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\begin{document}

\title{Prevalence, main serovars, and antimicrobial resistance profiles of
Non-typhoidal Salmonella in poultry samples from the Americas: A
systematic review and meta-analysis}



\author[1]{Daniel Diaz}%
\author[2]{Pavel Eduardo Hernandez-Carreño}%
\author[2]{Diana Zuleika Velazquez}%
\author[2]{Miguel Angel Chaidez-Ibarra}%
\author[2]{Arnulfo Montero-Pardo}%
\author[3]{Francisco Antonio Martinez-Villa}%
\author[4]{Adrian Canizalez-Roman}%
\author[5]{Vianney Francisco Ortiz-Navarrete}%
\author[6]{Rene Rosiles}%
\author[2]{Soila Maribel Gaxiola}%
\author[7]{Francisco Jimenez-Trejo}%
\affil[1]{Universidad Nacional Autonoma de Mexico}%
\affil[2]{Universidad Autonoma de Sinaloa - Escuela de Medicina Veterinaria y Zootecnia}%
\affil[3]{Instituto Mexicano del Seguro Social}%
\affil[4]{Universidad Autonoma de Sinaloa Facultad de Medicina}%
\affil[5]{CINVESTAV Departamento de Fisiologia Biofisica y Neurociencias}%
\affil[6]{Universidad Nacional Autonoma de Mexico Facultad de Medicina Veterinaria y Zootecnia}%
\affil[7]{Instituto Nacional de Pediatria}%


\vspace{-1em}



  
  \date{May 21, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Poultry and poultry-derived products such as meat and eggs are among the
main sources of non-typhoidal \emph{Salmonella} (NTS) transmission to
the human. Therefore, we performed a systematic review and used
random-effects meta-analyses to 1) estimate the prevalence of NTS in
poultry samples from birds, products and subproducts, and environmental
samples, 2) examine the diversity and frequency of their serovars, and
3) estimate the prevalence and profiles of antimicrobial resistance
(AMR) in NTS isolates reported in studies from the Americas. We included
157 studies from 15 countries comprising 261,408 poultry samples and
estimated an overall pooled prevalence of 17.9\% (95\% CI: 10.8--26.3)
in birds, 21.8\% (17.7--26.1) in products and subproducts, and 29.5\%
(24.2--35.1) in environmental samples. At the national level, the
prevalence of NTS was heterogenous across countries with the highest
values in Mexico, the USA, and Canada. In total, 131 serovars were
identified from 13,388 isolates, Heidelberg, Kentucky, Enteritidis, and
Typhimurium were the most prevalent in the overall top 10 ranking (range
6.5--20.8\%). At the national level, Enteritidis and Typhimurium were
identified in most of the countries, though with national differences in
their ranks. The prevalence of AMR increased from 24.1\% for 1
antibiotic, to 36.2\% for 2-3 antibiotics, and 49.6\% for [?] 4
antibiotics. Kentucky, Heidelberg, Typhimurium, and Enteritidis were the
serovars with the highest prevalence of AMR and tetracycline,
ampicillin, streptomycin, ceftiofur, and amoxicillin-clavulanic acid
were the top five antibiotics to which NTS isolates were resistant. In
conclusion, NTS was distributed through the avian production chain with
high and heterogenous values of prevalence in poultry samples. Besides,
there were distinctive patterns of serovars distribution across
countries and an alarming prevalence of AMR among zoonotic serovars.%
\end{abstract}%



\sloppy


\textbf{Prevalence, main serovars, and antimicrobial resistance profiles
of Non-typhoidal \emph{Salmonella} in poultry samples from the Americas:
A systematic review and meta-analysis}

Daniel Diaz\textsuperscript{a,b,*}, Pavel Eduardo
Hernandez-Carre\selectlanguage{ngerman}ño\textsuperscript{b}, Diana Zuleika
Velazquez\textsuperscript{b}, Miguel Angel
Chaidez-Ibarra\textsuperscript{b}, Arnulfo
Montero-Pardo\textsuperscript{b}, Francisco Antonio
Martinez-Villa\textsuperscript{c}, Adrian
Canizalez-Roman\textsuperscript{d}, Vianney Francisco
Ortiz-Navarrete\textsuperscript{e}, Rene Rosiles\textsuperscript{f},
Soila Maribel Gaxiola\textsuperscript{b,*}, Francisco
Jimenez-Trejo\textsuperscript{g}

\textsuperscript{a} Centro de Ciencias de la Complejidad, Universidad
Nacional Autónoma de México, Ciudad Universitaria, Ciudad de México
04510, México

\textsuperscript{b} Facultad de Medicina Veterinaria y Zootecnia,
Universidad Autónoma de Sinaloa, Culiacán Rosales 80246, Sinaloa, México

