Key Points

• There was a significant change in the

distribution of JEV last year in Australia, with

a geographic expansion from the northern tip

of the country to four southern states.

• JEV is a mosquito-borne flavivirus belonging to

Flaviviridae, the same family as West Nile

virus, Yellow fever, Murray Valley encephalitis

virus, and Saint Louis encephalitis. There is

one serotype and five genotypes.

12,13

• Globally, JEV has been detected in 30

species of mosquitoes belonging to the

genera Aedes, Anopheles, Armigeres,

Coquillettidia, Culex, and Mansonia. 13, 14

• The primary mosquito JEV vectors are Culex

tritaeniorhynchus (in Asia) and Culex

annulirostris (in Australia). 14

• Culex pipiens, Culex quinquefasciatus, and

Aedes japonicus, native U.S. mosquito

species, may be important vectors if JEV

were introduced into the United States.6,14, 18

• JEV is a zoonotic vector-borne disease that

can affect multiple mammalian and bird

species including pigs, horses, donkeys,

cattle, sheep, goats, dogs, cats, egrets,

herons as well as reptiles, amphibians, and

humans.5

• During the 2021-2022 season, pigs on more

than 79 swine farms across four Australian

states (South Australia, Victoria, New South

Wales, and Queensland) were impacted by

JEV; over 40 JEV human infections led to

seven deaths. 7, 9, 20, 22, 27

• JEV was detected for the first time

during the 2022-2023 season in New

South Wales in November 2022.11

• JEV is found throughout Asia,

Australia, and Western Pacific Islands;

however, Australia observed a

geographic spread of JEV last year.

5,12

 Concerns for US Animal Health

• JEV would have a major impact to

livestock health, trade, and production

as well as public health.

10

• Potential pathways into the United

States are through the introduction of

infected mosquitoes and migratory

waterbirds and globalized livestock

trade.

4,10, 17

• Competent mosquito vectors and hosts

are present in the United States. It has

been demonstrated that North

American domestic pigs are susceptible

to JEV infections.16, 17

• JEV primarily affects pigs and horses

with between 50 to 70% reproductive

losses and low mortality in adult pigs,

piglets experiencing up to 100%

mortality: and encephalitis in horses with

a 0.3% to 30% mortality rate.

5, 9, 10, 12, 14

• Animals infected with JEV, other than

pigs and birds, may exhibit clinical

signs, but are considered dead-end

hosts where they do not have a high

enough viremia for mosquitoes to pick

up the virus and transmit to another

host.11

• JEV control options include treating

infected animals, chemical control of

vector species, and landscape

modifications such as screening barns

to reduce vector contact. 5,18

Emerging Risk Notice

 

May 2023

Japanese Encephalitis Virus (JEV)
Epidemiology

Distribution

• JEV is distributed throughout Indonesia, 

Thailand, Cambodia, Korea, Japan, India, 

Malaysia, Brunei, Singapore, China, Taiwan, 

Philippines, Papua New Guinea, Vietnam, 

Bangladesh, Pakistan, and northern 

Australia. It has never been detected in the 

United States.

3, 12

Hosts

Wildlife Hosts

• Ardeid birds (e.g., herons, egrets; natural 

reservoirs), can maintain high JEV viremia 

levels that infect mosquitoes in both rural and 

urban areas. 1,2, 8,5 

• There are at least 90 different bird species 

that may be involved in the transmission of 

JEV. They have a lifelong immunity to the 

virus after the initial viral infection.6 

• JEV has been detected in feral swine in 

Australia in Victoria Daly, Litchfield, Tiwi 

islands, West Arnhem, Marrakai-Dougalis 

Daly areas. They can maintain high JEV 

viremia levels that can infect mosquitoes 9,10, 21

Domestic Hosts

Swine

• Domestic pigs (amplifying hosts) can maintain 

high JEV viremia levels that infect mosquitoes in 

both agricultural and metropolitan areas. 1,2, 8,5

• Swine symptoms include fetal abnormalities 

and abortions including mummified and 

stillborn fetuses, weakness with neurological 

signs, boar infertility, fluid retention, and 

congested testicles. 10,14, 18

• Reproductive loss in domestic swine can be 

from 50 to 70% with negligible mortality in 

adult swine.

5 

• High viral titers in swine can last 2 to 4 

days. Clinical symptoms can develop in 

as few as 3 days. Direct transmission via 

nasal shedding of live virus secretions 

between pigs has been demonstrated. 

Infected pigs that survive have a longlasting immunity to the virus.6, 14, 15

• Viral transmission from pig to pig via 

genetic materials (e.g., semen) is rare.

10

• JEV can be detected in boar semen.18

Equines

• Horses are dead end hosts not capable of 

transmitting virus to mosquitoes or other 

animals.10

• Incubation period is 4 to 14 days in horses. 

• Horse symptoms include fever, jaundice, 

lethargy, anorexia, and neurological signs

may include incoordination, impaired 

vision, difficulty swallowing.

