Prevalence, species identification, and antibiotic resistance of Staphylococci in dogs visiting veterinary clinics in Vietnam

TL;DR

This study found that many dogs in Vietnam carry antibiotic-resistant Staphylococci, with higher resistance in diseased dogs, raising concerns for public health.

Contribution

The study provides new insights into the prevalence and resistance profiles of Staphylococci in dogs in Vietnam.

Findings

Staphylococcus was isolated from 71.2% of dog samples, with higher rates in diseased dogs and skin samples.

S. pseudintermedius was the most common species, and 60.5% of isolates were multidrug-resistant.

Resistance genes like aacA-aphD and mecA were frequently detected, indicating widespread antibiotic resistance.

Abstract

Staphylococci are important commensal and opportunistic bacteria found in various animals, including dogs and humans. The emergence of antibiotic-resistant Staphylococci is a growing global concern, including in Vietnam. This study aimed to investigate the prevalence, species distribution, and antibiotic-resistance profiles of Staphylococci isolated from dogs visiting veterinary clinics in Vietnam. A total of 309 Staphylococcus strains were isolated from 410 nasal and skin samples collected from both healthy and diseased dogs between December 2021 and December 2023 in Ho Chi Minh city. The isolation rate of Staphylococcus spp. was 71.2% (95% confidence interval [CI]: 66.6%–75.6%), with 78.9% (95% CI: 73.6%–83.7%) in diseased dogs, 56.9% (95% CI: 48.4%–65.2%) in healthy dogs, 80.1% (95% CI: 74.3%–85.1%) in skin samples, and 60.3% (95% CI: 52.9%–67.5%) in nasal samples. Species identification indicated that S. pseudintermedius was dominant, followed by S. aureus. Other species identified included S. epidermidis and S. schleiferi. Antibiotic susceptibility testing showed complex resistance patterns. Approximately 91.3% of isolates were resistant to at least one antibiotic, and 60.5% were multidrug-resistant (resistant to three or more antibiotics). A total of 215 antibiotic-resistance phenotypes were observed, with 85 phenotypes showing resistance to more than ten different antibiotics. Isolates from diseased dogs exhibited higher antibiotic-resistance rates than those from healthy dogs. Several antibiotic-resistance genes were identified, with aacA-aphD being the most prevalent, followed by tetK, gyrA, mecA, msrA, dfrA, and ermA. These findings highlight the widespread presence of antibiotic-resistant Staphylococci in dogs and emphasize the necessity for ongoing surveillance of antibiotic-resistance evolution in animals and its implications for human health.

Figures

Summary of sample distribution and isolation of Staphylococcus spp. from 410 healthy and diseased dogs visiting the veterinary clinics in Ho Chi Minh city.

Antimicrobial susceptibility profile of 309 Staphylococcus spp. isolates from healthy and diseased dogs.The percentage of resistant (green) and sensitive (gray) isolates to 20 antibiotics, stratified by source: diseased and healthy dogs (upper panel) and nasal and skin samples (lower panel). (*) indicates statistically significant differences (P < 0.05) between groups (diseased and healthy samples, nasal and skin samples).

Heat map showing the percentage of Staphylococcus species and sample types exhibiting antibiotic resistance.

Upset plot showing the number of Staphylococcus spp. isolates by resistance phenotype.Black dots represent resistance to the corresponding antibiotic in the same row. The bar chart on the right shows the number of strains resistant to each antibiotic, and the bar chart above indicates the number of resistance phenotypes corresponding to the antibiotics in each column.

Sunburst chart illustrating the 215 antibiotic-resistance phenotypes among 309 Staphylococcus spp. isolates.

Comparison in the percentage of 309 Staphylococcus spp. isolates carrying resistance genes (blue) versus those not carrying resistance genes (gray), between diseased and healthy dogs (upper panel) and nares and skin samples (lower panel).(*) indicates statistically significant differences (P < 0.05) between groups (diseased and healthy samples, nasal and skin samples).

Equations

Full text

1. Introduction

Staphylococcus is a Gram-positive aerobic genus with more than 90 species and 30 subspecies [1]. These organisms are common commensals and opportunistic pathogens of the skin and mucosal membranes, temporarily colonizing the intestinal tract in humans and other animals [2]. Based on their ability to coagulate rabbit plasma, Staphylococci are categorized into two groups, coagulase-positive (CoPS) and coagulase-negative (CoNS) [1]. CoPS are well recognized as important pathogens; Staphylococcus aureus (S. aureus) is a primary or opportunistic pathogen in humans, and Staphylococcus pseudintermedius (S. pseudintermedius) is commonly found in canine microbiota and infections [3–5]. CoNS are part of the normal microbiota of humans and animals; however, they have been implicated in certain serious infections [6].

