3.1 ASF dynamic
In both simulated scenarios, ASF exhibited an infection dynamic which
could be characterized in four distinct phases (Guberti et al., 2019):
invasion (years 1-2), first epidemic wave (years 3-4), endemic phase
(years 5-7), second epidemic wave (years 8-10). During the initial
invasion phase, the disease remained rather localized with a low virus
prevalence, in average 0.01%, and a mild reduction in wild boar density
(Tab.2 and Fig. 2). This was also the phase during which the ASF virus
had the highest chances to disappear from the wild boar population: when
simulating only transmission through direct and carcass contacts, the
virus failed in reaching the endemic status in 40% of the model runs
after two years; when adding the survivor-mediated infection, the
extinction probability after the first two years was slightly reduced to
34% (Fig. 3).
During the first epidemic wave (years 3-4), the ASF virus exhibited a
rapid geographic spread and a progressive increase in both prevalence
and seroprevalence. In scenario 1 (two transmission pathways) the ASF
prevalence and seroprevalence were in average 1-2% (Tab. 2), with peaks
of 2.87% in virus prevalence and 6.52% in seroprevalence (Fig. 2).
When accounting for role of convalescents in disease transmission
(scenario 2), the average virus and seroprevalence slightly increased,
with peaks of 4.52% and 6.61%, respectively. During this phase, the
virus faded out in 1% of the iterations in both scenarios,
corresponding to a 59% persistence probability and 65% in scenario 2
(Fig. 3).
After spreading across the whole study area, ASF entered its endemic
phase in years 5-7, during which virus prevalence was in average lower
than 0.5% in both scenarios (Tab. 2). Seroprevalence progressively
decreased during the endemic years, averaging 1.79% and 2.11% in the
two scenarios, respectively (Fig. 2). This period was also the one
during which the population reached its lowest density, in average about
0.8-0.9 individuals / km2, depending on the scenario
(Tab. 2).
Despite the low virus prevalence, ASF had low probabilities to disappear
from the population during the endemic phase: extinction rate during
this period was 1% in scenario 1 and 2% in scenario 2, so that at the
end of seventh year the ASF virus was still present in 58% of the
iterations in scenario 1 (only direct and carcass-mediated transmission)
and in 63% of cases in scenario 2 (all three transmission pathways
included).
The last phase of the ASF dynamic occurred in years 8-10, during which a
second lower epidemic wave emerged (Fig. 2), with average prevalence and
seroprevalence ranging 1-2% and 2-3%, respectively. At the end of the
10-year simulated period, the ASF virus was still present in the wild
boar population in 52% of the iterations in scenario 1 and in 57% in
scenario 2 (Tab. 2, Fig. 3).
Overall, about 58% of the virus transmissions occurred directly between
an infected and a susceptible wild boar, 38% by mean of an infected
carcass, 4% due to a convalescent wild boar (Fig. 4a). These
proportions, though, were not constant over the 10 simulated years.
Direct transmission was relatively more frequent in the initial years of
the epidemic, when about 65% of the infections occurred using this
pathway; carcass-mediated transmission, instead, was relatively less
frequent at the beginning of the simulated period, when it accounted for
only about 20% of the infections, but became progressively more
important, especially during the endemic phase. In those years, in fact,
it represented 40% of all ASF transmissions. Moreover, carcass-mediated
virus transmission was strongly correlated to wild boar density. As
shown in Fig, 5, this infection pathway accounted for only about 20% of
all virus transmission when wild boar density was around 3 individuals /
km2, but it increased to 60% when density decreased
to 1.0 / km2 or lower values. The proportion of
survivor-mediated virus transmission remained rather constant during the
simulated period, ranging 2-4% of all infections. Finally, the
proportion of infections occurring within and between social groups was
also rather constant during the study period, with about 55% of the
virus transmission taking place within the same 3x3 km cell, the
remaining 45% between two neighbouring cells (Fig. 4b).
During the 10 simulated years, the ASF force of infection exhibited a
seasonal cycle, with a peak in spring and summer at about 1.05, after
newly born piglets entered the susceptible compartment. The force
progressively decreased during the following seasons, exhibiting
negative values (0.95 – 1.00) throughout winter, starting a new cycle
at the onset of the successive reproductive season. The temporal trend
in the force of infection is shown in Fig. S3 in the Online Supporting
Information.