Estimation of the Lethality Rate, Recovery Rate, and Case Fatality Ratio of Classical Swine Fever in Japanese Wild Boar: An Analysis of the Epidemics From September 2018 to March 2019.

Ryota Matsuyama,Takehisa Yamamoto,Ryosuke Omori,Yoko Hayama

doi:10.3389/fvets.2021.772995

Abstract

Understanding the morbidity and lethality of diseases is necessary to evaluate the effectiveness of countermeasure against the epidemics (e.g., vaccination). To estimate them, detailed data on host population dynamics are required; however, estimating the population size for wildlife is often difficult. We aimed to elucidate the morbidity and lethality of classical swine fever (CSF) currently highly prevalent in the wild boar population in Japan. To this end, we estimated lethality rate, recovery rate, and case fatality ratio (CFR) of CSF without detailed data on the population estimates of wild boar. A mathematical model was constructed to describe the CSF dynamics and population dynamics of wild boar. We fitted the model to the (i) results of the reverse transcription polymerase chain reaction (RT-PCR) test for the CSFV gene and the (ii) results of the enzyme-linked immunosorbent assay (ELISA) test for the antibody against CSFV in sampled wild boar. In the 280 wild boar sampled from September 2018 to March 2019 in the major CSF-affected area in Japan, the lethality rate and recovery rate of CSF per week were estimated as 0.165 (95% confidence interval: 0.081–0.250) and 0.004 (0–0.009), respectively. While the estimate of lethality rate of CSF was similar with the estimates in previous studies, the recovery rate was lower than those reported previously. CFR was estimated as 0.959 (0.904–0.981) using our estimate of recovery rate. This study is the first to estimate lethality rate of CSF from the dynamics of CSF epidemics in the wild boar population. Since the value of CFR is sensitive to the value of recovery rate, the accuracy in the estimate of recovery rate is a key for the accurate estimation of CFR. A long-term transmission experiment of moderately virulent strains may lead to more accurate estimation of the recovery rate and CFR of CSF.

Highlights

Classical swine fever (CSF) is one of the most important infectious diseases in the pig farming industry because of its significant impact on animal health and economic losses [1,2,3]
The disease is caused by the CSF virus (CSFV), which belongs to the genus Pestivirus of the Flaviviridae family, and its obligate host is the family Suidae (e.g., domesticated pigs and Eurasian wild boar (Sus scrofa)) [3, 4]
We propose a method for estimating the lethality and recovery rates of CSF when detailed data on the population dynamics of wild boar are not available

Summary

Introduction

Classical swine fever (CSF) is one of the most important infectious diseases in the pig farming industry because of its significant impact on animal health and economic losses [1,2,3]. The introduction of CSF to susceptible populations can sometimes result in substantial morbidity and mortality It is designated as a notifiable disease by the World Organisation for Animal Health [5]. The course of morbidity and mortality events varies with the virulence of the CSFV strains, age of the hosts, breeds (in pigs), and the living environment (domestic or wild animals) [2, 6]. Infections with highly virulent strains in vulnerable hosts may cause high mortality [2, 10], and the case fatality ratio (CFR) among young and adult domestic pigs may reach 80–100% [11]. Considering that the experimental infection of moderately virulent CSFV (genotype 2.3) resulted in only the sub-clinical course of CSF infection in 8 month-old wild boar [12], lethality lower than 90% may occur in field conditions. The actual morbidity and lethality rates in wild Sus scrofa populations are not well-understood

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