1. Introduction
Globally,
Salmonella spp. are known to cause high morbidity, mortality and economic losses in the poultry industry [
1,
2,
3,
4]. Additionally,
Salmonella spp. are zoonotic agents with the potential to cause infection and disease in humans who consume improperly cooked contaminated poultry products [
5,
6,
7,
8]. In an effort to prevent, control or eradicate potential infection by
Salmonella spp. and other pathogens in the poultry industry, antimicrobial agents are used for prophylaxis, therapy and as growth promoters [
9,
10,
11,
12,
13]. In most developed countries, the use of antimicrobial agents is strictly controlled or banned for specific reasons in poultry through the enforcement of existing laws and regulations [
5,
6,
14]. However, in most developing counties including South Africa, although laws exist to control the use of antimicrobial agents, there are challenges in their enforcement thereby resulting in their inappropriate use (overuse or abuse) [
15,
16]. In addition to the unregulated use of antimicrobial agents in South Africa, the Fertilizers, Farm Feed, Agricultural and Stock Remedies Act (Act 36, 1947) exists which legalizes the use of some antimicrobial agents, including tetracyclines, sulfonamides and trimethoprim. The Act also permits the purchase of over-the-counter antimicrobial agents without a prescription for use in the livestock industry [
13,
17,
18]. This practice is likely to contribute to the development of resistant pathogens [
13,
19] or the occurrence of antimicrobial residues in animal tissues with public health implications [
20,
21].
In South Africa, over the years, outlets of the informal chicken market have emerged as popular venues for the population in the townships to access retailed chicken which is perceived to taste better than those from the commercial retail outlets. Furthermore, their prices are competitive and affordable, and they are conveniently located [
22,
23,
24]. However, since these outlets are unregulated by health personnel and are considered illegal, concerns have been raised regarding their roadside locations, lack or inadequacy of potable water, and inadequate wastewater and solid waste disposal [
22,
25,
26]. Sanitary practices have been reported to be poor, posing health risk to consumers of products from these outlets [
22,
23,
27]. The chickens slaughtered at the informal market outlets usually originate from commercial poultry farms [
23,
28] and small poultry enterprises. Therefore, the findings on the chickens processed at the outlets may reflect the occurrence of antimicrobial resistance at the farms from where they originated. There is also the possibility of carcass contamination being affected by practices at the outlets which may cause cross-contamination of processed chickens.
The rising threat of antimicrobial resistance (AMR) has prompted the development of national action plans whose five strategic objectives include, among others, optimization of surveillance and early detection of AMR for reporting local, regional, and national resistance patterns to optimize empirical and targeted antibiotic choices [
29,
30].
There are a few reports of bacterial pathogens on chickens from the outlets of the informal market in Gauteng province, such as the prevalence and molecular characterization of
Staphylococcus aureus isolates [
22,
31] and the isolation of
Salmonella [
32]. However, to date, there is no published documentation of the antimicrobial resistance profiles of
Salmonella in chickens processed at outlets of the informal market in South Africa.
The current study determined the antimicrobial resistance profiles of Salmonella isolates recovered from chickens slaughtered, processed, and retailed at outlets of the informal market in Gauteng province. In addition, the study investigated the potential effect of township sources of chickens, types of samples processed and the serovars of isolates of Salmonella and their antimicrobial profiles. Another objective of the current study was to assess the potential effect of the practices at the outlets of the informal market on cross-contamination of carcass, cloacal swabs, and carcass drips by Salmonella. It is anticipated that the results from this study will contribute towards addressing AMR challenges in the country.
3. Discussion
Our study demonstrated, for the first time in South Africa, a very high prevalence, 94.9% (93/98) of resistant
Salmonella on chicken carcasses purchased from the informal market in South Africa. Of the 170
Salmonella isolates recovered from carcass swabs, cloacal swabs and carcass drips, all (100%) exhibited resistance to antimicrobial agents. This study strategy was used because of the unique situation in ‘wet markets’, such as the informal market outlets in Gauteng province which have limited physical infrastructure, lack a potable water supply, have poor drainage, and inadequate wastewater and solid waste disposal [
25]. Most importantly, as reported earlier [
25], slaughtered chickens are rinsed in infrequently changed water in drums and buckets. The implication is that there is a high potential for cross-contamination among several carcasses with resistant
Salmonella during the rinsing process. Considering that chickens in the informal market are bagged fresh in nylon bags for sale, the rationale of the study was therefore to collect three types of samples (carcass swabs, cloacal swabs and carcass drips) from each bag. This approach allowed a comparison of the serovars and resistance patterns of
Salmonella from the three sources to provide evidence of cross contamination. Additionally, the cloacal swab samples were likely to be indicative of the prevalence and resistance patterns from the farms from where the live chickens originated. It cannot be over-emphasized that the high prevalence of resistant
Salmonella in the chickens sampled, poses a risk of infection or clinical salmonellosis in consumers of improperly cooked
Salmonella-contaminated chickens. There are also potential therapeutic and public health implications for workers and implications for workers at these outlets.
