Malaria – Mosquitoes and Disease Transmission: A Global Perspective

Malaria – Mosquitoes and Disease Transmission: A Global Perspective

Malaria – Mosquitoes and the diseases they spread have been responsible for killing more people than all the wars in history, they belong to the group of insects known as diptera (flies). Since ancient times, mosquitoes have been appreciated as the source of various ailments afflicting humans.

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Mosquito-borne diseases are flourishing worldwide with a disproportionate effect on children and adolescents, these conditions are responsible for substantial global morbidity and mortality (White, 2004; WHO, 2006; Milner, et al., 2008).

Comprising approximately 3500 species, Mosquitoes are found beyond the tropical and subtropical regions of the world with which they are classically associated (Reiter, 2001). In the course of the District’s operation about 10 species are commonly found in the country, eight of the species accounts for over 99%of complaints from the public. There are three commonly groups – Anopheles   (malaria, filariasis), Aedes (yellow fever, dengue) and culex (West Nile, Japanese encephalitis (White, 2004).

Most female mosquitoes take blood meals from vertebrate to obtain the necessary nutrition to produce their eggs, (Robert, 2001; White, 2004), injecting saliva into the host animal. Those which regulary feed on humans, and in which pathogens can complete an obligatory life cycle phase and multiply in the mosquito’s salivary glands, can be important vectors of human diseases (White, 2004).

Mosquitoes breed in water, occasionally depositing eggs directly on water, but generally using a variety of moist surfaces, tree holes and containers (White, 2004). Human activities, such as production of a large amount of environmental debris that holds water pools (disposable bottles and cans and discarded tires) and storage of water on or around living premises, may markedly increase available mosquito-breeding sites.

Malaria in particular, continues to impact a major disease burden on infants and young children in endemic region (WHO, 2000; Jones, et al.,; Fegan, et al.,). There are 350 – 500 million cases of malaria annually, with at-least 1 million deaths affecting mostly infants and young children.

Dengue virus has expanded its range and is responsible for 50 – 100 million infections annually, with thousands of deaths, mostly from its severe dengue haemorrhagic fever.

West Nile virus emerged in the Americas, becoming endemic throughout the region (Campbell, et al., 2002).

Japanese encephalitis virus has expanded its range in the Indian subcontinent and Australasia affecting children less than 10 years (Oya, et al., 2007).

Due to coordinated control strategies, filariasis, a parasitic disease causing elephantiasis, has become less common and is the subject of a global eradication campaign directed by the World Health Organization (WHO).


Malaria is a mosquito – borne disease that affects more tan 500 million people annually, causing between 1 and 3 million deaths, most common in tropical and subtropical climates. Malaria is a protozoa infection of red blood cells and ranks as the most significant parasitic disease affecting humans (White, et al., 2004). The overwhelming majority of malaria infections are caused by P. falciparum and P. vivax (Baird, 2005).

Before the 20th century, malaria was found throughout most of the nonpolar world, including North America and Northern Europe. It has been eradicated and reduced in some places (Mexico, the Middle East, North Africa and China). Malaria transmission has traditionally been considered restricted to certain temperature ranges (below 16 0 C parasite development within the mosquito cannot take place) and altitude below 2000m (White, 2004). However, malaria distribution may be changing as a result of global climate change, with increased average temperatures in the East African highlands being cited as a possible explanation for the return of malaria transmission to highland areas>2000m in recent years (Alsop, 2007).

P. falciparum causes the overwhelming majority of severe malaria cases and thus is the chief contributor of global malaria morbidity and mortality. P. vivax and P. ovale have the ability to form a dormant hepatic form known as a hypnozoite that may cause disease. P. malariae is a relatively rare form of malaria, responsible for perhaps 1% cases worldwide.

Plasmodium Life Cycle

The malaria parasite exhibit an alternative of two different host organisms in a whole life cycle: an invertebrate (the Anopheles mosquito) and a superior vertebrate (Man)

Vertebrate Phase

Human infection with malaria also known as the asexual or schizogonic cycle. It begins with a mosquito bite and the associated injection of infectious sporozoites. The sporozoites  invades the blood stream and reach the liver within a few minutes (one hour at most) where they initiate the pre-erythrocytic phase of development. This results in a large schizonts, containing tiny merozoites. The duration of this phase depend on the specie of  plasmodium: 5 to 6 days for P.falciparum to 10 to 14 days for P. vivax to occasionally much longer (approx 1 month) for P. malariae (White, 2004). At maturity, the schizont ruptures, liberating merizoites into the circulation and invade red blood cells and develop into male and female gametocytes.