\textsuperscript{c} Unidad de Medicina Familiar No. 46, Instituto
Mexicano del Seguro Social, Culiacán Rosales 80178, Sinaloa, México

\textsuperscript{d} Centro de Investigación Aplicada a la Salud Pública,
Facultad de Medicina, Universidad Autónoma de Sinaloa, Culiacán Rosales
80246, Sinaloa, México

\textsuperscript{e} Departamento de Biomedicina Molecular, Centro de
Investigaciones Avanzadas, Unidad Zacatenco, Ciudad de México 07360,
México

\textsuperscript{f} Departamento de Nutricio\selectlanguage{english}n Animal y Bioqui\selectlanguage{english}mica,
Facultad de Medicina Veterinaria y Zootecnia, Universidad Nacional
Aut\selectlanguage{ngerman}ónoma de México, Ciudad Universitaria, Ciudad de México 04510, México

\textsuperscript{g} Instituto Nacional de Pediatría, Ciudad de México
01480, México

PEHC, pavel.hernandez.mvz@uas.edu.mx; DZV, velazquez.mvz@gmail.com;
MACI, migcha3@gmail.com; AMP, arnulfo.montero@uas.edu.mx; FAMV,
tps48\_@hotmail.com; ACR, canizalez@uas.edu.mx; VFON,
vortiz@cinvestav.mx; RR, toxrosiles@yahoo.com.mx;FJT,
trejofj@hotmail.com

ORCID: DD:\emph{https://orcid.org/0000-0003-2302-1982}

* Correspondence: DD, Centro de Ciencias de la Complejidad (C3),
Universidad Nacional Autónoma de México, Avenida Universidad 3000,
Copilco, Coyoacán, Ciudad de México, México. Tel: +52 228-125-9501
ddiaz@ciencias.unam.mx; SMG, Laboratorio de Parasitología, Facultad de
Medicina Veterinaria y Zootecnia, Universidad Autónoma de Sinaloa,
Boulevard San Ángel 3886, Predio Las Coloradas, Culiacán Rosales 80246,
Sinaloa, México. Tel: +52 667-102-8476; soilagaxiola@uas.edu.mx;

\section*{Abstract}

{\label{abstract}}

Poultry and poultry-derived products such as meat and eggs are among the
main sources of non-typhoidal \emph{Salmonella} (NTS) transmission to
the human. Therefore, we performed a systematic review and used
random-effects meta-analyses to 1) estimate the prevalence of NTS in
poultry samples from birds, products and subproducts, and environmental
samples, 2) examine the diversity and frequency of their serovars, and
3) estimate the prevalence and profiles of antimicrobial resistance
(AMR) in NTS isolates reported in studies from the Americas. We included
157 studies from 15 countries comprising 261,408 poultry samples and
estimated an overall pooled prevalence of 17.9\% (95\% CI: 10.8--26.3)
in birds, 21.8\% (17.7--26.1) in products and subproducts, and 29.5\%
(24.2--35.1) in environmental samples. At the national level, the
prevalence of NTS was heterogenous across countries with the highest
values in Mexico, the USA, and Canada. In total, 131 serovars were
identified from 13,388 isolates, Heidelberg, Kentucky, Enteritidis, and
Typhimurium were the most prevalent in the overall top 10 ranking (range
6.5--20.8\%). At the national level, Enteritidis and Typhimurium were
identified in most of the countries, though with national differences in
their ranks. The prevalence of AMR increased from 24.1\% for 1
antibiotic, to 36.2\% for 2-3 antibiotics, and 49.6\% for \selectlanguage{english}[?] 4
antibiotics. Kentucky, Heidelberg, Typhimurium, and Enteritidis were the
serovars with the highest prevalence of AMR and tetracycline,
ampicillin, streptomycin, ceftiofur, and amoxicillin-clavulanic acid
were the top five antibiotics to which NTS isolates were resistant. In
conclusion, NTS was distributed through the avian production chain with
high and heterogenous values of prevalence in poultry samples. Besides,
there were distinctive patterns of serovars distribution across
countries and an alarming prevalence of AMR among zoonotic serovars.

\textbf{Keywords:} Infectious diseases, Foodborne disease, Food safety,
Multi-drug resistance, Salmonellosis, Zoonosis.

\textbf{1. INTRODUCTION}

\emph{Salmonella} are Gram negative rod-shaped bacilli that belongs to
the family \emph{Enterobactericeae} . The \emph{Salmonella} genus
includes only two species, one of which is \emph{Salmonellaenterica}
that is divided into six subspecies that includes\emph{S. enterica}
subsp. \emph{enterica} (Popoff, Bockemuhl, \& Gheesling, 2004).
According to the Kauffmann-White scheme,\emph{Salmonella} strains are
serologically classified into more than 2,610 serovars based on their
specific antigenic reaction (Guibourdenche et al., 2010). \emph{S.
enterica} subsp. \emph{enterica} is of medical interest because causes
an infectious disease that can be classified into: enteric fever
(typhoidal) that affects only humans and non-typhoidal \emph{Salmonella}
(NTS) that affects both humans and animals (Barrow \& Methner, 2013).
Indeed, NTS is considered among the main pathogen bacteria that cause
foodborne disease (FBD), which causes a significant public health,
economic and social burden worldwide, particularly in low- and
middle-income countries (World Health Organization, 2021).

