Dermatoses affecting desmosomes in animals: a mechanistic review of acantholytic blistering skin diseases

Common superficial and deep cutaneous bacterial infections in domestic animals: A review

The skin covers the external surface of animals, and it is constantly exposed to and inhabited by different microorganisms, including bacteria. Alterations in the skin barrier allow commensal and/or pathogenic bacteria to proliferate and penetrate deep into the lower layers of the skin. Being the first barrier to the external environment, the skin is prone to injuries, allowing the penetration of microorganisms that may lead to severe deep infections. Companion animals, especially dogs, are prone to bacterial infections, often secondary to allergic dermatitis. When environmental conditions are unfavorable, horses, cattle, sheep, and goats can develop superficial infections, such as those caused by Dermatophilus congolensis. Deep inflammation is commonly caused by Mycobacterium spp., which results in granulomatous to pyogranulomatous dermatitis and panniculitis. Likewise, bacteria such as Nocardia spp. and Actinomyces spp. can cause deep pyogranulomatous inflammation. Bacteria that lead to deep necrotizing lesions (eg, necrotizing fasciitis/flesh-eating bacteria) can be severe and even result in death. This review includes an overview of the most common cutaneous bacterial infections of domestic animals, highlighting the main features and histologic morphology of the bacteria, cutaneous structures involved, and the type of inflammatory infiltrates.

Heterozygous ATP2A2 missense variant identified in a Shih Tzu with Darier disease

Darier disease is caused by heterozygous loss of function variants in the ATP2A2 gene encoding the endoplasmic/sarcoplasmic reticulum Ca²⁺ pump ATP2A2. Defective intracellular calcium signaling in the epidermis results in a loss of desmosomal adhesion and the development of characteristic skin lesions. In this study, we investigated a Shih Tzu that developed erythematous papules on the ventrum and, over time, the dorsal neck and a nodule in the right ear canal with secondary ear infection. Histopathologic examination demonstrated discrete foci of acantholysis affecting suprabasal layers of the epidermis. Whole genome sequencing of the affected dog identified a heterozygous missense variant, p.N809H, affecting an evolutionarily conserved amino acid residue of the ATP2A2 protein. The highly characteristic clinical and histopathologic findings together with a plausible variant in the only known functional candidate gene establish the diagnosis of canine Darier disease in the studied dog and highlight the potential of genetic analyses as complementary diagnostic approach in veterinary medicine.

Inheritance of Monogenic Hereditary Skin Disease and Related Canine Breeds

The plasticity of the genome is an evolutionary factor in all animal species, including canines, but it can also be the origin of diseases caused by hereditary genetic mutation. Genetic changes, or mutations, that give rise to a pathology in most cases result from recessive alleles that are normally found with minority allelic frequency. The use of genetic improvement increases the consanguinity within canine breeds and, on many occasions, also increases the frequency of these recessive alleles, increasing the prevalence of these pathologies. This prevalence has been known for a long time, but mutations differ according to the canine breed. These genetic diseases, including skin diseases, or genodermatosis, which is narrowly defined as monogenic hereditary dermatosis. In this review, we focus on genodermatosis sensu estricto, i.e., monogenic, and hereditary dermatosis, in addition to the clinical features, diagnosis, pathogeny, and treatment. Specifically, this review analyzes epidermolytic and non-epidermolytic ichthyosis, junctional epidermolysis bullosa, nasal parakeratosis, mucinosis, dermoid sinus, among others, in canine breeds, such as Golden Retriever, German Pointer, Australian Shepherd, American Bulldog, Great Dane, Jack Russell Terrier, Labrador Retriever, Shar-Pei, and Rhodesian Ridgeback.

Trunk-dominant and classic facial pemphigus foliaceus in dogs - comparison of anti-desmocollin-1 and anti-desmoglein-1 autoantibodies and clinical presentations

BACKGROUND

Canine trunk-dominant pemphigus foliaceus (PF) is mentioned rarely in the literature.

HYPOTHESIS/OBJECTIVES

The goal of this study was to provide clinical description of trunk-dominant PF and to demonstrate the prevalence of serum antikeratinocyte, anti-desmocollin-1 (DSC1) and anti-desmoglein-1 (DSG1) antibodies, and determine their diagnostic value in this particular PF phenotype.

MATERIALS AND METHODS

Clinically relevant information was collected from 31, 25 and 34 dogs with trunk-dominant and facial PF and superficial pyoderma (SP), respectively. Sera from these dogs were tested for antikeratinocyte, anti-DSC1 and anti-DSG1 antibodies using indirect immunofluorescence on canine tissues and DSC1- and DSG1-transfected cells. Sera from healthy dogs and dogs with clinically irrelevant diseases served as controls.

RESULTS

Footpad involvement and grouped/polycyclic lesion organisation were identified as features of both PF phenotypes, and not of SP. Antikeratinocyte immunoglobulin (Ig)G was not specific for canine PF. By contrast, antigen-specific IgG was detected only in PF sera; anti-DSC1 IgG in 100% and 58% of dogs with facial and trunk-dominant PF, respectively, and anti-DSG1 IgG in 7% of dogs with trunk-dominant PF only.