Globally, the impacts of antimicrobial resistance on human and animal health are increasing [7–9]. Staphylococcus spp. are capable of developing antibiotic resistance, including multidrug resistance (MDR), and can serve as reservoirs of drug resistance genes [10]. In particular, the emergence and spread of methicillin-resistant S. aureus (MRSA) has important ramifications on public health [11]. Additionally, several other Staphylococcus spp. have also developed methicillin resistance [12]. Resistance to linezolid, an antimicrobial effective against MDR Gram-positive pathogens, has been discovered in Staphylococcus spp. in dogs. Several studies in dogs revealed that Staphylococcus isolates often exhibit MDR [8,13], including resistance to beta-lactams, clindamycin, tetracycline, fluoroquinolones, and trimethoprim-sulfamethoxazole [14–16].

Transmission of S. aureus between humans and dogs has been reported in several countries [17,18], with dogs acting as reservoirs for some cases. Dogs are natural carriers of S. pseudintermedius, with the bacterium being frequently isolated from the nostrils, oropharynx, and perianal region. Staphylococci have progressively acquired MDR during adaptation and have been identified as zoonotic pathogens, particularly posing risks to immunocompromised individuals [19–21]. Therefore, monitoring Staphylococci and antimicrobial resistance is essential for epidemiological studies, enabling the tracking of species distribution and the evolution of antibiotic resistance across spatial and temporal contexts. This knowledge supports formulating infection control strategies, informs antibiotic stewardship efforts, and strengthens coordinated interventions in veterinary and human healthcare sectors. Ultimately, such measures are crucial in mitigating the broader impact of antimicrobial resistance [22].

In Vietnam, concerns regarding MDR Staphylococcus spp. have gained prominence. MDR S. aureus showed a prevalence rate of 51.8% in children [23], with 13.39% of those with pneumonia being infected with MRSA [24]. In adults, the prevalence was 76.5% [25], with more than 63 MRSA isolates and seven methicillin-resistant coagulase-negative Staphylococcus (MRCoNS) strains detected in Ho Chi Minh city [26]. The MRCoNS strains (S. haemolyticus and S. cohnii) displayed intermediate resistance to linezolid and carried the cfr gene. S. aureus was recently recognized to be responsible for 15.8% of the bloodstream infections detected in a teaching hospital, with MDR being identified in 57.0% of the cases [9].

The current understanding of the distribution and antimicrobial resistance patterns of Staphylococcus spp. in domestic animals in Vietnam is limited. Existing studies on MRSA in production animals have shown a 59.6% (59/99) prevalence in healthy and diseased pigs [27] and 1.5% (6/400) in raw milk of cows with subclinical mastitis [28]. However, studies in pet animals, such as dogs, which are in close contact with humans, are lacking. This study aimed to address these critical research gaps by (i) investigating the prevalence and species distribution of Staphylococci and (ii) determining the antibiotic susceptibility of the bacteria isolated from healthy and diseased dogs visiting veterinary clinics in Vietnam.

2. Materials and methods

2.1. Ethics approval

All experimental protocols were approved by the Institutional Animal Care and Use Committee of Can Tho University (ethics approval No. DT2021-04/KNN). Swab sampling was performed according to the guidelines in the Regulation on Animal Experimentation of Can Tho University, Vietnam. Informed consent was obtained from all dog owners through verbal explanation of the purpose and requirements of the study.

2.2. Study areas, study period, and sample collection

The cross-sectional study was conducted from December 2021 to December 2023 on dogs visiting ten veterinary clinics in Ho Chi Minh city, Vietnam. The veterinary clinics are located in the central area of Ho Chi Minh city and its neighbouring districts, including Animal Health Laboratory and Treatment Division of Ho Chi Minh City. Animals that met the sampling criteria and had the owner’s consent were randomly sampled until the sample size was reached. A minimum sample size of 138 was calculated using Cochran’s formula [29]:

$$[eqn]$$

where:

n: Sample size.

z: The standard normal value for the desired confidence level (z = 1.96 in this study).

d: The maximum difference between observed and actual infection rates (no more than 5%).

p: Estimated infection rate (10%).

Dogs included in this study were divided into two groups: (a) healthy dogs visiting for vaccination or routine health check-ups with no history of illness and normal clinical examination; and (b) dogs presenting to the clinics with clinical signs of respiratory disease (nasal discharge with cloudy or purulent mucus, dry or wet cough, or dyspnoea) or skin disease (itching, increased skin pigmentation or redness, foul odor, increased mucus secretion with or without pus at the inflammation site, hair loss, small pimples on the skin and scaly or keratinized skin).

Samples were taken from the two study groups, with one swab collected from either the nares or skin of each dog. In healthy dogs, an equal number of samples were collected from each of the clinics, with the sample site determined using a simple random sampling method, while in diseased dogs, samples were obtained from the site of infection. The sampling procedure of dogs with diseased skin followed the guidelines by Hillier et al (2014) [30] and was performed by veterinarians. Each sample was placed in a labelled tube containing sterile Stuart Amies solution and stored in containers at 2–8°C. The samples were immediately transported to the Laboratory of Microbiology, Faculty of Veterinary Medicine and Animal Husbandry, HUTECH University, for bacterial isolation.

