1. Introduction
Poultry production in Zambia is one of the most important activities in the livestock sector. The chicken population is estimated at 94 million broilers,15 million village chickens, and 5.8 million layers [
1]. Moreover, poultry products in Zambia, like in other developing African countries, form an integral part of the human diet in many households, as it is a cheaper source of animal protein and is easier to produce compared to other foods of animal origin [
2,
3].
As the Zambian population progressively expands, the demand for meat and other foods increases, leading to food security problems [
1]. As a result, the government relies on agricultural industries to heighten animal production and address food insecurity. These industries usually raise large numbers of animals by boosting production through extensive farming methods that involve Antimicrobial Growth Promoters [
4]. Although such livestock intensification approaches are essential for alleviating food shortages, they are also associated with the emergence and spread of antimicrobial resistance (AMR) [
5]. This is further exacerbated by poorly monitored animal husbandry practices that result in frequent infections requiring antimicrobial use, leading to AMR. Besides, farmers sometimes deliberately underdose their livestock because of the high cost associated with antibiotics, worsening the problem.
AMR can be defined as the inability of bacteria parasites and viruses to respond to medicines making infection treatment difficult [
6]. AMR may be encoded chromosomally, but the production of plasmid-mediated extended-spectrum lactamases (ESBLs) is more common. There are nine ESBL classes, but common ones include variants of the CTX-M-type and derivatives of SHV-1, TEM-1 and TEM-2 [
7]. ESBL-producing
Enterobacteriaceae, resistant to third generation cephalosporins like cefotaxime (CTX), are dreaded profoundly because of their extensive geographic distribution and adverse health impacts. While ESBLs are more prevalent among hospital isolates, poultry has emerged as an important reservoir for possible zoonotic transmission [
7]. This reservoir includes ESBL-encoding genes harboured by commensal and pathogenic strains and may disseminate to humans via two main mechanisms. Firstly, ESBL genes may be transmitted by horizontal gene transfer, and the treatment implications depend on the pathogenicity of the recipient bacterial strain [
8]. Of greater concern is the direct transmission of disease-causing pathogens by clonal expansion, potentially leading to clinical disease and treatment failure. Therefore, understanding the zoonotic transmission of poultry associated ESBLs requires a multipronged approach that considers both healthy and diseased chickens. In Zambia, most studies have focused on non-pathogenic bacteria isolated from asymptomatic chickens, leaving a gap in the ESBL status among sick chickens. Thus, this study was undertaken to estimate antimicrobial usage, isolate bacterial pathogens and confirm the presence of ESBL-encoding genes by PCR and sequencing in bacteria isolated from poultry.
3. Discussion
The documentation of antimicrobial importation and sales data is vital as it serves as a basis for intervention programs and policy decisions. In this study, antibiotic disk selection was based on sales data from March 2021 to February 2022 from one of the country's largest wholesale and retail outlets of animal pharmaceuticals. The highest group of antibiotics sold belonged to the tetracyclines, followed by the penicillins, which corresponds to the AST results that showed that 68.8% of the isolates were resistant to tetracyclines, 62.5% to amoxicillin, and 56.2% to ampicillin. The high levels of tetracycline and penicillin resistance observed in this study are similar to what has been reported previously [
9]. This could be attributed to huge quantities of tetracyclines and penicillins being imported and sold in the country, as reflected in the sales data (
Figure 2). These results also concur with a study done in Tanzania and Cameroon where tetracyclines, penicillins and sulfonamides were the most used antimicrobials in poultry production [
10,
11].
The cumulative rise in AMR could firstly be attributed to the use of antibiotics for infection control rather than treatment of disease by poultry farmers. This is usually done by introducing antibiotics in the first week of the chick’s life to counter infections that may rise due to breaches in biosecurity [
12].In Zambia, farmers avoid economic losses from infections by medicating healthy chickens upto to the sixth week despite being aware of the withdrawal periods [
13]. Secondly, despite large agro shops dispensing antimicrobials by prescription, farmers still have access to antimicrobials in smaller outlets in the central business district [
14]. Furthermore, the lack of knowledge by farmers is a major attribute contributing to the development of MDR as some farmers are of the belief that the use of different antibiotics lowers the chances of AMR development [
15].
