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MALARIA DISEASE

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       Malaria is a parasitic disease caused by species of the genus Plasmodium, and only Anopheles spp. mosquitoes can transmit the disease.  Humans may be infected by only four species of the parasite:  Plasmodium falciparum, P. vivax., P. malariae and P. ovale.  The sexual cycle of the parasite occurs in the mosquito, and therefore Service (2008) explained that it is best to consider the mosquito as the definitive host and humans as the intermediate host.

Plasmodium vivax - Life Cycle

 

Plasmodium Sexual Cycle

 

       Mosquito females while feeding on blood ingest the Plasmodium male and female gametocytes.  They then enter the mosquito's stomach where they develop cyclically, which includes a sexual cycle called sporogony.  Only the gametocytes survive in the stomach, while all asexual forms die.  The male gametocytes (called "microgametocytes") develop a flagellum which is the male gamete.  These microgametes move about freely while they fertilize the female gametes (called "macrogametes").  The result is a zygote that increases in size to become the "ookinete".  The ookinete passes through the wall of the mosquito's stomach to reach the outer membrane where it becomes spherical and transforms into an "oocyst".  The oocyst's nucleus then divides several times producing many spindle-shaped "sporozoites."  About eight days later the mature oocyst ruptures, which releases thousands of sporozoites into the mosquito's haemocoel.  sporozoites are then carried in the mosquito's haemolymph to all parts of its body, but most enter the salivary glands about 9-14 days later. 

 

       The cycle length varies with temperature and different Plasmodium species.  At 30-deg. Centigrade sporogony in Plasmodium falciparum requires nine days, while at 25-deg. C. 10 days and at 20-deg. C. 23 days. At temperatures below 17-deg. C. the cycle cannot be finished.  With Plasmodium vivax sporogony develops quicker, being completed in 9 days at 25-deg C. and 16 days at 20-deg. C. (Service 2008).  For graphic details please see <Plasmodium Cycle>

 

       With completion of the cycle the mosquitoes can now infect humans with sporozoites.  Service (2008) noted that a single oocyst may produce over 1,000 sporozoites, but in heavier infections there could be 60,000-70,000 sporozoites in the mosquito's salivary glands.  However, sometimes only less than 10 will be transmitted to a human during a mosquito bite.  Also the percentage of female mosquitoes that have sporozoites in their salivary glands is variable both seasonally, species wise and by locality.  The figure is usually 1-5 percent, but the mosquito can remain infective for its entire life.

 

Mosquito Vectors

 

        A discussion of the principal mosquito Anopheles species that transmit malaria to humans and animals in the major regions of the world follows.  Many details were given by Matheson (1950) and Service (2008) that are valuable for a broad understanding of the complexity and diversity of the principal vectors in North America, whereas Service (2008) also stressed other world regions.  The Anopheles mosquitoes differ from those of the Culex genus in several ways (see Differences), and the adults of some important Anopheles vectors may be identified from Wing Patterns.

 

UNITED STATES & CANADA

 

       North America north of Mexico has very few vectors of malaria, and reservoirs of Plasmodium are very scarce, existing only temporarily when malaria-infected individuals enter the continent from abroad.  The screening for diseases of legal immigrants has kept the incidence of malaria and other diseases low, but the steady increase of illegal immigration without screening now poses a significant threat to public health.  A discussion of two mosquito species of principal importance follows:

 

       Anopheles freeborni -- Range is from southern Oregon, California to New Mexico and West Texas.  Larvae are found in fresh water seepages, irrigation ditches, rice fields, streams and in open sunlight.

 

       Anopheles quadrimaculatus -- Range is from Massachusetts west through Ontario to Minnesota south to central Texas, the Gulf of Mexico doast and east to the Atlantic coast.  Larvae are found where vegetation is usually abundant in lakes, ponds, impounded waters, freshwater marshes, swamps, bayous, grassy pools and among driftwood.

 

SUB-SAHARAN AFRICA

 

Anopheles gambiae Complex

 

       Out of the almost 490 identified species of Anopheles mosquitoes, only 73 are known to serve as vectors of malaria.  Of these about 42 are important vectors, which varies in different geographical areas.  Service (2008) and Matheson (1950) pointed out that some vectors occur in species complexes that comprise almost identically appearing species, which can be identified only by their chromosomal banding patterns, by biochemical procedures or by molecular methods.  Also, mosquito species within a complex can differ in behavior, distribution and vector status.  The Anopheles gambiae complex in sub-Saharan Africa is one of the most important, and are discussed as presented by Service (2008) and Matheson (1950) as follows:

 

       Anopheles gambiae -- Of the seven species that make up the Anopheles gambiae complex, A. gambiae is the most important.  The larvae occur in sunlit pools, hoofprints, borrow pits and rice fields.  Adult mosquitoes draw blood from humans both indoors and outdoors, and also feed on domesticated animals.  They rest primarily indoors, but also outdoors.  Other vectors in this complex are Anopheles arabiensis, Anopheles melas, Anopheles merus and Anopheles bwambae.  Anopheles quadriannulatus is also present but they feed primarily on cattle.

