Vector diseases and impact of climate change

What causes vector-borne diseases?

Vector-borne diseases have a climate dimension, hence specific vector diseases are specific to a certain area and a certain population. A critical premise of epidemiology is that disease and other health events do not occur randomly in a population, but are more likely to occur in some members of the population than others because of risk factors that may not be distributed randomly in the population.

Medically, a vector is a living organism – like a tick or mosquito – that transmits an infectious agent from an infected animal to a human or another animal. Any live agent that carries and transmits an infectious pathogen to another living creature is referred to be a disease vector in epidemiology. These agents include parasites and bacteria. Ronald Ross made the first significant finding of a disease vector in 1897 when he dissected a mosquito and found the malaria pathogen.

More than 17% of all infectious illnesses are vector-borne, and they account for more than 700,000 yearly fatalities. These illnesses disproportionately afflict the poorest people and are more prevalent in tropical and subtropical regions. Numerous nations have seen large epidemics of dengue, malaria, chikungunya, yellow fever, and zika since 2014, which have affected populations, claimed lives, and taxed health systems. Other illnesses including leishmaniasis, lymphatic filariasis, and chikungunya inflict lifelong morbidity, chronic misery, disabilities, and sporadic stigmatization.

Vectors can transmit infectious diseases either actively or passively:

Biological vectors, such as mosquitoes and ticks may carry pathogens that can multiply within their bodies and be delivered to new hosts, usually by biting. Mechanical vectors, such as flies can pick up infectious agents on the outside of their bodies and transmit them through physical contact. Examples of vector borne diseases are: Lymphatic filariasis; Leishmaniasis; Crimean-Congo hemorrhagic fever; Chagas disease (American trypanosomiasis); Onchocerciasis; Trypanosomiasis, human African (sleeping sickness); Plague; Schistosomiasis; Dengue and severe dengue; Yellow fever; Zika virus; Malaria; Japanese encephalitis etc. Many of vector-borne diseases are preventable, through protective measures, and community mobilization.

How is climate change increasing the risk of vector-borne diseases?

As vector-borne diseases have a climate dimension and due to climate change, more locations will be appropriate for vectors. The geographic range of habitats where vectors, such as mosquitoes and ticks, may live and spawn can expand as a result of warmer temperatures. Increased rainfall may result in more standing water, which would increase the number of vector breeding grounds. By creating pools of stagnant water from streams of rushing water, droughts can also foster breeding. In Addition to this, the season for disease transmission is extended in warmer climes. The climatic and environmental conditions for the spread of many diseases are becoming better due to climate change. The length of the seasons during which diseases are transmitted may also lengthen as a result of this, increasing the possibility of spread of those diseases.

Changes in temperature also can have an impact on how vectors behave. Increased temperatures, for instance, alter the way mosquitoes bite, limiting the effectiveness of defenses like bed nets. A virus spread by mosquitoes called dengue produces a flu-like disease and, in more serious instances, may result in hospitalization and even death. Although it is primarily found in tropical and subtropical settings, it has spread worldwide since 1990, especially in South Asia, sub-Saharan Africa, and Latin America and the Caribbean.

In Asia, Europe, Central and South America, and sub-Saharan Africa, “dengue risk will increase with longer seasons and a wider geographic distribution, potentially putting additional billions of people at risk by the end of the century,” according to an Intergovernmental Panel on Climate Change 2022 report.

Other regions will become too hot for them to dwell while certain spots are warming to the point that vectors may thrive there. According to research conducted in laboratories, Aedes aegypti mosquito larvae start to die when the water temperature rises over 34°C, while adult mosquitoes begin to die when the air temperature reaches 40°C. Mosquitoes have been observed hiding in small-scale habitats, such as cement tanks, tires, or domestic pitchers, where there is little chance of ambient temperature change.

It’s not just mosquitoes which are alarming mosquitoes though. Another carrier of several infections, including dangerous zoonotic viruses, are ticks. Winters that have been warmer during the past ten years owing to global warming have favored tick proliferation.

Ticks that transmit the virus that causes tick-borne encephalitis have spread into Asia’s and Europe’s northern subarctic areas. Ticks that transmit tropical diseases were also discovered to have survived the winter in Germany in 2019. This finding could be a sign that these ticks are preparing to colonize new areas in the north.

Climate change is increasing the risk of vector-borne diseases, so, the question arises, what can be done to reduce the risk?

In essence, the primary solution, of course, is to solve or combat climate change itself. The sooner we act to mitigate the impacts of climate change – by transitioning from using fossil fuels such as coal or gas to clean, renewable energy like solar or wind – the better off we’ll be in the future. But this change basically takes generational time, resources and commitment. That is why it’ll take a long time for the threat of climate change to go away. But vector diseases are problems that require immediate attention, hence mitigation of climate change by itself won’t be sufficient. We must also take necessary steps such as the following to adjust to the risks:

• Ensure that everyone has access to care and illness management

• Accelerate vaccine development with new technologies.

• Prioritize wetland management and the elimination of vector breeding sites close to populations.

• Explore novel vector control methods.

• Improve disease surveillance, including health education and community surveillance

• Minimize vector exposure (for example, by using window and door screens, protective clothing, insecticide, or habitat avoidance) (for example, the Wolbachia-infected mosquitoes) 

As climate weather experts stress that the need for robust lifesaving early warning system UN, (2022), it should take consideration long term strategies immediately to alleviate with. Meanwhile climate science is increasingly able to show that many of the extreme weather events that we are experiencing have become more likely and more intense due to human-induced climate change. Petteri Taalas (2022) attentively said that we have seen this repeatedly this year, with tragic effect, thus, it is more important than ever that we scale up action on early warning systems to build resilience to current and future climate risks in vulnerable communities. And of course, climate change is major responsible facts to increasing the risk of vector-borne diseases, the sooner we act to alleviate the impacts of climate change, the better off we’ll be in the future. Beside taking mitigation to adapt to the risks, provide universal access to care and disease management is essence, indeed.

(Shrestha is an enthusiast towards community-level health issues, and a MBBS , 2nd Year student at the Nepal Medical College Jorpati. )