Does warmer world spell doom for health?


As temperatures rise driven by climate change, there are fears that disease-causing vectors like mosquitoes will conquer previously uninhabitable regions. But does a warming world signal a rise...

Wednesday March 18 2020

One uneventful cold February day this year, farmers in Ol Kalau, Nyandarua County, woke up to the ghastly scenes of their cattle losing their unborn calves and dying. A strange phenomenon they had never dealt with.

Tests at the national veterinary laboratory in Kabete, Kiambu County confirmed that the livestock was suffering from Rift Valley Fever, a disease that was unusual in that location. The disease, which got its name from the place it was discovered in 1939 – the Rift Valley – is transmitted to livestock by the bite of its carrier the Aedes mosquito. From that initial cow, it sweeps through the herd, causing abortions in nine out of 10 cows, a phenomenon that scientists call a “storm”.

Rift Valley Fever is a zoonosis, which means that it can be transmitted from livestock to humans who come into contact with the infected animals. It is something that farmers in Garissa have dealt with from time to time, with the most recent and largest ever recorded outbreak being the 1997 and 1998 outbreak that infected nearly 28,000 cattle and caused at least 170 human deaths.
But in Nyandarua, it was a strange phenomenon. Rift Valley Fever thrives in Garissa which has a weather pattern that deeply contrasts Nyandarua’s – warm with periodic rain and flooding – the perfect conditions for the mosquitoes that carry the disease.
Malaria, another disease transmitted through the bite of Anopheles mosquitoes infected with Plasmodium, the malaria-causing parasite, was previously zoned into four regions in Kenya, according to the National Malaria Strategy (2009 to 2017). Endemic – places with altitudes ranging from zero to 1,300 metres, around Lake Victoria and in the coast, with climatic conditions (rainfall, temperature and humidity) suitable for survival of malaria mosquitoes and transmission of malaria all year. Seasonal in arid and semi-arid areas of the northern and south-eastern parts of the country, which experience intense malaria transmission during the rainy season, with pools created during the rainy season and high temperatures providing the perfect breeding conditions. In the western highlands, transmission is also seasonal, with increase in minimum temperatures (around 18 degrees) during the long rains favouring and sustaining mosquito breeding, while in low-risk malaria areas – the central highlands, including Nairobi – where prevalence is less than one per cent as compared to eight to 27 per cent in endemic zones, temperatures are usually too low to allow the completion of the vector sporogonic cycle when the malaria parasite multiplies in the mosquito. While malaria transmission has been associated with the endemic zones, the National Malaria Strategy notes that increasing temperatures and changes in the hydrological cycle associated with climate change are likely to lead to an increase in the areas suitable for mosquito breeding, with the introduction of malaria transmission in areas where it had not existed before.

The United Nations-backed Intergovernmental Panel on Climate Change estimated that Kenya and other countries in the equatorial belt, such as Uganda and Cameroon, will experience an increase in temperature by an approximately 1.4 degrees Celsius. This, scientists say, is revising weather patterns making it possible for disease-causing vectors such as mosquitoes to make a home in places that were previously uninhabitable for them.
According to Eric Ochomo, who studies insects that cause human diseases at the Kenya Medical Research Institute-Centers for Disease Control collaborative programme (KEMRI-CDC), mosquitoes thrive at between 16 to 40 degrees Celsius.
“Environmental temperatures between 28 to 33 degrees Celsius are optimal for growth of mosquito larvae. Therefore, as more places attain temperatures nearing the 30 degrees Celsius, larval habitats become more productive and more mosquitoes are produced,” said Dr Ochomo.

Data from the Economic Survey released by the Kenya National Bureau of Statistics in April showed that maximum annual temperature was 27.9 degrees Celsius, while minimum annual temperature was 17.2 degrees Celsius.
During the rainy season of March to May (long rains) and October to December (short rains), warmed air heats the ocean leading to evaporation and the air hold more water than cooler air. The rain then falls in torrents causing flooding, and leaving pools of water that provide ample breeding sites for mosquitoes.

At 20 degrees Celsius, the malaria parasite takes 26 days to develop, while at 25 degrees Celsius, it takes 13 days. Moreover, at these optimal temperatures, mosquitoes mature faster and bite and transmit malaria faster.
Therefore, as temperatures rise, mosquitoes are expanding into areas that were uninhabitable for them in the past, and along with them, they carry the diseases they transmit. Aedes mosquitoes which transmits dengue fever and yellow fever have a wider range, but rarely convey the virus in colder places that fall below 10 degrees Celsius.
However, scientists say that the spread of the disease cannot be attributed to climate change alone.
In a previous interview, infectious diseases expert Dr Njenga Kariuki said that people’s encroachment into forests and animals’ habitats make them come into contact with the disease carriers (vectors).
“People are getting closer to disease-causing vectors because of intensified farming, construction and even hunting, increasing the probability of infections,” he said.
Even Dr Mark Nanyingih, an infectious diseases epidemiologist and research officer at KEMRI-CDC is slow to peg the Rift Valley Fever infections in Nyandarua to warming of the atmosphere, but he does not rule it out.

There could be other factors that would aid the temperatures. The disease may have been brought from a neighbouring county through movement of livestock, as well as economic activities such as quarries and fish ponds that provide breeding sites.
The researchers advise a strengthening of vector control, guided by the country’s elaborate early warning system.
Dr Ochomo adds that the fight against mosquito transmitted infections will require the input of every Kenyan.
“Being able to observe simple sanitation practices such as disposing of tins and plastics responsibly, covering all water containers, clearing drainages and draining of stagnant water will go a long way in controlling the mosquitoes we see around us.
“If you are sick, seek immediate diagnosis and treatment and if you LIVE in a malaria-endemic setting, use your bed net.”

Aside from promoting the thriving of mosquitoes, various research papers have listed other ways that global warming affects health. Floods wash sewage and other disease-causing pathogens into the supplies of drinking water which cause cholera. Floods also flush fertiliser into water sources leading to the growth of killer algae blooms that have a negative effect on the health of ecosystems. These algae are highly toxic when inhaled, but also contaminate fish which cause diseases when consumed.
As the algae bloom, they also support the uncontrolled growth of pathogens such as Vibrio cholerae which causes cholera.