Spatiotemporal distribution of insecticide resistance in Anopheles culicifacies and Anopheles subpictus in Sri Lanka.

The malaria situation in Sri Lanka worsened during the 1990s with the emergence and spread of resistance to the drugs and insecticides used for control. Chloroquine resistance has increased rapidly over this period, but adverse changes in malaria transmission are more closely associated with insecticide use rather than drug resistance. Insecticide susceptibility tests were routinely carried out in key anopheline vectors across the country for more than a decade. These sentinel data were combined with data collected by other research programmes and used to map the spatial and temporal trends of insecticide resistance in the main vectors, Anopheles culicifacies and A. subpictus, and to examine the relationship between insecticide resistance, changes in national spraying regimens and malaria prevalence. Both species had widespread resistance to malathion, the insecticide of choice in the early 1990s. Both species were initially susceptible to the organophosphate and pyrethroid insecticides used operationally from 1993, but some resistance has now been selected. The levels of malathion and fenitrothion resistance in A. subpictus were higher in some ecological regions than others, which may be related to the distribution of sibling species, agricultural pesticide exposure and/or environmental factors. The study highlights that the emergence and spread of insecticide resistance is a constant threat and that active surveillance systems are vital in identifying key vectors and evidence of resistance.

Insecticide resistance in Anopheles sacharovi Favre in southern Turkey.

We report the resistance to 12 insecticides of specimens of Anopheles sacharovi, both in laboratory cultures and those collected in the malarious areas of Adana, Adiyaman, Antalya, Aydin, and Mugla in southern Turkey. Mortality was higher 24 h after exposure than immediately after exposure but was unaffected by temperature (24 degrees C or 29 degrees C) or the position of the test kit (horizontal or vertical). In Adana, Adiyaman and Antalya, A. sacharovi was susceptible only to malathion and pirimiphos-methyl. In Aydin it was susceptible to both these insecticides as well as to dieldrin, lambda-cyhalothrin, and etofenprox; and in Mugla it was susceptible to dieldrin, fenitrothion, lambda-cyhalothrin, cyfluthrin and etofenprox, as well as to malathion and pirimiphos-methyl.

Method to estimate relative transmission efficiencies of Anopheles species (Diptera: Culicidae) in human malaria transmission.

A mathematical expression was derived to estimate the relative malaria transmission efficiency of an anopheline species with respect to a standard well-characterized species for which all vector parameters can be sufficiently determined. The method is particularly useful in situations where multiple anopheline species contribute to human malaria transmission and requires the estimation of the man-biting rate, the sporozoite rate, and the human malaria incidence. Under stable conditions of vector abundance, the average sporozoite rate in a species during a transmission season would by itself reflect its relative transmission efficiency. This "efficiency" then was used to calculate the "effective human-biting rate"; i.e., the human-biting rate of that species if it were to have ecological properties identical to those of the standard species. The standard well-characterized species then could be used with the effective human-biting rate of all species to quantify transmission, thus overcoming the need to measure vector parameters for all anopheline species contributing to transmission. An expression also was derived to calculate the relative contribution made by each species to malaria transmission. The usefulness of this method was illustrated using entomological and epidemiological data from Kataragama, Sri Lanka.

Clustering of malaria infections within an endemic population: risk of malaria associated with the type of housing construction.

The occurrence of malaria infections due to Plasmodium vivax and P. falciparum was monitored in a population of 3,023 people living in six contiguous villages in Kataragama, an area of endemic malaria in southern Sri Lanka, over a period of 17 months. The annual incidence of malaria in this population during the study period was 25.8%. Malaria attacks were clustered, occurring more frequently than expected in certain individuals and housing groups and less frequently than expected in others. In one of these villages, the distribution of cases was examined in relation to locality and to the type of house construction. There was a strong association between the malaria incidence and house construction, independent of location. The risk of getting malaria was greater for inhabitants of the poorest type of house construction (incomplete, mud, or cadjan (palm) walls, and cadjan thatched roofs) compared to houses with complete brick and plaster walls and tiled roofs. Houses that were better constructed had a significantly lower malaria incidence rate (10.5%) than those that were poorly constructed (21.2%; P less than 0.01, by Student's t-test). There was also a significantly higher number of indoor resting mosquitoes collected from the poorly constructed houses than from those better constructed; the average (geometric mean) of mosquito densities found in houses of better versus poor construction were 0.97 and 1.89 per collection in the dry season, and 1.95 and 3.42 per collection in the wet season, respectively (P less than 0.05 in both seasons). This indicated that the higher malaria risk associated with poorly constructed houses was at least partly due to higher human-mosquito contact among their inhabitants.

