Introduction

Pyoderma is a pyogenic skin bacterial infection and the most common skin disease seen in dogs. Staphylococcus pseudintermedius is a commensal as well as the most frequent bacterial pathogen causing bacterial pyoderma in the dog.

Underlying Causes for Bacterial Pyoderma

Bacterial pyoderma is always secondary to an underlying cutaneous or systemic disease that disrupts the skin’s defense mechanisms.

• Hypersensitivity dermatitis (adverse food reactions, atopic dermatitis, flea bite hypersensitivity)
• Demodex spp., Sarcoptes scabiei, fleas)
• Endocrinopathies: hypothyroidism, hyperadrenocorticism, diabetes mellitus
• Follicular dysplasia
• Keratinisation disorders (sebaceous adenitis, zinc responsive dermatosis)
• Malignancies

Diagnosis of bacterial pyoderma.

Clinical Lesions

• Pustules
• Crusts
• Epidermal collarettes
• Papules
• Focal to multifocal alopecia
• Erosions, ulcers
• Nodules and draining tracts

Infections due to methicillin resistant Staphylococcus pseudintermedius (MRSP) and methicillin susceptible Staphylococcus pseudintermedius (MSSP) look the same except MRSP infections do not respond to appropriately selected empirical antibiotic.

Cytology

A clinical diagnosis is made using a combination of consistent clinical lesions and cytological evidence of bacteria with suppurative inflammation, i.e., degenerate neutrophils with intracellular cocci. Sampling techniques include direct and indirect (e.g., cotton buds) impression smears and adhesive tape cytology. The slides are stained with a Romanowsky stain such as Diff-Quik, and examined under the microscope using the 100x oil immersion field.

Methicillin Resistant Staphylococcus pseudintermedius (MRSP)

MRSP infections are a major concern worldwide. MRSP infections are resistant to the antibiotic methicillin, which is used to deal with B lactamase producing staphylococci. Oxacillin is a replacement for methicillin in laboratories due to availability and stability. Cefoxitin is used to test for methicillin resistance in Staphylococcus aureus but is not reliable in diagnosis of MRSP.

Methicillin resistance is due to the mecA gene, which encodes for modified penicillin binding protein (pbp2a) that has low affinity for all B lactam antibiotics. The mecA gene is contained within a mobile genetic element called the staphylococcal cassette chromosome mec (SCCmec), which can be transferred between and within staphylococci species. Horizontal gene transfer is frequent in S. pseudintermedius and multidrug resistant MRSP evolved rapidly through acquisition of a very limited number of mobile gene elements and mutations.

MRSP Clones

Molecular testing using multilocus sequence typing (MLST) has allowed differentiation of MRSP into clones, which enables researchers to monitor the spread of MRSP worldwide. The major clones worldwide are the ST 71 in Europe, ST 68 in USA and ST45 in South East Asia.

Prevalence

The prevalence of MRSP in dogs varies geographically and on the studied populations. Published studies in Asia reported frequencies as high as 28% in Thailand and 66.5% in Japan. The prevalence of MRSP in healthy dogs is generally low.

Transmission

The suggested routes of transmission of S. pseudintermedius between dogs include vertical transmission from bitches to puppies (including milk) and horizontal transmission between dogs.

Risk Factors for Developing MRSP Infections

• Increased number of antibacterial drug prescriptions
• Exposure to multiple drug classes (especially beta-lactams)
• Recent (30 days) antibiotic usage
• Misuse/excessive use of antibiotics
• Breeding dogs
• Hospitalisation/veterinary visits
• Concurrent immunomodulatory therapy
• Male? (Finland)

When to Culture for MRSP? Modified from Hillier et al. 2014

• Less than 50% improvement in lesions within 2 weeks of appropriate systemic antimicrobial therapy
• Appearance of new lesions consistent with bacterial pyoderma 2 or more weeks after starting appropriate systemic antimicrobial therapy
• Presence of residual lesions and cytological evidence of bacterial pyoderma after 6 weeks of appropriate systemic antimicrobial therapy
• Prior history of multidrug resistant infection in affected dog or from in contact dog in same household
• Recent history of hospitalisation
• Breeding dogs?

Bacterial Culture Collection and Antibiotic Susceptibility Testing

A culture swab is used to collect samples from the surface and superficial lesions. Primary lesions such as intact pustules are preferred over secondary lesions such as epidermal collarettes and crusts. For deeper lesions such as nodules and draining tracts, the skin surface should be surgically prepared and a deep tissue biopsy collected for bacterial culture and antibiotic susceptibility testing.

Antimicrobial susceptibility testing is most commonly performed using the disk diffusion or dilution methods. While methicillin resistance is identified using oxacillin antibiotic testing, the gold standard to diagnose methicillin resistance is polymerase chain reaction (PCR) detection of mecA gene. When there is a clinical suspicion for MRSP, the author request for an extended panel of antibiotics including oxacillin. This is because some MRSP may show apparent in vitro susceptibilities to beta lactams, i.e., cephalexin and amoxicillin-clavulanate acid but are ineffective in vivo.

