What's in a mosquito bite? How warmer climates spread disease

Dr. Gamou Fall (left) and lab technician Arame Ba in the cellular isolation room in the Pasteur institute of Dakar, Senegal, 2023

Dr. Gamou Fall (left) and lab technician Arame Ba in the cellular isolation room in the Pasteur institute of Dakar, Senegal, 2023. Chiara Luxardo/Handout via Thomson Reuters Foundation

What’s the context?

Global warming and shifts in agriculture are helping mosquitoes to thrive and spread West Nile virus to Europe from Africa

  • Global warming, land use change, allow diseases to spread
  • Infected birds carry West Nile virus to Europe from Africa
  • Floods, rice paddies, help mosquitoes to thrive

ORISTANO, Italy/DJOUDJ, Senegal – Living on the edge of wetlands on the Italian island of Sardinia, Anna Rita Cocco is mourning the loss of her elderly father who died in a coma within weeks of a fatal mosquito bite.

"My father was full of life and used to walk for miles each day. I was expecting him to die at some point, but not suffering like that, taken from me by a mosquito," she said her late father, Bernardino, who died aged 80.

Italy was only declared malaria-free by the World Health Organization (WHO) in 1970, but now other lesser-known mosquito- or tick-borne diseases are on the rise.

A complex mix of global warming, changes in land use and more movement of people and goods are contributing to the spread of illnesses - such as dengue or Lyme disease - to new regions in a worsening trend, the U.N. panel of scientists says.

Migratory birds infected by mosquitoes and flying over 2,000 miles (3,200 km) from Senegal in west Africa have been identified as carriers of the West Nile virus that killed Cocco's father in Sardinia, where summers are becoming hotter.

The Mediterranean island, and northern areas of Italy where the virus is also spreading, are both suffering more extreme events of floods and droughts.

Abundant water helps mosquitoes to breed, while more drought and fewer trees constrict the migratory birds' ecosystems, forcing them into closer contact with each other, enabling some diseases to spread.

"People don't seem to be aware of the threat," said Cocco.

Habitat changes in Senegal 

Epidemiologists identify habitat change as one of the main factors behind the intercontinental leap of the West Nile virus.

First identified in 1937 in the West Nile region of Uganda, the disease has spread within Africa and to other continents. Almost 3,000 people have died in the United States alone since a first outbreak in New York in 1999. 

In the red clay houses of Maka Diama village in northwest Senegal, women make soap from plants that grow in a nearby river, which they sell to tourists and local hotels, and cook rice grown in surrounding paddy fields.

Recent years have seen huge changes in this wetland region teeming with crocodiles and migratory birds, most notably a leap in rice production, driven by government efforts to reduce Senegal's reliance on imported rice.

Barrages built near the coast to retain and safeguard freshwater supplies from salty sea water have slowed river flows, and fertilisers used for rice paddies have encouraged the growth of river plants.

This push for greater food self-sufficiency has tripled rice production to 1.3 million tonnes over a decade. But changes in land use have upset the delicate wetland habitats, helping mosquitoes which lay eggs in stagnant water.

"There are so many mosquitoes here these days," said Arame Diop, one of the village soap-makers. "Far more than there used to be." Diop's family already sleep under mosquito nets to avoid malaria, which is endemic in Senegal.

Assane Gueye Fall, an entomologist at the National Livestock and Veterinary Research Laboratory in the capital, Dakar, said Senegal's policies sought to improve food and water security.

"But to solve a problem, they created another," he said of what he called the "explosion of mosquitoes" and of disease.

Carried on a wing

The long-distance carriers of West Nile are birds that receive the virus from bites by infected mosquitoes and then fly on their migratory routes, to be bitten once more by mosquitoes that then spread illness to people and other animals, mainly horses.

Flamingos, herons, storks and birds of prey are among many migratory species found in the wetlands of the Djoudj National Bird Sanctuary, a UNESCO World Heritage Site near Maka Diama.

Commandant and Veterinary Doctor Ndao in Guembeul Natural Reserve. In the backdrop and artificial birdhouse inhabited by Sterne Hensel birds. St Louis, Senegal, January 2023

Commandant and Veterinary Doctor Ndao in Guembeul Natural Reserve. In the backdrop and artificial birdhouse inhabited by Sterne Hensel birds. St Louis, Senegal, January 2023. Chiara Luxardo/Handout via Thomson Reuters Foundation

Commandant and Veterinary Doctor Ndao in Guembeul Natural Reserve. In the backdrop and artificial birdhouse inhabited by Sterne Hensel birds. St Louis, Senegal, January 2023. Chiara Luxardo/Handout via Thomson Reuters Foundation

Navigating his boat slowly, captain Ibrahima Ndao, the park conservationist, explained how abrupt changes in land use are impacting the wetlands. Pelicans are swooping all around, catching fish for their young.

