivity produced by the low-density carriers accounted for only 2.16% of the total infectivity produced by all the carriers, suggesting that these carriers are of minor importance in the transmission of filariasis. The change of microfilarial count over time among untreated mf-positive subjects was not remarkable during a 60-252-day observation period. However, the low-density carrier group showed a mean increase of 36%, the younger such carriers (under 30 years old) showing a 132% increase. The production of low-density carriers by DEC-C single-dose treatment (6 mg/kg body weight) was not as great as expected.

The efficacy of annual single-dose treatment with diethylcarbamazine citrate against diurnally subperiodic bancroftian filariasis in Samoa.

Treatment of subperiodic bancroftian filariasis, which is endemic in Samoa, with diethylcarbamazine citrate (DEC-C) in single doses of 4 mg/kg, 6 mg/kg, and 8 mg/kg body weight was evaluated using the nuclepore filtration method (with 1 ml blood) and compared in terms of efficacy against the microfilariae (mf) and side-reactions produced. The 6 mg/kg single-dose treatment assessed at six months showed that the effect of DEC-C to eliminate microfilariae was closely associated with the pre-treatment microfilarial level. The treatment cured nearly 60% of the low-density carriers with /=501 mf/ml. However, the percentage decrease in the microfilarial count, which averaged 89.3%, did not seem to differ greatly according to the level of the pre-treatment count. The age group 20-29 years showed a poorer response to the treatment compared with the other age groups. When the different dosage regimens (4 mg/kg, 6 mg/kg and 8 mg/kg) were compared at 6 and 12 months after treatment, the 6 mg/kg regimen was found to be more effective than the 4 mg/kg regimen in reducing the microfilarial count, and it produced fewer adverse reactions than the 8 mg/kg regimen. The comparison between the annual single-dose treatment at 6 mg/kg and the six-monthly two doses/year treatment at the same dosage (total 12 mg/kg/year) showed that the latter had little advantage over the former, thus indicating the effectiveness of the single-dose treatment for longer than six months.

Comparison of methods for the detection of microfilariae of Wuchereria bancrofti in Western Samoa.

The 1 ml nuclepore filtration technique (NP method), 60 c.mm finger-prick or venous blood smear method and the counting chamber method were compared for their effectiveness in detecting microfilariae. The loss of microfilariae by the NP method was small enough to be disregarded. The 60 c.mm blood smear method was shown to be as efficient as the 60 c.mm NP method, when using finger-prick blood. The finger-prick blood contained 32% more microfilariae than venous blood when compared by the 60 c.mm blood smear method. Probably reflecting this difference of mf count between finger-prick and venous blood, 1 ml NP method using venous blood detected only 12.4 times more microfilariae than 60 c.mm NP method using finger-prick blood, despite the fact that the former requires 16.7 times more blood than the latter. No difference of mf count was shown between the 60 c.mm blood smear and 60 c.mm counting chamber methods, when using finger-prick blood.