Studies on the spatial distribution of anopheline mosquito larvae were conducted in 302 villages over two transmission seasons in Eritrea. Additional longitudinal studies were also conducted at eight villages over a 24-mo period to determine the seasonal variation in anopheline larval densities. Eight anopheline species were identified with Anopheles arabiensis predominating in most of the habitats. Other species collected included: An. cinereus, An. pretoriensis, An. d'thali, An. funestus, An. squamosus, An. adenensis, and An. demeilloni. An. arabiensis was found in five of the six aquatic habitats found positive for anopheline larvae during the survey. Anopheles larvae were sampled predominantly from stream edges and streambed pools, with samples from this habitat type representing 91.2% (n = 9481) of the total anopheline larval collection in the spatial distribution survey. Other important anopheline habitats included rain pools, ponds, dams, swamps, and drainage channels at communal water supply points. Anopheline larvae were abundant in habitats that were shallow, slow flowing and had clear water. The presence of vegetation, intensity of shade, and permanence of aquatic habitats were not significant determinants of larval distribution and abundance. Larval density was positively correlated with water temperature. Larval abundance increased during the wet season and decreased in the dry season but the timing of peak densities was variable among habitat types and zones. Anopheline larvae were collected all year round with the dry season larval production restricted mainly to artificial aquatic habitats such as drainage channels at communal water supply points. This study provides important information on seasonal patterns of anopheline larval production and larval habitat diversity on a countrywide scale that will be useful in guiding larval control operations in Eritrea.

Summaryobjective To assess the effectiveness of impregnated mosquito nets, indoor residual spraying and larval control relative to the impacts of climate variability in the decline of malaria cases in Eritrea.methods Monthly data on clinical malaria cases by subzoba (district) in three zobas (zones) of Eritrea for 1998-2003 were used in Poisson regression models to determine whether there is statistical evidence for reduction in cases by DDT, malathion, impregnated nets and larval control used over the period, while analysing the effects of satellite-derived climate variables in the same geographic areas.results Both indoor residual spraying (with DDT or malathion) and impregnated nets were independently and significantly negatively associated with reduction in cases, as was larval control in one zoba. Malaria cases were significantly positively related to differences in current and previous months' vegetation (NDVI) anomalies. The relationship to rainfall differences 2 and 3 months previously was also significant, but the direction of the effect varied by zoba. Standardized regression coefficients indicated a greater effect of climate in the zoba with less intense malaria transmission.conclusion The results support the view that both indoor residual spraying and impregnated nets have been independently effective against malaria, and that larval control was also effective in one area. Thus climate, while significant, is not the only explanation for the recent decline in malaria cases in Eritrea. If appropriate statistical approaches are used, routine surveillance data from cases attending health facilities can be useful for assessing control programme success and providing estimates of the effectiveness of individual control measures. Effectiveness estimates suitable for use in cost-effectiveness analysis have been obtained.

Eritrea has a successful malaria control program, but it is still susceptible to devastating malaria epidemics. Monthly data on clinical malaria cases from 242 health facilities in 58 subzobas (districts) of Eritrea from 1996 to 2003 were used in a novel stratification process using principal component analysis and nonhierarchical clustering to define five areas with distinct malaria intensity and seasonality patterns, to guide future interventions and development of an epidemic early warning system. Relationships between monthly clinical malaria incidence by subzoba and monthly climate data from several sources, and with seasonal climate forecasts, were investigated. Remotely sensed climate data were averaged over the same subzoba geographic administrative units as the malaria cases. Although correlation was good between malaria anomalies and actual rainfall from ground stations (lagged by 2 months), the stations did not have sufficiently even coverage to be widely useful. Satellite derived rainfall from the Climate Prediction Center Merged Analysis of Precipitation was correlated with malaria incidence anomalies, with a lead time of 2-3 months. NDVI anomalies were highly correlated with malaria incidence anomalies, particularly in the semi-arid north of the country and along the northern Red Sea coast, which is a highly epidemic-prone area. Eritrea has 2 distinct rainy seasons in different parts of the country. The seasonal forecasting skill from Global Circulation Models for the June/July/August season was low except for the Eastern border. For the coastal October/November/December season, forecasting skill was good only during the 1997-1998 El Niño event. For epidemic control, shorter-range warning based on remotely sensed rainfall estimates and an enhanced epidemic early-detection system based on data derived for this study are needed.

A parasitological cross-sectional survey was undertaken from September 2000 through February 2001 to estimate the prevalence of malaria parasitemia in Eritrea. A total of 12,937 individuals from 176 villages were screened for both Plasmodium falciparum and Plasmodium vivax parasite species using the OptiMal Rapid Diagnostic Test. Malaria prevalence was generally low but highly focal and variable with the proportion of parasitemia at 2.2% (range: 0.4% to 6.5%). Despite no significant differences in age or sex-specific prevalence rates, 7% of households accounted for the positive cases and 90% of these were P. falciparum. Multivariate regression analyses revealed that mud walls were positively associated with malaria infection (OR [odds ratio] = 1.6 [95% CI: 1.2, 2.2], P < 0.008). For countries with low and seasonal malaria transmission, such information can help programs design improved strategic interventions.

Abstract. Entomologic studies were conducted in eight villages to investigate the patterns of malaria transmission in different ecologic zones in Eritrea. Mosquito collections were conducted for 24 months between September 1999 and January 2002. The biting rates of Anopheles arabiensis were highly seasonal, with activity concentrated in the wet season between June and October in the highlands and western lowlands, and between December and March in the coastal region. The biting rates in the western lowlands were twice as high as in the western escarpment and 20 times higher than in the coastal region. Sporozoite rates were not significantly different among villages. The risk of infection ranged from zero on the coast to 70.6 infective bites per year in the western lowlands. The number of days it would take for an individual to receive an infective bite from an infected An. arabiensis was variable among villages (range ‫ס‬ 2.8-203.1 days). The data revealed the presence of only one main malaria transmission period between July and October for the highlands and western lowlands. Peak inoculation rates were recorded in August and September (range ‫ס‬ 0.29-43.6 infective bits/person/month) at all sites over the two-year period. The annual entomologic inoculation rates (EIRs) varied greatly depending on year. The EIR profiles indicated that the risk of exposure to infected mosquitoes is highly heterogeneous and seasonal, with high inoculation rates during the rainy season, and with little or no transmission during the dry season. This study demonstrates the need to generate spatial and temporal data on transmission intensity on smaller scales to guide targeted control of malaria operations in semi-arid regions. Furthermore, EIR estimates derived in the present study provide a means of quantifying levels of exposure to infected mosquitoes in different regions of the country and could be important for evaluating the efficacy of vector control measures, since Eritrea has made significant steps in reducing the burden of malaria based on the Roll Back Malaria initiative of the World Health Organization.