CONCLUSIONS

Trunk-dominant PF shares DSC1 as a major autoantigen with facial PF. The ability to detect anti-DSC1 IgG is lower in trunk-dominant PF, yet despite the lower sensitivity, the positive predictive value and accuracy of this particular anti-DSC1 IgG test are high. A negative test result, however, cannot exclude the diagnosis, and characteristic clinical features such as footpad involvement and/or grouped/polycyclic lesions must be considered when distinguishing trunk-dominant PF from its most relevant differential diagnosis: SP.

Successful management of feline pemphigus foliaceus with pentoxifylline and topical hydrocortisone aceponate

The treatment regimen for feline pemphigus foliaceus (PF), an autoimmune disease caused by auto-antibodies against proteins of the desmosome junction, usually includes high doses of oral or parenteral immunosuppressive drugs, typically glucocorticoids. This case adds to a growing body of evidence that topical hydrocortisone aceponate is effective for the treatment of feline PF, and demonstrates the practical use of a non-invasive diagnostic method for histopathology when owners refuse a biopsy to support a clinical diagnosis of PF. Finally, this case highlights an international trend of owner-initiated treatment of feline infectious peritonitis (FIP) using unlicensed, unregistered drugs.

Gene Expression Analysis in Four Dogs With Canine Pemphigus Clinical Subtypes Reveals B Cell Signatures and Immune Activation Pathways Similar to Human Disease

Pemphigus is a group of autoimmune-mediated mucocutaneous blistering diseases characterized by acantholysis. Pemphigus has also been recognized in dogs and shares similar clinical characteristics and variants with human pemphigus. While relationships between human and canine pemphigus have been reported, gene expression patterns across species have not been described in the literature. We sought to perform gene expression analysis of lesional skin tissue from four dogs with various forms of pemphigus to examine gene expression during spontaneous disease in dogs. We found increased T and B cell signatures in canine pemphigus lesions compared to controls, as well as significant upregulation of CCL3, CCL4, CXCL10, and CXCL8 (IL8), among other genes. Similar chemokine/cytokine expression patterns and immune infiltrates have been reported in humans, suggesting that these genes play a role in spontaneous disease. Direct comparison of our dataset to previously published human pemphigus datasets revealed five conserved differentially expressed genes: CD19, WIF1, CXCL10, CD86, and S100A12. Our data expands our understanding of pemphigus and facilitates identification of biomarkers for prediction of disease prognosis and treatment response, which may be useful for future veterinary and human clinical trials.

The Pharmacodynamics Study of Insect Defensin DLP4 Against Toxigenic Staphylococcus hyicus ACCC 61734 in Vitro and Vivo

Staphylococcus hyicus (S. hyicus), as the main pathogen of exudative epidermitis (EE) in piglet, can cause a wide variety of diseases, ranging from bovine mastitis, chicken arthritis and even human sepsis, which has brought serious threats to animals and human. The potential threat of S. hyicus infection to both public and animal health has aroused great concern. The aim of our study was to explore the efficacy of insect defensin DLP4 against S. hyicus ACCC 61734 in vitro and in vivo. The in vitro efficacies of DLP4 against S. hyicus ACCC 61734 showed high antibacterial activity (0.92 μM), a long postantibiotic effect (9.54 h), a synergistic effect with ceftriaxone, penicillin and amoxicillin, a stable bacteriostatic effect, and intracellular bacteriostatic activity against S. hyicus ACCC 61734 in HaCaT cells. Besides, the antibacterial mechanism of DLP4 against S. hyicus ACCC 61734 was explored for the first time, which indicated that the antibacterial effect of DLP4 was related to its ability to destroy cell wall and generate membrane vesicles. The in vivo therapeutic effect of DLP4 was evaluated through mouse abscess model, and the results showed that DLP4 could effectively alleviate the mouse skin abscess by inhibiting bacterial proliferation and regulating cytokines. This study first demonstrated that DLP4 may be a promising therapeutic agent against S. hyicus ACCC 61734 infection.

Beneficial effect of oclacitinib in a case of feline pemphigus foliaceus

Pemphigus foliaceous (PF) is a pustular, immune-mediated skin disease characterised by acantholytic cells and commonly treated with high doses of glucocorticoids. This report describes one case of feline PF successfully controlled using oral oclacitinib, suggesting a possible therapeutic alternative to glucocorticoids in some cases.

Deep pemphigus (pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus) in dogs, cats and horses: a comprehensive review

Pemphigus is the term used to describe a group of rare mucocutaneous autoimmune bullous diseases characterized by flaccid blisters and erosions of the mucous membranes and/or skin. When the autoantibodies target desmosomes in the deep layers of the epidermis, deep pemphigus variants such as pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus develop. In this article, we will review the signalment, clinical signs, histopathology and treatment outcome of pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus in dogs, cats and horses; where pertinent, we compare the animal diseases to their human homologue. Canine, feline and equine pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus have many features similar to the human counterpart. These chronic and often relapsing autoimmune dermatoses require aggressive immunosuppressive therapy. In animals, the partial-to-complete remission of pemphigus vulgaris and pemphigus vegetans has been achieved with high dose glucocorticoid therapy, with or without adjunct immunosuppressants; the prognosis is grave for paraneoplastic pemphigus.

