Resistance of early midgut stages of natural Plasmodium falciparum parasites to high temperatures in experimentally infected Anopheles gambiae (Diptera: Culicidae).

We studied the effects of high temperature, 30 and 32 versus 27 C on early Plasmodium falciparum development in Anopheles gambiae experimentally infected with gametocytes from 30 volunteers with mean density of 264.1 gametocytes/microl blood (range: 16-1,536/microl). From several batches of mosquitoes, fed by membrane feeding, midguts of individual mosquitoes were dissected at 24 hr for ookinete enumeration and at 7 days to quantify oocysts. There were temperature-related differences in mean ookinete intensity per mosquito midgut, with 9.71 +/- 1.6 at 27 C, 9.85 +/- 2.32 at 30 C, and 3.89 +/- 0.81 at 32 C. The prevalence of oocyst infection decreased with an increase in temperatures from 15.9 to 8.5 to 6.4% at 27, 30, and 32 C, respectively. The average oocyst intensities for the infected mosquitoes increased with temperatures from 2.9 at 27 C to 3.5 at 30 C, and to 3.3 at 32 C. However, the success of infections was reduced at 30 and 32 C, and resulted in greater losses during consecutive inter-stage parasite development. The most significant impact of high temperatures occurred at the transition between macrogametocytes and ookinetes, whereas the transition between ookinetes and oocysts apparently was not affected. In contrast to other reports, exposure of mosquitoes infected with natural parasites to high temperatures did not eliminate preoocyst stages, as has been observed from laboratory studies using the NF-54 strain of P. falciparum. This observation of parasite resistance to high temperatures is consistent with the natural situation in tropical environments where perennial malaria transmission occurs during hot dry seasons.

The development of Plasmodium falciparum in experimentally infected Anopheles gambiae (Diptera: Culicidae) under ambient microhabitat temperature in western Kenya.

The effect of microhabitat temperature variation on the early development of Plasmodium falciparum in experimentally infected Anopheles gambiae s.s. (Diptera: Culicidae) was studied. Batches of mosquitoes were fed artificially on gametocyteamic blood obtained from human volunteers and then held in five environmental conditions described as: (1) incubator maintained at constant temperature of 28 +/- 1 degrees C as control; (2) temperature unregulated laboratory environment; (3) screen house; (4) grass thatched mud house and (5) corrugated iron roofed mud house. Both the grass and iron roofed mud houses were real houses found in the village communities around the ICIPE Research Centre in Mbita Point, Suba District south-western Kenya. The temperature and relative humidity of these holding environments were recorded over the study period. Mosquitoes were dissected after 24 h and 7 days to enumerate ookinetes and oocysts stages, respectively in their midguts. The mean temperature observed in the temperature-unregulated laboratory (28 degrees C) was significantly higher than the temperature of the screen house (24 degrees C) while the mean temperature observed in the iron roof mud house (27 degrees C) was comparable with that in the grass-thatched mud house (27 degrees C) although the iron roof house experienced more variation (coefficient of variation, C.V., = 9.6%) and higher peaking temperatures than the grass-thatch house. The mean relative humidity for the laboratory and screen house were 23% and 32.5%, respectively, much lower than relative humidity in the incubator (73%). Relative humidity of the grass thatch hut (42%) and Iron roof hut (51%) were also lower than those of the incubator. The ookinete intensities for mosquitoes in the screen house (10.11 +/- 1.79 ookinetes/midgut) were not statistically different (P = 0.41) from those held in the laboratory (7.50 +/- 1.19 ookinetes/midgut) or in the incubator (9.89 +/- 1.47 ookinetes/midgut). The holding environments influenced the oocyst infection rates (P = 0.04) that increased from 8.4% in the screen house to 10.2% in the laboratory. The highest infection rate (12.5%) was observed in mosquitoes held in the incubator. However, the mean oocyst intensities in mosquitoes did not differ under these environments (P = 0.58). In the 'real village house' environments, the mean ookinete intensities were not statistically different between groups of mosquitoes compared to the incubator (P = 0.86). The oocyst infection rates observed in the highly fluctuating iron roof house were 9.4% as compared to 9.0% and 6.9% in the more stable and constant habitats of grass thatch house and incubator, respectively. Results show that the natural microhabitats did not influence the infections rates in mosquitoes (P = 0.62). These findings indicate that the variation in temperatures prevailing in western Kenya particularly inside the village houses do not impede the development of malaria parasites in A. gambiae mosquitoes.

Comparative field evaluation of the Mbita trap, the Centers for Disease Control light trap, and the human landing catch for sampling of malaria vectors in western Kenya.

The mosquito sampling efficiency of a new bed net trap (the Mbita trap) was compared with that of the Centers for Disease Control miniature light trap (hung adjacent to an occupied bed net) and the human landing catch in western Kenya. Overall, the Mbita trap caught 48.7 +/- 4.8% (mean +/- SEM) the number of Anopheles gambiae Giles sensu lato caught in the human landing catch and 27.4 +/- 8.2% of the number caught by the light trap. The corresponding figures for Anopheles funestus Giles were 74.6 +/- 1.3% and 39.2 +/- 1.9%, respectively. Despite the clear differences in the numbers of mosquitoes caught by each method, both the Mbita trap and light trap catches were directly proportional to human landing catches regardless of mosquito density. No significant differences in parity or sporozoite incidence were observed between mosquitoes caught by the three methods for either An. gambiae s.l. or An. funestus. Identification of the sibling species of the An. gambiae complex by a polymerase chain reaction indicated that the ratio of An. gambiae Giles sensu stricto to An. arabiensis Patton did not vary according to the sampling method used. It is concluded that the Mbita trap is a promising tool for sampling malaria vector populations since its catch can be readily converted into equivalent human biting catch, it can be applied more intensively, it requires neither expensive equipment nor skilled personnel, and it samples mosquitoes in an exposure-free manner. Such intensive sampling capability will allow cost-effective surveillance of malaria transmission at much finer spatial and temporal resolution than has been previously possible.