‘A successful fight against the dengue virus must begin with the water supply.’
Early in the morning, the port of Dar es Salaam in Tanzania resembles a hornet’s nest. Colourful containers are loaded onto cargo ships from heavy metal bridges, as if stacking Lego bricks. Forklift trucks drive crates from all over the world criss-crossing the site. A deafening noise emanates from a factory hall. The ‘port of peace’, as Dar es Salaam translates, is an important centre for global goods flows. As in other African countries, the Chinese have recently invested in the expansion of the port infrastructure here. They are interested in the copper deposits in neighbouring Zambia. The valuable metal rattles along the rails to the port of Dar es Salaam, is shipped to China and used in electric cars.
It doesn’t take much imagination to picture a stowaway arriving here in the early 1950s, silently but dangerously: the yellow fever mosquito (Aedes aegypti). It transmits various viruses to humans, including the dangerous dengue virus. Researchers such as Jeffrey R. Powell of the Yale School of
Medicine believe that the mosquito was transported to the New World around 1500 from what is now Angola in West Africa on Portuguese slave ships. Three hundred years later, Aedes aegypti also conquered the Mediterranean region by sea, spreading to Asia, the South Pacific and Australia at
the end of the 19th century.11
The economic metropolis of Dar es Salaam recorded its first dengue outbreak in 2010. Since then, hardly a year has passed without the dengue virus claiming numerous victims among the city’s population. ‘The dengue virus will become the biggest health threat in the Tanzanian metropolis over the next 20 years,’ says Tanzanian Frank Tenywa. The researcher at the Ifakara Health Institute (IHI) has been studying the spread of the mosquito in the city for the past three years. Around 180,000 mosquitoes were caught in his traps, 5 % of which belonged to the Aedes aegypti species. ‘The yellow fever mosquito has adapted wonderfully to an urban environment thanks to its long period of co-evolution with humans,’ says Tenywa. Indeed, the mosquito is extremely undemanding. It only needs a small amount of water to breed: a car tyre filled with water, a flower pot, a sad knothole.
Urban health authorities are struggling to keep up with the spread of the mosquito and the transmission of the dengue virus. Unlike the malaria mosquito, the yellow fever mosquito also bites during the day. The approximately 8 million people of Dar es Salaam cannot escape the infectious
bites of the yellow fever mosquito. And even common insecticides are ineffective against this insect. Scientific work by Frank Tenywa and the Ifakara Health Institute in particular shows that Aedes aegypti is resistant to conventional insecticides such as pyrethroids and chlorine compounds.
However, the real reason for the spread of the dengue virus is not only the mosquito’s ingenious ability to adapt to the urban environment. It is simply the poverty of many residents of Dar es Salaam.
Viewed from the famous bird’s-eye perspective, the metropolis appears as an endless patchwork quilt of corrugated iron, concrete and brick, interspersed with rural-looking green spaces. Here and there, a minaret stretches upwards. The population boom has been unabated for decades. Alongside Luanda (Angola), Yaoundé (Cameroon) and Kumasi (Ghana), Dar es Salaam is one of the fastest-growing cities on the continent. Attracted by the hope of a better life, the city is a magnet for Tanzania’s impoverished rural population. The urban infrastructure can hardly keep up with the influx of people. The roads are hopelessly congested, as is the sewage system, and constant access to clean drinking water is a privilege of the wealthy.
‘A successful fight against the dengue virus must begin with the water supply,’ says Frank Tenywa. In many neighbourhoods of the city, drinking water only flows sporadically from the water pipes. When this fortunate circumstance occurs, people fill their drinking water containers and leave them open in the kitchen: an ideal breeding ground for Aedes aegypti! The open drinking water containers in households have sometimes led to the dengue mosquito no longer being active only during the rainy season, but throughout the year. Frank Tenywa therefore advocates the sale of sealable water containers and an awareness campaign for the residents of Dar es Salaam. As a scientist, he also advises the political authorities, particularly on the control of the dengue virus. ‘With the increase in dengue outbreaks, politicians have also become more attentive in recent years,’ he says. This is because the virus is not only harmful to the health of Dar es Salaam’s inhabitants, but also poisonous for tourism.
Controlling the spread of the mosquito is a race against time. The ‘Qdenga’ vaccine currently available in Switzerland is intended for travellers who have already contracted the virus. It is not intended for the local population in Africa. In addition, climate change – in the form of torrential rainfall and flooding – is benefiting the mosquito in the long term. ‘More investment is therefore needed in research, in better surveillance systems for the virus and in improving the general living conditions of the affected population,’ says Frank Tenywa.