A mathematical model for Plasmodium vivax malaria transmission: estimation of the impact of transmission-blocking immunity in an endemic area.

We have developed a multi-state mathematical model to describe the transmission of Plasmodium vivax malaria; the model accommodates variable transmission-blocking/enhancing immunity during the course of a blood infection, a short memory for boosting immunity, and relapses. Using the model, we simulated the incidence of human malaria, sporozoite rates in the vector population, and the level of transmission-blocking immunity for the infected population over a period of time. Field data from an epidemiological study conducted in Kataragama in the south of Sri Lanka were used to test the results obtained. The incidence of malaria during the study period was simulated satisfactorily. The impact of naturally-acquired transmission-blocking immunity on malaria transmission under different vectorial capacities was also simulated. The results show that at low vectorial capacities, e.g., just above the threshold for transmission, the effect of transmission-blocking immunity is very significant; however, the effect is lower at higher vectorial capacities.

Characteristics of malaria transmission in Kataragama, Sri Lanka: a focus for immuno-epidemiological studies.

Parasitological and entomological parameters of malaria transmission were monitored for 17 months in 3,625 residents in a Plasmodium vivax malaria endemic region in southern Sri Lanka; the study area consisted of 7 contiguous villages where routine national malaria control operations were being conducted. Malaria was monitored in every resident; fever patients were screened and 4 periodical mass blood surveys were conducted. An annual malaria incidence rate of 23.1% was reported during the period: 9.3% was due to P. vivax and 13.8% was due to P. falciparum; there had been a recent epidemic of the latter in this region, whereas the P. falciparum incidence rate in the previous 10 years had been negligible. There was a wide seasonal fluctuation in the malaria incidence, with the peak incidence closely following the monsoon rains. The prevalence of malaria due to both species detected at the 4 mass blood surveys ranged from 0.98% (at low transmission) to 2.35% (at peak transmission periods). Adults and children developed acute clinical manifestations of malaria. Entomological measurements confirmed a low degree of endemicity with estimated inoculation rates of 0.0029 and 0.0109 (infectious bites/man/night) for P. vivax and P. falciparum, respectively. Several anopheline species contributed to the transmission, and the overall man biting rates (MBR) showed a marked seasonal variation. Malaria at Kataragama, typical of endemic areas of Sri Lanka, thus presents characteristics of "unstable" transmission. Malaria was clustered in the population. There was a low clinical tolerance to P. falciparum malaria, to which most had only been at risk, compared to P. vivax, to which most had had a life-long exposure.

Pesticide selection pressure on Anopheles subpictus in Sri Lanka: comparison with two other Sri Lankan anophelines.

Adult Anopheles subpictus from Sri Lanka show a broad spectrum of resistance towards organophosphate insecticides but not to carbamates in contrast to the broad resistance to organophosphates and carbamates reported earlier for An. nigerrimus. In both species the frequency of resistance to malathion and fenitrothion increased between 1980 and 1987, despite the ban on agricultural use of these two compounds and the restriction of malathion to indoor residual spraying in malaria control since 1977. In contrast, An. culicifacies shows only low level specific resistance to malathion at a very low frequency. As An. subpictus breeds to a large extent in paddy fields which are highly contaminated by agricultural pesticides, and is highly endophilic, selection for resistance theoretically could occur through both agricultural and anti-malarial pesticide use. However, the anti-malarial use of malathion may have been less important, taking into consideration the low level of resistance of An. culicifacies which is also highly endophilic but breeds to a negligible extent in paddy fields.

Modulation of human malaria transmission by anti-gamete transmission blocking immunity.

Natural Plasmodium vivax malaria infections in man evoke anti-gamete transmission blocking antibodies which influence the infectivity of malaria patients to the vector mosquito. In this study, entomological, immunological and parasitological data obtained through the monitoring of an epidemic of human vivax malaria in Sri Lanka were used in a mathematical simulation to assess the effect of naturally induced transmission blocking immunity on malaria transmission. A mathematical model to describe malaria transmission accounting for transmission blocking immunity was developed from the basic differential equations originally stated by R. Ross and the epidemic was simulated using the available data. An attempt was made to predict the monthly malaria incidence by means of the mathematical simulation, with and without accounting for transmission blocking immunity. A plausible mathematical solution of the epidemic could be obtained when transmission blocking immunity was accounted for, and it was not possible to obtain such a plausible solution in the absence of immunity. Thus, the postulated occurrence of transmission blocking immunity was essential to describe adequately this malaria epidemic, indicating that, at least in epidemic situations, naturally occurring transmission blocking immunity has a controlling influence on malaria incidence.

