Introduction
Mosquitoes are vectors of numerous infectious agents (parasites and viruses) that transmit diseases from animals to humans and humans to humans. Most mosquito-borne diseases are fatal or result in comorbidities that last for a few years or throughout the lifespan of affected individuals. Mosquitoes are reported as the deadliest vectors transmitting illnesses in humans by WHO (World Health Organization).
The emergence and resurgence of infectious pathogens transmitted through mosquitoes have raised an alarming alert on global public health. Recent studies suggest that mosquito-borne infections are not only primary health problems in endemic areas like Africa and Asia but also major health threats in developed countries like the United States of America and European countries due to an increase in uncontrolled urbanization, international travel, global trade and relocation of the population from endemic areas.
To tackle such health threats and lower their incidence rates, international health agencies like Centres for Disease Control and Prevention (CDC), local governments, and research scientists are working ceaselessly to upgrade existing vector-control methods and discover new techniques and approaches in reducing transmission.
What Are the Illnesses Transmitted by Mosquitoes?
Malaria and dengue are the most prevalent mosquito-borne diseases that pose a major health burden in endemic areas. However, recent WHO reports suggest that the incidence of encephalitis caused by a variety of mosquito-vectored viruses is far more increasing in developed countries like the US. The predominant species of mosquito that transmit disease are Aedes sp., Anopheles sp., and Culex sp.
Why Is It Important to Control Mosquitoes?
Understanding that mosquitoes act as carriers of various life-threatening pathogens and transmit fatal illnesses, reducing the vector population is considered the most effective approach for controlling disease transmission by mosquitoes.
Global health regulatory bodies such as WHO, CDC, and local authorities have formulated multiple protocols and guidelines for vector control based on the geographical and environmental factors that favor mosquito existence in prevalent areas.
Following are some facts about the behavioral patterns of mosquitoes that further help in comprehending mosquito-control techniques.
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There are over 3000 types of mosquito species present worldwide. Among them, only some bite animals and humans.
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A few mosquito types bite animals or humans but do not transmit diseases. Such types are termed 'nuisance mosquitoes.'
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Only adult female mosquitoes bite to retreive blood from animals and humans because they require blood to produce eggs.
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When mosquitoes suck in blood from animals or humans harboring pathogens (parasites and viruses), they get infected and become vectors of such causative organisms.
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The pathogens remain alive or complete a part of their life cycle inside the mosquito body before they are injected into another animal or human through mosquito bites, and thus transmission of infection continues.
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Only a minimum number of mosquitoes is needed to trigger a massive outbreak in a particular area.
Mosquito control is an essential initiative executed for the wider protection of public health and to prevent vector transmission worldwide.
The predominant reasons to establish mosquito-control practices both indoors and outdoors are,
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To reduce the population of mosquitoes that serve as vectors of infectious diseases and to control outbreaks of mosquito-borne illnesses, which are major public health threats.
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To control nuisance mosquitoes that disturb people's normal activities at homes, indoor settings, parks, and other recreational places.
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To eradicate mosquito population in areas of economic importance such as places of tourism value, real estate business ventures, and livestock and poultry farms where mosquitoes harm their economic production.
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To hamper and minimize mosquito adaptive behaviors, which they incorporate in them genetically to survive and evolve in their changing environment.
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To remove mosquito types that not only carry but also act as secondary hosts for mutations of pathogens resulting in the emergence of new virulent strains.
What Are the Diverse Techniques Employed in Mosquito-Borne Disease Control?
The basic fundamental step to control mosquito-borne illnesses is public awareness and prevention-control programs designed with scientific inputs. This includes large-scale surveillance of endemic areas and mosquito behavior and population monitoring. Epidemiological studies conducted on outbreak areas have helped identify the type of mosquito species that act as vectors and environmental factors that influence its life cycles, such as climatic conditions, associated flora and fauna, and adaptive mechanisms.
The three major components of mosquito-borne disease control are,
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Physical barriers.
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Chemical barriers.
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Biological barriers.
1) Physical Barriers
The prime strategy to prevent mosquito biting is avoidance which creates physical inaccessibility for the vector to reach the host. Surveillance and identification of breeding grounds such as stagnant water, backwaters, ditches, construction debris, ponds, and drains that collect rainwater and eliminate them help reduce population.
Removing unwanted vegetation around homes and waterbodies also prevents the breeding of mosquitoes. Using wire mesh for doors and windows, mosquito nets are some of the physical barrier methods.
Recent advances in physical barrier methods include using Long-Lasting Insecticide-Treated Nets (LLINs) designed using fabric technology. These nets are coated with synthetic insecticides and are used as an efficient vector-control tool. They are more durable and also reduce environmental side-effects caused due to spraying of insecticides.