Overall, across the six townships, the prevalence of resistant
Salmonella was particularly high to erythromycin (99.6%), spectinomycin (92.6%), streptomycin (82.5%), oxytetracycline (78.0%), and doxycycline (76.1%), which are commonly used by poultry farmers (commercial and backyard) and veterinarians in the country. The detected high prevalence of resistant
Salmonella in our study has implications for the poultry farms in Gauteng province because all the chickens (broilers, culled breeders and spent hens) processed at the informal market outlets originated from these farms. The overall high prevalence of resistant
Salmonella may be indicative of improper use or misuse of antimicrobial agents for prophylaxis, growth promotion and therapy which may be as a result of noncompliance by farmers or failure by the authorities to enforce existing legislations [
9,
10,
11,
12,
13]. The outcome of such a practice is an increase in the prevalence of resistance among pathogens, including
Salmonella, and therefore resultant therapeutic failure. High prevalence of resistance to antimicrobial agents has been documented for
Salmonella and other pathogens in the livestock industry in South Africa [
17,
18,
19,
33], and in other countries [
34]. Additionally, infection of poultry with resistant
Salmonella on farms has the potential to cause high morbidity and mortality in chickens as earlier reported [
1,
2,
3,
4,
35]. Furthermore, the potential zoonotic spread of resistant
Salmonella to workers has been reported [
36,
37,
38].
The high prevalence of resistance to the antimicrobial agents such as erythromycin, spectinomycin, streptomycin, tetracycline, and doxycycline by
Salmonella in our study agrees with published reports of poultry-associated
Salmonella by others [
39,
40,
41]. It is pertinent to mention that in South Africa, the tetracyclines are the most commonly used or over-used antibiotics in the livestock industry, particularly in poultry, to treat bacterial infections such as
Salmonella spp. and
Escherichia coli, among others [
15,
16]. This is due to the fact that they are comparatively inexpensive, readily available as over-the-counter veterinary drugs [
13,
18], and their use is allowed by the South Africa Fertilizers, Farm Feed, Agricultural and Stock Remedies Act (Act 36, 1947). The high prevalence of resistance to tetracycline and doxycycline among
Salmonella detected in our study is therefore not unexpected. The prevalence of resistance to six antimicrobial agents (tetracycline, doxycycline, streptomycin, spectinomycin, and sulfamethoxazole-trimethoprim) found in
Salmonella on chickens sampled and processed at the outlets in our study varied significantly across the townships as a result of the origin of the live chickens. This may be due, in part, to the variable use or abuse of the antimicrobial agents on the farms from where the chickens were sourced. This aspect did not form part of our study. The lack of compliance with legislation governing the type and number of antimicrobial agents used in the livestock and human medicine in South African may be a causative factor in the development of antimicrobial resistance of the
Salmonella isolates studied [
17,
18,
42], as similarly reported by others elsewhere [
43,
44]. Another potential contributing factor to the significantly different prevalence of resistance to antimicrobial agents among
Salmonella across townships is the practice of contractors purchasing live chickens from commercial broiler and layer farms across the province and supplying them to the outlets of the informal market [
22,
28]. Geographical location and farm size are some of the factors that have been reported to significantly facilitate the carriage and spread of
Salmonella [
45,
46,
47,
48].
The variable prevalence of resistant Salmonella on these farms will therefore be reflected in our findings at the outlets. This explanation is further supported by the finding of similarly high prevalence (100.0%) of resistant Salmonella in the cloacal swab samples which also varied significantly across the townships and measured the carriage of the pathogen in the flocks from where they originated. This is because cloacal swabs of chickens were not subjected to potential cross-contamination experienced by carcass swabs and carcass drips during processing at the outlets.