Non-typhoidal salmonellosis is commonly manifested as mild to moderate
gastroenteritis characterized by vomiting, nausea, diarrhea, and
abdominal pain (Foley \& Lynne, 2008). Usually, these symptoms developed
6-72 h after ingestion of the bacteria (Centers for Disease Control and
Prevention, 2004) and are resolved within 2-7 days because they are
self-limiting and thus do not require treatment. However, NTS can
develop into a more severe illness that invade normally sterile sites
that results in bacteremia, meningitis, and other focal infections,
which are denominated invasive NTS (Crump, Sjolund-Karlsson, Gordon, \&
Parry, 2015). Only in 2017, there were 535,000 cases of invasive NTS
worldwide that caused 77,500 deaths worldwide that affected mostly
children under five years of age, elderly people, and people with HIV
infection (Stanaway et al., 2019).

In the human, NTS is acquired mainly through contaminated food of animal
origin such as poultry products and poultry-derived products, which are
recognized as common sources of transmission (Goncalves-Tenorio, Silva,
Rodrigues, Cadavez, \& Gonzales-Barron, 2018). In poultry, this pathogen
bacteria can be found in the carcasses, raw meat, and eggs (eggshell,
salk, and yolk) (Foley, Lynne, \& Nayak, 2008), as well as in live birds
from poultry farms (El-Sharkawy et al., 2017), and in the environment
related to poultry production and slaughter (Manoj \& Singh, 2015).
Thus, NTS is widely distributed in the poultry production chain and
serves as an important reservoir for the dissemination of the pathogen
to the humans (Howard, O'Bryan, Crandall, \& Ricke, 2012).

From the perspective of public health, the subspecies
\emph{enterica}concentrates the most important zoonotic serovars of NTS
that include Typhimurium and Enteritidis, which are frequently reported
as a cause of FBD associated to the consumption of poultry products
(Park et al., 2014). Previous studies have reported contrasting patterns
of serovars predominance across the regions of the world: in poultry
from Asia, Latin America, Europe, and Africa, Enteritis was predominant,
and Kentucky and Sofia were most prevalent in North America and Oceania
(Ferrari et al., 2019), whereas in poultry samples from Europe,
Enteritidis, Infantis, and Typhimurium were the most frequent serovars
(Antonelli, Belluco, Mancin, Losasso, \& Ricci, 2019). Nevertheless, the
pattern of predominance and distribution of the serovars might be prone
to change due to the increase in the international trading of poultry
and poultry-derived products (Manoj \& Singh, 2015) in conjunction with
the implementation of control measures that have induced the expansion
of previously less common serovars (Antunes, Mourao, Campos, \& Peixe,
2016). Besides, the emergence and spread of NTS serovars with
antimicrobial resistance (AMR) is a major public health concern given
that the abusive use of antibiotics in intensive livestock systems
increase the rate of appearance of AMR (Antunes et al., 2016) and
because antibiotics are crucial for successful treatment of invasive
infections.

In consequence, it is necessary to implement effective intervention
strategies focused on reducing and controlling the dissemination of NTS
from poultry to humans. Besides, estimating the burden of the most
important FBD, such as NTS that cause a great impact on public health,
is an essential component in the effort to reduce its negative effect
(World Health Organization, 2021). In this context, identifying and
estimating the magnitude of \emph{Salmonella} serovars with AMR in
poultry and their products capable of causing FBD in humans might
provide an initial step in the identification of sources of exposure for
the dissemination of NTS because these products are recognized as the
main source of foodborne salmonellosis (Antunes et al., 2016) . This is
particularly important in regions of the world with a high level of
poultry production and consumption, such is the case of the Americas
that concentrates 42.3\% of the poultry meat production at the global
level (FAO, 2019). Therefore, we conducted a systematic review and
meta-analysis to 1) estimate the prevalence of NTS in poultry samples
from birds, products and subproducts, and environmental samples, 2)
determine the diversity and frequency of their serovars, and 3) estimate
the prevalence and profiles of AMR in NTS isolates reported in studies
from the Americas.