2.3. Identification of Staphylococcus species using bacterial isolation and PCR.

Each swab was streaked onto blood agar base, Baird–Parker agar (BPA) supplemented with egg yolk tellurite, and mannitol salt agar (MSA) plates (Oxoid, UK). The isolation procedure was performed following methods described in previous studies [4]. Plates were incubated aerobically at 37°C for 24 hours, and the colonies with typical morphology were chosen for further investigations. Typical morphology of colonies was determined as follows: on MSA, colonies were round, convex, yellow or white, with the surrounding area turning yellow or displaying no color shift. On BPA agar, colonies were round, convex, black, with or without a halo around them. On blood agar, colonies were round, large, milky white, with or without hemolysis. One to two colonies with different morphology were selected from each sample. These colonies were purified on nutrient agar and underwent Gram staining. Oxidase and catalase tests were then performed on these colonies. The procedures of Gram staining, oxidase and catalase tests were performed as described in Markey et al. (2013) [2]. In the case of negative oxidase and positive catalase tests, the colonies were transferred into Brain Heart Infusion broth (HiMedia Laboratories, India) supplemented with 30% glycerol and stored at −20°C for further investigations.

Staphylococci were confirmed and identified at the species level by conventional and multiplex PCR (S1 Fig). For DNA extraction, a 500-μL aliquot of each culture suspension was utilized, following the Tris-EDTA-NaCl-Triton X100 method [31]. Seven primer pairs were used in the multiplex PCR to determine Staphylococcus species. Multiplex PCR was carried out in a total volume of 20-μL reaction mixture, which consisted of 3 μL of genomic DNA template, 13 μL of the GoTaq^®^ Colorless Master Mix (Promega, USA), 2 μL of DEPC-treated water and 1 μL each of 10 μM forward and reverse primers as indicated in S1 Table. PCR cycling conditions were carried out as previously described [32–36] with 94°C for 4 min, followed by 35 cycles of 94°C for 60 s, and annealing temperature as described in S1 Table. Resulting PCR amplicons were electrophoresed on 1.3% tris-acetate-EDTA agarose gels stained with ethidium bromide and visualized under UV transillumination. If the PCR result was positive, a selected colony was used for antibiotic susceptibility and coagulase testing. The coagulase test followed protocols as previously described [2].

2.4. Antibiotic susceptibility testing

The antibiotic susceptibility of the Staphylococcus isolates was tested using the disk diffusion method according to the antibiotic susceptibility testing standards of the Clinical and Laboratory Standards Institute [37]. Nineteen different antibiotic agents that are widely used for treatment in humans and animals in Vietnam were tested: amoxicillin (Ax, 10 µg), penicillin (Pn, 10 IU), ampicillin (Am; 10 µg), oxacillin (Ox, 10 µg), cefoxitin (Cn, 30 µg), amoxicillin/clavulanic acid (Ac, 20/10 µg), cephalexin (Cp, 30 µg), chloramphenicol (Cl, 30 µg), ciprofloxacin (Ci, 5 µg), levofloxacin (Lv, 5 µg), clindamycin (cL, 2 µg), tetracycline (Te, 30 µg), doxycycline (Dx, 30 µg), erythromycin (Er, 15 µg), azithromycin (Az, 15 µg), gentamicin (Ge, 10 µg), amikacin (Ak, 30 µg), sulfamethoxazole/trimethoprim (Bt, 23.75 µg:1.25 µg), and linezolid (Li, 30 µg). These antibiotic agents were purchased from Nam Khoa Biotek Co. Ltd. (HCM, Vietnam). S. aureus subsp. aureus ATCC 25923 was used as a control strain for susceptibility testing to antibiotics and quality control for commercial products used in this study. Isolates were classified as MDR when they demonstrated resistance to at least three distinct antibiotic categories [38].

Minimum inhibitory concentration (MIC) of vancomycin was determined using methods described in previous studies and CLSI standards [37,39]. Staphylococcus spp. were cultured for 24 hours and then suspended in sterile saline with the McFarland turbidity standard 0.5. A sterile cotton swab was used to spread the suspension evenly on Mueller-Hinton agar medium. ^NK^MIC Diffusion Strips (^NK^MIC.DS) of vancomycin at concentrations ranging from 0.01 to 128 µg (Nam Khoa Biotek Co. Ltd., HCM, Vietnam) were placed on the agar surface. The MIC results were determined by the values at the intersection points between the ^NK^MIC.DS vancomycin and zone of inhibition. The susceptibility of the bacteria was read based on the MIC values according to CLSI guidelines [37].

2.5. Determination of antibiotic resistance genes using PCR

Conventional PCR tests were performed to detect resistance genes of the representative Staphylococcus isolates (S2 Fig). Seven primer pairs were used to identify antibiotic resistance genes of Staphylococcus, including mecA, aacA-aphD, ermA, tetK, msrA, dfrA, and gyrA, which encode resistance to beta-lactams, aminoglycosides, erythromycin, tetracycline, macrolides, trimethoprim, and fluoroquinolones, respectively. Primers and PCR conditions used to detect these genes [40–45] are described in S2 Table.

