Porcine Japanese Encephalitis

Japanese encephalitis (JE), also known as epidemic B encephalitis or Japanese B encephalitis—commonly referred to as "B encephalitis"—is a severe mosquito-borne zoonotic viral disease caused by the Japanese encephalitis virus (JEV). China is a key region where JE is endemic, and the disease also poses a significant threat to the pig industry, causing one of its most devastating outbreaks. Infected pigs exhibit prolonged viremia with high viral titers, making them the most important amplifying hosts for JEV and a major source of infection for human cases of JE. This article provides a detailed review of recent research advancements in Japanese encephalitis virus in pigs, aiming to offer valuable insights for the rapid diagnosis and effective prevention and control of this disease.

This virus belongs to the Flaviviridae family and the Flavivirus genus. It is a single-stranded positive-sense RNA virus with a genome approximately 11 kb in length. The viral particles are spherical, measuring about 40–50 nm in diameter, and exhibit icosahedral symmetry. Their buoyant density in cesium chloride ranges from 1.24 to 1.25 g/cm³, with a sedimentation coefficient of 200 S. The virus has a molecular weight of roughly 3,000 kD, making it one of the smallest members within the Flaviviridae family. It is enveloped, with an outer layer featuring glycoprotein spikes. JEV is classified into three serotypes: La Gar, Nkayalna, and Mie (intermediate type), each displaying distinct biological characteristics—including differences in growth properties and virulence. Based on their genomic sequences, JEV viruses are further divided into five genotypes: GI, which itself is subdivided into G1a and G1b. Among these, JEV GI and JEV GIII are the predominant strains circulating primarily in Asian countries, while JEV GII represents the second most common genotype, mainly found in Indonesia, Singapore, South Korea, Malaysia, and Australia. In contrast, JEV GIV and GV are relatively rare strains.

1. Sources of Infection and Transmission Routes: This virus is primarily transmitted through the bites of mosquitoes carrying the virus. Pigs serve as the main hosts for viral replication and dissemination. After infection, the virus is mainly found in the central nervous system, cerebrospinal fluid, blood, spleen, testes, and brain tissues of stillborn fetuses. In endemic areas, the virus can be isolated from blood-feeding insects, particularly those belonging to the *Culex* and *Aedes* genera—most notably the Culex tritaeniorhynchus mosquito. The transmission cycle involves pigs, mosquitoes, and pigs once again. Moreover, Japanese encephalitis virus can replicate within mosquitoes and survive the winter there. Infected sows can transmit the virus vertically to their fetuses via the placenta.

2. Susceptible Animals: Pigs, horses, cattle, sheep, monkeys, poultry, and humans can all be infected with Japanese encephalitis. Many animals exhibit asymptomatic infections, and these asymptomatic carriers may serve as sources of infection during the viremic phase. Furthermore, pigs of all ages, breeds, and sexes are equally susceptible to this disease.

3. Epidemiological Characteristics: The disease exhibits seasonal patterns, closely tied to mosquito activity. It primarily prevails during summer and autumn, with the highest incidence occurring from July to September each year. Notably, the severity of the outbreak is greater in southern China compared to northern regions.

The incubation period for porcine encephalitis B typically lasts 3 to 4 days. This disease can lead to abortion in pregnant sows, resulting in stillbirths or weak piglets; orchitis in boars; and persistent high fever, reduced appetite, increased tendency to lie down and sleep, general lethargy, and neurological symptoms in growing pigs—such as trembling limbs, lameness, swollen joints, hindlimb paralysis, teeth grinding with foaming at the mouth, and limb spasms. In severe cases, pigs may exhibit head-shaking movements, circling behavior, and even blindly charge into obstacles, often succumbing to death within just two days. Pathological examinations reveal significant congestion, hemorrhage, and edema of the uterine lining in aborted sows. Boars show varying degrees of testicular swelling, usually unilateral but sometimes bilateral. Upon incision of the swollen testicles, fluid accumulation is observed between the tunica vaginalis and the testicular parenchyma, with the tunica cavity often filled with a large amount of yellowish-brown, opaque fluid. Stillborn fetuses and newborn piglets displaying neurological signs upon birth typically exhibit cerebral edema and hydropericardium during necropsy, along with distinct non-purulent encephalitis changes in the central nervous system.

Diagnosis: A preliminary diagnosis of Japanese encephalitis can be made based on epidemiological patterns and clinical symptoms, but definitive confirmation requires laboratory testing. Common diagnostic methods include virus isolation, serological assays, and qPCR. However, virus isolation is labor-intensive and time-consuming, limiting its practical use in clinical settings. Additionally, antigenic cross-reactivity among flaviviruses and the frequent occurrence of asymptomatic infections in both humans and animals further complicate diagnosis. Particularly with the ongoing promotion of vaccination programs, the reliability of JEV-specific antibody detection via serum tests has been challenged. In practice, serological diagnosis typically relies on the detection of specific IgM antibodies and a fourfold or greater increase in IgG antibody levels in convalescent-phase serum compared to acute-phase serum. Meanwhile, qPCR has emerged as the fastest, most accurate, and reliable method for pathogen identification, making it widely adopted in clinical diagnostics.

1. Eliminate mosquitoes and insects, and maintain环境卫生. Summer and autumn are peak seasons for mosquito breeding and activity. Install mosquito nets to prevent the proliferation of mosquitoes and flies, and regularly spray highly effective, low-toxicity insecticides to eradicate blood-sucking insects like mosquitoes. Additionally, keep the animal housing environment clean, hygienic, and dry to minimize mosquito breeding and reproduction.

2. To strengthen immunity and prevent the spread of the virus, animal facilities should scientifically formulate vaccination protocols based on the Japanese encephalitis (JE) epidemic situation, and appropriately select JE vaccines for immunization. Administering attenuated JE vaccine 1 to 2 months before the onset of an epidemic can effectively enhance pigs' disease resistance and deliver excellent preventive outcomes. Currently, China primarily uses three types of JE vaccines: mouse-brain inactivated vaccine, cell-culture inactivated vaccine, and cell-culture live-attenuated vaccine. Among these, the inactivated vaccine derived from either mouse brain or cell cultures has gained widespread acceptance. Notably, the SA14-14-2 strain, widely used in China, is regarded as highly effective against all genotypes of the virus.

Strictly implement harmless treatment methods to cut off transmission routes. When this disease occurs, farms must promptly adopt preventive and control measures such as isolation and culling, followed by thorough disinfection of the entire farm to prevent the outbreak from spreading. For diseased or dead pigs, as well as aborted fetuses, placentas, stillborn piglets, amniotic fluids, and other reproductive tract secretions, immediate harmless treatment is essential. Additionally, thoroughly disinfect all contaminated environments, including both indoor and outdoor areas of pigsties, as well as farming tools. It's worth noting that Japanese encephalitis virus isn't highly resistant to external conditions—exposure to 56°C for 30 minutes or 100°C for just 2 minutes can effectively inactivate it. However, under low-temperature conditions, the virus can survive for extended periods. Moreover, it is sensitive to changes in pH and can be efficiently neutralized by common disinfectants, such as 2% caustic soda or 3% Lysol.

3. Ensure thorough diagnosis and treatment to control secondary infections. There is no specific medication for this disease; severely affected pigs should be culled promptly, while those with milder symptoms can receive symptomatic treatment.

This disease is one of the most common arboviral infections affecting the human central nervous system. Currently, there is no specific treatment available, posing a serious threat to human health. Vaccinating pigs can effectively reduce the damage caused by JEV to the swine industry, while rapid and accurate detection of JEV is of paramount importance.

References

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