\textbf{2. METHODS}

\textbf{2.1 Protocol and objectives}

A protocol, available from the corresponding author upon reasonable
request, was developed \emph{a priori} in accordance to the Preferred
Reporting Items for Systematic reviews and Meta-analysis Protocol
(PRISMA-P) statement (Moher et al., 2015). The study was conducted
according to the Cochrane handbook for systematics reviews (Higgins et
al., 2019) and it is reported in agreement with the PRISMA statement
(Liberati et al., 2009). We conducted a systematic review and
meta-analysis to address the following questions: What is the prevalence
of NTS in samples of poultry from the Americas? Which are the main
serovars of NTS identified in each type of sample? What is the
prevalence and profile of antimicrobial resistance in NTS isolates of
poultry?

\textbf{2.2 Eligibility criteria}

We defined the eligibility criteria for the studies according to the POS
(Population, Outcome, and Study) approach of the PRISMA (Preferred
Reporting Items for Systematic reviews and Meta-analysis) statement
(Liberati et al., 2009). In \textbf{Table 1} are summarized the
definitions used. In brief, we included publications that met the
following inclusion criteria: 1) the studies reported the presence of
NTS in samples from poultry birds (included subpopulations such as
layers, broilers, and breeders at the individual or flock/collective
level), samples from products and subproducts of poultry, or
environmental samples from the ambient related to poultry production, 2)
the studies assessed the prevalence of NTS, the frequency and diversity
of the serovars, or the antimicrobial resistance profiles, and 3) the
studies were full text primary publications from the Americas published
in English, Spanish, or Portuguese in peer-reviewed journals from 1980
to March 2020. Reasons for exclusion were 1) reporting turkeys, ducks,
ostriches, pigeons, quails, or wild, song or, ornamental birds, 2) not
reporting any of the three types of samples defined, 3) reporting NTS
isolates previously obtained, 4) not reporting at least one of the
defined outcomes, 5) re-used data from a previous study, 6) study
outside the Americas. We did not include any grey literature (i.e.,
unpublished studies, thesis, conference proceedings, or reports) to
assure an adequate and comparable level of methodological quality among
the selected studies.

\textbf{2.3 Information sources and search methodology}

A single reviewer consulted PubMed, CAB Abstracts, Virtual Health
Library (VHL), Science Direct, Scopus, and Web of Science from 24
February to 02 March 2020 to find scientific literature. To find the
most relevant and specific publications, the reviewer conducted
separated searches for each outcome (prevalence, serovars, and
antimicrobial resistance). For this, we defined a common search term for
the population {[}(salmonell* OR salmonella OR salmonellosis) AND (avian
OR poultry OR broiler OR laying hen OR fowl OR chick){]} that was used
in conjunction with search terms for prevalence (prevalence OR
incidence), serovars (genotype OR serotyping OR serovar), and
antimicrobial resistance (resistance OR resistant). Full searches and
the number of recovered records for each database are presented
in\textbf{Supplementary Table 1} of the supplementary material. After
completing the searches, all the retrieved records were downloaded and
stored in a single library in EndNote x9 (Thompson Reuters, USA).

\textbf{2.4 Study selection process and data extraction}

To select the studies to be included in the narrative synthesis, one
reviewer first removed the duplicates automatically from the EndNote
library and then revised the records manually. Next, the same reviewer
performed the screening of all the remaining records by checking whether
the publications were related to the reviewed subject, first based on
the title, and second, based on the abstract. Once the screening was
completed, the selected publications were retrieved in full text to
assess their eligibility for final inclusion in the systematic review
and meta-analysis. To account for this, two independent reviewers used a
standardized questionnaire that was based on the eligibility criteria
(\textbf{Supplementary Table 2} ) and was piloted in 10\% of the
publications selected at random. Each time there was a discrepancy
between the reviewers, a third reviewer served as referee until
consensus was reached.

A single reviewer used a standardized questionnaire to extract data from
all the selected publications. Prior to be used, the extraction sheet
format was piloted in 10\% of the studies selected at random. The
extracted data included a) the main characteristics of the publication
(first author, country, publication year, and study design), b) the main
characteristics of the assessed population (poultry subgroup, sex, and
age), c) the type of sample reported (birds, products and subproducts,
or environmental), d) the diagnostic technique for NTS, and e) the
reported outcomes (prevalence, serotyping, or antimicrobial resistance).
For the publications that reported both data on the individual and
flock/collective level, we selected for extraction only data at the
individual level to avoid duplication of estimates for a single
publication and because we found less variation in the estimations made
at the individual when compared to the flock/collective level. Besides,
for the publications that assessed more than one type of sample, each
one of them was extracted separately and counted as one independent
study within a single publication. All the extracted data were
registered into an Excel spreadsheet (Microsoft, USA) and a codebook was
created to facilitate its handling.

