A disease vector is a living organism that transmits disease from humans or animals to humans. A disease vector spreads or transmits a disease-causing microorganism from an infected human or animal to a healthy human. Mostly vectors are blood-sucking insects and they ingest the disease-causing microorganism during a blood meal from an infected person or animal (host) and later inject into a healthy person during a subsequent blood meal. It is very important to note that the vector is not producing (cause) the disease but only spreads it. For example, that mosquito is a vector transmits the disease malaria by ingesting the plasmodium parasite into its stomach during a blood meal from a person suffering from malaria and transmits the disease by injecting the plasmodium parasite into a healthy person later (after a few days) during a blood meal from that person. Mosquitoes are the most common disease-vectors and ticks, flies, sandflies, fleas, triatomine bugs and some freshwater aquatic snails are other vectors.
Vector and the vector-borne diseases
Mosquito is Spanish for “little fly.” There are more than 3,500 species of mosquitoes. Both male and female feed mainly on fruit and plant nectar, but the female also needs the protein in blood to help her eggs develop. Hence, female mosquitoes take a blood meal before laying eggs. Hence, only female mosquitoes bite people. A mosquito can drink up to three times its weight in blood. Carbon dioxide in exhaled human breath, ammonia and lactic acid in body emanations, and 98.4 ˚F (36 ˚C) are the cues followed by mosquitoes to locate the source of blood, human. Female mosquitoes can lay up to 300 eggs at a time. Usually, the eggs are deposited in clusters – called rafts – on the surface of stagnant water. The average mosquito lifespan is less than two months. Males have the shortest lives, usually 10 days or less, and females can live about six to eight weeks, Females will lay eggs up to three times before they die.Mosquitoes do not cause diseases but spread several diseases by transmitting the parasite responsible for a particular disease from an infected source. Unless a mosquito has taken a blood meal from a source of infection or an infected person it does not transmit the disease. Hence, every mosquito bite does not cause a disease such as malaria. Mosquitoes are the deadliest insects, mass murderers that kill a large number of people all over the globe by spreading diseases. Though mosquitoes transmit a large number of diseases they do not transmit AIDS virus.
Diseases spread by mosquitoes
The word malaria comes from Italian mal (=bad) and aria (=air), was considered to be the cause of deaths of workers who dug swamps around Rome. Charles Louis Alphonse Laveran discovered the protozoan cause of malaria in 1880. Malaria is caused by parasites of the genus Plasmodium and usually transmitted by the bite of infected Anopheles mosquitoes.Malaria is one of the deadliest of the vector borne diseases that threaten the human race because of the heavy death toll of nearly one million people every year. It accounts for 250 million acute illnesses with 2300 deaths everyday (this was 3000 five years back) and most of them are children. Many children who survive an episode of severe malaria may suffer from learning impairments or brain damage. Pregnant women and their unborn children are also particularly vulnerable to malaria. Though there are four major types of malaria the one caused by the parasite Plasmodium falciparum is the most common human malaria and the principle vector of the parasite is the female Anopheles mosquitoes.
The malaria parasite enters the female Anopheles mosquito during its blood meal from a human being infected with malaria. The parasite that enters the mosquito’s stomach as gametocyte with the blood matures until it reaches the sexual stage where it can again infect a human host. The sporozoites escape the midgut rupturing the oocyst and invade the salivary glands, where they stay for possibly very long periods until injected into another vertebrate host when the next blood meal is taken. The plasmodium takes 10-14 days to invade the salivary glands of a mosquito, therefore when the mosquito takes her next blood meal 10 to 14 days later the parasite enters a person and the transmission cycle of the disease continues. Host/parasite/vector interactions are essential for the transmission of malaria. It has been observed that salivary glands are not depleted of sporozoites even in vectors that feed up to 15 times, which allows infected mosquitoes to remain potentially infectious for life.
Zika virus disease is a mosquito-borne virus it was first identified in Uganda in 1947 in monkeys. In 1952 it was identified in humans in Uganda and the United Republic of Tanzania. Though there are reports on outbreaks of Zika in Africa, the Americas, Asia and Pacific in 2015 Brazil reported Zika virus disease in association with microcephaly leading the fetus deformation. Zika virus is primarily transmitted to people through the bite of an infected mosquito from the Aedes genus, mainly Aedes aegypti in tropical regions. Unlike the mosquitoes that spread malaria, Aedes mosquitoes usually bite during the day, peaking during early morning and late afternoon/evening. This is the same mosquito that transmits dengue, chikungunya and yellow fever.Symptoms of zika virus disease are fever, skin rashes, conjunctivitis, joint and muscle pains and headache. These are similar to the symptoms of other mosquito-borne diseases such as dengue and chikunguniya.