10

• Horses may recover in 2 to 3 days. In a 

severe JEV infection, horses may display

high fevers, sweating, wandering, 

aggressive behavior, blindness, and 

muscle tremors. Severe JEV infections 

can result in coma and death. These 

symptoms are indistinguishable from 

West Nile virus (WNV) and other 

encephalitic diseases. 11, 18

• JEV isolations from horses have been 

reported in Japan, Hong Kong, Taiwan, 

and India.12

• There have been no confirmed cases of 

JEV in horses in Australia as of the 

publication of this notice, but there have 

been cases where the combination of 

clinical signs and test results suggest that 

JEV was possibly the cause of disease.11

•
• A 2003 to 2004 serological study completed in 

Nepal found JEV sero-prevalence in domestic 

pigs (48.11%), in ducks (26.79%), and in 

horses (50%).15

Cattle 

• Rare JEV clinical cases have been reported in 

cattle showing neurological signs, fever, 

depression, decreased appetite and, in some 

cases, death.5 

Vectors

• Highly competent mosquito species for JEV 

transmission are Aedes albopictus, Aedes 

vexans, Aedes vigilax, Anopheles tessellatus,

Armigeres subalbatus Culex annulus, Culex 

annulirostris (main vector in Australia), Culex 

bitaeniorhynchus, Culex fuscocephala, Culex 

gelidus, Culex pipiens, Culex pipiens pallens, 

Culex pseudovishnui, Culex quinquefasciatus,

Culex sitiens, Culex tarsalis, Culex 

tritaeniorhynchus, and Culex vishnui 14

• Mosquito species found in the U.S. that could be

JEV competent are Culex tritaeniorhynchus, 

Culex tarsalis, Culex annulirostris, Culex gelidus, 

Culex fuscocephala, Culex vishnui, Culex 

bitaeniorhynchus, Culex pseudovishnui, Culex 

whitmorei, Culex sitiens, Culex pipiens molestus, 

Culex pipiens pipiens, Culex quinquefasciatus, 

Culex salinarius, Culex nigripalpus, Culiseta 

inornata, Ochlerotatus dorasalis, Ochlerotatus 

nigromaculis, Ochlerotatus japonicus, Aedes 

albopictus, Aedes japonicus, Aedes vexans, 

Anopheles spp. and Mansonia uniformis.

6, 18

• Vertical transmission occurs between mosquitoes 

and their offspring and may be a mechanism that

allows for JEV survival through the winter 

months.

6

• JEV has been isolated from Culicoides species 

in China.5

Pathways

• Geographic expansion of JEV is likely 

due to human and pig population 

growth, and changes in land use that 

may support habitats that are preferred 

by both bird and mosquito species.2

• Introductory pathways to other regions may 

include bird migration, accidental 

transportation of mosquito vectors, human 

movement, and international travel.2, 9

• JEV infected mosquito vectors are most 

likely to enter the United States via 

aircraft or cargo ships based on a 

qualitative risk assessment completed in 

2018; followed by the authors’ 2019 

quantitative assessment, which 

determined the higher risk for infected 

mosquitoes to enter the United States is 

via aircraft. 13,19

Diagnostic Testing

• RT-PCR particularly on central nervous 

system tissue or fetal tissues. JEV is 

rarely isolated or detected by PCR in 

cerebrospinal fluid (CSF). Brain tissue in 

horses can be used for RT-PCR.23, 24,25

• Swine samples can include fetal tissues, 

fresh brain, tonsil, spleen, blood, or 

serum. Equine can include fresh brain, 

CSF, blood, or serum. Isolation/detection 

of JEV in horses is usually very low so 

serology can be used to detect 

antibodies. Currently, the diagnosis of 

JEV in pigs can be based on virus 

isolation of central nervous system 

tissues; viral RNA detection in samples 

such as blood, brain, and CSF; and/or via 

the detection of JEV specific antibodies in 

CSF or serum. In horses, the specimens 

collected for virus isolation or detection 

(nucleic acid or antigen) are portions of 

the corpus striatum, cortex, or thalamus 

of the brain. 
Blood and spinal cord samples can also be 

used. Due to the short viremia associated 

with JEV infection (4-5 days) there is a short 

window for virus detection by PCR in blood 

and serum samples. 5, 26

• Pigs seroconvert prior to onset of 

reproduction signs. High antibody titers in 

these animals in a single serum sample can 

be indicative of JEV infection. Detection of 

specific IgM and IgG in equine cerebrospinal

fluid or serum may be suggestive of a current

JEV infection.5

• The collection of oral/nasal secretion samples 

from domestic pigs using cotton ropes may be 

a method to detect JEV positive pigs and 

appropriate for JEV surveillance in a region. 

JEV detection in oronasal secretions has 

potential veterinary diagnostic applications.

17

• Samples are to be submitted to the National 

Veterinary Services Laboratories (NVSL). The 

NVSL Diagnostic Virology Laboratory (DVL) in 

Ames, IA maintains diagnostics for other 

flaviviruses, including West Nile Virus. The 

NVSL Foreign Animal Disease Diagnostic 

Laboratory (FADDL) at Plum Island, NY is 

evaluating front-line molecular and 

confirmatory diagnostic methods. 

Treatment

• There are no specific treatments for infected 

animals beyond treatment for symptoms.5, 18

• Live and inactivated vaccines are available for 

swine and inactivated vaccine for horses;

however, no licensed vaccine is available in 

the United States. 2,5

Prevention

• Vaccination of equine and swine in endemic 

areas may be effective. 17, 18

• Pyrethroids, organophosphates, and 

carbamates can be used for chemical control 

of adult mosquitoes.5,18 

• Landscape modifications may also reduce 

exposure to mosquito bites such as stabling 

animals in screened barns, insecticide

treated mosquito nets, elimination of waterin 

outside water-holding containers, and 

installation of fans in barns. 5,18
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McVey DS, Piaggio J, and Cernicchiaro N. 2018. 

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literature. Preventive Veterinary Medicine 154: 71-89.

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K. 2009. Past, present, and future of Japanese 

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