File:
<malaria.htm> <Medical Index> <General Index> Site Description Glossary <Navigate to Home>
MALARIA
DISEASE (Contact) Please
CLICK on underlined links for
details: MAIN
CATEGORIES 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 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. 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. 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. 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. 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. 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. 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. 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 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. = = = = = = = = = = = = = = = = = = = = 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. |