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.

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.

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.

Vectors of dengue fever/dengue haemorrhagic fever with advances in their control.

The most important vector of dengue in the Western Pacific Region is Ae. aegypti. On some occasions, Ae. albopictus, Ae. polynesiensis and several other species of the Ae. scutellaris complex have been suspected vectors in localities where Ae. aegypti was not found. Long-term vector control by the use of insecticides has been expensive and difficult to achieve. However, spray equipment and ULV insecticides have been obtained in many places for emergency use during outbreaks, and Abate larviciding also has been used in high-risk localities to help prevent outbreaks. Obtaining greater community participation supported by intensified health education and other acceptable measures is considered an important approach for achieving improved long-term control. Developing an acceptable and effective cover for domestic water storage containers is also recognized as a potentially important tool for suppressing vector populations.

A multidisciplinary study on bancroftian filariasis in Jakarta.

A study on bancroftian filariasis in Jakarta has indicated that one person in one year could be exposed to 223,000 bites of Culex pipens fatigans and to 1,941 infective-stage larvae of Wuchereria bancrofti. Blood surveys with 20 mm3 samples revealed a microfilaria rate of 6%. Although some cases of hydrocele (4% of 272 males examined) were found, there was little evidence of severe filarial disease in either males or females. Amongst wild-caught mosquitoes only 0.3% contained infective larvae, but much higher levels of vectorial competence were established under laboratory conditions. High daily vector mortality (30%) coupled with noticeable improvements in standards of living could have been important factors preventing an increase in endemicity.

Observations on the breeding habitats of Aedes aegypti (L.) in Jakarta, Indonesia.

A one-year study was done of the breeding habitats of Aedes aegypti (L.) and of the water storage habits of the inhabitants in four crowded districts of urban Jakarta. Immature mosquitoes were found in or near houses in containers of relatively clean water used for drinking or bathing purposes. An average of 185 containers were found per 100 houses, of which 60 were positive for Aedes immatures, resulting in a Container Index of 32%. The mean potential water storage capacity per house was 173 liters of which only 92 liters of water was actually being stored at any one time. Water jars were the most common containers found, but bak mandi (cuboidal or oblong concrete reservoirs) held more water per container. Total water storage per house in bak mandi was twice that of water jars and ten times that of drums, which were uncommon. Ant traps and other miscellaneous containers were unimportant for either water storage or mosquito production. A mean of 0.93 pupae per house was found, pupal production indoors being ten times than outdoors. Indoor water jars produced more pupae per house than all other containers combined. The infestation rate of covered containers was significantly higher than that of uncovered containers, perhaps because loose-fitting lids allowed entrance of gravid females to the attractive darkened interior of the container.

Ecological studies on Culex tritaeniorhynchus as a vector of Japanese encephalitis.

In the Republic of Korea, light-trap assessments and collections from cows and pigs after sunset were highly successful in monitoring the seasonal abundance of C. tritaeniorhynchus, the primary vector of Japanese encephalitis. The dates and duration of the population peaks at semirural suburbs of Seoul and Pusan were markedly different from those at a rural rice-growing site. The main breeding sites in these three areas were, respectively, swamps, marshes, and ricefields. The overall adult densities were the lowest at the rice-growing site where agricultural pesticides were extensively used. A short period of man-vector contact occurred at each study site at low densities when the natural population was at its peak. These vector monitoring techniques, coupled with information on antibodies in swine, represent a simple but valuable surveillance system.

Aerial applications of ultra-low-volume insecticides to control the vector of Japanese encephalitis in Korea.

As a suitable emergency measure to arrest epidemics of Japanese encephalitis in Korea, the ultra-low-volume method of spraying insecticide to control the mosquito vector Culex tritaeniorhynchus has been tested in 2 successive years over a 16-km (2) area, utilizing a large fixed-wing aircraft. Malathion concentrate applied at 0.36 litres/ha gave insufficient control of the parous (infective) females, and no reduction in total numbers of this species. Fenitrothion concentrate applied at 0.45 litres/ha resulted in a 77-87% reduction in total numbers and an 87-98% reduction in parous females over a 4-day period.