2.5 Parameter optimization
For several of the main parameters, known to play a role in ASF
persistence, there was no reliable estimate obtained in the field from
the affected wild boar populations. In particular, virus transmission
rates mediated by infected or convalescent individuals, or by wild boar
carcasses, are substantially unknown. A few experimental estimates exist
but with different virus genotypes (de Carvalho Ferreira et al., 2013;
Eblé et al., 2019) and obtained mainly on pigs and in controlled
conditions. The applicability of such estimates to the complex
eco-epidemiological conditions of a natural wild boar population are
therefore dubious. Similar considerations can be done for other crucial
parameters, such as the duration of carcass infectivity in the field or
the disease lethality rate in the field.
To overcome this limitation and provide the model with appropriate
values of the missing parameters, we used an optimization process based
on a series of demographic and epidemiological criteria regarding the
evolution of the disease in a wild boar population. In particular, we
optimized model parameters to mimic the evolution of the ASF spread in
Latvia during the period 2014-2019, as described in Oļševskis et al.
(2020). First, we selected all parameters for which we had no
field-based reliable estimate. They were the three ASF infection
probabilities (direct, carcass, convalescent), the duration of a carcass
infectious period, the duration of a convalescent infectious period,
disease lethality rate and the ratio between the infection rate within
and between wild boar social groups. Then, we ran 1,000 iterations of
our model, each time with a randomly selected value for each of the
parameters to be optimized. For each iteration, we reported the number
of days of disease persistence, the maximum virus prevalence and
seroprevalence during the 10-year period and the minimum observed
population density. Finally, we picked the iterations which satisfied
all of the following criteria, derived from the Latvian study case
(Oļševskis et al., 2020):
- The virus was still circulating in the population at the end of the
simulated study period
- The highest observed virus prevalence ranged 2-5%
- The highest observed seroprevalence ranged 5-10%
- Population density decreased by at least 70% during the epidemic
The parameter values which maximized the probability of all the four
criteria to be met were picked as input values for our model, thus
assuring the highest ASF persistence chances and a realistic
epidemiological and demographic evolution of the study system. The
values for each parameter, resulting from the optimization process, are
shown in Tab. 1, whereas the density probability functions associated to
each parameter are available in Figs. S1 and S2 in the Online Supporting
Information.