African swine fever endemic persistence in wild boar populations: key
mechanisms explored through modelling
Abstract
African swine fever (ASF), is a serious global concern from an
ecological and economic point of view. While it is well established that
its main transmission routes comprise contact between infected and
susceptible animals and transmission through contaminated carcasses, the
specific mechanism leading to its long-term persistence is still not
clear. Among others, a proposed mechanism involves the potential role of
convalescent individuals, which would be able to shed the virus after
the end of the acute infection. Using a spatially explicit, stochastic,
individual-based model, we tested: 1) if ASF can persist when
transmission occurs only through infected wild boars and infected
carcasses; 2) if the animals that survive ASF can play a relevant role
in increasing ASF persistence chances; 3) how hunting pressure can
affect the ASF probability to persist. The scenario in which only direct
and carcass-mediated transmission were contemplated had 52% probability
of virus persistence 10 years after the initial outbreak. The inclusion
of survivor-mediated transmission corresponded to slightly higher
persistence probabilities (57%). ASF prevalence during the endemic
phase was generally low, ranging 0.1-0.2%. The proportion of
seropositive individuals gradually decreased with time and ranged 4.5 –
6.6%. Our results indicate that direct and carcass-mediated infection
routes are sufficient to explain and justify the long-term persistence
of ASF at low wild boar density and the ongoing geographic expansion of
the disease front in the European continent. During the initial years of
an ASF outbreak, hunting should be carefully evaluated as a management
tool, in terms of potential benefits and negative side-effects, and
combined with an intensive effort for the detection and removal of wild
boar carcasses. During the endemic phase, further increasing hunting
effort should not be considered as an effective strategy. Additional
effort should be dedicated to finding and removing as many wild boar
carcasses as possible.