- Vector-borne diseases are human illnesses caused by parasites, viruses and bacteria that are transmitted by vectors.
- Vectors are living organisms that can transmit infectious pathogens between humans, or from animals to humans.
- Every year there are more than 700,000 deaths from diseases such as malaria, dengue, schistosomiasis, human African trypanosomiasis, leishmaniasis, Chagas disease, yellow fever, Japanese encephalitis and onchocerciasis.
A Losing Battle
- In a warming world, viruses and their mosquito vectors are fast acquiring traits to gang up on humans.
- The most serious diseases such as dengue, chikungunya, yellow fever and Zika are transmitted by just two mosquito species known as Aedes Aegypti and Aedes albopictus which are fast emerging throughout the world as a public health threat.
- A modeling study published in “Nature Communications” shows that the world became about 1.5 per cent more suitable per decade for the development of Aedes aegypti during 1950-2000 and the trend is likely to increase to 3.2-4.4 % per decade by 2050.
- According to Expert studies, By 2050, 49 % of the world’s population will live in places where Aedes aegypti and Aedes albopictus are present if greenhouse gas emissions continue at the current rates.
- Some studies also show that the mosquitos are turning into more efficient vectors.
- The virus also affects the behavior of the mosquitoes.
- Eg: Zika virus-infected females showed significantly increased movement compared to uninfected females during the egg-laying phase.
- Under current climate conditions and population densities, both mosquito species will continue to spread globally over the coming decades.
- Eg: Aedes aegypti is predicted to spread 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 while Aedes albopictus is forecast to spread widely throughout Europe and reach large areas of France and Germany over the next 30 years.
Climate Change: A Major Driver
- A study published in “Global Change Biology” shows that if climate change remains unmitigated, as many as 1.3 billion new people could be living in areas with temperatures suitable for Zika transmission by 2050.
- According to a mathematical model published in “Frontiers in Public Health” indicates that while global abundance of Aedes aegypti over the last century has increased by 9.5%, it is expected to increase by 20% under low carbon emission and 30% under high carbon emission scenarios by the end of this century.
- Climate change increases the vector population too, as mosquitoes are now able to breed through the year.
- Eg: Breeding behavior of the mosquitoes has also changed. Traditionally, it is believed that the mosquito species breed in clean water, but now studies suggest that they can also lay eggs in polluted water.
- While human-made conditions have made it easy for the mosquitoes to spread over larger areas, global warming has further aided in their proliferation as warm and wet environments are excellent places for mosquitoes to breed.
- A study published in “International Journal of Tropical Insect Science” concluded that constant exposure to insecticides during vector control campaigns have probably compelled the mosquito to adapt to new ecological niches.
- Several other factors are also responsible for the increasing menace caused by these mosquitoes which also includes environmental cues such as light pollution.
- Aedes aegypti has also developed resistance to common insecticide permethrin.
- Climate change has also affected the disease carrying capacity of mosquitoes.
- Eg: The findings show that the R0 for all arboviral diseases tracked has increased since 1950-54. In 2020, the no. of infections transmitted by Aedes aegypti was 13% higher than the 1950-54 baseline years and those spread by Aedes albopictus was 7% higher.
- (R0 is the expected number of secondary infections resulting from one infection)
- Hence, the researchers point out the importance of curbing climate change considering that after the next five to 15 years, expansion of these mosquito species will be driven by changes in climate, temperature and Urbanization that create new and favorable habitats for them to breed.
Target the Vectors
- The researchers point out that Aedes aegypti is heat-tolerant and the Aedes albopictus is heat-limited and in a warming world, year-round transmission potential from Aedes aegypti is likely to expand, particularly in South Asia and sub-Saharan Africa while transmission potential of Aedes albopictus is likely to decline substantially in the tropics.
- This suggests that while complete mitigation of climate change to a preindustrial baseline may protect almost a billion people from arbovirus range expansions, middle-of-the-road mitigation could produce the greatest expansion in the potential for viral transmission by Aedes albopictus.
- The viruses are very difficult to control because they move around with infected people, a large proportion of which do not experience symptoms so do not know they’re infectious.
- Controlling the vector is the only option available as mitigating Climate change would shift the burden of Aedes transmitted viruses from higher-income regions back onto the tropics, where transmission would have otherwise begun to decline due to rising temperatures.
- A study published in “PLOS Neglected Tropical Diseases” suggests that Controlling the vector and reducing the social and ecological conditions that allow it to thrive around people (dense, crowded urban areas with unreliable access to piped water and solid waste sanitation, housing with inadequate window and door screens) are some of the most effective and sustainable ways to control the disease.
- Other technological strategies include engineering the mosquito to be sterile or to be refractory to the viruses using Wolbachia or genetic engineering.
- The only way to survive in this complex situation is to identify the “mosquito affected hotspots”.
- Eg: Identifying the hotspots of one fever, dengue for instance, can help create a predictive map for future outbreaks of Zika and chikungunya.
- As Vector Borne disease is becoming a public health threat, it becomes necessary for a country to reduce its GHG Emissions as Climate change proves to be a major driver behind the expansion of such vectors.
About Zika Virus
- Zika is a viral infection, spread by mosquitoes.
- The vector is the Aedes aegypti mosquito, which also spreads dengue and chikungunya. Additionally, infected people can transmit Zika sexually.
- First identified in Uganda in 1947 in monkeys, Zika was detected in humans five years later.
- In 2015, a major outbreak in Brazil led to the revelation that Zika can be associated with microcephaly, a condition in which babies are born with small and underdeveloped brains.
What are the symptoms of Zika virus?
- Most people infected with the virus do not develop symptoms.
- When they are manifested, the symptoms are similar to those of flu, including fever, bodyache, headache etc.
- If the symptoms worsen, people should seek medical advice. Additional symptoms can include the occasional rash like in dengue, while some patients also have conjunctivitis.
- The incubation period (the time from exposure to symptoms) of Zika virus disease is estimated to be 3-14 days.
How do you treat Zika virus?
- Zika has no treatment or vaccine. The symptoms of Zika virus are mild and usually require rest, consumption of plenty of fluids, and common pain and fever medicines, according to the WHO.
- Governments take mosquito control measures such as spraying of pesticides, use of repellents etc. Because of the possibility of congenital abnormalities and sexual transmission, there is also focus on contraceptives.
- The WHO requires countries to counsel sexually active men and women on the matter to minimise chances of conception at the time of an outbreak.