Staphylococcus pseudintermedius Human Infection Cases in Spain: Dog-to-Human Transmission

Introduction

S taphylococcus pseudintermedius is an opportunistic pathogen that, along with Staphylococcus intermedius and Staphylococcus delphini, belongs to the S. intermedius group. This coagulase-positive species has been identified as infectious agent or colonizer in several animal species, and frequently causing pyoderma in dogs (Weese and van Duijkeren 2010). S. pseudintermedius has also been detected in humans and more specifically in people in contact with dogs (veterinarians or owners) (Gómez-Sanz et al. 2013a, 2013b).

The first case of an S. pseudintermedius infection in humans, confirmed by molecular identification, was an endocarditis case published in 2006 (van Hoovels et al. 2006). Since then, other human infections have been described, causing local lesions or even bacteremia, brain abscess, endocarditis, or pneumonia, among others (Weese and van Duijkeren 2010, Starlander et al. 2014). These human clinical cases are still rare and normally are associated with dog bite wounds, although it is important to remark that this bacterial species is many times misdiagnosed as Staphylococcus aureus or S. intermedius, and therefore, S. pseudintermedius cases can be underreported (Börjesson et al. 2015). In the past years, the interest about this microorganism has increased because of its potential zoonotic and also in part because of the emergence of methicillin-resistant S. pseudintermedius (MRSP). The aim of this study was to perform the genetic characterization of human infection cases caused by S. pseudintermedius, and to determine the possible pet-to-human transmission.

Case Studies

Four clinical cases caused by this microorganism were identified in Miguel Servet University Hospital, Zaragoza (Spain) from August 2014 to May 2015. The four patients gave their written consent to be included in this study. Patient 1 was hospitalized with a surgical wound infection of ventral hernia repair and both S. pseudintermedius and Escherichia coli strains were isolated from a wound sample. This patient had no contact with animals and fosfomycin was the antibiotic chosen for treating the infection (Table 1).

In the remaining three patients, S. pseudintermedius was the only pathogen identified in the tested clinical samples. Patient 2 had contact with dogs although she was not the owner. She showed left lower extremity cellulitis and was treated with amoxicillin/clavulanic acid. The other two patients were pet owners and lived with their dogs in the same household (patient 3 had one dog and patient 4 had two dogs). Nasal samples of these healthy animals were obtained and S. pseudintermedius was recovered from the three dogs (Table 1). Patient 3 suffered cancer and presented a nail exudate and he was treated with gentamicin. Patient 4 was immunocompromised and had a foot ulcer; surgical drainage and cloxacillin were chosen as treatment. All four patients evolved favorably.

Seven S. pseudintermedius strains were obtained from the four patients and from the three dogs (Table 1). Strains were recovered in blood agar and colistin nalidixic agar plates (Oxoid, England), and they were identified by MALDI-TOF MS (Bruker^®, Biotyper 3). Antimicrobial susceptibility to 21 antibiotics (ampicillin, amoxicillin/clavulanic acid, cefotaxime, imipenem, penicillin, cefoxitin, oxacillin, tetracycline, azithromycin, erythromycin, clindamycin, quinupristin/dalfopristin, ciprofloxacin, levofloxacin, moxifloxacin, gentamicin, fosfomycin, trimethoprim-sulfamethoxazole, linezolid, rifampicin, and teicoplanin) was determined by using Microscan system and agar disk diffusion method (CLSI 2015).

The presence of macrolide-lincosamide [erm(A), erm(B), erm(C), erm(T)], tetracycline [tet(K), tet(L), tet(M), tet(O)], and trimetoprim (dfrS1, dfrD, dfrG, dfrK) resistance genes was analyzed by PCR (Gómez-Sanz et al. 2013a). The virulence genes lukS/F-I, siet, expB, sec_canine , and expA were also tested by PCR (Lozano et al. 2015). Clonal relatedness of all strains was studied by pulsed-field gel electrophoresis (PFGE) of chromosomal DNA digested with the SmaI enzyme. PFGE was run using pulsed time ramping from 2 to 5 s for 24 h at 5.6 V/cm (Perreten et al. 2010). Moreover, multilocus sequence typing (MLST) was performed in all strains (http://pubmlst.org/spseudintermedius).

Characteristics of the seven S. pseudintermedius strains of human and animal origin are shown in Table 1. Methicillin resistance was not identified in any of the strains. Methicillin-susceptible S. pseudintermedius (MSSP) strains are usually susceptible to all antimicrobials except to penicillin (Gómez-Sanz et al. 2013a). In our study, none of the seven strains was penicillin resistant. However, resistance to several other antibiotics (tetracycline, trimethoprim-sulfamethoxazole, ciprofloxacin, erythromycin, and/or clindamycin) was detected and the resistance genes erm(B), tet(M), and dfrG were identified. In a few studies, MSSP with multiresistance phenotype has been described (Gómez-Sanz et al. 2013a, 2013b, McCarthy et al. 2015).

In cases 1 and 2, both strains (C8187 and C8368) showed resistance to tetracycline, harbored the gene tet(M), and presented different PFGE patterns (Supplementary Fig. 1; Supplementary Data are available online at www.liebertpub.com/vbz). In cases 3 and 4, animal and human strains recovered from the same household presented similar antimicrobial resistance phenotype and genotype and identical PFGE profile (pattern B in case 3 [C8188 and C8477] and pattern C in case 4 [C8189, C8478, and C8479]) (Supplementary Fig. 1 and Table 1). These results could be indicative of dog-to-human transmission as S. pseudintermedius is a normal colonizer of dogs. Other possible cases of S. pseudintermedius transference among humans and their pets have been previously identified (Laarhoven et al. 2011, van Duijkeren et al. 2011). In those reports and in this study, similar PFGE patterns were observed in strains recovered from dogs and their owners. However, it is also important to consider that in some studies different clonal lineages of S. pseudintermedius have been identified in members of the same household (Laarhoven et al. 2011). Moreover, direct or indirect patient-to-patient transmission of this microorganism has been suggested at a tertiary hospital as no animal source was identified (Starlander et al. 2014). However, in our study, it seems unlikely a transmission of these strains within the hospital.

All strains were typed by MLST and the sequence type (ST) ST241 and three new STs (ST521, ST719, and ST720) were identified. These new STs presented new allele combinations and were submitted to S. pseudintermedius MLST database (http://pubmlst.org/spseudintermedius). An important genetic diversity, as well as a high rate of novel lineages among MSSP strains, has been previously identified (Gómez-Sanz et al. 2013b). All strains contained the virulence genes lukS/F-I and siet. Remarkably, the gene expB was only identified in the strain of patient 2. This gene is related to dermatitis in dogs (Lozano et al. 2015), although this patient did not have domestic animals.

In our country, the species S. pseudintermedius has been previously identified as colonizer in humans (Gómez-Sanz et al. 2013a, 2013b, Lozano et al. 2015); in most of the cases, these humans were pet-owning household members. However, to our knowledge, this is the first report of human infections by S. pseudintermedius in Spain.

Conclusions

S. pseudintermedius animal-to-human transmission has been suggested in this study. MSSP strains isolated from lesions of patients and from nasal samples of their dogs living in the same household showed identical PFGE patterns and similar resistance phenotype and genotype. Four different STs were identified in the four infection cases, three STs being new ones. The role of S. pseudintermedius as a zoonotic pathogen is potentiated, although it needs more research.