Research Article: The Risk of Dengue Virus Transmission in Dar es Salaam, Tanzania during an Epidemic Period of 2014

Date Published: January 26, 2016

Publisher: Public Library of Science

Author(s): Leonard E. G. Mboera, Clement N. Mweya, Susan F. Rumisha, Patrick K. Tungu, Grades Stanley, Mariam R. Makange, Gerald Misinzo, Pasquale De Nardo, Francesco Vairo, Ndekya M. Oriyo, Ken E. Olson.

Abstract: BackgroundIn 2010, 2012, 2013 and 2014 dengue outbreaks have been reported in Dar es Salaam, Tanzania. However, there is no comprehensive data on the risk of transmission of dengue in the country. The objective of this study was to assess the risk of transmission of dengue in Dar es Salaam during the 2014 epidemic.Methodology/Principal FindingsThis cross-sectional study was conducted in Dar es Salaam, Tanzania during the dengue outbreak of 2014. The study involved Ilala, Kinondoni and Temeke districts. Adult mosquitoes were collected using carbon dioxide-propane powered Mosquito Magnet Liberty Plus traps. In each household compound, water-holding containers were examined for mosquito larvae and pupae. Dengue virus infection of mosquitoes was determined using real-time reverse transcription polymerase chain reaction (qRT-PCR). Partial amplification and sequencing of dengue virus genome in infected mosquitoes was performed. A total of 1,000 adult mosquitoes were collected. Over half (59.9%) of the adult mosquitoes were collected in Kinondoni. Aedes aegypti accounted for 17.2% of the mosquitoes of which 90.6% were from Kinondoni. Of a total of 796 houses inspected, 38.3% had water-holding containers in their premises. Kinondoni had the largest proportion of water-holding containers (57.7%), followed by Temeke (31.4%) and Ilala (23.4%). The most common breeding containers for the Aedes mosquitoes were discarded plastic containers and tires. High Aedes infestation indices were observed for all districts and sites, with a house index of 18.1% in Ilala, 25.5% in Temeke and 35.3% in Kinondoni. The respective container indices were 77.4%, 65.2% and 80.2%. Of the reared larvae and pupae, 5,250 adult mosquitoes emerged, of which 61.9% were Ae. aegypti. Overall, 27 (8.18) of the 330 pools of Ae. aegypti were positive for dengue virus. On average, the overall maximum likelihood estimate (MLE) indicates pooled infection rate of 8.49 per 1,000 mosquitoes (95%CI = 5.72–12.16). There was no significant difference in pooled infection rates between the districts. Dengue viruses in the tested mosquitoes clustered into serotype 2 cosmopolitan genotype.Conclusions/SignificanceAe. aegypti is the main vector of dengue in Dar es Salaam and breeds mainly in medium size plastic containers and tires. The Aedes house indices were high, indicating that the three districts were at high risk of dengue transmission. The 2014 dengue outbreak was caused by Dengue virus serotype 2. The high mosquito larval and pupal indices in the area require intensification of vector surveillance along with source reduction and health education.

Partial Text: Dengue is one of the most important mosquito-borne viral diseases in the tropics and subtropics. Although the exact global burden of dengue cases is unknown, recent estimates indicate that 390 million dengue infections occur annually. Of these, 96 million cases manifests clinically [1] with about half a million cases of dengue haemorrhagic fever requiring hospitalization [2]. Dengue is prevalent in Africa, though rarely reported. Statistics indicate that the disease has increased dramatically since 1980, with epidemics occurring in both eastern and western Africa [3–5]. The World Health Organization statistics indicate that 2.4% of the global dengue haemorrhagic fever cases occur in Africa and one-fifth of the population in the continent is at risk [6].

In this study, Aedes aegypti accounted for about one fifth of the man-biting mosquitoes caught in Dar es Salaam. These findings provide the most up-date of data on Ae. aegypti in the region since 1980s. Over one-third of the inspected house premises had water-holding containers positive for larvae or pupae of Aedes mosquitoes. The most common breeding containers for Ae. aegypti in Dar es Salaam were discarded plastic containers and tires. Kinondoni district accounted for over half of the Aedes positive water holding containers. Most of these water holding containers are man-made, with only a few natural breeding sites identified. Previous studies in Tanzania have shown that in most places Ae. aegypti breeds in both artificial and natural sites [14,20, 21]. Similar to our findings, a study by Trpis [14] reported that tires, wrecked motor cars, water-pots, coconut shells, snail shells and tins were the most productive containers in some areas of the City during the early 1970s. It has already been shown that a part of the Ae. aegypti population in East Africa is maintained in some biotypes such as automobile dumps and coral rock holes by continuous breeding [14,20]. Discarded tires and animal watering pans have been reported to be the two most common larval breeding sites elsewhere [22].



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