VECTOR OF MALARIA
Malaria or chill and
fever disease is an old age human disease and is still prevailing. This disease
is prevalent in all countries extending from 40⁰South
to 60 ⁰ North, but the tropical zone is endemic home of al
types of malaria. Malaria is caused by intracellular protozoal endoparasite
belonging to genus Plasmodium. Four species
of Plasmodium are responsible for
causing malaria in human – P. vivax, P.
Falciparum, P. malariae and P. ovale.
BIOLOGY OF VECTOR OF MALARIA
Out of about 45 species
of Anopheles found in India, only few
have been identified as malarial vector. They are Anopheles culicifacies, A.
fluviatilis, A. minimum, A. philippinensis, A. stephensni, A. sundaicus and
A. leucosphyrus.
DISTRIBUTION
They
are cosmopolitan in distribution. The area of distribution of Anopheles varies with species. A. philippinensis are found in plains, A. sundaicus and A. stephensi are found in coastal regions, while A. fluviatilis inhabits the foot hill
regions. Out of above mentioned species of Anopheles,
A. fluviatilis is highly anthrophilic i.e. have high preference
for human blood and thus are the most important vector of malaria in man.
HABITS AND HABITAT
Mosquitos
are nocturnal, troublesome and irritating insect. They inhabit near human
population. Only female Anopheles
sucks the blood which is required for oviposition. Male Anopheles is short lived and lives on nectars of flowers and plant
juices. Anopheles are known to
hibernate in adult form when conditions are unfavourable. Normally they do not
fly far away from their breeding grounds. But instances have been reported of Anopheles having flying range of 11 km.
the life span of female Anopheles is
8-34 days, as it is on the stake of environmental conditions.
GENERAL CHARACTERS
Anopheles mosquito are thin bodied with long delicate legs,
making an angle of 45 ⁰ with ground while
resting. Body divisible into head thorax and abdomen.
Head lies at anteriour extremity ob body, bears a pair of
large compound eyes, long palpi, a pair of antennae, long proboscis (piercing
and sucking type). Male lacks the biting apparatus. The antennae in males is of
bushy type (plumose) while the antennae in males is less bushy as compared to
that of male.
Thorax is round and bears a pair of spotted wings and three
pairs of long delicate legs, the beating of wings produce the notorious buzzing
sound.
Abdomen is long, narrow and made up of ten segments, the
last two segments are modified as external genitalia.
LIFE HISTORY
There
are four stages – Egg, Larva, Pupa and Adult. Breeding period extends
throughout the year. Copulation occurs during flying. Female lays about 80-100
eggs in early hours of morning in relatively clean and stagnant water. Each egg
is boat shaped bearing lateral floats. The ganotrophic
cycle (period between taking blood and laying eggs) is upto 48 hrs. egg hatches into larva within 2-3 days.
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| DIFFERENT STAGES OF LIFE CYCLE OF ANOPHELES MOSQUITO |
Larva or wriggler is a free
swimming stage. The body is divisible into head, thorax and abdomen, head bears
compound eyes, antennae, and simple eye each in pairs. Thorax is segmented
while abdomen is nine segmented. In 8th segment is present
respiratory siphon, and in 9th segment four respiratory gills are
present. The larva are bottom feeders. And their body lies parallel to water
surface. Larval period last up to 5 – 7 days.
Pupa is coma (,) shaped, active but non feeding stage. Body
divisible into cephalothorax and abdomen. A pair of small respiratory trumpets
are present on the cephalothorax. 9th abdominal segment bears a pair
of paddles and each paddle bears pair of bristles. Within 7 days imago or adult
mosquito emerges out from pupa.
MODE OF INFECTION
Plasmodium is digenetic (requires two hosts to complete its
life cycle – man and female Anopheles).
Man is the secondary host in which Plasmodium
spends it asexual phase of life while female Anopheles is the definite host in which it undergoes its sexual
cycle.
When
a female Anopheles bites a malaria
infected person, the micro and macrogametocytes from the patient enters the Anopheles’s gut, where after performing
sexual reproduction zygote is formed. The motile zygote penetrates out of the
gut wall to be changed into cystic stage (oocyte), inside the oocyte by the
process of sporogony sporozoites are formed and thus the parasite acquires its
infective stage (sporozoite). The whole process take upto 8-10 days. Such infected
mosquito when bites a healthy person, the sprorzoite stage of the parasite is
transmitted to the new host through the salivary secretion of the mosquito. So mosquitos
belonging to the genus Anopheles are
the vector of malaria.
CONTROL
The
best way to control malaria is keeping the population of its vector under
control, which is the primary weapon to control malarial infection. Use of
excessive insecticides has led to appearance of resistant varieties of
mosquito, and also environmental degradation.
1. ANTILARVAL MEASURES: not allowing water to
accumulate in places, proper drainage system and filling and levelling of ditches.
2. CHEMICAL CONTROL: it includes A) application of kerosene, diesel, fuel oil, etc in stagnant water
in a week.
3. Application of paris green (Copper acetoarsenite) in
mosquito breeding places. It leads to stomach poisoning of larva leading to
death.
4. BIOLOGICAL CONTROL: introduction of larvicidal
fishes such as Gambusia and Lebistes, in breeding places is helpful.
5. ANTI-ADULT MEASURES: killing of adult mosquitoes by
applying various insecticides.
6. PROTECTION AGAINST MOSQUITO BITES: use of mosquito
nets, repellents etc.