\textbf{2.5 Assessment of the risk of bias in individual studies}

One reviewer assessed the risk of bias using an adapted version of the
method published by Higgins et al. (2019). Each study was rated as
having or not (Yes or Not) risk of bias according to the following
criteria: for all the studies, 1) samples that tested positive to NTS
were confirmed by at least two diagnostic techniques; for the studies
that reported serovars, 2) all the NTS-positive samples were serotyped
and 3) the study presents a complete report of all the serovars that
were found; for the studies that reported antimicrobial resistance, 4)
all the NTS isolates were assessed for antimicrobial resistance, and 5)
the study presents the complete antimicrobial resistance profile. For
the studies that did not report the three outcomes, each criterion was
scored as not applicable (NA). To summarize the results, we present the
percentage of studies that had or not risk of bias per each criterion.

\textbf{2.6 Summary measures and statistical analysis}

We estimated the overall pooled prevalence of NTS and the prevalence of
antimicrobial resistance in poultry samples with a meta-analysis of
proportions using the Freeman-Tukey double arcsine transformation with
95\% exact confidence intervals (95\% CI) (Chaidez-Ibarra et al., 2021).
Due to expected heterogeneity across studies, we defined \emph{a
priori}a random effects model (DerSimonian \& Kacker, 2007). To estimate
the prevalence of NTS: first, we performed independent meta-analyses
according to the type of sample (birds, products, and environmental)
with subgroup analysis aggregating the studies by the level of sampling
(individual or flock/collective), and second, we performed independent
meta-analysis according to the type of sample aggregating the studies by
country (considering both individual and flock/collective data). To
estimate the prevalence of antimicrobial resistance, we performed
independent meta-analyses according to the number of antibiotics (1,
2-3, or \selectlanguage{english}[?]4) to which the NTS isolates were resistant and aggregated the
studies by the type of sample. The overall effect of the model was
assessed with the z statistic assuming an effect size = 0, whereas the
Cochran's Q statistic (\emph{X} \textsuperscript{2} test) was used to
assess significant heterogeneity across trials and the\emph{I}
\textsuperscript{2} statistic was used to measure the proportion of
variation in the effects caused by heterogeneity in the true effects
rather than sampling error (Diaz et al., 2019; Romo-Barron et al.,
2019). All analyses were performed on Stata 12 (StataCorp, TX, USA) and
the graphs were constructed on Prism 9 (GraphPad Inc., CA, USA). In all
the cases, we considered a value of \emph{p} \textless{} 0.05 as
significant.

\textbf{3. RESULTS}

\textbf{3.1 Study selection}

As shown in \textbf{Figure 1} , a total of 12,279 records were retrieved
from the electronic databases, of which 84.5\% were provided by VHL,
Scopus, and PubMed. In total, 4,815 publications remained after
duplicates were removed and once the screening of titles and abstract
was completed, 340 publications were retrieved in full text for final
eligibility. A total of 157 publications that met the inclusion criteria
were included in the quantitative synthesis. Among the 183 full text
publications that were excluded, the main reasons were: assessed SNT
isolates previously obtained (56.8\%), lacked the defined population
(21.3\%), and not a full text or other type of study different than the
defined (10.9\%). A list of the publications that were excluded and the
primary reason for exclusion are provided in \textbf{Supplementary Table
2} and a full reference list with all the included publications is
presented in \textbf{Supplementary Table 3} .

\textbf{3.2 Main characteristics of the studies}

The 157 publications included in our study were conducted in 15
countries from the Americas, of which 81.5\% (128/157) were from the
United States of America (USA), Brazil, and Canada. Colombia, Argentina,
and Mexico provided 4-6 studies each, whereas Trinidad and Tobago
contributed with three studies and Costa Rica, Ecuador, and Paraguay
provided two studies each. Finally, we found only one publication from
Chile, Guatemala, Puerto Rico, Uruguay, and Venezuela (\textbf{Fig. 2a}
). English was the predominant language of publication with 94.9\%
(149/157) and according to the study design, cross-sectional studies
were the most frequent with 77.7\% (122/157) of the publications,
followed by surveys, surveillance studies, and experiments (19, 7, and 4
publications, respectively) (\textbf{Fig. 2b} ). The cumulative
distribution according to the year of publication showed that 53.3\% of
the studies were published during the last decade since 2010
(\textbf{Fig. 2c} ). As depicted in \textbf{Figure 2d} , the
publications used a mixture of diagnostic techniques to detect NTS in
the samples, 43.9\% (69/157) of them reported up to three diagnostic
tools, whereas 37.6\% (59/157) used at least two different diagnostic
techniques. Among the publications, bacterial culture was the most
frequent diagnostic tool with 95.5\% (150/157), followed by serological
and biochemical tests (117 and 83). Only 22.3\% (35/157) of the
publications reported the use of PCR for the molecular confirmation of
NTS in the positive samples and used the following genes: \emph{inv} A,
RNAr 16S,\emph{bla} \textsubscript{CMY}, \emph{bla}
\textsubscript{CMY-2},\emph{SpvC} , \emph{mdh} y \emph{ompC} .