2.6. Statistical analysis

Data collation, processing, and initial manipulation were performed using Microsoft Excel. Aggregate data were analyzed using SPSS 22.0 statistical software. The Chi-square test (χ2) and Fisher’s exact test (when the expected frequency was less than 5) were used to compare proportions, and a p-value of less than 0.05 was considered statistically significant. The prevalence of Staphylococcus spp. isolation and the corresponding 95% confidence interval (95% CI) were calculated. Odds ratios and their 95% CI were also calculated to assess the association between the prevalence of Staphylococcus spp., multidrug-resistant isolates, and risk factors. Graphs and visualizations were generated using R software (version 4.3.2) with the ggplot2 package.

3. Results

3.1. Staphylococcus spp. distribution

The distribution of Staphylococcus spp. isolated from healthy and diseased dogs in Ho Chi Minh city, Vietnam, is shown in Fig 1. The prevalence and species distribution are presented in Table 1 and S3 Table. A total of 309 Staphylococcus spp. were isolated from 410 dogs visiting veterinary clinics.

Table 1: The prevalence and species distribution of Staphylococcus spp. isolated from 410 healthy and diseased dogs visiting veterinary clinics in Ho Chi Minh city, Vietnam.

Summary of sample distribution and isolation of Staphylococcus spp. from 410 healthy and diseased dogs visiting the veterinary clinics in Ho Chi Minh city.

The isolation prevalence in diseased dogs was 78.9% (210 isolates/266 samples; 95% CI: 73.6%–83.7%), compared with 56.9% in healthy dogs (82 isolates/144 samples; 95% CI: 48.4%–65.2%), with OR = 2.84 (95% CI: 1.8–4.4). Among all the Staphylococcus isolates, 80.6% (249/309) belonged to CoPS and 19.4% (60/309) to CoNS. Additionally, four distinct Staphylococcus spp. were identified: S. pseudintermedius (154 isolates), S. aureus (25 isolates), S. schleiferi (9 isolates), and S. epidermidis (3 isolates). The remaining 118 isolates were of unknown species.

The distribution of Staphylococcus spp. across anatomical locations is summarized in Table 1 and S3 Table. A higher prevalence was observed in skin samples (181 isolates/226 samples; 80.1%; 95% CI: 74.3%–85.1%) compared with the nasal samples (111 isolates/184 samples; 60.3%; 95% CI: 52.9%–67.5%) in both healthy and diseased dogs (S3 Table). In diseased dogs, prevalence was significantly greater in skin samples (135/154; 87.7%; 95% CI: 81.4%–92.4%) than in the nasal samples (75/112 isolates; 66.9%; 95% CI: 57.4%–75.6%), with OR = 3.51 (95% CI: 1.88–6.52). Factors shown to be associated with the increased prevalence of Staphylococcus spp. include health status, age and management practices (S4 Table).

3.2. Antibiotic susceptibility

In total, 309 Staphylococcus isolates were selected for testing antibiotic susceptibility (Fig 2 and S5 Table). Approximately 91.3% (282/309 isolates) exhibited resistance to at least one antibiotic. More than 50% were resistant to amoxicillin, penicillin, ampicillin, tetracycline, erythromycin, and azithromycin (Fig 2). Staphylococcus spp. isolated from diseased dogs displayed a higher proportion of resistance to amoxicillin, penicillin, ampicillin, tetracycline, gentamicin, ciprofloxacin, and sulfamethoxazole-trimethoprim than those from healthy dogs. The proportion of antibiotic resistance to amoxicillin/clavulanic acid from nasal isolates (17.1%, 20/117 isolates) was higher than those from skin samples (8.3%, 16/192 isolates). Most Staphylococcus isolates remained sensitive to amikacin, cephalexin, amoxicillin/clavulanic acid, cefoxitin, and levofloxacin. None of the isolates were resistant to linezolid and vancomycin.

Antimicrobial susceptibility profile of 309 Staphylococcus spp. isolates from healthy and diseased dogs.The percentage of resistant (green) and sensitive (gray) isolates to 20 antibiotics, stratified by source: diseased and healthy dogs (upper panel) and nasal and skin samples (lower panel). () indicates statistically significant differences (P < 0.05) between groups (diseased and healthy samples, nasal and skin samples).*

Additionally, Staphylococcus isolated from dogs with respiratory disease showed a higher proportion of resistance to amoxicillin, ampicillin, penicillin, ciprofloxacin, levofloxacin, erythromycin, gentamicin, and sulfamethoxazole/trimethoprim compared with those isolated from healthy dogs (P < 0.05). Similarly, Staphylococcus isolates from the skin of dogs with dermatitis demonstrated a higher proportion of resistance to penicillin compared to those from healthy dogs (S6 Table).

