Transmission of malaria is highly heterogeneous and is clustered even in areas of moderate transmission with groups of households, termed hotspots, maintaining high transmission throughout the year. Hotspots provide a reservoir of parasites for mosquitoes that spread the infection outside the immune populations in the wet season. Hotspots are not well defined. In this study we aimed to identify and describe the characteristics of hotspots in a moderate transmission setting in northern Tanzania.
A complete household survey was carried out in 4 villages in Misungwi district, from September to November 2010. Every household was visited and mapped by GPS. From consenting households morbidity, malaria risk and demographic data were collected. All people present were further consented to give a dried blood spot that was stored and later analysed for P. falciparum using a nested PCR. Passive surveillance of fever cases at 3 health facilities were tested with an RDT to confirm malaria. These were treated with an ACT and visited at home in order to establish the GPS position. Cross sectional survey and health facility surveillance data were combined and analysed for clustering. SaTScan software (Kulldorff, 2010) was used to determine clusters of malaria PCR positive individuals by looking at household exposure.
Data was obtained from 11 sub-villages, with the total number of people screened 3,067 (34% of total) coming from 643 Households. Overall PCR detectable parasiteamia was detected in 1,058 (34.5%) individuals. Five hotspots were identified with a median Risk Ratio (RR) of 2.03 (0- 2.78). Odds were double of a passively detected case to come from other people who were parasite +ve (OR 1.94,95%CI 1.55-2.45). The proportion of PCR +ve was higher in HH that had at least one case reported by passive case detection 50% compared to a median of 33%, and odds of a case coming from a hotspots were triple compared to outside a hotspot (OR 3.35,95%CI 1.42-7.89).
This study demonstrates the existence of hotspots and coldspots in an area of moderate malaria transmission. We characterized the households and population of hotspots: they are poorer, more likely to have eaves and thatched roofs, more remote, located nearer to breeding sites. We have also shown that hotspot households appear more immune to malaria on a cross sectional survey suggesting stability over time. Hotspots have more parasitaemia across all ages, mean parasite density is less and sero-prevalence is higher.
Gibson Kibiki (KCMC) & Daniel Chandramohan (LSHTM)