Over the past few years, AMR among the
Enterobacteriaceae family has skyrocketed in worldwide [
16]. Studies pertaining to AMR have frequently been reported in
E. coli and
Salmonella from healthy chickens [
2,
17].In this study, the most predominant species isolated was
Enterobacter (43/103,47%) which showed multidrug resistance to tetracyclines, sulphonamides, ampicillin and amoxicillin. Sulfonamide and penicillin resistance are commonly reported among
Enterobacteriaceae in other studies [
18], probably due to the overuse of these two drug classes in poultry. For instance, previous studies have revealed a corelation between tetracycline concentration and resistance in the environment [
19], as well as a relationship between tetracycline usage and resistance in poultry [
20].
In our study, the prevalence of
E. coli was 19.4% (20/103), which was lower than what has been reported elsewhere. For instance, Ibrahim et al (2019) and Ameen-Ur-Rashid et al (2016) found about 34% and 35%
E. coli isolates in diseased chickens in Jordan and Pakistan, respectively [
21,
22]. Also, a higher percentage of
E. coli (75.5%) isolates was observed in a study by Engy Ahmed Hamed et al [
23]. Despite the lower
E. coli prevalence, 60% (3/5) of the tested
E. coli isolates exhibited MDR, involving tetracyclines, penicillins, and suphomonamides. Since this observation could suggest the clonal expansion of one strain or the horizontal transfer of an MDR mobile element (e.g., a plasmid), detailed characterization by whole-genome sequencing will be required to confirm the hypothesis.
Ten out of the 103 (9.7%) were
Salmonella. A recent study on the Copperbelt Province in Zambia reported a higher
Salmonella prevalence of 17.7% in commercial poultry farms [
24]. Interestingly, the four randomly analyzed
Salmonella strains in our study showed susceptibility to all tested antibiotics. While this finding could suggest a lower AMR burden in
Salmonella from diseased chickens, the sample size was too low to allow valid prevalence estimation. Therefore, future studies must target larger numbers to account for rare phenotypes and genotypes.
Over the recent years,
Enterobacter has emerged as the third
Enterobacteriaceae showing resistance to third generation cephalosporins (3GCs) after
E. coli and
Klebsiella [
18]. This study revealed the presence of CTX resistance not only in
E. coli but
Enterobacter as well. One out of eight (12.5%)
Enterobacter and 3/6 (50%)
E. coli showed significantly high CTX MICs of at least 512 µg/ml. Globally, 3GCs are administered to the parent flock or day-old chicks, which contributes to increased levels of CTX resistance [
25]. Additionally, CTX resistance could result from the selection pressure created by other antibiotics if ESBL genes coexist with other AMR genes on the same mobile genetic element [
8]. Also, the administration of antibiotics through feed and water has contributed to increased levels of resistance as this allows the uptake of antimicrobials by both infected and healthy chickens [
26]. Such oral treatment regimens in chickens are prone to contamination with antimicrobials either by application or exposure to excreted faeces from treated chickens.
This is the first study to describe the presence of ESBL-encoding genes in diseased chickens in Zambia. Studies in healthy chickens in Zambia have reported high prevalence of the
blaCTX-M gene among CTX-resistant
E. coli. Accordingly, our study also revealed the presence of
blaCTX-M in all CTX-resistant
E. coli, although none of the
Enterobacter strains harboured this gene. Conversely, all
Enterobacter isolates exhibited the
blaTEM gene. Interestingly, the
blaCTX-M and
blaTEM genes coexisted in two-thirds of the CTX-resistant
E. coli isolates. This could be attributed to the presence of the genes on the same plasmid, as reported previously in Lusaka, Zambia [
27] and in rural Nepal were
E.coli co-harboured the genes
blaCTX-M and
blaTEM [
28]. Studies done in Zambia reported 13% prevalence of
blaCTX-M in
E. coli isolated from market ready chickens [
2]. Another study done on commercial poultry farms in Zambia’s Copperbelt Province revealed 12.8% occurrence of
blaCTX-M in
Salmonella [
24].