The main characteristics of the 157 publications included in the
quantitative synthesis are summarized in \textbf{Supplementary Table 4}
. The publications were characterized by reporting a mixture of the
three defined outcomes: 149/157 reported the prevalence of NTS, 94/157
included the serotyping of the isolates, and 46/157 assessed the
antimicrobial resistance profiles of the NTS isolates. Besides, the 149
publications that reported the prevalence of NTS included up to three
types of samples and we extracted each sample as an independent study
within a publication; therefore, there were 194 independent studies that
were distributed according to the type of sample in products and
subproducts (90), environmental (61), and birds (43).

\textbf{3.3 Risk of bias assessment}

As shown in \textbf{Figure 2e} , 81.5\% of the publications reported at
least two diagnostic tools to detect NTS-positive samples and thus were
judges as not having risk of bias. Among the 94 publications that
serotyped the NTS isolates, 87.2\% were judges as not having risk of
bias because the studies serotyped all the isolates. For these same
publications, 74.5\% reported the complete diversity of serovars found
and thus were free of risk of bias. A total of 84.8\% of the 46
publications that assessed the antimicrobial resistance profile of the
isolates were free of risk of bias because they included all the
NTS-positive samples. However, 54.3\% of these 46 publications had risk
of bias because they did not report the complete antimicrobial
resistance profiles for all the assessed isolates.

\textbf{3.4 What is the prevalence of NTS in samples of poultry from the
Americas?}

As summarized in \textbf{Table 2} , the prevalence of NTS varied both
according to the type of the sample assessed and the country. The pooled
overall prevalence of NTS was 17.9\% (95\% CI: 10.8 to 26.3) in 43
studies that reported 24,113 samples of birds from 11 countries with a
significant test of effect size (Z=7.5, \emph{p} =0.00) and substantial
heterogeneity as judged by the \emph{I} \textsuperscript{2}
(99.5\%,\emph{p} =0.00). For these samples, the subgroup analysis
revealed a lower prevalence of 9.8\% of NTS in individual data (3.8 to
18.2, n=25) in comparison to the value of 33.8\% (16.6 to 53.3, n=18)
estimated in flock data (\textbf{Supplementary Figure 1} ). At the
national level, the prevalence of NTS ranged from 23.2 to 50\% in
samples of birds reported in studies from Mexico, the USA, Uruguay, and
Venezuela. In contrast, the prevalence of NTS was lower in studies from
Brazil, Chile, Paraguay, and Trinidad and Tobago where the estimations
varied between 0.8 and 3.7\% (\textbf{Table 2} ).

In 90 studies that reported 173,354 samples of products and subproducts
from 10 countries, the overall pooled prevalence of NTS was 21.8\% (17.7
to 26.1) with a significant test of effect size (Z=16.8, \emph{p} =0.00)
and a significant variation attributable to heterogeneity (\emph{I}
\textsuperscript{2}=99.6\%, \emph{p} =0.00). According to subgroup
analysis, the prevalence estimations were consistent between the
individual (21.2\%, 17.7 to 26.1, n=78) and collective data (28.1\%,
12.6 to 46.4, n=12) of these samples (\textbf{Supplementary Figure 2} ).
The studies from Canada showed the highest pooled prevalence of NTS in
samples from products and subproducts, while prevalence values ranging
from 31.2 to 34.4\% were estimated in studies from Guatemala, Mexico,
and Puerto Rico. Finally, Argentina, Brazil, Costa Rica, and Trinidad
and Tobago had prevalence values below-average (\textbf{Table 2} ).

Regarding the environmental samples, the pooled prevalence of NTS was
29.5\% (24.2 to 35.1) in 61 studies that comprised 63,941 samples from
eight countries. There was a significant test of effect size
(Z=17.1,\emph{p} =0.00) and substantial heterogeneity across studies
(\emph{I} \textsuperscript{2}=94.4\%, \emph{p} =0.00), and the subgroup
analysis showed a contrasting difference of 21.9 and 52.4\% at the
individual (17.6 to 26.6, n=43) and collective (41.7 to 62.9, n=18)
level, respectively (\textbf{Supplementary Figure 3} ). At the national
level, the studies from Canada and the USA had prevalence values above
the overall estimation, whereas with estimations varying 3.3 to 9.0\%,
the studies from Argentina and Costa Rica showed the lowest prevalence
of NTS in environmental samples (\textbf{Table 2} ).

\textbf{Which are the main serovars of NTS identified in each type of
sample?}

In total, 131 NTS serovars were identified from 13,388 isolates from
poultry samples reported in 94 publications from 13 countries
(\textbf{Fig. 3} ). The USA and Canada contributed both with the highest
percentage of isolates (84.0\%) and the greatest diversity of NTS
serovars (68 and 57, respectively). Likewise, Brazil and Colombia
provided a great diversity of serovars (53 and 40, respectively),
whereas the remaining countries reported a lower diversity of NTS
serovars (range 1-19). A detailed list of the distribution of the 131
NTS serovars per country is provided in \textbf{Supplementary Table 5} .

