Abstract
SummaryMortality data are routinely collected for many livestock and poultry species, and they are often used for epidemiological purposes, including estimating transmission parameters. In this study, we infer transmission rates for African swine fever virus (ASFV), an important transboundary disease of swine, using mortality data collected from nine pig herds in the Russian Federation with confirmed outbreaks of ASFV. Parameters in a stochastic model for the transmission of ASFV within a herd were estimated using approximate Bayesian computation. Estimates for the basic reproduction number varied amongst herds, ranging from 4.4 to 17.3. This was primarily a consequence of differences in transmission rate (range: 0.7–2.2), but also differences in the mean infectious period (range: 4.5–8.3 days). We also found differences amongst herds in the mean latent period (range: 5.8–9.7 days). Furthermore, our results suggest that ASFV could be circulating in a herd for several weeks before a substantial increase in mortality is observed in a herd, limiting the usefulness of mortality data as a means of early detection of an outbreak. However, our results also show that mortality data are a potential source of data from which to infer transmission parameters, at least for diseases which cause high mortality.
Highlights
These data have often been used for epidemiological purposes, for example, as part of syndromic surveillance (Alba et al, 2015; Tapprest et al, 2016; Torres et al, 2015), for detecting outbreaks of disease (Backer, Brouwer, van Schaik, & van Roermund, 2011; Bos et al, 2007) or to assess the impact of an epidemic (Perrin et al, 2010)
African swine fever (ASF) is one of the most important infectious diseases of swine (Costard, Mur, Lubroth, Sanchez-Vizcaıno, & Pfeiffer, 2013; Sanchez-Vizcaıno, Mur, Gomez-Villamandos, & Carrasco, 2015). It is caused by African swine fever virus (ASFV) and many strains result in the death of almost 100% of infected pigs (Blome, Gabriel, & Beer, 2013; Guinat, Gogin, et al, 2016)
African swine fever virus infection was confirmed in all nine herds by virus isolation from randomly collected blood and tissue samples taken from dead pigs
Summary
Incidence and testing data for ASFV in the RF are routinely collected by the Federal Research Center for Virology and Microbiology (FRCVM). Data on pig mortality were obtained for nine pig herds in which ASFV was detected through routine surveillance between 2010 and 2014 (Table S1; see Figure 1). African swine fever virus infection was confirmed in all nine herds by virus isolation from randomly collected blood and tissue samples taken from dead pigs It is maintained in a sylvatic warthog–tick cycle in subSaharan Africa, once introduced to domestic pigs, ASFV is transmitted through direct contact and fomites (Costard et al, 2013). The transmission rate (b), latent and infectious period parameters (lE, kE, lI, kI) and natural mortality rate (rM) were estimated for each herd independently. Seven parameters were estimated by fitting the model to the mortality data for each of the nine pig herds. An exponential prior was used for the natural mortality rate rM (mean 0.0002, calculated from daily mortality data obtained from 34 ASFV-free pig herds in the areas surrounding the outbreak herds). To explore the sensitivity of parameter inferences to prior assumptions, parameters were estimated for each herd using a further five combinations of priors (i.e., in addition to informative priors for all parameters): informative priors for all parameters, except the transmission rate; informative priors for all parameters, except the transmission and mortality rates; informative prior for the transmission rate only; informative prior for the mortality rate only; and non-informative priors for all parameters
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