Infections caused by bacteria of the genus Staphylococcus present a significant challenge for veterinarians. These opportunistic pathogens regularly infect humans and animals. Recent years have seen the emergence of methicillin-resistant Staphylococcus aureus (MRSA) strains with high zoonotic potential [7,8], which have led to high morbidity and mortality rates in animals and humans [9,10].

Leonard and Markey [7] reported that the primary driver of antibiotic resistance in this bacterial genus is the irrational use of antibiotics and the increasingly close relationship between humans and pets [11]. Historically, penicillins and other β-lactam antibiotics have been used to treat Staphylococcus infections, but the production of β-lactamase by some bacteria reduced the efficacy of these drugs [7], leading to a decline in their use. Methicillin was developed to inhibit β-lactamase, but strains of S. aureus resistant to methicillin were soon identified [8,10]. MRSA strains are resistant to all β-lactam antibiotics, including penicillins, cephalosporins, combinations with clavulanic acid, and carbapenems, mechanisms associated with the folate metabolism, serine-modified cross-bridges that thicken their cell wall [9,12]. Antibiotic resistance in animals is often linked to the overuse and misuse of these medications and the close relationship between humans and animals [13]. Therefore, the monitoring and reporting of animal infections is essential, particularly in rural areas with limited access to veterinary care and pharmacological treatments.

This paper reports a case involving a pansusceptible strain of S. aureus isolated from a mare in a rural context where veterinary services are scarce, showing how geographical and socio-economic factors can influence or maintain the low prevalence of antibiotic resistance.

As part of the veterinary assistance continuous activity in Costa Rica’s indigenous communities from the Escuela de Medicina Veterinaria, Universidad Nacional de Costa Rica (project 0284-24), the Roca Quemada community in Turrialba was visited on September 16, 2023. This study evaluated 186 animals treated when required, including a mare named Sarna. Sarna was a 17-year-old mixed-breed (sin raza definida, SRD) mare presenting a deep, pyogranulomatous, ulcerated, and necrotizing lesion on the back. She had no previous treatments, and follow-up was not performed because of the difficult access to the area where the horse's owners live and the absence of telephone contact for monitoring. The owner had cared for the horse for 1.5 months. Sarna weighed 222 kg, had a dark reddish coat (retinto), and was not working because of the lesion. Her behavior was positive, and her diet consisted of pasture. The body condition score was 1.5.

The mare was in a good mood and was reported to have a good appetite. She had a large skin lesion at the level of the cross; ulcerated, purulent, and necrotized (Fig. 1). Therefore, a general objective examination was carried out. For sample collection, the area was cleaned with gauze and chlorhexidine, and the location of the injury was then scraped with a scalpel to collect affected tissue and crust. The material was placed into a sterile tube, which was kept at room temperature until transported to the Bacteriology Laboratory of the School of Veterinary Medicine of the Universidad Nacional de Costa Rica (Escuela de Medicina Veterinaria, Universidad Nacional) for culture, identification, and antibiotic susceptibility profiling. The sample was processed as soon as possible, considering the working hours and field conditions, which was achieved within 2 days. The culture yielded abundant growth, was pure, and sample collection was performed using an aseptic technique, without duplication.

