JAPANESE Encephalitis

 

Japanese Encephalitis is a viral disease. It is transmitted by infective bites of female mosquitoes mainly belonging to Culex tritaeniorhynchus, Culex vishnui and Culex pseudovishnui group. However, some other mosquito species also play a role in transmission under specific conditions. JE virus is primarily zoonotic in its natural cycle and man is an accidental host. JE virus is neurotorpic and arbovirus and primarily affects central nervous system

Sign and symptoms 

  • JE virus infection presents classical symptoms similar to any other virus causing encephalitis
  • JE virus infection may result in febrile illness of variable severity associated with neurological symptoms ranging from headache to meningitis or encephalitis. Symptoms can include headache, fever, meningeal signs, stupor, disorientation, coma, tremors, paralysis (generalized), hypertonia, loss of coordination, etc.
  • Prodromal stage may be abrupt (1-6 hours), acute (6-24 hours) or more commonly subacute (2-5 days)
  • In acute encephalitic stage, symptoms noted in prodromal phase convulsions, alteration of sensorium, behavioural changes, motor paralysis and involuntary movement supervene and focal neurological deficit is common. Usually lasts for a week but may prolong due to complications.
  • Amongst patients who survive, some lead to full recovery through steady improvement and some suffer with stabilization of neurological deficit. Convalescent phase is prolonged and vary from a few weeks to several months.
  • Clinically it is difficult to differentiate between JE and other viral encephalitis
  • JE virus infection presents classical symptoms similar to any other virus causing encephalitis.

TRANSMISSION

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.

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.
Sl. No. Species No. of Isolations State*
1 Cx. tritaeniorhynchus 79 TN, KA, KL
2 Cx. vishnui 30 TN, KA, WB
3 Cx. pseudovishnui 8 KA, GOA
4 Cx. bitaeniorhynchus 3 KA, WB
5 Cx. epidesmus 1 WB
6 Cx. fuscocephala 7 TN, KA
7 Cx. gelidus 8 TN, KA
8 Cx. quinquefasciatus 1 KA
9 Cx. whitmorei 4 TN,KA, AP, WB
10 An. barbirostris 1 WB
11 An. paeditaeniatus 1 KA
12 An. subpictus 9 TN, KA, KL
13 Ma. annulifera 2 KL, ASSAM
14 Ma. indiana 3 KL
15 Ma. uniformis 4 KA, KL
  * AP= Andhra Pradesh; TN= Tamil Nadu; KA= Karnataka; KL= Kerala; WB= West Bengal  
  • Life cycle consists of egg, four instars of larvae, pupa and adult. The whole cycle takes more than a month, however, duration depends on temperature and other ecological conditions
  • Culex vishnui subgroup is very common, widespread and breed in water with luxuriant vegetation mainly in paddy fields and the abundance is related to rice cultivation, shallow ditches and pools.
  • These vectors are primarily outdoor resting in vegetation and other shaded places but in summer may also rest in indoors.
  • They are in principally cattle feeders, though human and pig feeding are also recorded in some areas.

Diagnosis

Clinical:

Clinically JE cases present signs and symptoms similar to encephalitis of viral origin and cannot be distinguished for confirmation. However, JE can be suspected as the cause of encephalitis as a febrile illness of variable severity associated with neurological symptoms ranging from headache to meningitis or encephalitis. Symptoms can include headache, fever, meningeal signs, stupor, disorientation, coma, tremors, paralysis (generalized), hypertonia , loss of coordination.

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