· Japanese encephalitis
is a vector borne disease.
·
Several species of mosquitoes are capable of transmitting JE virus.
·
JE is a zoonotic infection. Natural hosts of JE virus include water birds of
Ardeidae family (mainly pond herons and cattle egrets). Pigs play an important
role in the natural cycle and serve as an amplifier host since they allow manifold
virus multiplication without suffering from disease and maintain prolonged
viraemia.
· Due to prolonged
viraemia, mosquitoes get opportunity to pick up infection from pigs easily.
· Man is a dead end
in transmission cycle due to low and short-lived viraemia. Mosquitoes do
not get infection from JE patient.
Japanese Encephalitis Vectors in India
· Japanese encephalitis
virus isolation has been made from a variety of mosquito species.
· Culicine mosquitoes mainly Culex vishnui group (Culex tritaeniorhynchus,
Culex vishnui and Culex pseudovishnui) are the chief vectors of JE in different
parts
of India.
The disease is transmitted to man by culex vishnui group. (C. Vishnui,
C. pseudovishnui, C. tritaeneorhynchus). Man to man transmission has not
been recorded so far.
1)Animal host : Pigs are a major source of reservoir of JE virus.
Infected pigs do not show any overt symptoms of illness and can transmit
the virus to human beings through mosquito bites. The pigs are considered
as "Amplifiers" of the virus. Cattle, buffalow, horses are the
animal hosts which can be infected.
2)Birds : Pond herons, cattle egrets, poultry ducks ardeid birds
appear to be involved in the natural history of JE virus.
3)Mosquito vectors : C. Vishnui, C. pseudovishnui, C. tritaeneorhynchus,
C. gelidus are the known vectors of JE virus.
Breeding places of mosquitoes :
Irrigated rice fields (Most important breeding place.)
Shallow ditches.
Pools and standing water.
There is no man- to - man transmission. Man is only an accidental
and dead end host. Children below the age 15 are often victims of the disease.
Chandipura Encephalitis : Through Sandflies. ( information
of sandflies is given below )
Sandflies are small insects, light or dark-brown in colour. They are smaller
than mosquitoes, measuring 1.5 to 2.5 mm in length with their bodies and wings
densely clothed with hair. Some 30 species of sand -flies have been recorded
in India. The important ones are: Phlebotomus argentipes, P. papatasii, P.
sergenti, and Sergentomyia punjabensis. (19).
General characters
The body of a sandfly is divided into head, thorax and abdomen,
(1) HEAD: The head bears a pair of long, slender and hairy antennae; palpi
and a proboscis. Only the females bite, the males live on vegetable juices.
(2) THORAX: The thorax bears a pair of wings and three pairs of legs. The
wings are upright, lanceolate in shape and densely hairy. The second longitudinal
vein on the wings branches twice, the first branching takes place in the middle
of the wing. This is a characteristic feature of the genus, Phlebotomus. The
legs are long and slender and out of proportion to the size of the body.
(3) ABDOMEN.: The abdomen has 10 segments and is covered' with hair. In the
female, the tip of the abdomen is rounded; in the male, there are claspers,
which are conspicuous and
attached to the last abdominal segment. .
Sandflies may be distinguished from mosquitoes by the following characteristics:
(1) Size: Sandflies are smaller than mosquitoes
(2) Wings: The wings of the sandfly are up-right and lanceolate in shape;
the second longitudinal vein branches twice, the first branching taking place
in the middle of the wing
(3) Legs: The legs of the sandfly are longer compared with the size of the
'body
(4) Hairs: Sandfly is a hairy
(5) Hopping: Sandflies hop about, and do not fly by choice.
Life history
The life history of the sandfly is characterised by complete metamorphosis,
having four stages: egg, larva, pupa and adult
(1) EGG: The eggs are laid in damp dark places in the vicinity of cattle sheds
and poultry. The eggs are comparatively large, and torpedo-shaped with longitudinal
wavy lines on the outside. The eggs hatch within 7 days.
(2) LARVA: The larvae are hairy maggots with a distinct head, thorax and abdomen.
The last abdominal segment carries two pairs of long stout hairs; one pair
is remarkably long. The larva feeds on decaying organic matter and becomes
a pupa in about 2 weeks.
(3) PUPA : The pupal stage lasts for about 1 week.
(4) ADULT: The average life of a sandfly is about 2 weeks.
Habits
Sandflies are troublesome noctural pests. Their bite is irritating and painful,
while their presence is scarcely observed. They infest dwellings during night,
and take shelter during day in holes and crevices in walls, holes in trees,
dark rooms, stables and store rooms. The females alone bite, as they require
a blood meal every third or fourth day for oviposition. Sandflies are incapable
of flying over long distances; they merely hop about from one place to another.
Sandflies are generally confined to within 50 yards of their breeding places.
SPECIES & Disease Transmitted
Phlebotomus argentipes -:Kala-azar
Sandflies are easily controlled because they do not move long distances from
the place of their breeding.
(1) INSECTICIDES: Resistance to DDT has not been demonstrated. A single application
of 1 to 2 g/m2 of DDT or 0.25 g/m2 of lindane has been found effective in reducing
sandflies. DDT residue may remain effective for a period of 1 to 2 years, and
lindane only for a period of 3 months (17). Spraying should be done in the
human dwellings, cattle sheds and other places.
(2) SANITATION: Sanitation measures such as removal of shrubs and vegetation
within 50 yards of human dwellings, filling up cracks and crevices in walls
and floors, and location of cattle sheds and poultry houses at a fair distance
from human habitations should receive attention.
