Transmission dynamics of hepatitis E virus in pigs: estimation from field data and effect of vaccination.

Hepatitis E is a viral disease that causes serious concerns for public health. Hepatitis E virus (HEV) genotype 3 is endemic in commercial pig farms worldwide that act as a reservoir. Pig-to-human transmission may occur when infectious animals enter the food chain at slaughter, through consumption of contaminated meat, direct exposure or use of by-products. To reduce the fraction of infectious animals at slaughter age and thus the risk for public health, it is important to understand the transmission dynamics of HEV in pig populations. In this study, we estimate the transmission rate parameter and mean infectious period of HEV in pigs from field data, using a Bayesian analysis. The data were collected in ten commercial pig herds that are each divided into three different age groups. Two transmission models were compared, assuming that animals are infected either locally by their group mates or globally by any infectious animal regardless of its group. For local and global transmission, the transmission rate parameters were 0.11 (posterior median with 95% credible interval: 0.092-0.14 day(-1)) and 0.16 (0.082-0.29 day(-1)), the mean infectious periods were 24 (18-33) days and 27 (20-39) days and the reproduction numbers were 2.7 (2.2-3.6) and 4.3 (2.8-6.9). Based on these results, global transmission is considered to be the more conservative model. Three effects of vaccination were explored separately. When vaccination is not sufficient to eliminate the virus, a shorter mean infectious period decreases the fraction of infectious animals at slaughter age, whereas a reduced transmission rate parameter adversely increases it. With a reduced susceptibility, vaccination of animals at a later age can be a better strategy than early vaccination. These effects should be taken into account in vaccine development.