 

       Anopheles arabiensis -- The larvae occupy the same habitats as Anopheles gambiae.  Adults draw human blood indoors and outdoors but also from cattle.  After feeding they rest either indoors or outdoors.  The species occurs in drier areas and is more apt to bite cattle and rest outdoors.

 

       Anopheles melas & Anopheles merus -- An. melas breeds in coastal salt waters of West Africa, while An. merus is a coastal saltwater species in East and southern Africa, and can also be found in inland saltwater habitats.  Both species feed on humans and rest indoors and outdoors, but they are regarded as secondary vectors.

 

       Anopheles bwambae -- This is a more rare species that breeds in warm mineral springs of Uganda.  It is not considered to be as important a vector even though it can transmit malaria in its restricted breeding area.

 

       Anopheles funestus -- The larvae inhabit the more permanent waters, especially those with vegetation, such as marshes, stream margins, rivers and ditches.  It prefers shaded areas.  Adults favor humans but will also draw blood from domesticated animals.  Feeding is both indoors and outdoors, but adults rest primarily indoors.

 

EUROPE, NORTH AFRICA, MIDDLE EAST

 

       Anopheles atroparvus -- This is one of 12 species in the Anopheles maculipennis complex.  Breeding is around sunlit pools and ditches that contain both fresh and brackish water.  Adults draw blood from humans and domesticated animals.  They regularly rest in stables, cowsheds and piggeries.  The adults hibernate in these and other shelters during winter, but will sometimes emerge to draw blood.

 

       Anopheles labranchiae -- Also in the An. maculipennis complex, breeding occurs in brackish water of coastal marshes or freshwater marshes, the banks of streams and ditches and rice fields.  They prefer sunlight.  Humans and domesticated animals are bitten both indoors and outdoors.  Adults rest in houses or animal shelters after feeding.  Adults hibernate in winter.

 

       Anopheles pharoensis -- Breeding is in marshes, ponds and water with grassy or floating vegetation and rice fields.  Adults draw blood from humans and animals both indoors and outdoors, but adults rest outdoors after feeding.  This species is especially active in Egypt.

 

       Anopheles sacharovi & #2-- Occurs in fresh or brackish water of coastal or inland marshes, pools and areas with vegetation.  They prefer sunlit habitats.  Humans and animals are bitten both indoors and outdoors.  Adults rest in houses or animal shelters.

 

       Anopheles sergentii -- They inhabit borrow pits, rice fields, seepage waters and slow-flowing streams and both sunny or partially shaded habitats.  Adults draw blood from humans and animals indoors and outdoors, but rest in houses and caves after feeding.

 

       Anopheles stephensi -- This can be an important vector in urban areas.  The distribution is wide from Egypt to India and China where it is often the principal vector of urban malaria.  Larvae breed in fresh, brackish or even polluted waters and in containers.  Adults draw blood indoors and outdoors and rest indoors afterwards. 

 

       Anopheles superpictus -- The species if found in flowing waters, pools and muddy areas.  Sunlit areas are preferred.  Humans and animals are bitten both indoors and outdoors, but after feeding adults rest primarily in dwellings, animal shelters and caves.

 

INDIAN SUBCONTINENT

 

       Anopheles annularis -- This is an important vector of malaria in India.  The larvae occur in ponds with vegetation, swamps and rice fields.  Adults draw blood from humans and cattle outdoors and indoors, and rest primarily outdoors.

 

       Anopheles culicifacies -- This is one of 5 species in the An. culicifacies complex.  It is the most important malaria vector on the Indian subcontinent.  Larvae are found in unpolluted habitats, irrigation ditches, wells and at the edges of streams, marshes rice fields and less commonly in brackish water.  Adults prefer domesticated animals but also draw blood from humans both indoors and outdoors.  Adults rest mainly indoors.  This is the principal malaria vector on the subcontinent.

 

       Anopheles fluviatilis -- There are two species in the An. fluviatilis complex.  The habitat is flowing waters, irrigation ditches and pools.  Humans and domestic animals are bitten both indoors and outdoors.

