B212 - JAPANESE ENCEPHALITIS

B212 - JAPANESE ENCEPHALITIS
Nature of the disease |
Japanese encephalitis (JE) is a mosquito-borne disease of man and animals that occurs over much of Asia. It is caused by a Flavivirus genus of the family Togaviridae. It is mainly associated with abortion in pigs and encephalitis in humans and horses. |
Classification |
OIE, List B disease |
Susceptible species |
Humans, pigs and horses, are the species at risk of
clinical disease.
Water birds (herons and egrets) are the main reservoir and amplifying hosts for the virus. Pigs are also important amplifying hosts. Unapparent infections, and very occasional clinical cases, occur in cattle, sheep and goats. |
Distribution |
JE virus is widespread throughout temperate and
tropical Asia.
It is also present in Australia, Norther Marianna Islands and Papua New Guinea. |
Clinical signs |
Pigs
Horses Three clinical syndromes are described:
Humans
|
Post-mortem findings |
There are no characteristic gross lesions in animals or aborted foetuses. Histological lesions are restricted to the central nervous system. |
Differential diagnosis |
In pigs:
In horses:
|
Specimens required for diagnosis |
For virus identification, specimens should be
collected from animals killed in the acute stages of the disease, or from
animals dead less than 12 hours. The brain should be removed and a range of
brain tissue samples collected. Full range of other tissues should be collected
into neutral buffered formalin for histopathology.
Blood samples should be collected for serological studies, using Virus Neutralisation, Haemagglutination inhibition and Complement Fixation Test. |
Transmission |
Culex
Mosquitoes are the biological vectors of JE. Infection builds up in
water birds and then in pigs and spills over to horses and humans where cases
tend to be sporadic. Both horses and humans are ‘dead end’ hosts. Major
epidemics occur when the virus moves into new areas.
The virus has been transmitted to gilts via infected sperm. |
Risk of introduction |
JE could be introduced via migrating water birds,
with infected mosquitoes, by the importation of viraemic pigs, or less likely
with infected semen.
While the introduction of JE is a possibility, it is difficult to assess whether it would establish. The recent experience in the Torres Strait between Australia and Papua New Guinea where sentinel animals have seroconverted two years running suggests that JE could establish in other countries in the region. |
Control / vaccines |
Options for controlling the disease include
eliminating the vectors, preventing amplification of the virus in birds and
pigs, or immunising at risk species.
Vector control is unlikely to be more than marginally successful, while reduction of avian reservoir hosts does not appear feasible. Some protection may be achieved through the use of insecticides and insect proofing house and stables. Live attenuated vaccines for pigs are used in Taiwan and Japan. This prevents infection and neutralises the role of pigs as amplifying hosts. A live attenuated vaccine for horses is widely used in China. An inactivated vaccine is licensed in Japan and Korea for use in humans. |
References |
|
Nature of the disease |
Japanese encephalitis (JE) is a mosquito-borne disease of man and animals that occurs over much of Asia. It is caused by a Flavivirus genus of the family Togaviridae. It is mainly associated with abortion in pigs and encephalitis in humans and horses. |
Classification |
OIE, List B disease |
Susceptible species |
Humans, pigs and horses, are the species at risk of
clinical disease.
Water birds (herons and egrets) are the main reservoir and amplifying hosts for the virus. Pigs are also important amplifying hosts. Unapparent infections, and very occasional clinical cases, occur in cattle, sheep and goats. |
Distribution |
JE virus is widespread throughout temperate and
tropical Asia.
It is also present in Australia, Norther Marianna Islands and Papua New Guinea. |
Clinical signs |
Pigs
Horses Three clinical syndromes are described:
Humans
|
Post-mortem findings |
There are no characteristic gross lesions in animals or aborted foetuses. Histological lesions are restricted to the central nervous system. |
Differential diagnosis |
In pigs:
In horses:
|
Specimens required for diagnosis |
For virus identification, specimens should be
collected from animals killed in the acute stages of the disease, or from
animals dead less than 12 hours. The brain should be removed and a range of
brain tissue samples collected. Full range of other tissues should be collected
into neutral buffered formalin for histopathology.
Blood samples should be collected for serological studies, using Virus Neutralisation, Haemagglutination inhibition and Complement Fixation Test. |
Transmission |
Culex
Mosquitoes are the biological vectors of JE. Infection builds up in
water birds and then in pigs and spills over to horses and humans where cases
tend to be sporadic. Both horses and humans are ‘dead end’ hosts. Major
epidemics occur when the virus moves into new areas.
The virus has been transmitted to gilts via infected sperm. |
Risk of introduction |
JE could be introduced via migrating water birds,
with infected mosquitoes, by the importation of viraemic pigs, or less likely
with infected semen.
While the introduction of JE is a possibility, it is difficult to assess whether it would establish. The recent experience in the Torres Strait between Australia and Papua New Guinea where sentinel animals have seroconverted two years running suggests that JE could establish in other countries in the region. |
Control / vaccines |
Options for controlling the disease include
eliminating the vectors, preventing amplification of the virus in birds and
pigs, or immunising at risk species.
Vector control is unlikely to be more than marginally successful, while reduction of avian reservoir hosts does not appear feasible. Some protection may be achieved through the use of insecticides and insect proofing house and stables. Live attenuated vaccines for pigs are used in Taiwan and Japan. This prevents infection and neutralises the role of pigs as amplifying hosts. A live attenuated vaccine for horses is widely used in China. An inactivated vaccine is licensed in Japan and Korea for use in humans. |
References |
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