Invertebrate Phase

This is referred to as the sexual or sporogonic cycle. It occurs almost entirely in the female Anopheles mosquito. It begins when the female Anopheles feeding on a patient with malaria, ingests human blood containing male (micro) and female (macro) gametocytes. The female gametocyte developes directly into a single microgamete. The male gametocyte however, undergo a very rapid nuclear division and develop eight flagellate filaments each containing a nucleus.These then seperate from the microgamete body and develop into macrogaetocytes. In the case of p. falciparum, where the gametocytes are crescentshaped, the first step of development is that the gametocytes assume a round form like other plasmodia parasitizing man. The whole process is influenced by temperature essentially (it is usually completed within twenty minutes). The microgamete produced by the flagellum moves toward the macrogametes and penetrates  it to form zygote or fertilized ovum which develops within 24 – 48 hours to slowly moving ookinete. The immobile ookinete with its outercovering is called oocyst. The oocyst matures and ruptures, setting free sporozoites in the insects haemocoele. They are carried to all parts of the mosquito’s body in circulation. The sporozoites concentrate in the insect’s salivary glands where they mature into infectious forms and then migrate to the salivary ducts, ready to be injected into man.


Malaria is restricted to areas where these mosquitoes can breed that is the tropics between 600 N and 400 S. It is of major importance in Africa, India, the far East and South America. Because of drug and insecticide resistance, malaria is now on the increase globally. About 35% of the world’s population is estimated to be infected, with some 10 million new cases annually and perhaps 2 million deaths. Increased air level means cases are regularly seen in the developed world, and unless the diagnosis is constantly borne in mind, vital treatment may be withheld, with fatal results. Malaria can also be transmitted by blood transfusion, needle accidents or very rarely, from mother to fetus (Minis et al., 2007).

Clinical Presentation

Faciparum malaria typically develops from days to weeks after exposure, while P. vivax and p. ovale may develop significantly later, due to their ability to form hepatic hypnozoites (Stauffer et al., 2003). P. malariae has a long incubation period (mean, 30 days).

Children and adolescents with malaria typically present with fever, rigors, headache, muscle aches, nausea and vomiting and considerable fatigue (Stauffer et al., 2003).

Nonspecific laboratory abnormalities are common with malaria, including elevated liver enzymes, thrombocytopenia, and neutropenia. Hyponatremia and hypoglycaemia, when present, are associated with a more severe clinical course (Stauffer, et al., 2003).


In the absence of re-infection, P. vivax, p. ovale and P. malariae are normally self – limiting infections, but relapses may occur. P. falciparum malaria, however is frequently fatal during the firs 2 – 3 weeks due to a variety of complications. Relapses may occur after months or even years, especially in P. vivax because of parasites that lie dormant in the liver (hyponozoites). Complicated P. falciparum malaria is most common in children aged between 6months and 5years and in pregnant, particularly primigravid, women (Cheesbrough, 1987).

Cerebral malaria with headache, neck stiffness, convulsion and coma. Anaemia, diarrhoea and vomiting may also occur.


The proper management of malaria depends on the diagnosis being swift and reliable. In many malaria endemic areas where diagnostic may be less available, clinical diagnosis of malaria is commonly employed (Font et al., 2001; Reyburn et al., 2004; Stauffer et al., 2003).

A variety of diagnostic tests exists for malaria. Historically, malaria has been diagnosed with blood smears; a minimum of three sets must be performed before malaria can be tentatively ruled out in an individual with a history of possible malaria exposure and clinical signs and symptoms potentially consistent with the disease (Stauffer, et al., 2003). Thick blood film evaluate the presence or absence of parasites, while the particular plasmodium species and its quantification may be determined on thin smears; parasitemias of > 5 % of visible red blood cells are associated with severe malaria (Griffith, et al., 2007; WHO, 2000). Blood smears may be more or less sensitive or specific depending on severe factors, including the quality of smear preparation and the experience of the individual interpreting the smear (Stauffer, et al., 2003).