 

       Anopheles minimus -- Occurs in the An. minimus complex.  Habitat is shaded areas around flowing waters, irrigation ditches and rice fields.  Feeding is primarily on humans but occasionally domestic animals.  Adults rest mostly indoors.

 

       Anopheles stephensi -- Habits are the same as in Europe.

 

       Anopheles sundaicus --In the An. sundaicus complex.  Habitat is salty or brackish water including lagoons, marshes and seepages with putrefying vegetation.  It is mainly a coastal species but may be found in freshwater inland pools in Java and Sumatra.  Sunlit areas are preferred.  Humans and domestic animals are attacked indoors and outdoors.  Adults rest mainly indoors.

 

       Anopheles superpictus -- Habits are the same as in Europe.

 

SOUTHEAST ASIA

 

       Anopheles aconitus -- Habitat in sunlit areas of rice fields, swamps, irrigation ditches, streams with vegetation.  Indoor and outdoor feeding on humans and cattle.  Adults rest indoors or outdoors.

 

       Anopheles anthropophagus -- Habitat in shaded pools and ponds, but sometimes in rice fields.  Humans are bitten indoors and adult mosquitoes rest indoors as well.

 

       Anopheles balabacensis -- Habitat in muddy and shaded forest pools, hoof prints and deep wells.  Adults feed on humans and cattle outdoors where they also rest.  Similar in appearance to Anopheles dirus, but with a more restricted distribution.  Occur in Sabah, Java, Borneo and the Philippines.

 

       Anopheles campestris -- Habitat in deep shaded waters such as wells and rice fields, but sometimes in brackish waters.  Outdoor and indoor biters on humans and animals.  Adults rest in similar places.

 

       Anopheles culicifacies -- Habits are the same as on the Indian subcontinent.

 

       Anopheles dirus -- Habitat in shaded pools, hoof prints and forest margins.  Adults feed on humans & domestic animals principally outdoors, and they remain outdoors after feeding.  Very similar to An. balabacensis, but with a wider distribution from western India to Southeast Asia.

 

       Anopheles flavirostris -- Habitat around shaded flowing waters.  Attacks domestic animals and humans inside or outside dwellings.  Resting is primarily outdoors.

 

       Anopheles fluviatilis-- Habits are the same as for the Indian subcontinent.

 

       Anopheles letifer -- Frequents shady areas by acidic and stagnant pools of water, swamps and ponds, particularly around coastal plains.  Adult mosquitoes draw blood primarily outdoors from humans and animals.  Adults rest outdoors also.

 

       Anopheles leucosphyrus -- Part of the Anopheles leucosphyrus complex and resemble An. balabacensis and An. dirus.  Prefers clear pools in forests.  Humans are bitten inside and outside dwellings, but the adults rest outdoors. 

 

       Anopheles maculatus -- Habitat in sunlit seepage waters, pools by strams, ditches and vegetation covered swamps.  Humans and animals are attacked outdoors, where the mosquito also rests after feeding.

 

       Anopheles minimus -- Habits are the same as on the Indian subcontinent.

 

       Anopheles nigerrimus -- The larvae occur in sunlit deep ponds, rice fields, irrigation ditches and marshlands with much vegetation.  Adults draw blood from humans and animals usually outdoors, where the mosquito also rests.

 

       Anopheles sinensis -- This is a common mosquito in China where it may be a more important vector than Anopheles anthropophagus.  The larvae inhabit rice fields, marshes, ditches and grassy ponds.  Adults draw blood from cattle and humans, indoors and outdoors, where the mosquito rests also.

      

       Anopheles subpictus -- Habitat in muddy pools near dwellings, gutters and in brackish water.  Mosquito bites animals primarily, but also humans both indoors and outdoors.  Adults rest indoors or outdoors after a blood meal.

 

       Anopheles sundaicus  -- Habits are the same as on the Indian subcontinent.

 

MEXICO & CENTRAL AMERICA

 

       Anopheles albimanus -- Habitat in sunlit fresh or brackish waters with floating or grassy vegetation.  Adults draw blood from humans and domestic animals both indoors and outdoors.  Adults regularly remain indoors after feeding.

 

       Anopheles albitarsis  -- Larvae usually occur in sunny ponds, large pools and marshes with filamentous algae.  Humans and domestic animals are both attacked.  Feeding is both outdoors and indoors, but resting is outdoors.

 

       Anopheles aquasalis  -- The tidal saltwater marshes and estuaries are the principal habitats, bot sunny and shady.  Adults draw blood from both humans and domestic animals indoors or outdoors.  Resting is usually outdoors.