The heat map in Fig 3 illustrates the proportion of antibiotic resistance among the various Staphylococcus spp. In nasal samples of diseased dogs, all S. aureus and S. pseudintermedius isolates displayed a similar resistance pattern, with over 60% of isolates resistant to amoxicillin, ampicillin, penicillin, tetracycline, erythromycin, and azithromycin. Similarly, all S. epidermidis isolates from nasal samples were resistant to azithromycin, erythromycin, chloramphenicol, amoxicillin, ampicillin, and penicillin. In the skin samples from diseased dogs, over 88.9% of S. aureus isolates were resistant to amoxicillin, ampicillin, and penicillin. In healthy dogs, the proportion of resistance of isolates ranged from 0% to 69.4% from nasal samples, and 0% to 67.4% from skin samples. Notably, S. pseudintermedius isolates showed considerable resistance to tetracycline, penicillin, ampicillin, and amoxicillin (Fig 3).

Heat map showing the percentage of Staphylococcus species and sample types exhibiting antibiotic resistance.

3.3. Antibiotic resistance phenotypes and MDR isolates

Fig 4 illustrates the antibiotic-resistance phenotypes of the 309 Staphylococcus spp. isolated. A total of 215 distinct antibiotic-resistance phenotypes were observed, with the most common being the Pn-Am-Ax (ten isolates), Pn-Am-Ax-Ox (nine isolates), and Pn-Am-Ax-Te (eight isolates). A majority of the isolates (60.5%, 187/309) exhibited MDR characteristics, defined as resistance to at least three classes of antibiotics (Fig 4 and S7 Table). Among them, 61.7% (140/227; 95% CI: 55.0–68.0%) were isolated from diseased dogs, and 57.3% (47/82, 95% CI: 45.9–68.2%) from healthy dogs. Out of the 215 antibiotic-resistance phenotypes, 85 exhibited resistance to ten or more distinct antibiotics (Fig 5). These were observed in 101 isolates belonging to different Staphylococcus spp., including S. aureus (7), S. pseudintermedius (71), S. epidermidis (1) and 22 others.

Upset plot showing the number of Staphylococcus spp. isolates by resistance phenotype.Black dots represent resistance to the corresponding antibiotic in the same row. The bar chart on the right shows the number of strains resistant to each antibiotic, and the bar chart above indicates the number of resistance phenotypes corresponding to the antibiotics in each column.

Sunburst chart illustrating the 215 antibiotic-resistance phenotypes among 309 Staphylococcus spp. isolates.

The prevalence of MDR was 68.0% (17/25) in S. aureus, 70.1% (108/154) in S. pseudintermedius, 57.1% (40/70) in other CoPS, 66.7% (2/3) in S. epidermidis, 22.2% (2/9) in S. schleiferi, and 37.5% (18/48) in other CoNS (S7 Table). For risk factor analysis, dogs sampled within 3 months of antibiotic use had a significantly higher rate of MDR Staphylococcus (86.7%; 95% CI: 79.2%–92.1%) compared to dogs with no antibiotic use (43.9%; 95% CI: 36.7%–51.3%), with OR = 0.12 (95% CI: 0.07–0.22; prior antibiotic use as reference). MDR prevalence was higher in S. aureus (68.0%; 95% CI: 46.5%–85.0%) compared with S. schleiferi (22.2%; 95% CI: 2.8%–60.0%), with OR = 0.13 (95% CI: 0.02–0.8; S. aureus as reference). It was markedly greater in S. pseudintermedius (70.1%; 95% CI: 62.2%–77.2%) compared with S. schleiferi (22.2%; 95% CI: 2.8%–60.0%) and other species (49.2%; 95% CI: 39.8%–58.5%) (S8 Table).

3.4. Detection of antibiotic resistance genes

The results of PCR-based detection of resistance genes in Staphylococcus isolates are summarized in Table 2. The genes—aacA-aphD, gyrA, and mecA—were more prevalent in the CoPS than in the CoNS. The most prevalent gene was aacA-aphD (200/309, 64.7%), identified in 172 CoPS and 28 CoNS. Additionally, 51.8% (160/309) of isolates were positive for tetK, including 134 CoPS and 26 CoNS. None of the CoNS isolates carried ermA, unlike six CoPS isolates. The mecA gene was present in 26.5% (82/309) of the isolates, including 73 CoPS and 9 CoNS.

Table 2: Distribution of antibiotic-resistance genes among Staphylococcus species.

Among the resistance-related genes detected, aacA-aphD was more prevalent in Staphylococcus spp. isolated from diseased dogs (70.0%) compared with healthy ones (50.0%), and more common in isolates from the skin (70.3%) than from nasal samples (55.6%), with a *P *< 0.05. Conversely, resistance-associated genes such as msrA, gyrA, and dfrA were more common in Staphylococcus spp. isolated from healthy dogs (42.7%, 40.2%, and 13.4%, respectively) compared with those isolated from diseased dogs (19.8%, 23.8%, and 6.2%, respectively; *P *< 0.05). Additionally, dfrA was more common in the strains isolated from nasal discharge (14.5%) than those present on skin (4.2%; P < 0.05) (Fig 6 and S9 Table).

