How much does direct transmission between pigs contribute to Japanese Encephalitis virus circulation? A modelling approach in Cambodia

Abstract

Japanese Encephalitis (JE) is the most important cause of human encephalitis throughout Asia and the Pacific. Although JE is a vector-borne disease, it has been demonstrated experimentally that transmission between pigs can occur through direct contact. Whether pig-to-pig transmission plays a role in the natural epidemiological cycle of JE remains unknown. To assess whether direct transmission between pigs may occur under field conditions, we built two mathematical models of JE transmission incorporating vector-borne transmission alone or a combination of vector-borne and direct transmission. We used Markov Chain Monte Carlo (MCMC) techniques to estimate the parameters of the models. We fitted the models to (i) two serological datasets collected longitudinally from two pig cohorts (C1 and C2) during two periods of four months on a farm on the outskirts of Phnom-Penh, Cambodia and to (ii) a cross-sectional (CS) serological survey dataset collected from 505 swine coming from eight different provinces of Cambodia. In both cases, the model incorporating both vector-borne and direct transmission better explained the data. We computed the value of the basic reproduction number R0 (2.93 for C1, 2.66 for C2 and 2.27 for CS), as well as the vector-borne reproduction number Rpv and the direct transmission reproduction number Rpp. We then determined the contribution of direct transmission on R0 (11.90% for C1, 11.62% for C2 and 7.51% for CS). According to our results, the existence of pig-to-pig transmission is consistent with our swine serological data. Thus, direct transmission may contribute to the epidemiological cycle of JE in Cambodia. These results need to be confirmed in other eco-climatic settings, in particular in temperate areas where pig-to-pig transmission may facilitate the persistence of JE virus (JEV) during cold seasons when there are no or few mosquitoes.

Modelling and assessment of combining gilt vaccination, vector control and pig herd management to control Japanese Encephalitis virus transmission in Southeast Asia

“…According to the sensitivity analysis results, model parameters had the same influence on model outputs in both breeding systems. In Diallo et al, the most influential parameter driving variations of the basic reproduction number (R 0 ) of two JE transmission models –one with vector borne transmission only and one incorporating both vector-borne and direct transmission, was the mosquito population size (61). Results of the sensitivity analyses in the present work are consistent with that: for both farming systems, the vector population size was the parameter that mostly influenced the maximal number of sows allowing epidemic die-out.…”

“…The epidemiological system was modeled by a host population (a pig herd) combined with a Culex mosquito vector population (mosquitoes). The dynamic of the epidemiological systems resulted from two distinct processes: the infectious process, operating in continuous time was adapted from (61), whereas the pig breeding process was modeled based on discrete events occurring instantaneously. A graphical representation of pig breeding processes and associated infectious dynamics is provided in Fig 1.…”

“…3c). As in (61), infected pigs were treated as immediately infectious, and we omitted the state “exposed”. Infected (viremic) pigs finally became immune ( R state) at a fixed rate γ (the average duration of viremia was thus1/ γ ).…”

“…The average of the four preceding values (14, 21, 6, 15), 14 days, was considered the average duration of I state in vectors. Considering the minor role of direct transmission in disease dynamic (61) a consecutive duration of 14 days with less than 1 viremic pig ( I state) would result, on average, in the death of all the infectious vectors, without any new infection of susceptible mosquitoes.…”

Targeting of the Nasal Mucosa by Japanese Encephalitis Virus for Non-Vector-Borne Transmission

“…Several in vivo experiments demonstrated that JEV can be excreted oronasally in infected pigs, independently of the route of inoculation, for a period of 5 days, with a maximum level of virus shedding reached after the end of viremia ( 8 – 12 ). Furthermore, mathematical modeling using data from outbreaks in Cambodian pig farms supported the possibility that direct transmission also occurs under field conditions ( 13 ). These findings represent a warning that JEV could have the potential to continue to circulate in the pig population by direct transmission in temperate regions during cold mosquito-free periods.…”