B212 - JAPANESE ENCEPHALITIS

IDevice Icon

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 
  • Adult non-pregnant pigs show no overt signs of infection
  • Pregnant sows may abort, produce mummified foetuses or give birth to stillborn or weak piglets.
  • Central nervous signs, occasionally seen in piglets up to six months of age.

Horses 

Three clinical syndromes are described:

  1. Transient type — fever anorexia, sluggish movement, congested or jaundiced mucous membranes, and uneventful recovery;
  2. Lethargic type— fluctuating fever, pronounced lethargy, anorexia, difficulty in swallowing, jaundice, petechial haemorrhages in visible mucous membranes, incoordination, staggering and falling, transient neck rigidity and radial paralysis, and usually recovery within a week;
  3. Hyperexcitable type — high fever, with aimless wandering, violent and demented behaviour, blindness, profuse sweating, muscle trembling, collapse and death.
  • Mortality rate in horses is generally about 5%, but may be as high as 30-40% in severe outbreaks.

Humans 

  • Fever
  • Central nervous system signs
  • About 25% of cases are fatal
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:
  • Nipah virus
  • Aujeszky’s disease
  • Porcine brucellosis
  • PRRS
  • Classical swine fever
  • Porcine parvovirus

In horses:

  • Equine encephalomyelitis (Western, Eastern and Venezuelan)
  • Rabies
  • Borna disease
  • Lead poisoning
  • Tetanus
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
  • Japenese encephalitis, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 1085-1086
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Office International des Epizooties, 2002
 
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 
  • Adult non-pregnant pigs show no overt signs of infection
  • Pregnant sows may abort, produce mummified foetuses or give birth to stillborn or weak piglets.
  • Central nervous signs, occasionally seen in piglets up to six months of age.

Horses 

Three clinical syndromes are described:

  1. Transient type — fever anorexia, sluggish movement, congested or jaundiced mucous membranes, and uneventful recovery;
  2. Lethargic type— fluctuating fever, pronounced lethargy, anorexia, difficulty in swallowing, jaundice, petechial haemorrhages in visible mucous membranes, incoordination, staggering and falling, transient neck rigidity and radial paralysis, and usually recovery within a week;
  3. Hyperexcitable type — high fever, with aimless wandering, violent and demented behaviour, blindness, profuse sweating, muscle trembling, collapse and death.
  • Mortality rate in horses is generally about 5%, but may be as high as 30-40% in severe outbreaks.

Humans 

  • Fever
  • Central nervous system signs
  • About 25% of cases are fatal
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:
  • Nipah virus
  • Aujeszky’s disease
  • Porcine brucellosis
  • PRRS
  • Classical swine fever
  • Porcine parvovirus

In horses:

  • Equine encephalomyelitis (Western, Eastern and Venezuelan)
  • Rabies
  • Borna disease
  • Lead poisoning
  • Tetanus
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
  • Japenese encephalitis, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 1085-1086
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Office International des Epizooties, 2002