Comparison in the percentage of 309 Staphylococcus spp. isolates carrying resistance genes (blue) versus those not carrying resistance genes (gray), between diseased and healthy dogs (upper panel) and nares and skin samples (lower panel).() indicates statistically significant differences (P < 0.05) between groups (diseased and healthy samples, nasal and skin samples).*

4. Discussion

This study revealed a high prevalence and species distribution of Staphylococcus spp. in nasal and skin samples from dogs, with a greater prevalence in diseased (78.9%) compared with healthy (56.9%) dogs (Table 1). These results are consistent with previous studies, such as those from China (60% in diseased and 41.79% in healthy dogs) [46], and Iran, where S. pseudintermedius was present in 50% of dogs with dermatitis compared with 40% in healthy dogs [47]. The enhanced prevalence of Staphylococcus spp. in diseased dogs may result from compromised immunity, which may exacerbate infection-associated conditions. Furthermore, damage to the epithelium or mucous membranes through trauma, foreign bodies, abrasions, injury to hair follicles or thermal damage, and hypersensitivity reactions alters the epithelial surface, reducing its integrity and leading to immune dysregulation or disruption of the local tissue structure. Many pathogenic Staphylococcus spp. possess diverse virulence factors that facilitate prolonged survival within tissues by helping evade the immune system and secreting factors that promote tissue necrosis, thereby enabling invasion of the skin and mucous membranes [48]. They also produce various toxins with superantigen properties that affect immunoregulatory and inflammatory cells, thereby exacerbating the symptoms of allergic diseases such as nasal itching, rhinorrhea, coughing, and swelling of the nasal mucosa [49]. A large proportion of Staphylococcus spp. isolated from the skin and mucous membranes have demonstrated a role in pyoderma [50, 51]. However, the detection rate of Staphylococcus spp. was not influenced by breed, gender, or location, but was markedly associated with health status, age, and management practices (S4 Table).

In this study, S. pseudintermedius was the most prevalent species in healthy and diseased dogs, followed by S. aureus. This was unsurprising, given S. pseudintermedius is recognized as the most common Staphylococcus spp. in dogs [13,46,50,51,52]. The predominance of S. pseudintermedius can be attributed to its presence in the normal bacterial flora, playing a significant role in bacterial diseases in dogs [4,53]. S. aureus was also isolated in this study, seen in diseased dogs with higher prevalence and antibiotic-resistance rates than those detected in healthy dogs. The frequent detection of S. aureus in dogs in the study is concerning, with dogs identified as potential carriers of the bacteria, posing a risk of infection or reinfection to vulnerable humans in close contact [4]. The propensity of S. aureus and other Staphylococcus spp. including S. pseudintermedius [12], to develop methicillin resistance and MDR, increases the potential for morbidity and mortality in both humans and dogs [26,54]. MRSA is resistant to the most commonly used antibiotics, increasing the mortality risk in infected animals. MRSA remains classified as a high-priority pathogen on the World Health Organization’s 2024 Bacterial Priority Pathogens List, and is recognized globally as one of the major causes of healthcare-associated and community-acquired infections [55–57].

The distribution of Staphylococcus spp. varies across studies. In Trinidad, the most common species isolated were the S. intermedius group (including S. pseudintermedius, S. intermedius, and S. delphini) [4], whereas S. aureus, S. pseudintermedius, S. intermedius, and more than 20 other Staphylococcus species have been detected in Australia [58]. However, in our study S. intermedius were not isolated, with the less common S. schleiferi detected in diseased dogs. The epidemiological understanding of S. schleiferi infections remains limited, as the true incidence has likely been underreported due to previous identification errors [59]. Further studies are needed to elucidate the clinical relevance of this species and its relationship with the distribution of Staphylococcus species seen in humans.

Antibiotic-resistant Staphylococcus spp. pose a substantial global challenge to human and animal health. The recent emergence of antibiotic-resistant Staphylococcus strains in veterinary medicine contributes to the current global health problem due to the widespread dissemination of the zoonotic superbugs. The antibiotic-resistance profile of Staphylococcus identified in this study revealed a complex pattern. A high prevalence of resistance (more than 75% of isolates) against several beta-lactam antibiotics, such as penicillin, ampicillin, and amoxicillin, was observed. In contrast, no resistance was detected to linezolid and vancomycin, the last resort for treating infections caused by MDR Gram-positive bacteria. This was despite linezolid-resistant CoNS being previously reported in human health settings in Vietnam [26,54]. Between 20% and 42% of study isolates were resistant to cefoxitin, and oxacillin, antibiotics used to identify MRS, with oxacillin being the antibiotic of choice for detecting MRS in several studies in Vietnam [24,26] and globally [21]. Cefoxitin, a second-generation cephalosporin, is generally not used as the first-line treatment in veterinary medicine as it is costly and primarily reserved for severe infections or those resistant to beta-lactam antibiotics. However, the increasing market availability of products containing ceftriaxone (a third-generation cephalosporin) may lead to a rise in resistance to this agent in Vietnam. Several Staphylococcus spp. strains in our study showed resistance to antibiotics prohibited in veterinary use such as chloramphenicol (approximately 33.3%, 103/309 isolates) and those used infrequently in veterinary medicine, such as cephalexin, cefoxitin, ciprofloxacin, levofloxacin, clindamycin, and erythromycin (S5 Table). Development of resistance in bacteria with little or no exposure to the specific antibiotic could be attributed to the vertical or horizontal transfer of antibiotic resistance-conferring genes in the environment or cross-resistance to antibiotics of the same class [60]. Furthermore, this study revealed that diseased dogs harbor more antimicrobial-resistant bacteria, which may be related to specific diseases or drugs commonly administered to diseased dogs.

