on pig premises relevant for disease
transmission between farms is low. Although farmers selling their
products on the premises may often receive visits from external
visitors, these are not likely to visit other pig farms on the same day.
An intermediate frequency was assigned to wild boar as porcine disease
pathway, even though specifically considering their role for
between-farm transmission only and not for disease introduction. Indeed,
in Switzerland areas densely populated with wild boar overlapped with
regions with high density of farms with outdoor access for pigs , which
may facilitate farm-to-farm spread of pathogens via wild boar movements.
Factors modifying the relevance of disease
pathways
The aforementioned role of outdoor access for pigs as modifying factor
for the exposure to wild boar is one example of how farm characteristics
should be considered when interpreting our findings. Production type is
another important factor. While upstream farms in the production chain,
such as breeding farms, are traditionally considered a bigger threat for
disease spread via pig trade, among our sample of farmers we observed
that also fattening and closed-cycle production farms could potentially
infect other farms via indirect disease pathways (Table 2, quotes from
interviews 1, 4, 5, 6, 13, 17 and 19). When modelling the transmission
of PRRSV, indeed found that fattening farms were involved in local
disease spread. Another example of production type influencing contact
pathways is that of carcass collection on the farm. This pathway was
found by the experts to be frequent, but since carcass collection
centers mainly offer this service for carcasses weighing 200 kilograms
or more, it may more often occur on farms owning old breeding animals.
Following biosecurity protocols effectively mitigates the risk of
disease transmission, especially for the indirect pathways that cannot
be eliminated even during outbreaks, such as veterinarian visits . Trust
might be an important aspect, when considering indirect pathways. A
higher trust in colleagues or external collaborators might be associated
with lower biosecurity (e.g., if biosecurity measures are not observed
by the trusted party). Oppositely, the trust or distrust placed in
colleagues and external collaborators might serve as a protective factor
for disease spread. Among the interviewed farmers, this could be
observed in several situations involving both colleagues (e.g., trusting
colleagues to dispose of pig carcasses from their farm, or not trusting
them to use their equipment) and external collaborators (e.g., trusting
feed advisors to enter the stables, or not trusting carcass collection
services to come on the premises). In future research it might be
advantageous to consider psychological factors, such as trust, as
vulnerability or protective factors.
The Mental Models
Approach
The use of an adapted MMA in this context was found to present multiple
advantages. First, the MMA provided the flexibility to be able to
include further pathways in every step of data collection. Thanks to
that, we gained knowledge on pathways that had not been considered in
past works. Second, accessing the combined knowledge of farmers and
experts proved to be a powerful tool for evaluating the importance of
indirect disease pathways, as both deal with issues related to pig
production and health, but from different perspectives. Third,
interviewing farmers by means of qualitative interviews allowed us to
simultaneously identify pathways and learn about farmers’ attitudes and
perceptions towards them. Open face-to-face interviews were very
effective in this process, as they greatly facilitated the building of
trust towards interviewers and the sharing of information on sometimes
frowned upon farming practices . Finally, expert elicitation enabled the
generalization of findings from the limited sample of interviewed
farmers to the country context, and to assess their relevance for the
spread of three diseases of national and international concern.
Study limitations
The limitations of our study are mainly those associated with
qualitative assessments of the pathways that are typically undertaken
relying on limited samples. The number of elicited experts is small, but
justifiable given the size of the country and the limited number of
available experts in the field, as previously reported by another expert
elicitation study in Switzerland . We coped with this limitation by
selecting an expert sample as diversified as possible, and we provided
the counts of single expert answers given during the workshop, which can
be used to interpret with caution the relevance of pathways with low
agreement among experts. The small sample of farmers did not allow for
the investigation of the association between the responses provided by
farmers and farm characteristics. A further quantitative study may be
considered to draw further conclusions in these regards. Nonetheless,
some of the associations are relatively straightforward, as in the case
of the wild boar pathway and outdoor access for pigs and can be
extrapolated by using national agricultural and other databases.
Finally, the presented frequencies of disease pathway occurrence are
based on expert opinions given in the absence of outbreaks in the
country. This may significantly vary through behavior change of farmers
and professionals in reaction to disease incursion. This shift in
behavior must be considered when modelling disease spread after first
detection.