Key Steps for Successful Treatment of Bacterial Pyoderma

• Correct diagnosis of pyoderma
• Use topical antibacterial therapy and if necessary select an appropriate systemic antibiotic
• Correct antibiotic administration i.e., correct dose and frequency until clinical cure
• Diagnosing and treating any underlying disease(s) causing bacterial pyoderma

Treatment Considerations

Topical Antibacterial Therapy

The skin lesions are readily accessible to topical therapy. Topical antibacterial therapy can be effective as a sole treatment against surface and superficial bacterial pyoderma regardless of methicillin susceptibility.

Benefits of Topical Antibacterial Therapies

• May avoid the need for systemic antibiotics
• More rapid resolution of infection
• Reduce duration of systemic antibiotics required
• Break up biofilms, remove crusts, debris, bacteria and allergens from the skin
• Restore normal skin structure and function against infections
• Control against reinfections when underlying disease is being investigated and managed
• Reduce environmental contamination
• Reduce risk of transmission to other dogs and humans

Topical antibacterial options include 2–3% chlorhexidine, benzoyl peroxide, bleach (sodium hypochlorite), miconazole, fusidic acid, mupirocin, triclosan, bacitracin and polymixin B. They are available in shampoos, conditioners, lotions and sprays for more generalised infections, and creams, lotions, gels and wipes for more localised infections.

Systemic Antibiotic

These are indicated for generalised, severe or deep bacterial pyoderma, if the dog is not amendable to topical therapy or owner unable to perform topical therapy. The results of antibiotic susceptibility testing guide the veterinarian in antibiotic selection. The final choice of antibiotic would depend on several factors including availability, safety, costs and patient factors (e.g., concurrent disease or drug therapy, drug reactions). Guidelines for selecting antibiotic and their doses have been published (Hillier et al. 2014) and will be discussed during the presentation.

Duration of Treatment

• Superficial bacterial pyoderma: 3 weeks or 1 week beyond clinical resolution
• Deep bacterial pyoderma: 4–6 weeks or 2 weeks beyond clinical resolution
• If treatment duration is less than 3 weeks, then the patient should be examined prior to stopping antibiotics to ensure resolution of infection

Controlling Against Reinfections

Bacterial pyoderma is always secondary to underlying diseases. Veterinarians play an important role in identifying and managing these underlying diseases so that we can avoid reinfections, repeated antibiotic treatments and reduce risks of development of antibiotic resistance.

Prognosis

Fortunately, there is no difference in treatment outcomes between dogs treated for MSSP and MRSP with both groups having an overall good prognosis. Dogs can continue to carry MRSP for more than one year after clinical resolution of infection. These dogs can pose a risk to susceptible in contact animals (both dogs and cats) and humans, as well as contaminating the environment.

Transmission to In-Contact Dogs and Owners

Staphylococcus pseudintermedius do not typically colonise humans although we can be transient carriers, especially veterinary professionals or those in close contact with dogs. Dogs and their owners can harbour genetically identical strains of S. pseudintermedius including MRSP.

Veterinarians and owners should focus on reducing direct and indirect spread of MRSP through direct contact and contamination of environments respectively. It is reasonable to restrict MRSP infected dogs from contact with other dogs and humans, especially those who are immune-compromised, until they receive treatment and show clinical improvement. Veterinarians should also educate owners on the importance of hand hygiene and how to decontaminate the environment.

In dogs, MRSP carriage has been identified as a risk factor for developing surgical site infections after tibial plateau levelling osteotomy (TPLO). Routine infection control practices are effective in controlling the potential transmission of staphylococci and MRSP between animals and to owners, veterinary and nursing staff. Personal protective equipment (PPE) prevents contamination of clothing and skin and subsequent transmission to other animals and staff.

References

1.  Bannoehr J, and Guardabassi L. Staphylococcus pseudintermedius in the dog: taxonomy, diagnostics, ecology, epidemiology and pathogenicity, Veterinary Dermatology, 2012; 23:253–e52.

2.  Morris DO, Loeffler A, Davis MF, Guardabassi L and Weese JS, Recommendations for approaches to methicillin resistant staphylococcal infections of small animals: diagnosis, therapeutic considerations and preventative measures, Veterinary Dermatology, 2017, 28:304–e69.

3.  McCarthy AJ, Harrison EM, Stanczak-Mrozek K et al. Genomic insights into the rapid emergence and evolution of MDR in Staphylococcus pseudintermedius, The Journal of Antimicrobial Chemotherapy, 2015, 70(4):997–1007.

4.  Hillier A, Lloyd DH, Weese JS et al., Guidelines for the diagnosis and antimicrobial therapy of canine superficial bacterial folliculitis (antimicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases), Veterinary Dermatology, 2014, 25: 163–e43.