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Climate Change and the Mosquito Invasion


By Maneka Gandhi

Do people really understand climate change and global warming? A few days of unseasonal chill brings back the sceptic in everyone – if the earth is getting hotter then why are we experiencing low temperatures, is a common question.
That the earth is getting hotter is no longer a moot question. It is. But will the heat kill you? Yes. But not directly. You can still walk barefoot on the grass without getting your feet burnt!
You will get killed by the viruses, the insects, the tsunamis and cyclones, the jellyfish in the oceans, the lack of water.
The new kings of the earth are getting ready to take their thrones: mosquitoes.
Just as birds flock to warmer areas when winter settles in, wild creatures seek out weather that suits them. But a changing climate is moving that comfort zone for many animals, including disease-carrying mosquitoes.
Mosquitoes are the deadliest creatures in the world. Their ability to carry and spread disease to humans causes millions of deaths every year. The worldwide incidence of dengue has risen 30-fold in the past 30 years, now infecting as many as 400 million people a year, with a quarter of them sick enough to be hospitalised, and more countries are reporting their first outbreaks of the disease. Zika, dengue, chikungunya, and yellow fever are all transmitted to humans by the Aedes aegypti and Aedes albopictus mosquitoes, to name just two species. (Malaria spread by the Culex mosquito needs a whole article to itself).
And, global warming is letting them take over the world.
Published in Nature Microbiology in March 2019, Scientists from Boston Children’s Hospital, Oxford University, University of Washington, London School of Hygiene and Tropical Medicine and the Universite Libre de Bruxelles (Belgium) have combined all the available data using climate, urbanisation, migration and human travel and made prediction models of the likely spread of two key disease-spreading mosquitoes — Aedes aegypti and Aedes albopictus. The models forecast that by 2050, 49 percent of the world’s population will live in places where these species are established, if greenhouse gas emissions continue at current rates. “We find evidence that, if no action is taken to reduce the current rate at which the climate is warming, pockets of habitat will open up across many urban areas with vast amounts of individuals susceptible to infection,”
The team gathered data on the distributions of Aedes aegypti and Aedes albopictus over time, in more than 3,000 locations, starting from the 1970s. They mapped current locations which they considered suitable as mosquito habitats, then projected their suitability in 2020, 2050 and 2080, based on various climate models, projections of urban growth and other variables. They included human migration and travel patterns, using census data and mobile phone records.
Their findings: Aedes aegypti has spread north at a relatively constant rate, about 150 miles per year. Aedes albopictus spread most quickly between 1990 and 1995. In Europe, Aedes albopictus has spread faster, advancing from 62 miles per year to 93 miles per year in the past five years.
Aedes aegypti is predicted to spread not just within its current tropical range, but also in new temperate areas in the US and China, reaching as far north as Chicago and Shanghai, respectively, by 2050. Aedes albopictus is forecast to spread widely throughout Europe over the next 30 years. It’s also expected to arrive in northern US and the highland regions of South America and East Africa. If climate change isn’t curbed by 2050, the spread is predicted to be even greater. Zika virus started two years ago in South America and has already spread very fast, despite border control measures.
Climate change is the next great health threat, says Prof Paul Auerbach co-author of the book Enviromedics: The Impact of Climate Change on Human Health. As the globe warms, mosquitoes will roam beyond their current habitats, shifting the burden of diseases, like malaria, dengue, chikungunya and West Nile virus, to temperate and colder regions.
Stanford biologist Erin Mordecai, and her colleagues from Stanford Medical School have also forecast how climate change will alter where mosquito species are most comfortable, and how quickly they spread disease. Economic development and cooler temperatures have largely kept mosquito-borne diseases out of wealthier Northern Hemisphere countries, but climate change will tip the scales in the other direction.
Their findings: malaria is most likely to spread at 25 degrees Celsius (78 degrees Fahrenheit) while the risk of Zika is highest at 29 degrees Celsius (84 degrees Fahrenheit). Carriers of dengue need the warmest climates, so will continue to plague hot regions such as sub-Saharan Africa. Mosquitoes that carry the West Nile virus, however, prefer a more temperate climate, so will migrate to cooler regions as the climate warms.
