The buzzing crisis: Why mosquito numbers are rising

By  Hellen Shikanda

Health and Science Reporter

Nation Media Group

You may have noticed more mosquitoes lately. They are not the shy type. They do not care about you trying to swat them. They perch on one part of your body, get hit, and simply move to the next. Their bites leave you scratching for a long time. Their buzzing is irritating, even during the day.

On social media, different theories have emerged about what could be causing the surge.

But let us start with the basics. Is there really a surge?

Dr Eric Ochomo, an entomologist (a scientist who studies insects) at the Kenya Medical Research Institute (Kemri), based at the Centre for Global Health Research, says they have noticed an increase in mosquito numbers. Kemri scientists run active surveillance of mosquitoes across about 60 sites in the country.

If you are in Nairobi, for instance, there are two main types of mosquitoes currently giving you a hard time, especially if you do not have a repellent. They are the Aedes and Culex mosquitoes. What scientists know so far is that these two species, at least in Nairobi, do not transmit any disease.

Dr Ochomo explains that the Aedes mosquito appears mostly during the daytime. They are bigger in size, black and white, and therefore easy to notice from a distance. Their bites are quite painful, and even after biting, they do not fly far away.

"They are lazy mosquitoes and can be quite stubborn," he tells the Nation.

He explains that they breed well in open water tanks, old tyres, shallow containers and open sewers.

The Culex mosquitoes, on the other hand, are loud and come out mostly at night. They are also easy to notice because of their large size.

The malaria-causing mosquitoes (Anopheles), which are not yet being recorded in Nairobi, are discreet and tiny.

Dr Ochomo explains that high temperatures in the country could be driving the surge in mosquito numbers, and the situation could worsen with climate change, which pushes temperatures even higher over time. Mosquitoes, he says, thrive in warm conditions.

However, he says the main driver of the increase is a sanitation problem.

"When you don't drain water and it is nice and warm, mosquitoes find a breeding ground," he says.

A mosquito matures from egg to larva to adult within just seven days. Mosquitoes lay large numbers of eggs, up to 500 at a time. If the temperature is warm, breeding accelerates further.

"Imagine if all those eggs become adults. They will be everywhere," he says.

Breeding grounds

He says counties need to grasp the situation now and deal with the sanitation problem first, because climate change is already happening.

In Nairobi, a number of high-rise buildings are going up to meet the demand for housing as people move from rural areas to urban centres. On construction sites, temporary soak pits are used, and Dr Ochomo explains that as long as that water is there and it is stagnant, mosquitoes are likely to breed.

He also says older sewerage systems that result in overflows should be upgraded to match the current population, to prevent the pooling of dirty water.

This is not just a Nairobi problem. Over the past 10 years, the burden of mosquitoes has also increased in Turkana County.

"Turkana is traditionally very hot. In the last couple of years, there has been development in that county and boreholes are being sunk. The challenge is that the wastewater is not managed very well and the boreholes end up being perfect breeding grounds for mosquitoes," he explains.

As a result, scientists have started seeing an increase in malaria transmission in Turkana County over time.

In places like coastal Kenya and parts of northern Kenya, Aedes mosquitoes – the same type currently present in Nairobi – are known to transmit diseases such as dengue and chikungunya.

"In Nairobi, we haven't seen that," he assures.

If you have an open container with water at your home, car tyres, water tanks that are left open, or even an empty bottle, Dr Ochomo advises you to drain them and ensure they are dry.

A study published last year in the scientific journal BMC also found that future urbanisation in Africa will likely increase Aedes mosquito populations, and that climate change will exacerbate the situation. The study investigated whether urbanisation can influence disease vectors by changing their larval habitat, microclimates and host abundance.

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