2) Chemical Barriers
Mosquito repellents are the primary defense practice to avoid mosquito bites. They are useful both indoors and outdoors.
Some of the commonly used repellents are-
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DEET (N, N-diethyl-meta-toluamide) - commonly used standard repellant.
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Picaridin (Odorless and non-sticky)
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Lemon oil of eucalyptus (not recommended for younger children)
The use of dichlorodiphenyltrichloroethane (DDT) as a preventive measure has been in practice for many years. But current studies prove that mosquitoes are becoming DDT-resistant, which leads to the development of new insecticides.
Current chemical techniques involve,
- Larviciding - It is done by adding methoprene-containing pesticides to stagnant water where mosquitoes breed and larvae form. Larvae ingest these chemicals and are killed by the toxicity of the pesticides.
- Adulticiding - Insecticides like 2 % pyrethroids and 95 % organophosphates are sprayed using peridomestic spraying techniques. Insecticide concentrations depend on the percentage of active ingredients and the toxicity level of the targeted mosquitoes. Through spraying methods, thermal fog, and cold fog (ultra low volume treatment), pesticide droplets are released into the air to eradicate the mosquito population.
Over the past decades, chemical methods have tremendously helped in controlling the transmission of mosquito-borne diseases in endemic areas and in outbreak times. Also, newer chemicals are discovered with minimal harmful side effects. However, the emergence of insecticide-resistant mosquitoes is an alarming threat and is a major obstacle in vector control.
3) Biological Barriers
Research studies on genetics, insect biology, and bioengineering have led to a better understanding of host-parasite interactions and the development of biological vector control programs. This includes genetic modifications of vectors, introducing predator agents in target environments, vaccine research, and using bacterio-insecticides that inhibit the development of mosquito resistance.
- Biological Predator Agents - Biological agents like elephant mosquitoes and fish predators introduced in aquatics have larvicidal effects. Certain species of fungi (Coelomomyces, Beauveria, Lagenidium, Culicinomyces, Metarhizium, Entomophthora) also exhibit larvicidal activity against vector species.
- Bacterial-Insecticides - Bacteria like Bacillus thuringiensis israelensis, Bacillus sphaericus, etc., are used as larvicidal agents. On introducing these bacteria into the larval population, the bacterial toxins destroy the larvae. Studies show that mosquitoes infected with the bacterium Wolbachia have reduced the capacity to transmit pathogens since the bacterium inhibits the multiplication of pathogens inside the vector.
- Sterile Insect Technique (SIT) - It is a genetic modification method specific to certain mosquito species. A large population of genetically engineered sterile male mosquitoes is released in the target area to compete with wild types. Female mosquitoes mate with genetically modified mosquitoes (GMMs) and yield minimal progeny, thus reducing vector population over generations.
- Introducing a Lethal Dominant Gene (RIDL) - Release of insects with lethal dominant gene) in modified mosquitoes causes the death of female mosquitoes. The use of RIDL is a promising tool to completely replace DDT-resistant mosquitoes.
- Vaccine Development - The discovery of vaccines, particularly malarial vaccines, inhibit the protozoan life cycle in humans. They effectively protect young children and are widely used by people who frequently travel to endemic areas. Recent advances in vaccine development include using biomolecules that directly target vectors.
Scientists have developed a new serum that directly inhibits a biological component in the mosquito's saliva at the deposition site, thus preventing the pathogen's entry into the bloodstream.
Dengue tetravalent live vaccine [CYD-TDV; licensed(2015) in Mexico], inactivated alum-adjuvant JE vaccine [strain used SA14-14-2; approved(2009) North America, Australia, European countries], and Vero-cell derived JE vaccine (Kolar strain- 821564XY; India) are some of the newer vaccines in current clinical practice against mosquito-borne diseases.
Other future perspectives in vector control include genetically modifying vector microbiome, applying CRISPR-Cas genetic tool to edit the genome of the vectors, and projects on vector population genomics are novel strategies that would result in breakthrough developments in vector-disease control.
Conclusion
Globally, mosquito-borne illnesses have persisted as life-threatening public health concerns over recent decades. The resurgence of non-pathogenic microbes as more virulent pathogens and the emergence of newer pathogens using vectors as secondary hosts remain a perennial challenge to the clinical community and world health authorities.
Numerous vector-control programs have succeeded and failed since mosquitoes intrinsically show quick evolutionary modifications to the change in their surviving environment. Since most of these adaptations occur genetically, current research and vector-control technology target to alter their genome at the molecular level. Further research and understanding of vector-pathogen-host interactions would eventually lead to implementing newer and safe vector-control strategies, thus helping in successfully combating mosquito-borne illnesses.