Protection against mosquito bites is tan important measure to protect against Zika virus infection. Wearing clothes (preferably light-coloured) that cover as much of the body as possible is one of the measures. Other preventive measures include:
There are two ways of inclusion of the insecticide(s) in the net namely, impregnation and incorporation. Based on the method of application of chemical nets are classified into coated and impregnated nets. Impregnated nets are otherwise, coated nets wherein the insecticide is coated onto the fiber with the help of a binder. The binder fixes the insecticide onto the fiber. In the case of incorporated nets, the insecticide is mixed and infused into the fiber during the time of extrusion and the insecticide is part of the fiber matrix along with other processing chemicals. Usually polyester nets are coated or impregnated nets and polyolefin (polyethylene and polypropylene) nets are incorporated nets. Owing to higher processing temperature for polyester thermally sensitive active substances cannot be incorporated into the yarn and they can only be coated after extrusion of the yarn. Coating is therefore done on the semi-finished product whereas in incorporation technology the active substance is mixed with the raw materials before drawing (extrusion) the yarn. In the case of incorporated nets bulk of the active substance is in the core of the net fiber (like a reservoir) and is released slowly to the surface by diffusion controlled process until equilibrium is reached. Any active substance removed during washing is replenished from the reservoir in the core. It may be noted that active substances or synergist such as piperonyl butoxide (PBO) cannot be coated and they have to be only incorporated into the fiber. This is the reason for LLINs such as Permanent® -3 to use combination of polyester and polyethylene fabrics. The polyethylene roof has the liquid PBO along with deltamethrin incorporated while the polyester side panels are coated deltamethrin alone. LLINs. PBO LLINs such as VEERALIN®LN have PBO in the entire net because they use incorporation of PBO along with alpha-cypermethrin into polyethylene yarns.
As LLINs kill mosquitoes (the malaria vector) that land on them they reduce the vector population and thus control transmission of malaria. Owing to their effectiveness in controlling malaria transmission LLINs have emerged as a strategic tool in malaria control programme and they have been supplied by The Global Fund the Global Fund to Fight AIDS, Tuberculosis and Malaria (Global Fund), UNICEF, President Malaria Initiative (PMI) USA, NGOs and ministry of health (MOH) of several countries. As per Malaria Report 2017, a total of 582 million insecticide-treated mosquito nets (ITNs) were delivered globally between 2014 and 2016, and of this amount, 505 million ITNs were delivered in sub-Saharan Africa. This indicates that in the last two years 203 million more ITNs were supplied comparing to the preceding 3-year period (2011–2013). The global LLIN supply over the years is given in the figure below
(source: Net Mapping Project, John Milliner, Milliner Global Associates: http://allianceformalariaprevention.com/working-groups/net-mapping-project/).
Figure: Global supply of LLINs from 2004
LLINs or ITNs distribution started in India only from 2009 though a small number of them are distributed over the years and in the year 2017 alone 21.263 million ITNs are distributed this is about 6 million nets more than the total number of bed-nets distributed from2014 (2014-16). National Framework for Malaria Elimination in India 2016-2030 (zero indigenous case) has mentioned the mass distribution of ITNs in controlling malaria in India. Hence, the distribution of ITNs is expected to increase in India.
Figure: ITNs distribution in India
ITNs are distributed through mass distribution campaigns sub-Saharan Africa contributed in malaria control and this has contributed in reduction of malaria incidences and malaria related deaths. As per the World Malaria Report 2017:
Malaria related deaths of adults has decreased by about 50% from 2000 and by 22% since 2010 whereas death of children below five years of age has decreased by about 60% since 2000 and 29% since 2010. Though, the indoor residual sprays (IRS) and other vector control and malaria campaign activities have contributed to the reduction ITNs usage plays a significant role in achieving this.
Figure: Malaria deaths of adults and children below 5 years since year 2000.