Rising temperatures elevate the risk to Nairobi's malaria-free status

“There is no local transmission of malaria in Nairobi. If you have been a full time resident of Nairobi and you get sick with fever the probability that it is malaria is close to zero,” says Timothy Kibe, the Head of Malaria Unit for Nairobi County.

However, scientists fear that Nairobi ‘safe haven’ status against malaria is about to be challenged by a formidable enemy.

The disease remains as deadly as ever.

In Kenya, deaths from malaria have fluctuated from highs of 26,017 in 2010 to lows of 360 in 2013: shooting back up to 15,061 in 2015 and dwindling back to 219 in 2019.

Kenya has made remarkable strides in reducing malaria prevalence and incidence, resulting in a prevalence decline from 11 per cent in 2010 to six per cent in 2020, according to the Ministry of Health.

“This is due to various intervention strategies. They include the distribution of long-lasting insecticidal treated nets through mass campaigns every three years, routine programmes for vulnerable populations, including pregnant women and children, indoor residual spraying, seasonal malaria chemoprevention, as well as treatment with artemisinin-based combination therapies ,” says Mary Muthoni, the Principal Secretary (PS) for State Department for Public Health and Professional Standards at Ministry of Health.

In 2023, Nanny Hellen described to this journalist how cerebral malaria sent her adrift from reality; hitting her hard with extreme fever and intense body aches.

“I had been sick with malaria at a younger age but this was the first time I was experiencing the disease with this level of intensity,” she said. “I believed I was dying: I am happy I survived.”

Hellen — who at the time worked and lived in Kitengela (the south-eastern township that buds off the capital) — had travelled from western Kenya a few days before this episode.

Thousands like her come to the city carrying the malaria parasite, but, it appears, it’s not transmitted further to healthy people.

Damaris Matoke, a research scientist at Kenya Medical Research Institute (KEMRI), says that the mosquitoes in Nairobi are not malaria-transmitting-vectors.

“Malaria is transmitted through the infectious bite of the female anopheles mosquito. In Kenya, Anopheles gambiae and Anopheles funestus are to blame for most of malaria transmissions. But we also have Anopheles arabiensis, Anopheles merus, and Anopheles coluzzii – in different parts of the country — which can also transmit malaria,” she says.

With more than 15 years of experience working on malaria, Dr Matoke says there are several science-backed factors that scientists believe are keeping malaria out of Nairobi.

“Nairobi is highly polluted. Malaria mosquitoes prefer slow-moving fresh water. The water bodies in Nairobi do not represent the ideal habitat for these mosquitoes.

“But also, the climatic conditions do not allow mosquitoes to carry the parasite — that causes malaria — to term. The average temperatures in Nairobi are too low; preventing the parasite that causes malaria — plasmodium — to complete its developmental cycles.

“Therefore, assuming a mosquito capable of transmitting malaria bit a person who is carrying the parasite, and they pick it, the parasite hits a dead end and is not propagated further. There is therefore no perpetuation of malaria transmission.”

Dr David Tchouassi, a senior research scientist at International Centre of Insect Physiology and Ecology in Nairobi, says: “Malaria prefers warm tropical conditions which are good for vector survival and can sustain the parasite’s life cycle development. Nairobi is on high altitude (1,795m above sea level). It therefore experiences low temperatures that can drop to 10 degrees Celsius in May, June and July.

“However, in anticipation that climate change continues to bite and Nairobi steadily becomes warmer, we might see temperatures being sustained above 25 Celsius degrees at certain times of the year, and this could tip the scales.”

Being a febrile illness, medics in the city do generally seek travel history before deploying a malaria test.

“In the absence of travel to high-transmission regions — such as the Lake Victoria basin or the Coast — malaria is unlikely to be the cause of illness in a Nairobi resident,” says PS Muthoni.

So far the city remains gilded. Scientists are however seeing signs that climate change could upend status quo.

To a layman climate change sounds like a bogeyman created by the so called scientists. But evidence of global warming is all around us.

Picture this: In early 1900s the peak of Mt Kenya was almost entirely covered in glacier. A 2011 study by a team from Austria’s University of Innsbruck concluded that between 1934 and 2010 the much famed Lewis Glacier (on Mt Kenya) lost 90 per cent of its volume. By some estimates the mountain is on course to becoming completely ice-free by 2030.