In many settings, rapid diagnostic test (RDTs) based on the detection of species – specific histidine – rich proteins (HRPs) or lactate dehydrogenase exist which will readily both diagnose malaria and differentiate between falciparum and non-falciparum infections (Moody, 2 002; Reyburn, et al., 2007). One recently reported randomized clinical trial demonstrated 95.4 % sensitivity and 95.9% specificity in the detection of malaria using RDTs (Reyburn, et al., 2007). In many resources limited -settings, nonclinicians (such as community health  workers (CHWs)) are depended on to perform clinical tasks such as the evaluation of febrile children for the possibility of malaria (Harvey, et al., 2008). Molecular diagnostics may be particularly useful where malaria incidence is low and false-negative microscopy or RDT results are more likely to be accepted as valid (Mens, et al., 2007).

Malaria Treatment

          Children more than age 6 who have a history of malaria and reside, or have recently resided, in an area hyper or holoendemic for malaria may be considered semi-immune; they may be managed as outpatients if nontoxic- appearing and in a family context where outpatient evaluation and treatment is feasible (Stauffer, et al., 2003).

Historically, most malaria of all species were sensitive to drugs such as quinine and chloroquine (Griffith et al., 2007). The WHO has produced malaria treatment guide-line, and the  U.S. and U. K. guide lines have recently reviewed (Lalloo, et al., 2007).

The choice of specific antimalaria therapy depends on several determinations, including the severity of clinical presentation, the malaria species, the degree of parasitemia, the patients ability to take oral medications, and the availability of medications (Lalloo, et al., 2007). In much of the world, artemisinins are used in the treatment of malaria (Rosenthal, 2008). Artemisimins (including artesunate, artemether, dihydroartemisinin, and others) posses rapid action against all erythrocyte forms of plasmodium including gametocytes. For serve malaria, the WHO recommends intravenous artesunate as the treatment of choice in children and adolescents in areas of hypoendemic or unstable malaria.

During pregnancy, WHO recommends either artesunate or quinine during the first trimester, and artesunate as initial therapy during the second and third trimesters (WHO, 2000).

Malaria Prevention

In endemic, resource –limited areas, priority has been given to broad public health approaches, including the use of insecticide- treated bed nets (ITNs) and curtains, intermittent preventive therapy (IPT), and vector control along with prompt case detection and treatment.

Insecticide-treated bed nets (ITNs) function as a “ baited trap” where mosquitoes are attracted to the individual sleeping under the net and killed (WHO, 2006). In Kenya, where ITNs are chiefly distributed to woman and children under age 5, ITN coverage for children less than age 5 has increased from the 7 % in 2004 to 67 % in 2006; this improved level of ITN coverage has been associated with a 44 % reduction in child malaria deaths (Fegan, et al., 2007). ITNs are also associated with a reduction of malaria in pregnancy that yields important benefits for both mother and infant (Gamble, et al., 2007; Schwarz, et al., 2008).

Intermittent preventive treatment (IPT) one of a three-part approach to malaria control in pregnancy (intermittent preventive treatment during pregnancy IPTP) (Gamble et al., 2007). IPT involves the administration of a complete therapeutic course of an antimalarial treatment at set times to members of an at risk population. WHO recommends IPTP with SP (Sulfadoxine-pyrimethamine) on at least two occasions after the first trimester (WHO, 2004).

Vector control: A key notron in Roll back Malaria (RBM) initiative programme, vector reduction seems an obvious and important part of  the control of any vector-transmitted disease and generally considered the most effective measure in the disease’s prevention (WHO, 2006). This involve the use of insecticide dichloro-diphenyl trichloroethane (DDT), indoor residual spraying (IRS)( (WHO, 2006).

Use of repellents: Insect repellents containing N,N-diethyl-M-toluamide (DEET) to be placed on skin (Hayes, et al., 2004). To be used at no more than 30 % concentration with infants and not to be used on children less than age 2 months (American Academy of paediatrics, 2003).

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This article was extracted from a SEMINAR Research Work/Material Topic “ MOSQUITOES AND DISEASE TRANSMISSION: A GLOBAL PERSECTIVE”

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