  The global spread of avian influenza A viruses in domestic birds is causing
dramatic economic and social losses. Various mechanistic models have been
developed in an attempt to better understand avian influenza transmission and
to evaluate the effectiveness of control measures. However, no comprehensive
review of the mechanistic approaches used currently exists. To help fill this
gap, we conducted a systematic review of mechanistic models applied to
real-world epidemics to (1) describe the type of models and their
epidemiological context, (2) synthetise estimated values of AIV transmission
parameters and (3) review the control strategies most frequently evaluated and
their outcome. Fourty-five articles qualified for inclusion, that fitted the
model to data and estimated parameter values (n = 42) and/or evaluated the
effectiveness of control strategies (n = 21). The majority were
population-based models (n = 26), followed by individual-based models (n = 15)
and a few metapopulation models (n = 4). Estimated values for the transmission
rate varied substantially according to epidemiological settings, virus subtypes
and epidemiological units. Other parameters such as the durations of the latent
and infectious periods were more frequently assumed, limiting the insights
brought by mechanistic models on these. Concerning control strategies, many
models evaluated culling (n = 15), while vaccination received less attention (n
= 7). According to the reviewed articles, optimal control strategies varied
between virus subtypes and local conditions, and also depended on the
objective. For instance, vaccination was optimal when the objective was to
limit the overall number of culled flocks, while pre-emptive culling was
preferred for reducing the epidemic size and duration. Earlier implementation
of interventions consistently improved the efficacy of control strategies,
highlighting the need for effective surveillance and epidemic preparedness.
Potential improvements of mechanistic models include explicitly accounting for
various transmission routes, and distinguishing poultry populations according
to species and farm type. To provide insights to policy makers in a timely
manner, aspects about the evaluation of control strategies that could deserve
further attention include: economic evaluation, combination of strategies
including vaccination, the use of optimization algorithm instead of comparing a
limited set of scenarios, and real-time evaluation.
