Japanese encephalitis in small-scale pig raising in rural Cambodia : Seroprevalence, reproductive disorders and disease awareness

Sammanfattning: Japanese encephalitis (JE) is endemic in several Asian countries, including Cambodia. It is a neurologic disease caused by the mosquito-borne virus JEV, which infects a wide range of vertebrate hosts. In humans, infection is usually subclinical, but one out of around 250 infected people develop acute encephalitis. Around 20–30% of the severe clinical cases are fatal, and up to half of the survivors are faced with permanent neuropsychiatric sequelae. Japanese encephalitis is mainly considered a childhood disease of rural areas. Pigs serve as amplifying host and develop high-level viremias following infection. They rarely develop clinical disease, although infection in adult pigs can cause reproductive disorders, such as abortions, stillbirths and infertility. Thus, JEV could have a major impact on the profitability of pig rearing. Cambodia is a low-income, Southeast Asian country with 16.2 million people and 1.76 million pigs (2018 estimates). Pork is the most consumed type of meat, and many households keep pigs. Most pig producers are smallholders that live in rural areas, where JEV is likely to circulate. The pigs are usually kept near human dwellings, which increases the risk of infection in humans. In 2007, the incidence of clinical JE in Cambodia was 11.1 per 100,000 children under 15 years of age. Sub-national vaccination of children was initiated in 2009, and a national vaccination programme was established in 2016. The infection rate and seroprevalence in pigs has been estimated to be high in both rural and peri-urban areas in Cambodia. However, JEV circulation among pigs has only been studied in the southern part of the country. In this study, 139 pig smallholders were visited in rural parts of Kampong Thom, Preah Vihear, Ratanakiri and Stung Treng provinces, north-eastern Cambodia. They were interviewed about pig management, occurrence of reproductive disorders, knowledge of JE and mosquito-borne diseases in general, and use of JE preventive measures, such as mosquito protection and vaccination. Pigs over three months of age (to avoid interference of maternal antibodies) were sampled for blood, and sera were analysed for JEV antibodies. Information about sex, age, breed and history of reproductive disorders was recorded for each sampled pig. In total, 242 pigs were sampled. The apparent seroprevalence was 89.1% in pigs between three and six months of age and 100% in pigs over six months of age. In total, 93.0% of the pigs were positive. There were no signifycant associations (p<0.05) between serologic status and breed, housing system, proximity to rice fields, reproductive disorders, or protection of pigs from mosquitos. Province appeared to be associated with seroprevalence (p<0.001), but very few test results were valid for Preah Vihear, where the prevalence was lowest. All families used mosquito protection, almost all respondents knew that mosquitos can transmit diseases, and over two-thirds had heard of JE. The JE vaccination coverage in children appeared to be high in all provinces. The results of this study suggest that JEV transmission is intense, not only in southern Cambodia, but in north-eastern, rural areas as well. Fortunately, people are well-aware of mosquito-borne diseases and protect themselves from mosquito bites, and many children are vaccinated against JE. Nonetheless, it is important that national vaccination is continued.