The importance of drains for the larval development of lymphatic filariasis and malaria vectors in Dar es Salaam, United Republic of Tanzania

Abstract

Background: Dar es Salaam has an extensive drain network, mostly with inadequate water flow, blocked by waste, causing flooding after rainfall. The presence of Anopheles and Culex larvae is common, which is likely to impact the transmission of lymphatic filariasis and malaria by the resulting adult mosquito populations. However, the importance of drains as larval habitats remains unknown.

Methodology: Data on mosquito larval habitats routinely collected by the Urban Malaria Control Program (UMCP) and a special drain survey conducted in 2006 were used to obtain a typology of habitats. Focusing on drains, logistic regression was used to evaluate potential factors impacting the presence of mosquito larvae. Spatial variation in the proportion of habitats that contained larvae was assessed through the local Moran's I indicator of spatial association.

Principal findings: More than 70% of larval habitats in Dar es Salaam were human-made. Aquatic habitats associated with agriculture had the highest proportion of Anopheles larvae presence and the second highest of Culex larvae presence. However, the majority of aquatic habitats were drains (42%), and therefore, 43% (1,364/3,149) of all culicine and 33% (320/976) of all anopheline positive habitats were drains. Compared with drains where water was flowing at normal velocity, the odds of finding Anopheles and Culex larvae were 8.8 and 6.3 (p<0.001) times larger, respectively, in drains with stagnant water. There was a positive association between vegetation and the presence of mosquito larvae (p<0.001). The proportion of habitats with mosquito larvae was spatially correlated.

Conclusion: Restoring and maintaining drains in Dar es Salaam has the potential to eliminate more than 40% of all potential mosquito larval habitats that are currently treated with larvicides by the UMCP. The importance of human-made larval habitats for both lymphatic filariasis and malaria vectors underscores the need for a synergy between on-going control efforts of those diseases.

Figure 1. Study area and administrative units in Dar es Salaam, Tanzania.

Administratively, Dar es Salaam comprises three municipalities – Ilala, Kinondoni and Temeke – and is divided into 73 wards (22 in Ilala, 27 in Kinondoni, and 24 in Temeke), classified by the Tanzania National Bureau of Statistics (NBS) as urban, rural or mixed. The wards are further divided into smaller areal units called mitaa, which are subdivided into ten-cell units (TCU), the smallest administrative unit in the city. The map highlights the 15 wards included in the study (5 in each municipality), which comprise the targeted area for an urban malaria control effort.

Figure 2. Sketches of drain segments that compose one unique surveyed drain.

The Figure shows two hypothetical drains, indicating how different segments were identified and surveyed during the Drain Assessment Survey. A. The drain follows the same linear direction throughout its entire extension, but five drain segments are identified: three are open (AB, CD, and EF) and two are covered – car and/or pedestrian passage (BC and DE). B. The drain follows local areal characteristics, and each change in direction corresponds to a unique drain segment.

Figure 3. Percentage distribution of aquatic habitats that contained larvae by habitat type, 2005–2007.

Data on aquatic habitats containing Anopheles or Culex larvae were retrieved from the Urban Malaria Control Program (UMCP) routine larval habitat survey.

Figure 4. Monthly distribution of rainfall and percentage of drains and puddles that contained larvae, 2005–2007.

Data on aquatic habitats by type were retrieved from the Urban Malaria Control Program (UMCP) routine larval habitat survey. Rainfall information was provided by the Tanzania Meteorological Agency, and was lagged by 1-month. Therefore, mosquito larval presence in a month is compared with the amount of rainfall in the previous month.

Figure 5. Clustering pattern in the proportion of aquatic habitats classified as drains that contained larvae, 2005–2007.

Clusters in the proportion of aquatic habitats that contained larvae, utilizing the ten-cell unit (TCU) as the spatial unit of analysis, were assessed through the use of local Moran's I indicator of spatial association, with a first order queen neighborhood. Cluster significance was determined based on a normal distribution and corrected for multiple comparisons utilizing the false discovery rate procedure (as described in Data and Methods).