In this study, the resistance to amoxicillin, ampicillin, penicillin, and tetracycline was higher in isolates from nasal samples. Beta-lactams and tetracyclines are the preferred antibiotics in Vietnamese veterinary clinics for treatment of canine respiratory diseases [61]. Antibiotic selection is often influenced by habitual practices of veterinarians, with antimicrobial susceptibility testing being infrequent. Incomplete treatment and unnecessary antibiotic use can facilitate the emergence of antibiotic-resistant Staphylococcus [60,62]. Our findings also highlight the risk of emergence of Staphylococcus superbugs and their zoonotic potential, emphasizing the need to develop appropriate antimicrobial stewardship and prevention strategies. The 2023 World Organisation for Animal Health Annual Report on antimicrobial agents intended for use in animals identified penicillins as the most commonly used class of drugs in dogs and cats across 56 nations, followed by fluoroquinolones and tetracyclines [63]. Of these, the five most reported agents were lincomycin (28%), amoxicillin (25%), doxycycline (23%), cephalexin (7%), and streptomycin (6%), which may increase the risk of resistance to these antibiotics. Other studies similarly demonstrate a wide range of Staphylococcus resistance to antibiotics, with beta-lactams [4,64], clindamycin, and tetracycline [51,65] being less effective against Staphylococcus infections in dogs [17,18,66]. In China, the resistance rates of S. pseudintermedius to azithromycin and doxycycline exceeded 80%, while those to oxacillin and sulfamethoxazole-trimethoprim were 48.6% and 64.7%, respectively [67].

MDR Staphylococcus isolates constituted 60.5% of the isolates (187/309). Among these, only 26.5% (82/309) were associated with the presence of mecA. Although methicillin resistance is of particular concern due to its ability to affect the sensitivity to all beta-lactams and its association with resistance to other antibiotic classes, resistance to different antibiotics also plays a crucial role in clinical settings. This was evidenced by our findings of Staphylococcus isolates displaying complex resistance to more than 15 distinct antibiotics.

The representative genes conferring resistance to beta-lactams (mecA), aminoglycosides (aacA-aphD), macrolides (msrA, ermA), tetracycline (tetK), trimethoprim (dfrA), and fluoroquinolones (gyrA) were frequently found in S. aureus and S. pseudintermedius strains; however, ermA was not detected in S. epidermidis and S. schleiferi strains (Table 2). It is difficult to make comparisons with the varied findings seen in previous studies on the prevalence of resistance genes due to differences in geographic and temporal designs, as well as the research populations [51,64,68,69]. Further studies are necessary to determine whether S. pseudintermedius, S. epidermidis, and S. schleiferi act as reservoirs of the antibiotic resistance genes and exchange them with S. aureus in dogs and humans.

Our analysis of antibiotic resistance-associated phenotypes and genotypes revealed that Staphylococcus isolates carrying the same one or two resistance-conferring genes exhibited diverse phenotypes. For example, most isolates harboring mecA displayed varied resistance phenotypes across multiple antibiotic classes. Furthermore, the presence or absence of a single antimicrobial resistance-conferring gene did not consistently predict susceptibility or resistance to any specific antibiotic [70]. This finding suggests that resistance-associated phenotypes can be expressed through various mechanisms, and bacteria carrying certain resistance-related genes may not always express the corresponding phenotype. These results are similar to Bertelloni et al. (2021) in Italy [51], in which 12/50 (24%) of Staphylococcus isolates carried mecA; however, only seven exhibited the related phenotypes (four S. pseudintermedius, one S. aureus, one S. xylosus, and one S. chromogenes); 15 isolates were oxacillin-resistant but lacked mec (seven S. pseudintermedius, five S. xylosus, one each of S. simulans, S. haemolyticus, and S. capitis); five isolates carrying mecA were susceptible to methicillin. A study in India reported 21 MRSA and 12 MRCoNS isolates, yet only 6/21 MRSA and 2/12 MRCoNS were positive for the mecA gene (genotype) [64]. The higher rate of methicillin resistance determined by phenotype when compared with the presence of mecA suggests that additional mechanisms are responsible for the expression of MRS phenotypes, particularly in CoNS. Our study only identified seven antibiotic resistance-associated genes, limiting our full explanation of the phenotypes based on the genotypes. While a single phenotype can be determined by multiple genes, accurate resistance profiling requires gene sequencing to clarify the discrepancies between resistance-associated phenotypes and genotypes. Further in-depth genomic studies would be able to determine the underlying mechanisms affecting the association of genotype expression with phenotype, and their linkages to other factors. Mapping antibiotic genes in different locations and study populations should be conducted to further ascertain the status of antibiotic resistance of Staphylococcus spp. in Vietnam, adopting a One Health approach to monitoring antimicrobial resistance.