The sample was cultured directly in Agar Columbia (Liofilchem, Italy) supplemented with 5% each of sheep’s blood, Agar MacConkey (Neogen, USA), and Agar Mannitol Salt (Liofilchem). The plates were incubated for 24 hours at 37°C, following previously described methodologies [14]. After incubation, colonies of medium size (2-3 mm in diameter), with white and smooth edges, and evidence of hemolysis, were observed in the Agar Columbia. Yellow colonies were formed in the Agar Mannitol Salt, accompanied by areas of yellow coloration. No bacterial growth was observed in agar MacConkey. Gram staining was performed on the isolated colonies following the standard procedures [15]. The presence of Gram-positive cocci arranged in clusters was observed. The automated Vitek 2 Compact system ( bioMérieux, France) was used for bacterial identification and the antibiotic sensitivity test using the GP and AST-GP80 cards, respectively, according to the manufacturer's instructions. The AST-GP80 card included a variety of antibiotics, including beta-lactams (benzylpenicillin, amoxicillin-clavulanate, oxacillin, cephalothin, cefovecin, and ceftiofur), aminoglycosides (gentamycin, kanamycin, and neomycin), fluoroquinolones (enrofloxacin, marbofloxacin, and pradofloxacin), a macrolide (erythromycin), a lincosamide (clindamycin), a phenyl (chloramphenicol), tetracyclines (tetracycline and doxycycline), a nitrofuran (nitrofurantoin), and a folate pathway inhibitor (trimethoprim-sulfamethoxazole). In addition, the results show (Table 1) the minimal inhibitory concentration and its respective interpretation. The antimicrobial sensitivity results were interpreted using Clinical and Laboratory Standards Institute (CLSI) veterinary breakpoints when available; otherwise, human CLSI M100 standards were applied [16,17]. Quality control procedures were performed to ensure the accuracy of bacterial identification and antimicrobial susceptibility testing. The reference strains Staphylococcus saprophyticus ATCC BAA-750 and S. aureus ATCC 29213 were included in each analytical run to verify the bacterial identification and antimicrobial susceptibility tests, respectively, as recommended for the automated Vitek 2 Compact system (bioMérieux).

The sample collected from the lesion on the mare Sarna's back (at the level of the withers) revealed a deep, pyogranulomatous, ulcerated, and necrotizing appearance (Fig. 1). On September 22, 2023, the laboratory reported a high bacterial load of S. aureus, with the strain being susceptible to all antimicrobial agents included in the tested panel, a condition referred to as pansusceptible [18,19].

The genus Staphylococcus is widespread, encompassing approximately 50 species and subspecies. Many of these species are opportunistic pathogens in horses. They are part of the normal microbiota of the skin and nasopharynx of mammals and birds and can occasionally be found in the gastrointestinal tract [20,21]. Transmission between species is generally limited [22]. Bacterial dermatitis often begins with skin trauma, where an infectious microorganism is introduced into the dermis, triggering a host response to contain the pathogen [23,24]. This condition commonly occurs in areas subject to trauma, such as the limbs, the scrotum, or areas prone to scratching. In the case of horses, it can be exacerbated by friction from saddles or poorly maintained equipment, which is frequently observed at the level of the withers, as was the case with the mare in this study (Fig. 1), transverse processes of the lumbar vertebrae, and the base of the tail [25,26].

Access to veterinary services for people residing in indigenous areas in Costa Rica is challenging because the nearest facilities are several hours away from their homes. Often, accessing these services requires multiple forms of transportation, such as buses or boats, and long walks. Rodríguez-Barahona [1] reported that some individuals may walk up to two days for veterinary care. Consequently, many never access veterinary services or antibacterial products, or only manage occasional visits.

According to the American Academy of Microbiology in 2009 [27], antibiotic resistance emerges through mechanisms involving natural competition between microorganisms as part of a standard evolutionary phenomenon. On the other hand, antibacterial products can exacerbate this resistance, regardless of whether their application is appropriate. Although many attribute antibiotic resistance solely to irrational use, this perspective is not entirely accurate; the rate of resistance is related to the use of these products, including the quantity used and environmental factors. Poor sanitation and inadequate waste management can act as triggering factors because resistance can develop in any environment where antibiotics are present, whether in humans, animals, or the environment [28,29].

Resistance factors affect the treatment of infections by reducing the effectiveness of first-line antibiotics, necessitating more selective and expensive drugs. This resistance also increases morbidity and mortality in animals and the people close to them [29]. In addition, the improper use of antimicrobials exacerbates the transfer of resistance between bacteria [28].

A high incidence of MRSA has been reported in the European Union, with approximately 7,000 deaths annually [30]. In Brazil, studies have identified a high rate of bacterial isolations from horses, with strains showing resistance to a broad spectrum of antibiotics. These include penicillin, amoxicillin, ampicillin, streptomycin, neomycin, tobramycin, erythromycin, clindamycin, rifampicin, tetracycline, cephalothin, ceftriaxone, gentamicin, doxycycline, sulfamethoxazole, amoxicillin with clavulanic acid, amikacin, azithromycin, and chloramphenicol [31,32]. This widespread resistance highlights significant challenges in managing bacterial infections in veterinary settings and underscores the need for rigorous antimicrobial stewardship [33].

