S. hyicus has been globally recognized as the causative pathogen of EE in pigs for over 180 years, establishing it as a significant staphylococcal skin disease. Clinical manifestations are most severe in piglets aged 3–32 days, often leading to dehydration and potential mortality [19, 20]. While extensive research has been conducted on staphylococcal-induced EE [21–25], studies specifically targeting S. hyicus remains sparse, both nationally and globally.
In this study, we investigated the exfoliative toxins produced by S. hyicus isolated from pigs with EE. Prior studies in South Korea have documented swine EE and associated mortality caused by S. hyicus on farms in the Gyeongsang [10], Chungcheong [26], and Jeolla provinces [12], with some cases exacerbated by concurrent viral infections. However, in-depth studies of the exfoliative toxins produced by S. hyicus are limited. These toxins are key virulence factors of S. hyicus, and ExhA, ExhB, ExhC, ExhD, SHETA, and SHETB toxins facilitate skin exfoliation in pigs [3, 15, 16]. All variants of these exfoliative toxins induce blister formation in porcine skin by cleaving desmoglein-1, though human desmoglein-1 is resistant to these toxins [3, 8]. Although toxigenic strains of S. hyicus have been isolated from both healthy and diseased pigs, the isolation rate is higher in pigs affected by EE than in healthy pigs [16, 21, 27]. In this study, 52.9% of the isolates were identified as toxigenic, consistent with Andresen et al. [21], who reported that 47.1–66.7% of S. hyicus strains isolated from pigs with EE appeared toxigenic. However, these findings contrast with Russian [28] and Brazilian [29] studies reporting that approximately 90% of isolates are toxigenic. The highest detection rates in this study were for exhB and exhC (17.6% each), followed by exhD (11.8%) and exhA (5.9%). Although the number of isolates was insufficient for a robust comparison with other studies, previous research has shown variable detection rates of exfoliative toxins; for instance, 18–22% with the highest rate at exhB was observed in Denmark [30], 0.7–48.9% with the highest rate at exhA in Japan [27], 24.4–76.1% with the highest rate at exhC in Brazil [29], and 89.5% with the highest detection rate at exhD in Russia [28]. Additionally, studies within the same country have shown temporal changes in the distribution of toxin genes, such as a decrease in the prevalence of exhB [15, 29]. Therefore, distribution of exfoliative toxins and prevalence of toxigenic strains reported in the literature vary according to different countries and study periods.
Both toxigenic and nontoxigenic strains of S. hyicus have been reported to induce hyperkeratosis and inflammatory cell hyperplasia of the epidermis in pigs [7, 9, 31]. Consistent with this, our findings showed no correlation between the presence or type of exfoliative toxins and clinicopathological presentation of skin lesions. For instance, 47.1% of the S. hyicus isolates were identified as non-toxigenic strains, yet all were isolated from skin displaying mild to severe pathological lesions of EE. The absence of toxins in the isolates even in cases with severe skin lesions suggests the involvement of other virulence factors in EE, necessitating further research utilizing whole-genome sequencing to identify potential virulence determinants beyond exfoliative toxins involved in EE in pigs. Additional predisposing factors that may contribute to S. hyicus colonization and virulence in pigs include viral diseases, nutritional deficiencies, dermatophytosis, pityriasis rosea, parasitism, poor hygiene, inadequate ventilation, high humidity, trauma, and genetic predisposition [19].
The current study demonstrated a high diversity of both toxigenic and non-toxigenic S. hyicus strains in South Korea, irrespective of the year of isolation, season, region, age, or antimicrobial resistance pattern. Consistent with our finding, previous studies have reported significant diversity in the PFGE patterns of S. hyicus strains isolated from pigs [29, 31–33], with no clustering based on toxigenic strains or resistance profiles. Furthermore, various PFGE patterns have been identified on the same farm [32]. PFGE analysis of S. hyicus strains isolated from other animal species has shown diverse patterns and high variability in chickens and bovine milk [32, 34]. Given these studies, the high diversity observed in the PFGE results of this study appears to be inherent to the characteristics of S. hyicus. Therefore, PFGE results have limitations in cross-national comparative analyses for epidemiological research, necessitating the application of other molecular analysis techniques.
Antimicrobial susceptibility testing revealed that all S. hyicus isolates were 100% susceptible to ceftiofur and sulfonamides. However, the isolates demonstrated low susceptibility to penicillins (17.6–23.5%) and fluoroquinolones (29.4%). Among the macrolides, resistance to tilmicosin was 29.4%, while resistance to tylosin and tulathromycin was 76.5%. Consistent with our findings, other studies have shown low resistance rates to ceftiofur (0–0.97%) and sulfadimethoxine (1.9–5.2%) in Brazil and trimethoprim/sulfamethoxazole (9.7–25.8%) in Brazil and Japan among S. hyicus isolates from porcine EE [27, 29]. Despite the low resistance to ceftiofur, third-generation cephalosporins are classified as highest priority critically important antimicrobials (HPCIA) for humans and veterinary critically important antimicrobials (VCIA) for animals, according to the WHO (2024) [35] and WOAH (2021) [36]. Fluoroquinolones are also classified into the HPCIA and VCIA categories. However, our results indicated a higher fluoroquinolone resistance rate at 64.7% compared to 0–13.2% in European countries, except for one Brazilian study [13, 29, 37]. Thus, there is an urgent need to address the high rate of fluoroquinolone resistance. Penicillin resistance rates vary widely across different countries and study periods, including 25.0% in Germany and 76.8% in Japan, and even fluctuate within the same country over time [13, 27, 29, 37]. However, a direct comparison of the MDR results obtained in this study with previous Korean studies poses challenges due to differing antimicrobials and testing methods used, even though a previous Korean report indicated a 12.6% MDR rate [10]. Furthermore, MDR has been observed to increase over time [29] and is predominant in toxigenic strains [27]. Consistent with this, 82.4% of isolates were MDR, with 76.5% (n = 13) resistant to five or more antimicrobial subclasses, and all toxigenic strains were 100% MDR. These findings showed that S. hyicus isolates from EE exhibited increased resistance to most antimicrobials compared to the findings of previous studies. Ensuring bacteria do not develop resistance to antimicrobials is crucial for both animal and human health. Therefore, it is essential to base diagnoses on susceptibility tests rather than on clinical symptoms alone to select appropriate antimicrobials [19]. Likewise, developing a vaccine against S. hyicus should also be considered, as autogenous vaccines using strains isolated from affected herds have reduced metaphylactic antimicrobial treatment and lowered morbidity and mortality rates in weaned pigs [19, 38].
Despite pigs developing disease resistance with age, S. hyicus can still be recovered from older pigs’ skin, and these asymptomatic carriers can contaminate naïve herds [39]. Research has shown that suckling piglets are primarily infected by dams, some of whom are vaginally infected at birth [19]. Moreover, S. hyicus has been isolated from healthy pigs. However, this study included limited samples for the differential diagnosis of piglets with skin lesions. Therefore, further investigations are warranted to determine the overall distribution of S. hyicus based on clinical manifestations, age groups, and pig farm environments in South Korea.