4. DISCUSSION
Summary and implications of the
evidence
Despite the increased global awareness regarding food safety in recent
decades, FBD still cause a substantial public health burden (World
Health Organization, 2021) and NTS stands among the main foodborne
pathogens that contribute to this burden, particularly by the
consumption of contaminated poultry and poultry-derived products. In
consequence, it is fundamental to assess the prevalence and
characteristics of this foodborne pathogen throughout the poultry
productive chain. The current systematic review and meta-analysis
provide a comprehensive synthesis of evidence regarding the prevalence,
the diversity and frequency of serovars, and the AMR profiles of NTS
reported in poultry samples from the Americas.
The meta-analyses showed that the prevalence of NTS was the highest in
environmental samples. We performed an additional meta-analysis grouping
together studies that reported internal (farm installations and
infrastructure, poultry houses, eggshells, bedding, feces, dust, and
litter) or external (supplies, transport cages, slaughterhouse
installations, and processing facilities) sources from where the
environmental samples were taken. The results showed a similar
prevalence of NTS in internal (31.5%, 25.1 to 38.3) and external
(29.4%, 11.1 to 51.9) sources of environmental samples and thus
demonstrate that NTS is both broadly distributed and highly frequent
throughout the environment related to poultry production in the
Americas. This result agrees with previous studies that have
demonstrated a consistent presence of NTS in environmental samples
including poultry feed (Magwedere, Rauff, De Klerk, Keddy, & Dziva,
2015), farm infrastructure (Bhatia & McNabb, 1980), slaughterhouse
installations and processing facilities (Rivera-Pérez, Barquero-Calvo,
& Zamora-Sanabria, 2014), and leftovers such as litter or feces (Vaz,
Voss-Rech, de Avila, Coldebella, & Silva, 2017).
With prevalence values ranging from 3.3 to 36.7%, SNT was isolated in
the environmental samples from 8 out of 15 countries included in the
study. Even though the variability was high in the estimates among the
countries, this is an important result because the presence of NTS in
the internal and external environment related to poultry production
might facilitate the reintroduction of the pathogen to the productive
chain and thus become a persistent source of exposition for further
contamination (Mueller‐Doblies, Sayers, Carrique‐Mas, & Davies, 2009).
In a recent longitudinal study, Voss-Rech et al. (2019) found that once
NTS was detected in the litter samples during the harvest of the first
flock, then the pathogen was isolated from the environment of subsequent
flocks, whereas Marin, Balasch, Vega, and Lainez (2011) reported that
all the different environmental samples related with poultry production
were contaminated with NTS and that the prevalence of the pathogen
remained high even after cleaning and disinfection. Several studies have
assessed different control measures to prevent and reduce the incidence
of NTS in the environment of poultry production. For instance,
preventing contamination of the facilities where the animal feed is
produced and killing the pathogens by pelleting the feed (Jones, 2011),
inactivation of residual microorganisms in recycled litter by shallow
fermentation or windrowing (Vaz et al., 2017), and cleaning and
disinfection to avoid persistent contamination in the broiler houses
(Davies, Breslin, Corry, Hudson, & Allen, 2001). However, the ability
of NTS to persist for long periods means that every available tool must
be used to control the organism, and efforts must be sustained (Jones,
2011) and constant sampling of the environment should be performed.
Additionally, the presence of NTS in the environment related to poultry
production and processing increases the probability of
cross-contamination of the carcasses and poultry meat during
slaughtering (Volkova et al., 2010). In this regard, Carramiñana et al.
(1997) reported that the percentage of poultry carcasses contaminated
with NTS increased from 56.7% to 70% after the processing, whereas
Tozser et al. (2019) found that the 0% positivity to NTS in the body
surface of the birds before transportation increased up to 100% after
the slaughter and processing stages. Given that poultry meat is one of
the most frequent sources of the exposure of NTS to the human, the
cross-contamination from the poultry environment to the carcasses and
meat poses a serious threat to public health, thus the interventions
aimed at reducing the contamination with pathogenic bacteria are
fundamental and require appropriate knowledge regarding the presence and
dissemination of these pathogens at various steps during the processing
stage (Berrang et al., 2007). Nevertheless, more studies are needed to
fully understand which interventions are effective against NTS
contamination of the poultry-derived products in countries from the
Americas.