Santos et al. [31] conducted a study in Brazil involving samples from the nostrils of 50 asymptomatic horses for respiratory diseases at the Veterinary Hospital of the University of Brazil. They found that 84% of the samples were positive for S. aureus. The antimicrobial susceptibility of these isolates was evaluated, showing 100% resistance to ampicillin, oxacillin, penicillin G, clindamycin, and cefoxitin. Resistance rates of 68.75%, 81.25%, and 75% were observed for azithromycin, ciprofloxacin, and tetracycline and gentamicin, respectively. Lower resistance percentages were reported for erythromycin, skin and soft tissue infections, musculoskeletal infections, chloramphenicol, linezolid, rifampicin, and sulfamethoxazole + trimethoprim, highlighting the ability of an organism to develop resistance [31].

A study conducted in Buenos Aires, Argentina, from 2015 to 2016, collected nasal vestibule samples from healthy horses used for equine therapy, zootechnics, polo, parades, and racing. The study observed an MRSA phenotype in 5% of the animals (nine horses) and observed resistance to erythromycin and clindamycin in seven horses [20].

MRSA emerged in 1961 and was initially confined to hospital settings, referred to as hospital-associated MRSA (MRSA-HA) [34]. By the mid-1990s, MRSA infections began to appear in high-risk patients within the community, such as children, athletes, prisoners, military personnel, and the homeless, leading to the classification of these infections as community-associated MRSA (MRSA-CA) [34]. An awareness of MRSA in animals increased significantly in 2005 when it was detected in pigs, marking a shift towards recognizing the broader zoonotic potential of MRSA [26]. Subsequently, MRSA was identified in livestock species, such as cows, poultry, and horses, in several European countries, being referred to as livestock-associated MRSA and recognized as a zoonotic pathogen [21,22]. In horses, MRSA has been documented in various regions, including North America, Ireland, Japan, Austria, and the United Kingdom, with healthy carriers also reported in Denmark [20,25].

A study conducted in Canada and the United States between 2000 and 2006 included 115 horses ranging from 0 to 31 years of age. The study classified 50.9% and 49.1% of MRSA infections as MRSA-HA and MRSA-CA, respectively. The most prevalent infections observed were cutaneous and mucosal infections [27].

The isolation of a pansusceptible S. aureus strain from a mare in Roca Quemada highlights the contrast between the antibiotic sensitivity and the rising prevalence of MRSA in other regions, particularly in horses, suggesting that limited veterinary care and antibiotic exposure in rural areas may help prevent the emergence of resistant strains. These findings support the notion that minimal antibiotic use could maintain high susceptibility rates, underscoring the need for continued surveillance and responsible stewardship. Nevertheless, more research will be needed to explore how the absence of regular veterinary interventions influences antibiotic resistance in rural communities.

Limited access to veterinary services in the rural and indigenous regions of Costa Rica contributes to the persistence of chronic lesions in working equids, allowing bacterial infections to remain. In the case of the mare Sarna, the bacterial isolates from her chronic wounds were assessed for antimicrobial susceptibility. The pathogens remained sensitive to commonly used antibiotics. This susceptibility can be associated with the absence of indiscriminate antibiotic use in the region because controlled and limited administration reduces the selective pressure that typically leads to resistance. Therefore, combining veterinary supervision with regulated antibiotic use helps preserve the treatment efficacy and ensures that infections can be managed effectively when professional care is available [1,35,36].

The use of horses in labor-related contexts is common, often involving long working hours, excessive loads, and improper or excessive use of tack or other traction equipment, which are frequently made of abrasive materials [37]. These factors can lead to dermatological lesions due to mechanical sources, compromise the skin barrier, and predispose horses to bacterial colonization [38].

Although other causes, such as parasitic infections or neoplastic lesions, can occur, in this case, the localization of the lesion on the back strongly suggests a traumatic origin related to the use of a tack, which aligns with reports describing welfare problems and skin injuries associated with saddle use in working equids [39]. Nevertheless, the possibility that S. aureus may act as an opportunistic agent in such scenarios cannot be excluded.