According to a study by Dr Soeren Metelmann et al of the University of Liverpool, almost all of England and Wales could be warm enough for the aedes species by the 2060s. Metelmann and colleagues created a model that combines knowledge of the life cycle of the mosquito with UK climate predictions from NASA for the period 2060–2069. So far, tropical mosquitoes come with travellers to the UK in warm summer months and survive – and even breed – for a few weeks before disappearing in the winter. But his team’s research shows that in the near future such introductions could lead to the establishment of resident populations that survive the winter.
An international team, including Moritz Kraemer at the University of Oxford, has done an independent study that predicts that the mosquitoes will spread throughout Europe over the next 30 years. Originally from East Asia, the insects have been spreading across Europe since the 1970s and are now found as far north as Germany and south-east England. In the last decade there have been outbreaks of chikungunya in Italy, showing that the spread of such mosquito-borne viruses within Europe is possible. By 2080, they predict that the mosquito will be in 197 countries worldwide, with 20 of those detecting its presence for the first time.
The mosquito seems to operate like a heat-driven missile of disease. Scientists say the hotter it gets, the better the mosquito is at transmitting a variety of dangerous illnesses. Mosquitoes live 10-12 days and that is how long it takes a virus to grow in its gut, making it infectious. Warmer air incubates the virus faster in the cold blooded mosquito, so the insect has more time to be infectious. Higher temperatures speed up larval development, increasing the number of adult populations, the autumnal development of the immature and consequently the increase of eggs over winter. Warmer temperatures make the mosquito hungrier, so it takes more blood meals and spreads more infection. Warmer temperatures also increase the mosquito population.
Zika, for instance, has been declared a global public health emergency after being linked to brain deformities in babies in South America. Zika outbreaks depend on temperature and drought. Recife, Brazil, the largest city in the Zika-struck region, saw its hottest September-October-November on record, about 1.2 degrees Celsius (2.2 degrees Fahrenheit) above normal, according to NASA data.
Aedes albopictus, the vector of a known vector of chikungunya virus, dengue virus and dirofilariasis, has undergone a dramatic global expansion facilitated by human activities, in particular the movement of used tyres and ‘lucky bamboo’ (single reason for its spread in Belgium and the Netherlands)– those horrible sticks they put in glasses of water and give to chief guests. It was first reported in Europe in 1979 in Albania. In 1985, it was first reported in Texas, USA. Now it is in 32 US states, including Hawaii. In Latin America, it was first reported in Brazil in 1986. In Africa, it was first detected in 1990 in South Africa. It is now listed as one of the top 100 invasive species by the Invasive Species Specialist Group. This mosquito is already showing signs of adaptation to colder climates, which may result in disease transmission in new areas. It has already caused outbreaks of chikungunya in Italy and France and dengue in France and Croatia.
It’s not just the spread of disease. Aedes albopictus is currently considered a serious biting nuisance for humans, where it is significantly reducing the quality of life. Adult females bite aggressively, usually during the day, both indoors and outdoors. Its prevalence has been linked to a reduction in children’s outdoor physical activity time, a factor contributing to childhood obesity.
Traditionally the aedes albopictus needs a mean winter temperature of 0 degrees C to permit egg overwintering, a mean annual temperature of 11 degrees C for adult survival and activity, and about 500 mm of annual rainfall. A summer temperature of 25?30 degrees Celsius is required for optimum development. There are reports now of populations establishing in areas with lower mean temperatures (5?28.5°C) and lower rainfall (290 mm annually).
Aedes albopictus can be the vector of at least 22 other viruses, including yellow fever virus, Rift Valley fever virus, Japanese encephalitis virus, West Nile virus. So, its spread is significant to human survival.
Can a mosquito carry more than one disease at the same time? Can someone get more than one disease from a single bite? Studies done in Gabon and India show that humans can be infected with chikungunya and dengue at the same time. Coinfection poses a challenge for accurate diagnosis, particularly when symptoms, such as fever and aches, can be similar.
Remember this every time you eat meat or allow a tree to be cut – the two biggest reasons for climate change.

(To join the animal welfare movement contact gandhim@nic.in, www.peoplefor animalsindia.org)