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Chapter

38

Antimicrobial Resistance in Wild Boar in Europe:

Present Knowledge and Future Challenges

Rita T. Torres, Mónica V. Cunha, Tânia Caetano, Sónia Mendo, Emmanuel

Serrano, and Carlos Fonseca

Natural and Anthropogenic Niches of

Antimicrobial Resistance

Antimicrobials are essential for the treatment of bacterial infec-
tions in humans and animals. However, their intensive use
(and misuse) has severely increased the frequency of resistance
among clinical and environmental bacteria, with progressively
less effective therapies, and a post-antibiotic era is predicted
(Woolhouse & Farrar 2014). Antimicrobial resistance (AMR)
is nowadays an obstacle to the treatment of infectious diseases,
posing a significant threat to public health. Antibiotics under-
pin routine medical practice, and thus monitoring and report-
ing their occurrence must remain a priority for health agencies
worldwide. In terms of economy, their impact is huge and the
global human burden posed by infections is difficult to quantify,
but 25,000 deaths and €1.5 billion in economic losses are esti-
mated in the EU alone for each year (EMA & ECDC 2009). As a
result, the World Health Organization has identified AMR as a
global, emerging, unparalleled and growing problem for public,
animal, and environment health (WHO 2014).
When a microorganism that was susceptible to an antibiotic
is no longer sensitive to it, resistance becomes acquired, mak-
ing antibiotics less effective and limiting treatment options. This
acquired resistance phenotype contrasts with natural resistance
displayed by several bacteria, in which resistance mechanisms
probably evolved to protect these bacteria against their own
produced molecules and against antibiotics that are naturally
produced in their ecological niches by other competing micro-
organisms (D’Costa et  al. 2007). Indeed, soil environmental
bacteria have been producing antibiotics for probably 2 billion
years (D’Costa et al. 2011). Soil is thus considered to be the origi-
nal source of bacteria naturally producing most of the antimi-
crobial molecules used nowadays in medicine and veterinary
settings (D’Costa et  al. 2007). The first reports of resistance
have usually arisen during clinical trials and resistance typi-
cally increases during the lifetime of an antibiotic. In the early
beginnings of this acquired resistance phenomenon, alternative
classes of antibiotics to which bacteria had not yet developed
resistance were always around. Multiple drug resistance (MDR),
i.e. resistance to multiple antibiotics, were already reported in
the late 1950s and early 1960s among common enteric bacteria
(Escherichia coli, Shigella, and Salmonella), along with descrip-
tions in the 1970s of Staphylococcus aureus resistance to several
classes of antibiotics (methicillin-resistant S. aureus, MRSA).

Bacteria acquire resistance through mutations and horizon-

tal gene transfer of resistance determinants. Direct inactivation

of antibiotics (e.g. by β-lactamases), modification (i.e. muta-

tion) of cellular targets, modification of cell walls, increased

efflux of drugs from cells by use of efflux pumps, and changes

in gene expression or metabolic bypasses are examples of resist-

ance strategies/mechanisms that microorganisms employ (Levy

& Marshall 2004). Mutation and mobilization of genes encod-

ing resistance mechanisms, as well as adaptive resistance pheno-

types, are promoted by the same factors that promote antibiotic

usage, particularly prolonged, cumulative, low-level exposure.

Several factors have promoted resistance, including general-

ized introduction of broad-spectrum antibiotics, urbanization

with overcrowding and poor sanitation, increment of antibiot-

ics usage to treat infections, pollution, environmental wasting,

use of animal manure and widespread discharge of antibiotic

residues, changes in demography and increased life expectancy,

immunosuppression and increased opportunistic infection.

Overuse and/or inadequate antibiotic prescription, as well as

poor patient compliance to treatment, further aggravate the

problem (Laxminarayan et al. 2013).

The majority of the annual global production of antibiot-

ics is actually not used in human medicine, but in veterinary

medicine, as food additives to promote livestock growth (cat-

tle, pigs, and poultry) and as infection prevention in agriculture,

aquaculture, and horticulture. In fact, most of the antibiotics

produced every year go to the agriculture and veterinary sectors

(Laxminarayan et al. 2013).

The Wild Compartment of AMR

As human populations grow and transform landscapes, contact

with wildlife concurrently increases. Disease emergence and

extensive spread of old and new pathogens are important conse-

quences of this acceleration in interaction, with the majority of

emerging infectious diseases in humans now arising from wild-

life reservoirs (Jones et al. 2008). The recent Ebola epidemic in

West Africa is a stark reminder of the role that animal reservoirs

may play in public health, with 28,646 case reports and 11,323

documented human deaths (WHO 2016). In fact, the increasing

incidence of AMR in humans and livestock has been linked to

the emergence of AMR in wildlife (Jones et al. 2008; Wellington

et al. 2013). However, the scientific community remains largely

unaware of the complex transmission dynamics of AMR in the

environmental setting (Allen et al. 2010; Wellington et al. 2013).

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