Malaria is the most common disease in Africa’s largest slum, Kibera, in Nairobi, say health workers, but at a cool altitude of about 1,700m, the capital city has long been considered a non-malarial zone.

The incidence of malaria in Nairobi and the resurgence of ‘highland malaria’ in several African countries have become controversial issues in debates about health and climate change.

The third assessment report, published in 2001, by the Intergovernmental Panel on Climate Change, paid special attention to highland malaria. The report states that due to the life-cycle of the mosquito and its role as host of the malaria parasite, "at low temperatures, a small increase in temperature can greatly increase the risk of malaria transmission" and "future climate change may increase transmission in some highland regions, such as in East Africa".

However, the IPCC report continues, "there are insufficient historical data on malaria distribution and activity to determine the role of warming, if any, in the recent resurgence of malaria in the highlands of Kenya, Uganda, Tanzania, and Ethiopia".

Furthermore, two subsequent studies drawing on weather records at several highland locations in Africa, including tea estates in Kenya's Kericho region, published in Nature and Proceedings of the National Academy of Sciences, reach differing conclusions about whether temperatures were increasing and the occurrence of malaria.

The award-winning film by former US vice-president Al Gore, An Inconvenient Truth, says Nairobi used to be too cold for malaria-carrying mosquitoes, but now climate change is causing the disease to occur.

Paul Reiter, a malaria expert now with the Pasteur Institute, has taken issue both with the film and some of the IPCC reporting. In the International Herald Tribune in January, he wrote, "Gore's claim is deceitful on four counts. Nairobi was dangerously infested when it was founded; it was founded for a railway, not for health reasons; it is now fairly clear of malaria; and it has not become warmer."

And in a travel advisory, the US Centers for Disease Control and Prevention, states there is "no [malaria] risk in Nairobi".

A ‘non-urban’ disease?

Whatever the causes, and the scientific wrangles, medical staff working in Kibera are having to tackle malaria. “Malaria is the leading disease we face,” says George Gecheo, clinical officer in Kibera’s Ushirika clinic. Nurse Dorah Nyanja, who works in Senye Medical Clinic in the slum’s Soweto Market, adds: “I am treating more people per day for malaria than any other condition.”

Malaria researcher Dr John Githure, head of the human health division of the African Insect Science for Food and Health research centre (ICIPE), told IRIN: "Malaria is traditionally considered a non-urban disease as its parasite is carried by mosquitoes that prefer hot, clean and sunny areas to cool and polluted cities."

"It [malaria] is there," adds Githure, "but not everywhere and the increase is not always obvious. It depends where you live."

Githure says Nairobi was malaria-prone in the 1900s, when it had many swampy areas. "Over time, with colonial draining, stagnant water treatments, and the growth of the city, anopheles mosquitoes, those which carry the disease, left the area," he says. "In the 1970s, when public health authorities started to crumble and treatments stopped being properly done, mosquitoes came back. However, it was mainly the Culex mosquito, as it is adapted to polluted water. It is not a malaria vector."

He says anopheles mosquitoes have returned, but only in small umbers. "Furthermore, anopheles need high temperatures to live and develop from the eggs to adult age. If temperatures are low, like in Nairobi, their development will take longer - two to three weeks instead of seven to 10 days. Coolness in Nairobi also delays the development of the parasite in the anopheles mosquito, which only lives for about a month.

"Moreover, the parasite is not transmitted through the eggs but through human blood. So anopheles need to grow up, bite a human who has contracted the disease and then bite another one for local transmission to take place in Nairobi."

Githure also says the adaptive nature of the mosquito is a factor. "There still is a danger as temperatures continue to rise and anopheles can adapt to new environments. A minority have already adapted to polluted water, for example," he adds.

A travelling disease

Gecheo estimates that 80 percent of the people he treats for malaria in Kibera have travelled out of Nairobi, been infected and returned, with symptoms only appearing once they are back in the slum.

ICIPE Director Christian Borgemeister says slum dwellers frequently travel upcountry to visit relatives. "This is why [ethnic] Luo people suffer more from malaria," he says. "Near Lake Victoria, where they live, the disease is widespread. By being less exposed to malaria, many have lost the semi-immunity they used to have. This is why they contract it easily when travelling."

"Mosquitoes also move more," Githure says. "They can easily be stuck in a bus or a train and progress from one place to another. Malaria, more than ever, is a travelling disease."

Kibera has grown up next to the railway line, with trains from malaria-prone areas passing through daily. The socio-economic make-up of the slum is also a factor. Malaria tends to affect the more vulnerable - infants, pregnant women, the malnourished or those living with HIV. Infection rates in children are an important indicator, Githure says.

"Malaria is a child killer. As children travel less, we would be able to detect local transmission of the disease if the numbers of children with malaria began to increase," Githure says.

Preparation for a potential comeback

"Widespread local transmission in Nairobi would be a catastrophe as its inhabitants are not immunised [by natural exposure] at all," says Borgemeister.

Outside Nairobi, many programmes are tackling the disease. Ayub Manya, an officer in the Ministry of Health’s Malaria Control Division, says: "The new treatments we have started to give for free, the mosquito nets we have distributed and the awareness campaigns we have carried with NGOs have given some results."

In 2006, the government also started to distribute new Artemisinin-based combination therapies (ACTs) free throughout Kenya.

Manya says these programmes could be expanded to Nairobi if malaria transmission increased, but "with ACTs free of charge, and as it is still a low-risk area, advocating systematic mosquito net use would be excessive".