Overall, 84.3\% of the isolates were distributed among 10 NTS serovars,
Heidelberg, Kentucky, and Enteritidis were the most prevalent (20.8,
20.6, and 17.6\%, respectively), whereas Mbandaka, Schwarzengrund, and
Montevideo were less prevalent with values ranging from 2.2 to 2.4\%.
Outside the top 10 serovars, the category ``Others'' comprised the
remaining 121 serovars that were found in 2,103 isolates. As shown
in\textbf{Figure 3} , there was a heterogenous pattern in the top 10
ranking of the NTS serovars across countries, which additionally had
several serovars not included in the overall top 10 ranking.
Additionally, in Costa Rica, Ecuador, Mexico, and Trinidad and Tobago
their 1\textsuperscript{st} top serovar was not included in the overall
top 10 ranking list. Even though the serovars Heidelberg and Kentucky
were ranked as 1\textsuperscript{st} and 2\textsuperscript{nd} overall,
these serovars were reported only in 5 and 6 countries, respectively,
where their ranks varied across countries. In contrast, serovars
Enteritidis and Typhimurium (3\textsuperscript{rd} and
4\textsuperscript{th} overall) were identified in most of the countries
(10 and 8 respectively); thus, these two serovars had the broadest
distribution in poultry samples across countries from the Americas.
Furthermore, Enteritidis was the 1\textsuperscript{st} top-ranked
serovar in the USA, Brazil, Colombia, Paraguay, and Uruguay where its
prevalence ranged from 23.4 to 100\% and the
2\textsuperscript{nd}top-ranked serovar in Argentina, Chile, and
Ecuador. Similarly, Typhimurium was the 1\textsuperscript{st} top-ranked
serovar in Chile and the 3\textsuperscript{rd} ranked in Colombia.
Despite the serovars Hadar, Thompson, and Montevideo were ranked in the
top 10 overall, they were reported only in two countries each and thus
showed a narrow geographical distribution.

We constructed Sankey diagrams to examine and compare the distribution
of the top five NTS serovars according to the type of poultry sample
(\textbf{Fig. 4} ). Laying hens, broilers, and a mixture category that
grouped chicks and hens were the main sources of NTS isolates in animal
samples, whereas carcasses, eggs, and subproducts for consumption (such
as meat and processed products) provided most of the isolates for the
products and subproducts group. Finally, among environmental samples,
most of the isolates were obtained from production wastes (such as
litter and feces), breeding facilities, and production supplies (such as
water, feed, and litter). Serovars Enteritidis and Heidelberg were
identified in all the poultry samples, whereas the samples from products
and subproducts and environmental shared 4 out of 5 serovars
(Enteritidis, Kentucky, Heidelberg, and Typhimurium). Among the samples
from birds and products and subproducts, Enteritidis was the most
prevalent serovar with 32.4 and 22.8\%, respectively, followed by
Senftenberg and Kentucky (10.6\% for birds and 21.5\% for products and
subproducts). Heidelberg and Kentucky were the most prevalent serovars
in environmental samples (32.1 and 28.8\%, respectively).

\textbf{What is the prevalence and the profile of antimicrobial
resistance in NTS isolates of poultry?}

Of the 46 publications that assessed the AMR of the NTS isolates, 38/46
reported the prevalence in 3,078 isolates of which 2,223 were resistant
to at least one antibiotic. As depicted in \textbf{Figure 5a} , the
prevalence of AMR tended to be higher according to the increase in the
number of antibiotics to which the NTS isolates were resistant: 1
antibiotic, 24.1\% (14.7 to 34.9\%, n=440); 2-3 antibiotics, 36.2\%
(24.5 to 48.6\%, n=758); and \selectlanguage{english}[?] 4 antibiotics, 49.6\% (36.9 to 62.2\%,
n=1,025). Besides, there was a heterogenous pattern of prevalence to AMR
among the three types of samples because in samples from birds and
products and subproducts, resistance to \selectlanguage{english}[?] 4 antibiotics was highly
prevalent (61.2 and 48.7\%), whereas resistance to 2-3 antibiotics was
highly prevalent in environmental samples (47.1\%)
(\textbf{Supplementary Figure 4} ).

