SummaryBackground-Malaria, a non-fatal disease if detected promptly and treated properly, still causes many deaths in malaria-endemic countries with limited healthcare facilities. National malaria mortality rates are, however, particularly difficult to assess reliably in such countries, as any fevers reliably diagnosed as malaria are likely therefore to be cured. Hence, most malaria deaths are from undiagnosed malaria, which may be misattributed in retrospective enquiries to other febrile causes of death, or vice-versa.

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Abstract. Eight Indian laboratory stocks of Anopheles stephensi Liston could be grouped into three categories with, respectively, 14–22, 12–17 and 9–15 ridges on the egg‐floats. The mode number of ridges among the eggs laid by individual females in these stocks was 16–19,13‐16 and 10–14, respectively. The category with the highest egg‐float ridge number corresponded with the type‐form and the lowest with var. mysorensis Sweet and Rao; the new egg‐float category with ridge number modes of thirteen to sixteen was designated as ‘intermediate’. All three forms, i.e. type‐form, intermediate and mysorensis were observed in semi‐urban areas while only intermediate and mysorensis were seen in rural areas. Breeding experiments indicated no post‐copulatory barriers between the populations. Likelihood analysis of the results of crosses and back crosses indicated that variation in ridge number is controlled by more than one genetic factor. The stocks with different ridge numbers are best considered as ‘ecological variants’.

A five-year epidemiologic study of patients attending a malaria clinic in Delhi was conducted to find the relapse rate of infections with Plasmodium vivax, its seasonal correlation between the primary infection and subsequent relapses, the duration of the incubation period, and the patterns of relapse. By our definition, the relapse rate ranged from 23% to 44% depending on the duration of follow-up. The relapse pattern observed in the study clearly suggests the existence of both tropical and temperate zone types of P. vivax in the population characterized by distinct incubation periods and the possible existence of P. vivax subpopulations characterized by primary long incubation periods. The implication of different incubating forms of P. vivax on the epidemiology and control of malaria is also discussed. Plasmodium vivax malaria constitutes about 60-65% of total malaria cases in India with, pronounced morbidity particularly in the economically weaker sections of the society. 1 The clinicoepidemiologic picture of P. vivax is not well understood due to the phenomenon of latency/relapse. Due to the persistence of the hepatic or hypnozoite form of the parasite, relapses occur in P. vivax infections and it is difficult to predict their timing. 2 Plasmodium vivax exhibits two primary types of incubation/relapse patterns that apparently depend on their tropical or temperate zone origin. 3-5 The classic example of the tropical type is the Chesson strain (New Guinea-South Pacific), characterized by an early primary attack, followed by a short latent period before appearance of frequent relapses during the next year or more, whereas the St. Elizabeth (United States) strain of the temperate type exhibits an early primary attack, followed by a long latent period of 6-14 months, and thereafter succeeded by a series of relapses at short intervals. The present study is an attempt to understand the composition of P. vivax populations exhibiting different types of incubations in relation to the phenomenon of latency and relapses to elucidate their transmission dynamics for planning vector control strategies and chemotherapeutic measures in P. vivax foci. MATERIALS AND METHODS Study site. The malaria clinic of the Malaria Research Centre, at 2-Nanak Enclave, Delhi is located in northeastern Delhi. The clinic attracts patients mostly from 8-9 periurban villages that are 4-5 km from the clinic and have an area of approximately 25 km 2. The Yamuna River is located approximately 3-4 km from these villages. The inhabitants belong mainly to low socioeconomic strata and are employed in small-scale industries as laborers. The climate of Delhi is divided into three distinct seasons: summer (April-June), monsoon (July-October), and winter (November-March). The average temperature, rainfall, and relative humidity during the three seasons are as follows:

The repellent action of neem oil (extracted from the seeds of Azadirachta indica A. Juss) was evaluated on mosquitoes at two villages near Delhi, India. Kerosene lamps containing neem oil were burned in the living rooms, and mosquitoes resting walls or attracted to human bait were collected inside rooms from 1800 to 0600 h. Neem oil (0.01-1%) mixed in kerosene reduced biting of human volunteers and catches of mosquitoes resting on walls in the rooms. Protection was more pronounced against Anopheles than against Culex. A 1% neem oil-kerosene mixture may provide economical personal protection from mosquito bites.