Climate change is impacting water related diseases throughout the world and one of the most significant public health complications. Some mosquitoes are the important vectors of diseases like malaria, dengue fever, dengue hemorrhagic fever, chikungounia, yellow fever, filariasis etc. Vectors growth is influenced by many environmental factors like urbanization, deforestation, rainfall, travel and trade.

Climate Change and mosquitoes

According to WHO, there are over one million people die due to mosquito’s related diseases every year.  As WHO reported that 3.2 billion people (half of the world population) is at risk of malaria. Before 1970, it was epidemic in only 9 countries but in recent decade, its spreading throughout the world.

According to American mosquito control association, 40% of the world’s population is susceptible to malaria and one child is killed after every 40 second by malaria. Thousands of people in Pakistan were diagnosed with dengue fever since 2010. According to WHO, 16580 cases and 257 deaths were caused by dengue fever in Lahore while 5000 cases and 60 deaths from rest of the country since 2010.

Malaria is caused by biting female mosquitoes named Anopheles (Anopheles gambiae; the most active one) which transmit plasmodium into the blood of human and mostly active at night. Malaria is caused by transmission of parasites Plasmodium malariae, P. falciparum and other species. They lay eggs in stagnant water, pools etc. They complete their life cycle in 7-20 days with four different stages.

Transmission and severity of disease depends on areas and age of the person. High temperature affects the virulence capacity of Anopheles. Dengue fever is a mosquito related viral disease spread through biting of mosquito Aedes aegypti and A. albopictus which are active both in dusk and dawn. Dengue fever spread through four closely related viruses or serotypes 1-4. Unlike Anopheles, Aedes aegypti lays their eggs in clean water and must feed on blood to lay eggs.

High temperature kills mosquitoes but warmer temperature increases their biting capacity and reproduction. Symptoms appear 3-7 days after biting and last 3-10 days. This disease spread through mosquito bite and rarely transfer through organ transplants and blood transfusion.

These vectors related diseases are influenced by climatic factors like temperature, humidity, change in rainfall pattern, floods along with urbanization and anthropogenic activities. Climate controls the distribution, life cycle and efficiency of transmission of diseases.

Malaria and dengue fever flourish after monsoon especially during August to November. Warm and humid climate support the growth and transmission of parasites through vectors. Surface water, vegetation, mid-season temperature supports the growth and provides habitat for breeding.

With the changing rainfall pattern and increase in temperature may change the mosquito species, population density and geographic distribution. Low temperature (<10oC) kills the Aedes eggs, larvae and adults. Yi and coauthors (2014) reported the effect of global warming on mosquitoes and related diseases and their control strategies.

They reported that higher rainfall can increase vector population and transmission of diseases by providing more habitats and breeding places but not in all conditions as floods may have negative effects on increasing the density of population because floods may wash or destroy their eggs and larvae.

Numerous ways are used to control mosquitoes which include chemical, biological, environmental management, personal protective measures and physical methods. Chemical methods include the use of tested and recommended insecticides (Pyrethroids for killing adults and larvae) should be used one month before transmission time.

These should be used under supervision of experts and trained staff (a team of Entomologist, vector control supervisor and field staff). Chemical spray or aerial spray of chemicals by low flying aircrafts (to cover large area or when there is limited access by vehicles) should be accomplished at their habitats, resting sites and breeding places after regular intervals of 2-3 weeks in homes for killing vectors. Spray should be done on all sleeping rooms, washrooms, walls corners etc.

For dengue control; manmade habitats should be screened and Methoperene/Altosid (Briquets) and Diflubenzuron (Dimlin) should be applied. As reported by Yi and coauthors (2010), diesel oil is effective in killing larvae and pupae of the mosquitoes in small water resources but this can kill unwanted aquatic animals. They suggested golden bear oil but only available in USA.

They suggested various ways to control mosquitoes like mosquito’s traps, genetically modified male mosquitoes, mosquito counter device etc. As suggested in guidelines for control of vectors of public health importance (2010) by Ministry of Health that during epidemic or outbreak indoor fogging or space spraying is important for the control of dengue.

Larvicides should be applied on clean and stagnant water. In case of drinking water Abat/Temephos, Methoperene/Altosid (XR Briquets), Bacillus Thuringienesis (BT), Diflubenzuron (Dimlin) are recommended larvicides. When using for drinking water dose related manufacturer instruction should strictly apply. Alphacypermethrin 10%, Deltamethrin 1.5% and Sumilarv are commonly used insecticides for the control of mosquitoes.

Biological methods include growth of insecticide repellents plants in home, gardens and nearby places. These plants include basil (Ocimum bacillicum), lemon thyme (Thymus citriodorus), lemon grass (Cymbopogon citratus), Lemon balm (Melissa officinalis), sweet wormwood (Artemisia annua) and rosemary (Rosmarinus officinalis).

Crushed leaves can be applied on the skin as mosquito repellent and an effective remedy but this will be of short duration. Natural oils like Neem (Azadirachta indica), Soya bean (Glycine max) oil and catnip (Nepeta cataria) are effective remedy which can be used as mosquito repellent. 

Another biological method includes Bacillus thuringiensis a naturally growing bacterium that can eradicate larvae of the mosquito. This weapon can be used in standing water, ponds etc. which also safe to human beings and aquatic animals.

Various fish types like gold fish, cat fish are predators of mosquitoes, but a fresh water fish commonly known as mosquito fish (Gambusia affinis) is used in mosquito control program in California. This feed on larvae and eggs of mosquitos, other insects and fish eggs.

It is not recommended for water resources like lakes, rivers etc. containing other useful aquatic animals. Native fish or larvivorous fish can also be used for the control of mosquitoes as they are well adapted to the natural habitat. Dragonflies are good to control mosquito as adults feed on adult while larvae feed on mosquito’s larvae.

These need special environment like water, vegetation; rocks etc. are useful for small ponds, gardens in home for controlling mosquitoes. Blue bird (Sialia spp.), frogs and toads are excellent predator of mosquito and their larvae.

Environmental management is important especially the controlling of mashes, open drains, standing water in the open fields etc. controlling marshes, surface water, gardening and waste management.

Personal protection measures include personal protective clothing, bed nets (long lasting insecticide treated nets and curtains at the doors), use of gauze in the doors and lotions for insect repellents. Picaridin/Icaradine and DEET are best recommended repellents can be used in emergency.

Cloth can be treated with Permethrin to control mosquitoes at the recommended dose 1.25mg/m2 after every five washing. Physical methods include closing the doors especially in the morning and evening etc.

With the changing climate, more attention is required from the experts, health department, before spread of dengue and malaria in Pakistan this year and in future. It is advisable that malaria and dengue control program should be a part of national policy of health with strong commitment of resources and implementation of policies.

Awareness and educating people among the community is very important about vector borne diseases especially mosquito related diseases like malaria and and dengue fever. Awareness campaigns can be started from educational institutions, offices, corner meetings etc. Cleaning at household level with detergents, insecticides and other surface cleaning agents are highly recommended.

Media can play important role in spreading awareness through newspapers, TV programs, talk shows etc. Reduction of breeding sources and waste management campaign are very important at community level. Health protection campaigns should be the top priority.

This article is collectively authored by Toqeer Ahmed 1 and Muhammad Mukahtar 2. The authors work at the 1Centre for Climate Research and Development (CCRD), COMSATS University Islamabad, Islamabad Campus, Park Road Chak Shahzad, Islamabad and 2Department of Zoonotic and Vector-Borne Diseases & Epidemic Investigation Cell, Public Health Laboratories Division, NIH-Islamabad.