A total of 26/46 publications reported 50 serovars from 1,688 NTS
isolates that were resistant to at least one antibiotic. As shown
in\textbf{Figure 5b} , 10 serovars contributed with 79.2\% of the total,
among which Kentucky (29.5\%), Heidelberg (16.7\%), Typhimurium (9.8\%),
and Enteritidis (7.1\%) were the top four serovars most frequently
reported as resistant to at least one antibiotic. Outside the top 10
ranking, the category ``Others'' comprised 40 serovars from 331 NTS
isolates of which Typhimurium var. 5-, 4,5, 12:1:, Pullorum, Anatum,
Lithfield, Montevideo, Hadar, Ohio, Newport, and Senftenberg were
reported in 10 to 33 isolates each. In contrast, serovars such as
Havana, Idikan, Ouakam, Bredeney, Alachua, Corvallis, Istambul,
Livingstone, Worthington, and I 4,5, 12: - were reported only once
across the publications.

In total, 40/46 publications reported 15 groups of antibiotics that
included 57 antibiotics to which 8,911 NTS isolates were resistant. Each
group included between 1 and 11 antibiotics that varied in the frequency
of report (\textbf{Supplementary Table 6} ). In \textbf{Figure 5c} are
depicted the main six groups of drugs and their corresponding
antibiotics to which 90.6\% of the isolates were resistant.
Cephalosporins, penicillins, tetracyclines, and aminoglycosides were the
top four groups, which concentrated 71.9\% of the resistant isolates of
NTS. As shown in \textbf{Figure 5d} , with a prevalence that ranged from
5.8 to 16.2\%, tetracycline, ampicillin, streptomycin, ceftiofur, and
amoxicillin-clavulanic acid were among the top 10 antibiotics with the
highest overall prevalence of resistance in \emph{Salmonella} isolates.
In the bottom rank, gentamicin and trimethoprim-sulfamethoxazole were
the antibiotics with the lowest prevalence of resistance in NTS.
Regarding the three poultry samples, the profile of antimicrobial
resistant was heterogenous. Except for tetracycline, which was the
1\textsuperscript{st} top-ranked antibiotic among resistant isolates,
the remaining antibiotics varied in their position within the rank. For
instance, streptomycin that was the second most prevalent in birds,
ranked 4\textsuperscript{th} and 5\textsuperscript{th} in environmental
samples and products and subproducts. Likewise, ampicillin ranked
5\textsuperscript{th} in environmental samples, whereas in the two other
samples this antibiotic was among the top three, nalidixic acid that was
the third to last most prevalent overall, in environmental samples was
ranked 3\textsuperscript{rd}. Besides, in the three samples of poultry
there were 1-2 antibiotics that were not included in the overall top 10
ranking.

\textbf{4. DISCUSSION}

\subsection*{Summary and implications of the
evidence}

{\label{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-Perez, 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\selectlanguage{english}-Doblies, Sayers, Carrique\selectlanguage{english}-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, Carraminana 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, \& Bankuti, 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
(Carraminana, 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 \selectlanguage{english}[?] 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 (Carraminana 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
2\textsuperscript{nd}top-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
1\textsuperscript{st} top-ranked, whereas ampicillin varied between the
2\textsuperscript{nd} and 5\textsuperscript{th} 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
8\textsuperscript{th}top-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.

\textbf{ACKNOWLEDGEMENTS}

D. Diaz is a postgraduate student from the Doctorado en Ciencias
Agropecuarias de la Universidad Autonoma de Sinaloa and performed this
study in partial fulfillment of the requirements to obtain his Ph.D.
degree. This study did not receive any specific grant from funding
agencies in the public, commercial, or not-for-profit sectors.

\textbf{ETHICS STATEMENT}

The authors confirm that the ethical policies of the journal, as noted
on the journal's author guidelines page, have been adhered to. No
ethical approval was required as this is a review article with no
original research data

\textbf{CONFLICT OF INTEREST}

The authors declare no conflict of interest.

\textbf{DATA AVAILABILITY STATEMENT}

All data and material are available from the supplementary material that
can be accessed online at the journal website.

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\textbf{Figure Legends}

\textbf{Fig. 1.} PRISMA flow chart for the selection of studies included
in the systematic review and meta-analysis.

\textbf{Fig. 2} . a) geographical distribution of the 157 studies
included, b) distribution of the design of the study, c) cumulative
frequency according to the year of publication, d) distribution of the
diagnostic technique and the number of diagnostic tools used to detect
NTS in poultry samples, and e) summary of the individual risk of bias
assessment.

\textbf{Fig. 3} . Top 10 ranking and prevalence of NTS in poultry
samples from the Americas.

\textbf{Fig. 4.} Sankey diagrams for the distribution of the five main
serovars of NTS in samples from a) birds, b) products and sub-products,
and c) environmental.

\textbf{Fig. 5.} a) forest plot of the meta-analysis to estimate the
prevalence of antimicrobial resistance in poultry samples, b) top 10
ranking of the NTS serovars that showed antimicrobial resistance, c)
distribution of \emph{Salmonella} isolates according to the group and
antibiotic to which they showed resistance, and d) top 10 ranking of the
antibiotics that were reported in resistant isolates.\selectlanguage{english}
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