“Climate change is complex. Areas that used to be cold are becoming warm. We (scientists) are asking ourselves, how exactly is it going to influence malaria? A warming planet will create the right conditions for malaria to spread in areas it never existed. There is good evidence that Nairobi is warming,” says Tchouassi.

‘Will malaria move into Nairobi?’ we ask him.

He answered: “In science we can never predict with 100 per cent certainty. But looking at scientific modelling I would say it is only a matter of ‘when’.”

Kenya Malaria Indicator Survey 2020 warns as much: increasing temperatures and changes in the hydrological cycle are likely widen malaria transmission zones. 

Several surveys we looked at show presence of Anopheles gambiae mosquito in Nairobi: though in extremely low quantities that many scientists consider negligible.

Anopheles gambiae is however the least of the scientists’ concern.

In December 2022, Division for National Malaria Programme and scientists from KEMRI collected mosquitoes in 14 Counties – Marsabit among them – as part of routine surveillance exercise.

Of 59 mosquito samples (collected in Marsabit) tested by Polymerase Chain Reaction (PCR), 23 were confirmed to be Anopheles stephensi – an invasive species that is responsible for majority of malaria in South Asia: where it is native.

Dr Matoke was part of the surveillance exercise. She says: “Anopheles stephensi is like no other malaria parasite we have had in Kenya. It is a competent vector for Plasmodium falciparum – the parasite responsible for majority of malaria diagnoses in Kenya.

“This mosquito thrives in urban environments, breeding in water storage containers, drainage systems, construction sites, discarded tires, and other artificial water collections—common features in Nairobi.

“The worry is that if the vector reaches Nairobi, and multiplies to significant numbers, we will start seeing a jump in malaria incidents.”

The World Health Organisation (WHO) sent out a communique to all countries; advising for scale up in surveillance against Anopheles stephensi .

According to Matoke, Djibouti was on the cusp of being declared malaria free when, in 2012, Anopheles stephensi was detected. Today, majority of malaria transmissions in the country are due to the vector.

Nairobi, with its estimated six million residents, is the most populous in East Africa. It is also the most dense: overflowing into townships like Kitengela (Kajiado County), Thika (Kiambu County) and Athi River (Machakos County).

Periods of high malaria activity in the city – driven by A.stephensii – could be the disaster that scientists are warning against today.

“I won’t say it is going to happen soon. But the likelihood of malaria landing in Nairobi is high; it might actually be sooner than we think,” Matoke says.

PS Muthoni says the ministry is aware of the novel vector and is in no way waiting for it to proliferate and potentially cause trouble.

“The Ministry of Health, through the Division of National Malaria Programme and partners, has taken proactive steps to mitigate this threat.

“These include enhanced entomological surveillance to monitor mosquito populations in the country with timely reporting and response.

“While we remain focused on preventing malaria transmission in Nairobi, our existing capacity allows us to respond swiftly and strategically if the risk profile changes. The goal is not just to react, but to stay ahead.”

Kibe, from Nairobi County Malaria Unit says: “Most of the malaria that gets diagnosed in Nairobi area is of people who’ve travelled from malaria endemic zones.

“We have three peaks in a year. Our highest peak is in January. Then we peak again in May; but lower than January. We get our third and final peak in September.

“During a peak we record as high as 3,000 cases per month. As the peak tapers off the numbers come down to hundreds.

“If you study the peaks they coincide with schools opening. When schools close, children – and their families — travel to the villages where many contract malaria.”

Prof David Odongo, a specialist in medical microbiology at the University of Nairobi, says malaria kills by causing anaemia. Sometimes the parasites can cause blockage of blood vessels to vital organs: causing renal failure against kidneys and cerebral malaria when the vessels affected lead to the brain.

The Ministry of Health confirmed to Health & Science that Anopheles stephensi has now been detected in seven counties: Turkana, Marsabit, Elgeyo Marakwet, Isiolo, Wajir, Mandera, and Samburu counties

This story was produced with support from Internews’ Earth Journalism Network.