Academic journal article Bulletin of the World Health Organization

Anopheles Minimus: Its Bionomics and Role in the Transmission of Malaria in Assam, India

Academic journal article Bulletin of the World Health Organization

Anopheles Minimus: Its Bionomics and Role in the Transmission of Malaria in Assam, India

Article excerpt

Introduction

Anopheles minimus used to be the most important xector of malaria in India along the foothills of the Himalayas from Uttar Pradesh (Terai region) to the north-eastern region of the country (1-5). However, following the application of DDT and other residual insecticides by the national malaria control/eradication programme, A. minimus disappeared from Uttar Pradesh (6, 7), and was believed to have also from the north-eastern region (8-10). Subsequently, the role played in malaria transmission by other vectors, i.e., A. philippinensis and A. balabacensis, was reported (9-11). However, because of persistent transmission of malaria in certain pockets in the north-eastern region, studies were initiated under the Plasmodiiim falciparum containment programme to identify the vectors and their relative importance. As a result, A. minimus was re-recorded in the foothills of the following states: Nagaland (12), Mizoram (13), Arunachal Pradesh (14), and Assam (15, 16). A. minimus was also recorded and identified as a malaria vector in Sonapur PHC, district of Kamrup, Assam (17). As a follow-up to these preliminary observations, additional data have been collected de novo on the seasonal prevalence, sporozoite rates, feeding behaviour, and breeding habitats of A. minimus in order to design appropriate malaria containment methods using an integrated disease vector control approach. At the same time the bionomics of A. minimus were studied and data were collected on malaria incidence.

Materials and methods

Study area

Sonapur PHC (altitude, 0-150m) is located 25km east of Gwahati on the south bank of the Brahmaputra river, adjacent to the border with Meghalaya State. The area is predominantly tribal, low-lying and prone to flooding, and most of the villages are located in the foothills. There are paddy-fields in the plains interspersed with criss-crossing perennial streams. The primary occupation of the inhabitants is paddy-field cultivation (jhoom), and most of the people live under poor socioeconomic conditions. Typically, the houses consist of two or three rooms and are made of bamboo with thatched roofs; often there is a cattle shed close to each house. Rainfall in the area is heavy (1400-2200mm per annum) with premonsoon showers in April-May, and the maximum precipitation occurs in the period July-September. The relative humidity varies from 60% to 85%. Temperatures lie in the range 10-26[degrees]C in winter (November-February) and 23-33[degrees]C throughout the rest of the year.

Methods

Malaria incidence

To determine the morbidity from malaria, we carried out active fever surveillance at weekly intervals in 50 villages within Sonapur PHC over the period 1989-91. Blood smears were taken from all fever cases and examined for malaria parasites. Cases found to be positive were given antimalarial drugs in accordance with the policy of the national malaria eradication programme.

Day-resting collections

To identify the day-resting anopheline species and their relative proportions, collections were made in human dwellings (indoor) from 09:00 to 12:00 over the period October 1989 to September 1990 for a total of 311 man-hours. With the aid of a torch and a suction tube the insects were collected while they were resting on walls, clothes, and other articles within the houses. Subsequently, they were indentified using the pictorial keys produced by Wattal & Kalra (18) and graded for their abdominal condition and resting behaviour. The seasonal prevalences and man-hour densities of indoor resting A. minimus and A. fluviatilis were also determined.

Vector identification and host preferences

The anophelines collected indoors from human dwellings during day-resting catches were dissected in normal saline solution (0.9%) to detect any sporozoites. Data were collected for the period July 1989 to September 1990 to determine the seasonal infection rates. …

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