Discussions
S. pseudintermedius is a novel Staphylococcus specieidentified in the last decade (Devriese et al., 2005) and this organism has been commonly associated with skin and soft tissue infections (SSTIs) in pet dogs (Chuang et al., 2010; Savini et al., 2013; Stegmann et al., 2010), but few raccoon dogs cases have been described. As an important fur animal, raccoon dog is being widely raised in China and its SSTIs and zoonotic diseases caused by the pathogens are of particular concern (Wang et al., 2014; Xu et al., 2016; Wioletta and Maria, 2018). In this study, about 47% (425/896) of raccoon dogs were infected by S. pseudintermedius and 17.6% of them had sever SSTIs and 4.2% of them were dead in 4 weeks. This is a new threaten to raccoon dogs!
In this study, the clinical symptoms of the raccoon dogs were described according to the characteristics of the disease at different stages and the symptom including suppuration of skin or/and joints and severe respiratory dysfunction was recommended to be used as one of important basises of clinical diagnosis for S. pseudintermedius infection in raccoon dogs. In addition, the pathological lesions in many organs of the diseased raccoon dogs were also examined and described, which greatly enriches the clinical data of the disease.
It is very difficult to determine the pathogens of the disease in clinic, because SSTIs and respiratory dysfunction can be caused by many pathogens, especially staphylococcus (Rashid and Kravitz, 2015). In the past, this disease in pet dogs had been misdiagnosed as S. intermedius and other bacteria infection (Sasaki et al., 2007; Bannoehr et al., 2007; BőRjesson et al., 2015). The mass spectrometer detection has been recommended as an effective method for diagnosing it (Silva et al., 2015; Somayaji et al., 2016; Wang et al., 2017). According to the results in this study, this method can indeed do that from all the tissues samples, but it is noteworthy that there are 8 species of bacteria identified from the pus tissue on the skin, and some of them might severely interfere the correct judgment on the pathogens; whereas, much less in the visceral organs such as lung and kidney. Therefore, it is better to aseptically collect samples from deep tissues for detecting the pathogens using mass spectrometry.
Usually, three species of S. intermedius group (SIG) including S. pseudintermedius , S. intermedia and S. delphini can effect the judgment for the pathogens of SSTIs (Fitzgerald, 2009). To distinguish them, some biochemical reactions to some designated reagents should be tested. According to the previous reports, S. pseudintermedius is positive to maltose and trehalose, but negative to mannitol; S. intermedia is positive to trehalose and mannitol, but negative to maltose; S. delphini is positive to maltose and mannitol, but negative to trehalose (Bannoehr and Guardabassi, 2012; Sasaki et al., 2007; Devriese et al., 2005). The results of biochemical reactions of the isolated pathogen in this study were exactly the same as previously reported, so we deduced that the isolated pathogen should be S. pseudintermedius. This can supply a reference for distinguishing S. pseudintermedius infection without mass spectrometer in veterinary clinics.
In some previous reports, S. pseudintermedius has been concerned as a potential zoonotic pathogen (Stegmann et al., 2010; Somayaji et al. 2016b). To evaluate the pathogenicity of the isolated S. pseudintermedius from raccoon dogs on other animals, the mice were inoculated with 1.8×109 CFU of S. pseudintermedius and most of them developed skin abscess, ulcer, dyspnea, and sever pathological lesions in some important organs in 14 dpi, which were almost identical with those of the diseased raccoon dogs. The results suggest that the isolated S. pseudintermediuscan not only cause sever lesions in raccoon dogs, but also in mice. Although S. pseudintermediu has been previously reported as a zoonotic pathogen, yet its pathogenicity on cross-species is rarely evaluated. This study can supply some important data for that.
Some studies suggest that S. pseudintermedius can be divided into methicillin-sensitive S. pseudintermedius (MSSP) and methicillin-resistant S. pseudintermedius (MRSP) (Ruscher et al., 2010; van Duijkeren et al., 2011). Methicillin resistance is mediated by the mecA gene. The mecA gene is located in the mobile genetic element SCCmec , which encodes the penicillin binding protein (PBP) and has a low affinity for β-lactam antibiotics (Chambers et al., 1997; Bannoehr and Guardabassi, 2012; Ane et al, 2012) When the bacteria were confirmed to contain mecA gene by PCR method, it showed that the bacteria were methicillin-resistant (Chambers, 1997). In recent years, the incidence of MRSP has been increasing, which has brought more and more challenges to clinical treatment against it (Moodley et al., 2014; Mccarthy et al., 2015; Richards et al., 2018). In this study, the resistant gene of mecA was amplified from the isolated strain, which indicates that it is a MRSP strain, like previous reported other strains.
Interestingly, how many antibiotics are sensitive or resistant to it? The results of drug sensitivity test showed that the strain was highly sensitive to 12 antibiotics and most of them were quinolones and aminoglycosides, and a few of them were chloramphenicol, macrolides and polypeptides, these indicate that the isolated strain has not develop serious multi-drug resistance like the global epidemic MRSP strains, although it belonged to MRSP. In addition, according to the results of drug sensitivity test, the diseased raccoon dogs were treated with some of sensitivity drugs such as quinolones and most of them completely recovered in two weeks, this greatly helps the treatment of this disease in clinical practice.