The relative importance and distribution of Aedes polynesiensis and Ae. aegypti larval habitats in Samoa.

In preparation for a Filariasis Control programme in Samoa, during 1978 monthly larval surveys of the vector mosquito Aedes polynesiensis were carried out in four study villages in the main island of Upolu. A more extensive survey of larval habitat distribution was then made in twenty-two villages of Upolu and eighteen of Savai'i island, to determine the importance of habitat types according to their abundance, volume of water and whether their productivity was permanent or seasonal. Ae.aegypti larval densities and habitat distribution were also monitored and the occurrence of predatory Toxorhynchites amboinensis larvae in northern Upolu was recorded from forty-one collections. Aedes Breteau and container indices fluctuated with the pattern of rainfall in two coastal villages and an inland bush village, but not in a coconut plantation community. The five main Aedes larval habitat types encountered were: 200 litre water-storage drums, discarded tins and bottles, coconut shells, automobile tyres and treeholes. Aedes immatures occurred perennially in drums and tree holes, but breeding discontinued in tins, bottles and coconut shells during the driest month of July. For Ae. polynesiensis in Upolu the Breteau and container indices of 104.5 +/- SD 80.9 and 35.3 +/- 12.4 respectively were significantly higher than those in Savai'i: 33.1 +/- 25.0 and 24.3 +/- 20.0 respectively. Likewise for Ae.aegypti the Breteau and container indices of 50.8 +/- 32.5 and 23.9 +/- 15.6, respectively, were also significantly higher than those in Savai'i: 12.7 +/- 17.1 and 9.4 +/- 13.2 respectively. Habitat types greater or lesser importance were determined by plotting the percentage of each type of cotnainer utilized for Aedes breeding against the percentage of ech type amongst all larva-positive containers. Ae.polynesiensis preferred tree-holes but not water-storage drums. Ae.aegypti preferred drums and tyres; mixed populations of larvae of both species were commonest in these two types of habitat. Ae.polynesiensis occurred in every village. Ae.aegypti was encountered in all twenty-two villages surveyed in Upolu and nine of eighteen villages in Savai'i. Total larval surveys revealed that drums and tree-holes contained the highest numbers of Aedes larvae. The study provided criteria for planning a control programme.

Observations on Culex quinquefasciatus Say in relation to transmission of filariasis due to subperiodic Wuchereria bancrofti in Samoa.

Observations on Culex quinquefasciatus Say in Samoa during a study of the epidemiology and control of subperiodic bancroftian filariasis are reported. The man-biting rate of C. quinquefasciatus was comparable with that of Aedes (Finlaya) samoanus Gruenberg in one indicator village and lower in another. The house-frequenting behaviour of the two species was similar. Culex quinquefasciatus was active throughout the night with peak biting during midnight hours, and its survivorship, estimated by two methods, was higher than those of the main vectors, A. (Stegomyia) polynesiensis Marks and A. samoanus. Culex quinquefasciatus showed low susceptibility to subperiodic Wuchereria bancrofti, compared with the two main vectors, in laboratory feeding experiments on volunteers with three levels of microfilaraemia. This agrees with the observed very low natural infection rate, suggesting that C. quinquefasciatus is an inefficient vector of subperiodic W. bancrofti in Samoa.

Distribution of breeding and control of the filariasis vector Aedes samoanus in leaf axils of Pandanus in Samoa.

Water in leaf axils of the screwpine Pandanus was sampled for mosquito immature stages at seventy villages in Upolu, fifty-five in Savai'i and three in Manono, the main islands of Samoa. Ten plants in every patch of Pandanus plantation were sampled at each village. Among 23,049 mosquito larvae collected from Upolu, 77% were the filariasis vector Aedes (Finlaya) samoanus, 17.7% were Ae. (Fin.) oceanicus and 5.3% were Ae. (Fin.) tutuilae. Out of 6981 larvae taken in Savai'i, 23.2% were Ae. samoanus, 67.6% Ae. oceanicus and 9.2% Ae.tutuilae. When larval counts per plant were analysed for each district, Ae. samoanus was found to predominate in Pandanus in Upolu and Ae. oceanicus in Savai'i. However, the adult density of Ae.samoanus was higher in Savai'i and this was attributed to the large areas of forests with Freycinetia for Ae.samoanus breeding. In Pandanus in Savai'i the number of Ae.samoanus was negligible. In Upolu, with more urbanization and larger plantations, there was greater breeding of Ae.samoanus in Pandanus. Two control trials were conducted against Ae.samoanus larvae in Pandanus, one using a sand culture of the parasitic nematode Romanomermis culicivorax and the other with temephos, an organophosphate insecticide. While R. culicivorax did not adapt to the leaf axil habitat, all plants were without larvae for 5 weeks after treatment with temephos.