"There has been a significant expansion of paddy fields around the reservoir. But we have to make sure that the birds’ environment is preserved," he said.

"If the space of their environment is reduced it's easier for illnesses to spread," said Ndao, pointing out increased growth of plants along the banks, including those used by the women of Maka Diama to make soap.

Ndao stressed the importance of a "one-health approach" that looks at human and animal health as one issue.

In the capital Dakar, Babacar Ngor Youm, head of Senegal's National Parks within the Ministry of Environment and Sustainable Development, said "political choices" must be made to raise food production despite risks of harmful side-effects.

Food sovereignty is necessary "as the Ukraine war showed," he said, referring to a rise in world food prices after Russia invaded its neighbour.

"With urbanisation comes deforestation ... and if there's less space for the birds, illnesses spread more quickly," said Youm, who previously worked on an outbreak of avian flu in Djoudj park.

As in Europe, Senegal is also losing natural habitats through desertification exacerbated by climate change, forcing animals into closer contact with human communities.

Bridging the knowledge gap

Vector-borne diseases - such as malaria, dengue, Zika, yellow fever and West Nile - are seen by WHO as an increased threat in Africa, potentially affecting over 800 million people, some 70% of the population.

West Nile virus has gained a foothold in an increasing number of countries, from Australia to Venezuela.   

Because it is easy to confuse West Nile with a generic flu or other mosquito-borne illnesses, patients rarely get tested.

As a result, the impact of the virus in Africa is virtually unknown. Local populations also build resistance to the disease. With little information, it is also difficult to build models of how it might spread in Europe and elsewhere.

West Nile is often asymptomatic or mild, but one out of 150 people who contract the virus can develop severe neurological complication including meningitis, paralysis and even death.

Dams have been causing severe disruption in the water balance of the habitat of Djoudj National park, including the proliferation of invasive aquatic plants, reduced amplitude of water levels, and decrease/disappearance of some bird colonies. Senegal, 2023

Dams have been causing severe disruption in the water balance of the habitat of Djoudj National park, including the proliferation of invasive aquatic plants, reduced amplitude of water levels, and decrease/disappearance of some bird colonies. Senegal, 2023. Chiara Luxardo/Handout via Thomson Reuters Foundation

Dams have been causing severe disruption in the water balance of the habitat of Djoudj National park, including the proliferation of invasive aquatic plants, reduced amplitude of water levels, and decrease/disappearance of some bird colonies. Senegal, 2023. Chiara Luxardo/Handout via Thomson Reuters Foundation

In 2022, 12 European nations reported 1,335 locally acquired cases of West Nile virus - with a few others brought in by international travellers - and 104 deaths. It was the highest number of cases since a peak of more than 1,500 in 2018.

Italy suffered most in the European Union in 2022 with 51 deaths, ahead of 33 in Greece and five in Romania, according to the European Centre for Disease Prevention and Control.

A public health campaign is alerting people, and authorities are stepping up tests of birds and mosquitoes.

The main vector is the mosquito Culex, but what makes West Nile potentially endemic is that it can be transmitted by more than 50 species of mosquito and by ticks, said entomologist Fall in Dakar. By contrast, dengue, for example, relies on one or two species.

There is no West Nile vaccine available for humans, although one has been developed for horses.

"The expansion of West Nile cannot be stopped," said Fall. "This is why we need collaborative research between Africa and Europe."

"And prevention in animals and in humans must be considered as one."

This story was supported by the European Journalism Centre Global Health Project.

(Reporting By Sara Perria, editing by Alister Doyle)


Context is powered by the Thomson Reuters Foundation Newsroom.

Our Standards: Thomson Reuters Trust Principles


Tags

  • Climate and health

Featured

Rerooted: the future of crops

In this series, we explore how climate change and shifting consumer habits are forcing us to rethink the way we grow staple crops, from coffee to rice.

Crops including coffee and rice are shown in orange on white background in this illustration. The text reads: THE FUTURE OF CROPS, REROOTED. Thomson Reuters Foundation/Karif Wat


Get our climate newsletter. Free. Every week.

By providing your email, you agree to our Privacy Policy.


Latest on Context