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

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.

Measuring impact of vaccination among wildlife: The case of bait vaccine campaigns for classical swine fever epidemic among wild boar in Japan

“…Using the estimates of γ and μ d , case fatality ratio (hereinafter, CFR) of CSF among the wild boar was estimated. CFR is generally defined as the proportion of deaths from a certain disease (in this study, CSF) among total infected cases (infected wild boar, PCR(+)), and theoretically defined as CFR = lethality rate/(lethality rate + recovery rate + natural mortality rate) according to the method described by Omori et al [ 21 ] and Matsuyama et al [ 22 ]. We assumed that the natural mortality rate ( μ ) is constant and following to a reported value, 0.15 per year [ 23 ].…”

“…The estimate of the lethality rate from CSF, 0.149 per week, falls within the range of previously reported values for European wild boar, 0.021 per week to 0.562 per week [ 7 ]. Additionally, the estimate was included in the 95% CI of the estimated lethality rate in our previous study (0.165 per week with 0.081–0.250 as 95% CI), which was estimated for the same Japanese wild boar population before vaccination by using a different analytical method [ 22 ]. The estimates of recovery rate and CFR in the present study, 0.002 per week and 0.964, respectively, were also close to the estimates from our previous study (recovery rate, 0.004 per week; CFR, 0.959) [ 22 ].…”

“…Additionally, the estimate was included in the 95% CI of the estimated lethality rate in our previous study (0.165 per week with 0.081–0.250 as 95% CI), which was estimated for the same Japanese wild boar population before vaccination by using a different analytical method [ 22 ]. The estimates of recovery rate and CFR in the present study, 0.002 per week and 0.964, respectively, were also close to the estimates from our previous study (recovery rate, 0.004 per week; CFR, 0.959) [ 22 ]. Our estimates of a recovery rate were lower than reported values in European wild boar, which ranged from 0.01 per week [ 27 ] to 0.416 per week (calculated from a daily recovery rate, 1/[13 days]; an inverse of the infectious period [13 days] described in Table 1 in the “Annex B. Models” section of European Food Safety Authority [ 7 ]), suggesting low recovery and low survival rates of infected wild boar in Japan.…”

“…Our estimates of a recovery rate were lower than reported values in European wild boar, which ranged from 0.01 per week [ 27 ] to 0.416 per week (calculated from a daily recovery rate, 1/[13 days]; an inverse of the infectious period [13 days] described in Table 1 in the “Annex B. Models” section of European Food Safety Authority [ 7 ]), suggesting low recovery and low survival rates of infected wild boar in Japan. This can be explained by the difference in the virulence of the CSFV strain detected from European wild boar and that from Japanese wild boar [ 22 ]. The CSFV in Japan is reported as a moderately-virulent strain [ 28 ], suggesting that the virus can cause a longer infectious period compared with the high virulent strains reported in European countries [ 22 , 29 ].…”

“…This can be explained by the difference in the virulence of the CSFV strain detected from European wild boar and that from Japanese wild boar [ 22 ]. The CSFV in Japan is reported as a moderately-virulent strain [ 28 ], suggesting that the virus can cause a longer infectious period compared with the high virulent strains reported in European countries [ 22 , 29 ]. Theoretically, as the inverse of the infectious period is equivalent to the recovery rate, the longer infectious period of CSFV in Japan can cause a lower recovery rate among Japanese wild boar.…”