Pesticide resistance mechanisms produced by field selection pressures on Anopheles nigerrimus and A. culicifacies in Sri Lanka.

In Sri Lanka, Anopheles nigerrimus is resistant to a range of organophosphate and carbamate insecticides at both the larval and adult stages. Biochemical studies indicate that an alteration in acetylcholinesterase is the basis of resistance rather than increased metabolic breakdown of the insecticides. In contrast, A. culicifacies is resistant only to malathion and closely related compounds containing a carboxylate ester bond. Agricultural pesticides are the sole source of selection pressure for resistance in A. nigerrimus, while in A. culicifacies pressure arises predominantly from antimalarial spraying.

Factors affecting selection for multiple resistance in Anopheles nigerrimus in Sri Lanka.

Adult resting habits and breeding habits of Anopheles nigerrimus in Sri Lanka were investigated. This species is primarily exophilic, in contrast to endophilic An. culicifacies in the same areas. An. nigerrimus larvae were found in all habitat types sampled, in particular paddy fields where a large quantity of agricultural pesticides is used. This species was shown to be resistant to a range of insecticides including a number of organophosphates and carbamates. The adult resting behaviour precludes selection of resistance from indoor residual insecticide usage for malaria control and suggests selection from agricultural pesticide usage.

Fenitrothion (OMS 43) resistance in the taxon Anopheles culicifacies Giles.

Two colonies of the taxon Anopheles culicifacies Giles (one from Maharashtra State and one from Gujarat State, India) produced 28 and 64% mortality rates, respectively, on 1% fenitrothion for 2 hours. Six-hour exposures were required to achieve mortalities of greater than or equal to 96% in both colonies.

The nature of malathion resistance in a population of Anopheles culicifacies Giles.

Malathion resistance in a population of Anopheles culicifacies from Maharashtra State in India, which also showed resistance to a number of other organophosphorus compounds, was found to be dominant in its expression. Most of the crossing and back-crossing results involving a susceptible population of the same species from Sri Lanka indicated the possible involvement of more than one genetic factor. The existence of such a broad spectrum of resistance, and experiments involving the use of the synergists triphenyl phosphate and piperonyl butoxide both suggest the presence of at least two mechanisms, one involving the specific carboxylesterase and the other the less specific mixed-function oxidase system.

Ovarian development and biting frequency in Anopheles culicifacies Giles in Sri Lanka.

In order to determine the duration of the gonotrophic cycle in Anopheles culicifacies Giles in Sri Lanka, ovarian development in freshly blood-fed wild-caught females was studied in the laboratory and the findings were compared with mark-and-release recoveries in the field. In specimens of unknown parity at first capture probable feeding intervals of 1, 2 and 3 days (mean: 1.7 - 2.1 days) were found. The release of freshly blood-fed nullipars bred from larvae produced two recaptures from which it could be concluded that the time required to become parous need not be more than three days. These results are of consequence in the assessment of the efficacy of insecticide spraying as an anti-malarial measure in Sri Lanka and possibly in other countries in South Asia where A. culicifacies is the vector. An equation is proposed for deriving the mosquito's daily survival rate (p) from the proportion of parous females in biting samples.

Recent developments of insecticide resistance in some Turkish anophelines.

Susceptibility tests carried out since 1974 in both the field and the laboratory with Anopheles sacharovi, A. hyrcanus and A. maculipennis from Turkey have revealed resistance to organophosphate and carbamate insecticides in all three species, in the first two in the southern Chukurova plain and in the third in the northern area around Osmanjik. Resistance was indicated by significant survivals when exposed to dosages killing presumed susceptible strains of these and other anopheline species. The temperature at which the susceptibility tests were carried out had a marked influence on mortalities. Broad spectra of resistance were shown by all three species, though with interspecific differences, and are attributed to the extensive and intensive use of a wide range of chemicals for agricultural pest control. Of the insecticides readily available for house-spraying for malaria control, malathion would appear to be the one with the greatest potential usefulness.