Infections caused by opportunistic MDR pathogens constrain the choice of effective antibiotics for veterinary use. Our study shows that the carriage of MDR Staphylococcus in healthy dogs is common. MDR among the normal flora of clinically healthy dogs is concerning, since most had no record of prior antibiotic therapy. In this study, the higher prevalence of MDR Staphylococcus spp. isolated from dogs treated with antibiotics within the previous 3 months suggests that recent antibiotic administration may be a risk factor for acquiring MDR Staphylococcus spp.

Asymptomatic healthy dogs may also play a role in transmitting bacteria to humans. Staphylococcus spp. are a part of the normal bacterial flora of the skin and mucous membranes. However, under certain circumstances, the commensal bacteria can become pathogenic, causing serious infections [50,71,72,73]. Our study showed that both CoPS and CoNS were present in diseased and healthy dogs (Table 1), aligning with several previous studies [3]. While CoPS is considered to cause infections, both CoPS and CoNS have the potential to become pathogenic in animals and humans [74]. This is facilitated by a complex interplay between genetic and physiological mechanisms within the bacteria, host immune status, and environmental conditions. Under favorable conditions, commensal CoPS and CoNS can exploit opportunities to cause various infections.

Monitoring antibiotic use in veterinary clinics is essential to mitigate antibiotic-resistance in pets and to reduce the risk of zoonotic Staphylococcus infections in humans. More in-depth investigations are required to determine the evolution, molecular biology, and clinical impacts of MDR pathogens, particularly those with complicated antibiotic-resistance characteristics. Inappropriate antibiotic use drives selection pressure and accelerates the emergence of MDR Staphylococci. Veterinary clinics should implement practices for antimicrobial stewardship, including the use of relevant epidemiological data to guide effective antibiotic treatment choices, as well as monitoring antibiotic resistance among microbes isolated from their patients [75].

This study successfully reported the prevalence, species identification, and antibiotic resistance of Staphylococci in dogs in Vietnam. However, the correlation between the presence of Staphylococci and disease severity remains unclear. Another limitation may be the lack of molecular epidemiological analysis, which would improve the integration of our data with the findings from other studies and help better understand the global profile of canine-resident Staphylococci. Further investigations are necessary to uncover risk factors contributing to the prevalence of antibiotic resistance and the relationship between antibiotic-resistant Staphylococcus strains occurring in dogs and their owners. Further studies should elucidate the mechanism underlying antibiotic resistance, the process by which antibiotic resistance-related genotypes are converted to phenotypes, and the roles of Staphylococcus in causing pathogenic conditions in infected dogs. Nevertheless, it provides crucial information about Staphylococcus infections of veterinary importance.

5. Conclusions

This study details the prevalence, species distribution, and antibiotic-resistance profiles of Staphylococci isolated from nasal and skin samples of healthy and diseased dogs in Vietnam. The identified Staphylococcus species encompassed four species: S. pseudintermedius and S. aureus belonging to CoPS, and S. epidermidis and S. schleiferi belonging to CoNS, with S. pseudintermedius being the predominant. The prevalence of antibiotic-resistant Staphylococcus strains was substantial, exhibiting complex antibiotic resistance phenotypes. This was characterized by a notable number of MDR isolates and the presence of multiple antibiotic resistance genes. The urgent implementation of monitoring and prevention strategies for antibiotic-resistant Staphylococci in dogs and other companion animals is essential to reduce the burden and minimize potential risks to both animal and human health.

Supporting information

S1 FigSingleplex PCR and multiplex PCR for species identification of Staphylococcus isolates.(TIF)

S2 FigSingleplex PCR and multiplex PCR for identification of antibiotic-resistance genes of Staphylococcus isolates.(TIF)

S1 TableNucleotide sequences of primers used for PCR detection of Staphylococcus species.(DOCX)

S2 TableNucleotide sequences of primers used for PCR detection of antibiotic-resistance genes of Staphylococcus species.(DOCX)

S3 TablePrevalence and species distribution of Staphylococcus spp. isolated from 410 healthy and diseased dogs.(DOCX)

S4 TableRisk factors associated with Staphylococcus spp. in dogs.(DOCX)

S5 TableNumber and percentage of Staphylococcus isolates showing antibiotic resistance by health status and anatomical locations.(DOCX)

S6 TableNumber and percentage of Staphylococcus isolates showing antibiotic resistance.(DOCX)

S7 TableNumber of multidrug resistant Staphylococcus isolates per species.(DOCX)

S8 TableRisk factors associated with MDR Staphylococcus spp.(DOCX)

S9 TableNumber and percentage of Staphylococcus isolates showing antibiotic-resistance genes by health status and anatomical locations.(DOCX)

S1 FileMinimal data set to consist of the data required to replicate findings reported in the article.(XLSX)