The detection
blaCTX-M genes in diseased chickens has public health implications as transmission of drug-resistant pathogenic strains to humans may cause hard-to-treat severe infection. It is assumed that not only is poultry a zoonotic risk to humans, but poultry also acts as a reservoir for ESBL-producing bacteria. Several studies suggest the transmission of
blaCTX-M-producing
Enterobacteriaceae between humans and animals through horizontal gene transfer and clonal expansion [
29,
30]. For instance, the
blaCTX-M gene has been detected on plasmids shared by human and poultry
E. coli strains [
31]. Also, similar
blaCTX-M-positive
E. coli strains have been found in humans and poultry, suggesting clonal dissemination. This transmission could be attributed to the poor handling of poultry in abattoirs or the increase in backyard poultry barns.
The increase in the presence of
blaCTX-M could be attributed to the general increase in antimicrobial usage over the recent years. In our study, amplicon sequencing of
blaCTX-M revealed the predominance of
blaCTX-M-15 (5/6, 83%) among the CTX-resistant
E. coli isolates. These findings are in line with the fact that
blaCTX-M-15 is the most widely spread ESBL genotype globally [
32]. The emergence of
blaCTX-M-15 has been attributed to the clonal spread of the
E. coli O25b:H4-ST131 pandemic clone. However, we did not perform multilocus sequence typing on our strains.
MDR bacteria pose significant danger to the public as common infections which were once easily treatable become fatal owing to the expensive nature of stronger antimicrobials. The
blaCTX-M-15-positive
E. coli isolates analysed in this study also portrayed a MDR phenotype which included tetracyclines and sulfonamides. The observed MDR could be attributed to the fact that in Zambia, farmers are still using antibiotics to optimise their production. This is supported by antibiotic sales data that shows that large amounts of antibiotics belonging to various classes are sold indiscriminately (
Figure 3). Moreover, MDR can also be selected by only one antibiotic since AMR genes usually reside together on mobile genetic elements, allowing for simultaneous selection by a single drug [
33].
Unlike
E. coli, the clinical relevance of
Enterobacter in poultry has not been well documented in Zambia, which can be attributed to the fact that
Enterobacter rarely causes disease in immunocompetent chickens [
18]. Data from this study has shown that
Enterobacter does indeed carry the gene
blaTEM, which may in turn be transmitted to humans. The MDR phenotype profile of the
blaTEM-positive
Enterobacter is alarming and calls for the urgent need for diagnostics before dispensing antimicrobials. This will allow for the more prudent use of antimicrobials and in turn limit the spread of AMR. Since
Enterobacter rarely causes disease in immunocompetent chickens, its presence probably goes unnoticed. Therefore, appropriate biosecurity measures in poultry houses would play a vital role in preventing its spread.
Study limitations
The study lacked MLST and whole-genome-based comparison analysis to assess the possibility of transmission between humans and poultry. In addition, this study only assessed a limited number of AMR genes. WGS would identify other AMR genes, mutations, serogroups, phylogroups, virulence genes, plasmids and mobile genetic elements. The identification of various virulence factors would allow the confirmation of the pathogenicity of the strains.
Author Contributions
“Conceptualization, C. Chileshe, W. Muleya, B. M Hang’ombe.; methodology, C.Chileshe, W. Muleya, B. M Hang’ombe.; software, C.Chileshe, M. Shawa and J.Ndebe; validation, C. Chileshe, N. Phiri and M. Shawa.; formal analysis, C. Chileshe, C.S. Khumalo and M. Shawa.; investigation, C. Chileshe and M. Shawa ; data curation, C. Chileshe ,J. Ndebe, M.Shawa.; writing—original draft preparation, C. Chileshe ,M. Shawa and N. Phiri .; writing—review and editing, W.Muleya, B Han’gombe,C. Nakajima, K. Masahiro and H. Higashi; visualization C.Chileshe and M.Shawa, ; supervision W. Muleya, B. Hangombe, H Sawa and Y. Suzuki. All authors have read and agreed to the published version of the manuscript.”.