Similarly, high prevalence of resistant
Salmonella has been reported for chickens slaughtered in the outlets of the ‘wet market’ in other countries, such as in the cecal contents of chickens slaughtered in ‘pluck shops’ in Trinidad [
46] where 100.0% of the
Salmonella isolates were resistant to antimicrobial agents which included 10 of the 16 used in the current study and in India where 100.0% (51/51) of
Salmonella isolates from the rectal swabs in poultry exhibited resistance to 16 antimicrobial agents [
49].
Equally of potential therapeutic importance was the detection of a very high (94.0%) prevalence of MDR isolates of
Salmonella. Other studies of the pathogens of poultry in South Africa have similarly reported a high prevalence of MDR in
Salmonella isolates in chickens and other livestock, thus highlighting their therapeutic and public health implications [
42,
50,
51,
52,
53,
54,
55,
56]. The prevalence of MDR in the current study is higher than was reported, also using the disc diffusion method, for
Salmonella from chickens (caeca) in Trinidad, 14.3% [
46], Portugal, 75.0% [
57], China, 80.0% [
58], and in the USA, 92.0% [
59], but lower than that reported for isolates in Nepal, 100% [
60], Mauritius, 100% [
61] and Trinidad, 100.0% [
41] and globally [
34].
It is of public health significance that all (100.0%) the 54 isolates of
Salmonella from carcass swabs exhibited resistance to one or more antimicrobial agent, and more importantly that 72.2% of these were MDR isolates, thereby posing a potential for zoonotic spread of resistant
Salmonella and a possible risk of therapeutic failure in workers at the informal market. There are reports of infections by
Salmonella and other zoonoses associated with exposure of workers on farms and during processing of slaughtered livestock [
36,
37,
38]. This is important considering that workers at the unregulated, illegal outlets of the informal market have been reported to practice poor hygiene in unsanitary environments [
22,
23], thus contributing to the cross-contamination of carcasses and exposure of the workers to the pathogen at these outlets. The possible exposure of consumers to improperly cooked chickens that are contaminated with resistant
Salmonella is a food safety concern that can lead to salmonellosis with therapeutic implications, as documented by others [
6,
7,
8].
The prevalence of resistance to six antimicrobial agents (tetracycline, doxycycline, streptomycin, spectinomycin, and sulfamethoxazole-trimethoprim) of
Salmonella on chickens sampled and processed at the outlets in our study varied significantly across the townships as a result of the origin of live chickens. This may be due, in part, to the variable use or abuse of the antimicrobial agents on the farms from where the chickens are sourced. This aspect did not form part of our study. The lack of compliance with legislation governing the type and number of antimicrobial agents used in the livestock and human medicine in South African may be a causative factor in the development of antimicrobial resistance of
Salmonella isolates studied [
17,
18,
42], as similarly reported by others elsewhere [
43,
44].
It was of interest to have detected that the resistance pattern (E-OXT-S-SPE-DO) was predominant among isolates from the three types of samples and was detected at frequencies of 42.6%, 41.1% and 43.3% among carcass swabs, cloacal swabs and carcass drips, respectively. The differences were found not to be statistically significantly, which may be suggestive of cross-contamination of resistant Salmonella among the three types of samples. However, among the isolates which exhibited MDR, the frequency was statistically significantly (p = 0.0308) lower for carcass swabs (72.2%) compared with cloacal swabs (92.8%) and carcass drip (80.0%). These findings suggest that cross contamination may not be the only factor involved and therefore follow-up investigation may be required.
In our study the prevalence of antimicrobial resistant
Salmonella in carcass drips was generally higher to antimicrobial agents compared to isolates from carcass swabs and cloacal swabs. It has also been reported that the prevalence of
Salmonella was significantly higher than that found in either the carcass or cloacal swabs of the same chickens sampled from the informal market in Gauteng province [
32].
A considerable variation of the prevalence of resistant
Salmonella was detected within and between the nine serovars identified in the current study. Of relevance is that statistically significant differences were detected in the prevalence of resistance to tetracycline, doxycycline, streptomycin, and gentamycin among the serovars of
Salmonella. This may reflect the differences in the antimicrobial resistance of
Salmonella serotypes and the frequency of use or abuse of antimicrobial agents across the farms from where the chickens originated. Our findings agree with the reports of other studies conducted in Trinidad [
41,
46], Spain [
62], Iran [
63], USA [
64], Korea [
65], and Malaysia [
66], where different prevalence of resistant
Salmonella was detected among the serovars isolated.