Epidemiological studies on Japanese encephalitis in Kyoto City area, Japan. III. Seasonal prevalence of virus infections in several pig populations shown by virus recovery from engorged Culex tritaeniorhynchus summorosus

Spatial statistical and environmental correlation analyses on vector density, vector infection index and Japanese encephalitis cases at the village and pigsty levels in Liyi County, Shanxi Province, China

Background

In the eco-epidemiological context of Japanese encephalitis (JE), geo-environmental features influence the spatial spread of the vector (Culex tritaeniorhynchus, Giles 1901) density, vector infection, and JE cases.

Methods

In Liyi County, Shanxi Province, China, the spatial autocorrelation of mosquito vector density, vector infection indices, and JE cases were investigated at the pigsty and village scales. The map and Enhanced Thematic Mapper (ETM) remote sensing databases on township JE cases and geo-environmental features were combined in a Geographic Information System (GIS), and the connections among these variables were analyzed with regression and spatial analyses.

Results

At the pigsty level, the vector density but not the infection index of the vector was spatially autocorrelated. For the pigsty vector density, the cotton field area was positively related, whereas the road length and the distance between pigsties and gullies were negatively related. In addition, the vector infection index was correlated with the pigsty vector density (PVD) and the number of pigs. At the village level, the vector density, vector infection index, and number of JE cases were not spatially autocorrelated. In the study area, the geo-environmental features, vector density, vector infection index, and JE case number comprised the Geo-Environment-Vector-JE (GEVJ) intercorrelation net system. In this system, pig abundance and cotton area were positive factors influencing the vector density first. Second, the infection index was primarily influenced by the vector density. Lastly, the JE case number was determined by the vector infection index and the wheat area.

Conclusions

This study provided quantitative associations among geo-environmental features, vectors, and the incidence of JE in study sties, one typical northern Chinese JE epidemiological area without rice cultivation. The results highlighted the importance of using a diverse range of environmental management methods to control mosquito disease vectors and provided useful information for improving the control of vector mosquitoes and reducing the incidence of JE in the northern Chinese agricultural context.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05305-8.

How Central Is the Domestic Pig in the Epidemiological Cycle of Japanese Encephalitis Virus? A Review of Scientific Evidence and Implications for Disease Control

Despite the existence of human vaccines, Japanese encephalitis (JE) remains the leading cause of human encephalitis in Asia. Pigs are described as the main amplifying host, but their role in JE epidemiology needs to be reassessed in order to identify and implement efficient control strategies, for both human and animal health. We aimed to provide a systematic review of publications linked to JE in swine, in terms of both individual and population characteristics of JE virus (JEV) infection and circulation, as well as observed epidemiological patterns. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement to select and analyze relevant articles from the Scopus database, 127 of which were included in the review. Pigs are central, but the implication of secondary hosts cannot be ruled out and should be further investigated. Although human vaccination cannot eradicate the virus, it is clearly the most important means of preventing human disease. However, a better understanding of the actual involvement of domestic pigs as well as other potential JEV hosts in different JEV epidemiological cycles and patterns could help to identify additional/complementary control measures, either by targeting pigs or not, and in some specific epidemiological contexts, contribute to reduce virus circulation and protect humans from JEV infection.

Month-of-birth effect on further body size in a pig model

Previous studies unanimously confirmed the existence of a dependence of human body size on the month of birth. The cause of the phenomenon has not been identified yet, although some possible causes were proposed e.g. seasonal changes of climatic and nutritional conditions. This study explored the issue in an animal model of 20,513 pigs. We found that body weights of 6-month-old pigs were the highest for subjects born in February, but for 2-month-old pigs the peak fell in May. Any statistical correlation between the month of birth and later body weight may be induced by (1) a long-term effect of the month of birth on further growth potential (LTE), or by (2) a short-term effect of seasonal factors differentiating the growth rate (STE), so we developed a mathematical method to separate the effects. The analysis proved that (1) the observed correlations resulted only from the STE, with May-June being the months of the highest growth tempo, and that (2) there was no significant LTE. The short-term effect was responsible for differences between patterns of weight for 2- and 6-month-old animals by the month of birth: since a pig monthly gain of weight increases with age, it is favorable for it to be born in February to attain the greatest weight at the age of 6 months, whereas 2-month-old piglets are heaviest when born a month or two before the May/June optimum for growth. The lack of a long-term effect of the month of birth on pigs' weight supports the hypothesis of the cultural character of factor(s) responsible for the relationship between the month of birth and later body size in humans.

Japanese encephalitis for a reference to international travelers

Japanese encephalitis (JE) is an inflammatory disease in the central nervous system caused by infection with Japanese encephalitis virus (JEV). JE is a disease with a high fatality rate and endemic and epidemic in East, Southeast, and South Asia. High morbidity is noted in children living in the endemic area. JEV is maintained mainly between vector mosquitoes and pigs in nature. The risk of JE increases as the number of vector mosquitoes increases. The expansion of JEV-endemic area depends on irrigated rice field and pig farming. These environments that are suitable for infectious cycle of JEV exist widely in Asia today. The effective and safe vaccine is available in endemic countries and for international travelers. JE vaccination is strongly recommended to those who visit the JEV-endemic regions, especially in the rainy season.

Epidemiological studies on Japanese encephalitis in Kyoto City area, Japan. IV. Natural infection in sentinel pigs

Natural infection with Japanese encephalitis virus in three sentinel pigs held in separate experimental huts was examined daily by virus recovery from blood samples of the pigs and from mosquitoes after incubation for about 7 days from their blood feeding and by HI antibody titration of the blood samples. After a period of low infection rates, below 6%, for about two weeks in engorged Culex tritaeniorhynchus summorosus, high mosquito infections of over 30% from each viremic pig occurred for two to three days. The pigs may be probably have been bitten by many infected but not infective mosquitoes in a period of about 10 days before infection.