Ecology, Conservation and Management of Wild Pigs and Peccaries

Chapter 38: Antimicrobial resistance in wild boar in Europe

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ungulate as an important carrier and spreader of resistant bac-
teria in Europe: for example, Portugal (Poeta et al. 2007; Dias
et al. 2015), Spain (Navarro-Gonzalez et al. 2012, 2013), Czech
Republic and Slovakia (Literak et  al. 2010), and Germany
(Schierack et al. 2009).
Most of the studies have focused on determining the antimi-
crobial susceptibility of specific isolated bacterial species but not
on the whole commensal bacterial community from wild boar.
These studies were concentrated in three bacterial groups: the
Gram-negative bacteria Escherichia coli and Salmonella spp. and
the Gram-positive Enterococcus spp., while one also focused on
Campylobacter spp. (Navarro-Gonzalez et al. 2014). The choice
of these bacterial species is mostly linked to their relevance as
human food-borne pathogens, but previous work has also sug-
gested their potential as reservoirs of antibiotic resistance genes
(Capita & Alonso-Calleja 2013).
E. coli is part of the mammal gut microbiota and can easily be
disseminated in different ecosystems. This facilitates the direct
comparison of its resistance phenotypes in distinct environments

and host animals (Literak et al. 2010). Salmonella spp. is the main

cause of food-borne disease outbreaks and the second most com-

monly reported cause of zoonotic diseases in humans in Europe

(~6.2 million cases of human infection in the EU each year;

Havelaar et al. 2013). Salmonella spp. is one of the most prevalent

causes of human gastroenteritis in industrialized countries, with

a broad range of hosts, including ungulates (Navarro-Gonzalez

et al. 2012). Most importantly, Salmonella is one of the zoonotic

pathogens that can be transmitted between animals and humans,

with reports of outbreaks tracked down to wildlife (Mentaberre

et al. 2013). Enterococci are common in the gastrointestinal tract

of humans and most animals. Enterococci have variable patho-

genic potential, but their capability to gain virulence traits and

withstand antimicrobial challenge gives them a special role in

nosocomial (hospital-acquired) infections, such as vancomycin-

resistant enterococci, potentially causing meningitis, septicae-

mia, or endocarditis (Fisher & Phillips 2009).

Across Europe, studies reported a high variability in the

prevalence of the three bacterial groups mentioned. Salmonella

Figure 38.1 Illustrative representation of the potential role of wild boar as vehicle and reservoir in AMR transmission.
Wild boar is located on the interface of urban, semi-natural and natural compartments, and promotes the interplay among these different compartments. Human
activity has significantly transformed landscape features and promoted overlap of ecological niches, once distinctly separated. Some species, such as wild boar,
have been highly successful to adapt and thrive in this new reality. As a consequence, they establish the link between the different sources of antibiotic resistance
genes in a complex web, creating selective pressures, enhancing adaptation processes and dissemination of resistance genes. A clear example of selective pressure
is the downstream effect exerted by anthropogenic-related antibiotics on bacterial resistance. After their use by humans in the urban compartment, either in clinical
or household settings, antibiotics are released through urban sewage systems. Some antibiotic residues will not be entirely removed by wastewater plants, and
may end up in the sewage sludge, or be released to water courses. Sewage sludge can eventually be released into the environment as fertilizer in agricultural soils
or through irrigation with wastewaters. Antibiotics can also be used in the semi-natural compartment, particularly in intensive livestock production, agriculture, and
aquaculture. These can go directly into the environment, or through the application of manure, which also contains antibiotic residues from antimicrobial use in
animals as food additives. These antibiotic-resistant bacteria can be transferred to surface water or groundwater and be released within the environment. Once in
the natural compartment, wild boar is exposed, through various sources, to foreign commensal and antibiotic-resistant microorganisms that have the potential to
cross-talk with the species’ own microbiota, creating a window of opportunity for cross-selection and clonal spread of tolerant bacteria. Wild boar thus potentially
becomes a significant vector of AMR, creating the risk of ecological spread as it moves between the different compartments. (A black and white version of this figure
will appear in some formats. For the colour version, please refer to the plate section.)

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