We found substantial heterogeneity in the estimates at the national
level. The disparate number of studies included for each country and the
difference in the number of samples assessed could have increased the
uncertainty of the estimations and thus cause part of the heterogeneity,
this variability might reflect different husbandry practices and the
absence of effective control measures across the poultry production
chain (Antunes et al., 2016; Brochu et al., 2021). Despite successful
control of NTS has involved actions such as culling and vaccination, the
currently available vaccines have played a minor role in the control of
fowl typhoid as they offer short-lived protection against clinical
disease and limited or variable protection against infection with field
strains (World Organization for Animal Health, 2018). Therefore, other
containment methods including improved hygiene, increased biosecurity,
segregated hatching, competitive exclusion treatment, and monitoring and
removal of infected flocks could also be applied (EFSA Panel on
Biological Hazards et al., 2019). Nevertheless, the differential
implementation of these control measures caused by the variability in
the poultry husbandry practices across countries from the Americas could
explain part of the heterogeneous prevalence seen in these countries. In
this regard, in several countries from South America the imposition of
strict controls on the environment and hygiene of poultry husbandry is
restricted due to high ambient temperatures (Barrow, Jones, Smith, &
Wigley, 2012), thus causing differences in the incidence of NTS
concerning countries with improved hygiene and management practices.
Finally, national programs of poultry health, current legislation, and
farmers’ perceptions are also factors capable of affecting the
monitoring and control of the pathogens in poultry as well as the
biosecurity measures needed to avoid their spreading (de Oliveira
Sidinei, Marcato, Perez, & Bánkuti, 2021; Reis et al., 2021).
Our summary of evidence showed that Enteritidis and Typhimurium were
identified in most of the countries, where they ranked among the top
three serovars, thus coinciding with several studies that report
Typhimurium and Enteritidis as the main serovars in poultry samples
(Carramiñana, Rota, Agustin, & Herrera, 2004; El-Sharkawy et al.,
2017). The serovar distribution and prevalence found in countries from
the Americas differ with the pattern reported in some European
countries, which report as main serovars Infantis in Hungary (Tozser et
al., 2019), Hadar, Anatum, and Mbandaka in France (Le Bouquin et al.,
2010), and Enteritidis, Hadar, Virchow, and Ohio in Spain (Marin et al.,
2011). However, the variety and prevalence of NTS serovars are expected
to differ among the studies from different regions and types of farms
(Andino & Hanning, 2015). Despite the great amount of NTS isolates
included in the studies, the serovar diversity was rather low because
only 131 serovars were identified; therefore, more studies are necessary
to increase our knowledge regarding the NTS serovar diversity and
frequency present in the poultry production chain of the countries from
the Americas. Especially given that many serovars are restricted to a
single region of the world that generates distinct profiles both within
and between regions (Galanis et al., 2006).
Enteritidis was the serovar most prevalent in birds samples (mainly
laying hens) and products and sub-products (mainly eggs and carcasses),
which concurs with previous reports of salmonellosis outbreaks caused by
consumption of eggs and poultry meat contaminated with this NTS serovar
in the USA (Jackson, Griffin, Cole, Walsh, & Chai, 2013) and Europe
(EFSA, 2019). Besides Typhimurium and Enteritidis, Heidelberg and
Newport have also been highlighted as NTS serovars capable of causing a
great burden of FBD due to poultry and poultry-derived products (Jajere,
2019) and our results confirm this for Heidelberg because this serovar
was consistently found among the top five serovars in each of the three
samples assessed in our study. Even more, Heidelberg was the most
prevalent serovar found in environmental samples taken from production
wastes and breeding facilities. Thus, the pattern of NTS serovars found
across the different sources for each type of sample demonstrates the
presence of the three most common serovars frequently associated with
FBD caused by poultry and poultry-derived products.
Despite the global tendency to both reduce the indiscriminate use of
antibiotics and increase awareness regarding their negative effect, our
results demonstrate a high and alarming prevalence of multiresistance to
antibiotics in NTS isolates. The pooled prevalence of AMR to 2-3
antibiotics was 36.2% with the highest values in birds and
environmental samples, whereas resistance to ≥ 4 antibiotics reached a
pooled estimate of 49.6% with even a higher prevalence of 61.2% in
birds. These results not only determine the magnitude of the current AMR
status in the poultry husbandry in the Americas but also emphasize the
major public health issue linked to the presence and dissemination of
NTS multiresistant strains throughout the poultry production chain that
ultimately might reach to the human. This pattern of multiresistance
could be a consequence of the combined use of antibiotics, which
although is prohibited in several countries, remains as a common
practice in several regions and countries.
