VECTOR OF MALARIA (A GENERALIZED STUDY)

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.

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 8^th segment is present respiratory siphon, and in 9^th 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. 9^th 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.