 

       Anopheles darlingi  -- Habitat is in freshwater marshes, lagoons, swamps, and lakes, stream borders where there is vegetation.  Larvae are mostly in shaded areas.  This species feeds mostly on humans indoors and remains there after feeding.

 

       Anopheles pseudopunctipennis -- Habitat in pools, seepage waters and along streams.  They prefer habitats with algae and sunlight.  Feeding is indoors or outdoors on humans and domestic animals.  Resting is outdoors.

 

       Anopheles punctimacula -- Small pools, swamps, stream borders and in shade serve as habitats.  Humans and animals are both attacked indoors or outdoors, where mosquitoes also rest after feeding.

 

SOUTH AMERICA

 

       Anopheles albitarsis  -- Habits same as Mexico & Central America

 

       Anopheles aquasalis -- Habits same as Mexico & Central America

 

       Anopheles bellator -- The larvae are found only in the leaf axils of bromeliads.  Humans are bitten during the day in shaded forests and also at night when they enter dwellings.  Adults rest outdoors.  These mosquitoes will also attack domestic animals.  They are common in Trinidad, Venezuela, Surinam, Guyana and Brazil.

 

       Anopheles cruzii -- Larvae occur in bromeliad axils.  They feed on humans both indoors and outdoors, and rest in either locality.  They are especially active along the coasts of Brazil.

 

       Anopheles darlingi -- Habits are the same as Mexico & Central America

 

       Anopheles nuneztovari -- Part of the Anopheles nuneztovari complex.  Larvae occur in muddy water of pools, hoof prints and small ponds, particularly around towns.  They prefer sunlight.  Feeding is primarily on animals, but in northern Colombia and western Venezuela adults will also attack humans both indoors and outdoors.  They rest outdoors after feeding.

 

       Anopheles punctimacula -- Habits are the same as Mexico & Central America

 

AUSTRALASIA

 

       Anopheles farauti -- Part of the Anopheles farauti complex and An. punctulatus group.  The larvae are regularly found in semi-permanent waters such as swamps, ponds, and lagoons and at the borders of slow-flowing streams, but sometimes in puddles and pools.  The water may be both brackish and fresh and in the sun or shade.  Adults draw blood from humans and animals both indoors and outdoors.  Adults prefer to rest outdoors, but will also enter dwellings.

 

       Anopheles koliensis -- Part of the An. punctulatus group.  Larvae occur in marshy pools, irrigation ditches, pools at the borders of forest streams and frequently in sunny habitats.  Adults draw blood from humans primarily, but also animals.  Resting is mainly indoors but occasionally outdoors.

 

       Anopheles punctulatus -- Part of the An. punctulatus group.  The larvae inhabit temporary and sometimes muddy pools, hoof prints and ditches, frequently in sunlit localities.  Adults prefer to draw blood from humans rather than animals and rest indoors or outdoors after feeding.

 

 

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 Key References:     <medvet.ref.htm>    <Hexapoda>

 

Beier, J. C.  1998.  malaria parasite development in mosquitoes.  Ann. Rev. Ent. 43:  519-43.

Curtis, C. F. & H. Townson.  1998.  Malaria: existing method of control and molecular entomology.  British Med. Bull. 54:  311-25.

Gilles, H. M. & D. A. Warrell.  2002.  Essential Malariology, 4th ed, Edward Arnold Publ., London.

Harbach, R. E.  2004.  The classification of the genus Anopheles (Diptera: Culicidae): a working hypothesis of phylogenic relationships.

       Bull. Ent. Res. 94:  537-53.

Matheson, R. 1950.  Medical Entomology.  Comstock Publ. Co, Inc.  610 p.

Beier, J. C.  1998.  Malaria parasite development in mosquitoes.  Ann. Rev. Ent. 43:  519-43.

Najera, J. A.  2001.  Malaria control: achievements, problems and strategies.  Parasitologia 43 (1-2): 1-98.

Service, M.  2008.  Medical Entomology For Students.  Cambridge Univ. Press.  289 p

Legner, E. F.  1995.  Biological control of Diptera of medical and veterinary importance.  J. Vector Ecology 20(1): 59_120.

Legner, E. F..  2000.  Biological control of aquatic Diptera.  p. 847_870.  Contributions to a Manual of Palaearctic Diptera, Vol. 1, Sci.  Herald,

        Budapest.  978 p.

Spielman, A.  2006.  Ethical dilemmas in malaria control.  J. Vect. Control 31:  1-8.

Wernsdorfer, W. H. & I. McGregor.  1988.  Malaria: Principles and Practice of Malariology. Vol. 1&2: Churchill Livingstone, Edinburgh.