Kentucky, Heidelberg, Typhimurium, Enteritidis, and Infantis were the
top five NTS serovars with the highest prevalence of AMR in the poultry
samples from the Americas, which partially coincide with several studies
in which the highest prevalence of AMR was found in the serovars
Enteritidis, Typhimurium, and Infantis (Carramiñana et al., 2004;
El-Sharkawy et al., 2017; Kunadu, Otwey, & Mosi, 2020). The presence of
these isolates with AMR in the poultry production chain is a concern
given that several actors from this husbandry and supply chain
frequently handle equipment, animals, and products without the necessary
protections and thus increase the risk of resistance transfer and spread
to commensal bacteria (Reis et al., 2021). Besides, even though the
serovar Heidelberg is not frequently reported among the main resistant
serovars, we found that Heidelberg was the 2ndtop-ranked serovar with AMR in the Americas and this is an important
result for public health given that Heidelberg is one of the serovars
that can induce systemic complications in people, especially children,
the elderly or immunocompromised people (Silva, Milbradt, Zamae,
Andreatti Filho, & Okamoto, 2016).
Cephalosporins, penicillins, tetracyclines, and aminoglycosides
concentrated 71.9% of the resistant isolates and thus were the four
main groups of antibiotics to which NTS was resistant in poultry samples
from the Americas. This profile partially contrast with recent secondary
studies, which reported that quinolones and beta-lactams in Europe
(Antonelli et al., 2019) and sulphonamides, quinolones, and
tetracyclines in Brazil (Voss-Rech et al., 2017) were the main groups of
antibiotics to which NTS isolated from poultry and poultry-derived
products were resistant. Even though the specific pattern of AMR was
distinct among the poultry samples, tetracycline was consistently the
1st top-ranked, whereas ampicillin varied between the
2nd and 5th top-ranked. Such a
result should probably reflect the fact that tetracyclines and
penicillins were the largest selling antimicrobials, but their use
gradually reduced while cephalosporins increased (Kim, Seo, Jeon, Lim,
& Lee, 2018), thus causing an increase in AMR to cephalosporins in NTS
from poultry as have been reported in NTS serovar Typhimurium, though
with concurrent resistance to ciprofloxacin and ceftriaxone increasing
in other serovars (Wong, Zeng, Liu, & Chen, 2013). AMR to ceftriaxone
and ciprofloxacin in NTS isolates from poultry is of major interest to
public health, because these two antibiotics in conjunction with
azithromycin are the key drugs of choice for the treatment of invasive
NTS infections. However, according to our profile of AMR, except for the
samples from birds in which ceftriaxone was 8thtop-ranked, none of these three antibiotics were among the top 10
antibiotics to which NTS was resistant.
4.2 Limitations
We detect several limitations in our study: 1) unpublished studies were
not included to maintain a comparable level of methodological quality
among the studies, which could introduce bias because only published
studies were searched and included, 2) despite several databases were
searched, we only found studies for 15/35 countries from the Americas,
in consequence the epidemiological landscape found might not be
representative for the countries that were not included, 3) our study
only included poultry samples from broilers, laying hens, and
reproducers and thus the epidemiological landscape for NTS in ducks,
turkeys, quails, and ostriches still needs to be assessed in countries
from the Americas, and 4) to provide an overall summary of the studies,
we grouped a broad variety of poultry samples into three discrete
categories that avoided the meta-analysis of the specific sources of NTS
contamination within each particular stage or condition.
4.3 Conclusion
Our systematic review and meta-analysis both confirm the presence of NTS
throughout the poultry productive chain in countries from the Americas
and determines the magnitude of the prevalence of this pathogen that
causes a high burden of FBD. Despite we found a reduced diversity of NTS
serovars among the three types of poultry samples, the presence of
zoonotic serovars capable of causing outbreaks was consistent across
countries. Besides, the results demonstrated a high and alarming
prevalence of AMR in the NTS isolates from poultry samples, which in
addition showed an increasing trend towards higher prevalence of
multiresistance. Furthermore, we found that the serovars with the higher
prevalence of AMR were those commonly reported in salmonellosis
outbreaks associated with poultry. Taken together, these results confirm
the growing concern of NTS as a public health problem caused by the
consumption and exposure to contaminated poultry and poultry-derived
products. Additionally, our results highlight the need for appropriate
control measures against NTS in the entire poultry productive chain to
prevent further emergence and dissemination of multiresistant serovars
to the human, which threatens the successful treatment of invasive
infections caused by this pathogen.