Natural infections of Wuchereria bancrofti in Aedes (Stegomyia) polynesiensis and Aedes (Finlaya) samoanus in Samoa.

Seven years after the 2nd mass treatment of the population with diethylcarbamazine, transmission of subperiodic Wuchereria bancrofti was studied in four villages in Samoa during one year by means of biting catches of Aedes polynesiensis and A. samoanus. 2 villages were coastal, one inland bush and the other an inland coconut plantation community. Overall infection and infective rates from 6702 Ae. polynesiensis were 0.84 and 0.27% respectively, and the infection rate from 2858 Ae. samoanus, collected in 10-minute catches from 24 sites, was 0.65%. No infective Ae. samoanus was found in these samples. 12-hour all-day catches in the 2 coastal villages confirmed active transmission by Ae. polynesiensis. 12-hour all-night catches in the same 2 villages recorded high transmission by Ae. samoanus although there was little evidence of local breeding. The annual transmission potential for Ae. polynesiensis and Ae. samoanus was high in one of the coastal villages and low in the other. A total of 221 infected Ae. polynesiensis and 40 Ae. samoanus were recorded. Of the 72 infective Ae. polynesiensis, 59.1% contained 1 to 2 larvae each (median density 1.4); 70% of the 40 infected Ae. samoanus had 1 to 2 larvae (median density 1.1). From the proportion of infective Ae. polynesiensis the mean probability of survival was estimated as 0.917.

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.

Filariasis transmission in Samoa. II. Some factors related to the development of microfilariae in the intermediate host.

The developmental period of microfilariae of sub-periodic Wuchereria bancrofti in laboratory-bred Aedes polynesiensis and Ae. samoanus was shorter in the warm season (December to May) than in the cool season (June to November). In the warm season the microfilariae reached the 'sausage' stage in three days, cylindrical second stage in seven days and the infective stage in 12 days after the infecting meal. During the cool season the incubation period was extended to 14 days. Microfilariae persisting in a carrier recently treated with diethylcarbamazine citrate readily infected Ae. polynesiensis and Ae. samoanus and developed into the infective stage, but the proportion which became infected was significantly less than the proportion infected on an untreated carrier having comparable microfilarial density. Under the conditions of the experiments the survival of infected Ae. polynesiensis and Ae. samoanus was dependent on physical conditions. The average level of infective worm burden did not appear to affect the mortality of the vectors.

Filariasis transmission in Samoa. I. Relation between density of microfilariae and larval density in laboratory-bred and wild-caught Aedes (Stegomyia) polynesiensis (Marks) and wild-caught Aedes (Finlaya) samoanus (Gruenberg).

Transmission experiments of laboratory-bred and wild-caught Aedes polynesiensis and wild-caught Ae. samoanus on carriers with different levels of microfilaraemia showed that the percentage of mosquitoes infected, the average number and range of larvae found in each infected mosquito were directly proportional to the microfilarial densities in the carrier at the time of feeding. There was no difference between the results for laboratory-bred and wild-caught mosquitoes. Aedes polynesiensis fed on low carriers (around 20 Mf ml-1 and less) gave an average infection rate of 4.9%. Each infected mosquito harboured only one larva. A plot of the probit of percentage Ae. polynesiensis positive against the logarithm of microfilarial density showed a linear relationship. Using the regression line, at 95% confidence interval, a microfilarial density of 1 ml-1 would give an infection rate of 0.22-2.51%, a density of 0.1 Mf ml-1 an infection rate of 0.01-1.0%. The concentrating capacity of Ae. polynesiensis ranged from 0.70 to 4.74. As microfilaria densities decreased, concentration increased. The microfilarial intake, the subsequent worm burden and concentrating capacity were less in Ae. samoanus than Ae. polynesiensis. There was no evidence of any association between microfilarial density and concentration in Ae. samoanus.