Visualising virulence factors: Trichophyton benhamiaes subtilisins demonstrated in a guinea pig skin ex vivo model

BACKGROUND

Dermatophytoses rank among the most frequent communicable diseases in humans, and the zoonotic transmission is increasing. The zoophilic dermatophyte Trichophyton (T.) benhamiae is nowadays one of the main causes of tinea faciei et corporis in children. However, scientific data on molecular pathomechanisms and specific virulence factors enabling this ubiquitous occurrence are scarce.

OBJECTIVES

To study tissue invasion and the expression of important virulence factors of T. benhamiae, isolates that were recovered from two groups of hosts (humans vs. guinea pigs (GP)) using an ex vivo skin model.

METHODS

After confirmation of species identity by ITS sequencing, CFU suspensions of dermatophyte isolates (n = 20) were applied to the skin infection model and cultured. Employing specific immunofluorescence staining techniques, the expression of subtilisin 3 and 6 and metallocarboxypeptidase A was analysed. The general mode of invasion was explored. Results were compared with biopsies of naturally infected GP.

RESULTS

All isolates were successfully recovered and proliferated well after application to the infection model. Progressive invasion of hyphae through all skin structures and destruction of explants were observed with early events being comparable to natural infection. An increasing expression of the examined virulence factors towards the end of culture was noticed but no difference between the two groups of isolates.

CONCLUSIONS

For the first time, important in vivo markers of dermatophytosis were visualised immunohistochemically in an ex vivo skin infection model and in skin biopsies of GP naturally infected with T. benhamiae. More research on the underlying pathomechanisms of dermatophyte infection is urgently needed.

ATP2A2 SINE Insertion in an Irish Terrier with Darier Disease and Associated Infundibular Cyst Formation

A 4-month-old female Irish Terrier presented with a well demarcated ulcerative and crusting lesion in the right ear canal. Histological analysis revealed epidermal hyperplasia with severe acantholysis affecting all suprabasal layers of the epidermis, which prompted a presumptive diagnosis of canine Darier disease. The lesion was successfully treated by repeated laser ablation of the affected epidermis. Over the course of three years, the dog additionally developed three dermal nodules of up to 4 cm in diameter that were excised and healed without complications. Histology of the excised tissue revealed multiple infundibular cysts extending from the upper dermis to the subcutis. The cysts were lined by squamous epithelium, which presented with abundant acantholysis of suprabasal keratinocytes. Infundibular cysts represent a novel finding not previously reported in Darier patients. Whole genome sequencing of the affected dog was performed, and the functional candidate genes for Darier disease (ATP2A2) and Hailey-Hailey disease (ATP2C1) were investigated. The analysis revealed a heterozygous SINE insertion into the ATP2A2 gene, at the end of intron 14, close to the boundary of exon 15. Analysis of the ATP2A2 mRNA from skin of the affected dog demonstrated a splicing defect and marked allelic imbalance, suggesting nonsense-mediated decay of the resulting aberrant transcripts. As Darier disease in humans is caused by haploinsufficiency of ATP2A2, our genetic findings are in agreement with the clinical and histopathological data and support the diagnosis of canine Darier disease.

Feline pemphigus foliaceus: original case series and a comprehensive literature review

BACKGROUND

Since the first description of feline pemphigus foliaceus (PF) more than 30 years ago, numerous case reports have been published, while larger case series have remained rare. This large body of information, if extrapolated, could address clinical discrepancies and expand our knowledge about the treatment of feline PF. This manuscript reviews cases of feline PF published between 1950 and 2016 and adds additional 35 original cases to provide further insight into the clinical aspect and treatment outcome of this disease.

RESULTS

Feline PF, while being a primary acantholytic pustular dermatosis, presents most often with crusts and erosions that predominantly affect the face and feet. More than half of cats with active disease exhibits non-dermatological signs such as lethargy, fever and/or anorexia. The prognosis of feline PF is good as the majority of cats rapidly achieve disease control even with the most basic treatment such as glucocorticoid monotherapy. Most PF-affected cats, however, require long-term treatment and, like other autoimmune diseases, feline PF has a tendency to relapse spontaneously or with treatment changes.

CONCLUSIONS

Therefore, despite the overall good prognosis cats with PF can be given, owners should be informed and prepared for these circumstances, which may reduce the risk of euthanasia in the case of disease relapse, and improve treatment compliance.

Current and Newly Emerging Autoimmune Diseases

There are many autoimmune diseases that are recognized in domestic animals. The descriptions of diseases provide examples of the magnitude of immune targets and the variable nature of autoimmune diseases. Other autoimmune diseases that are recognized in dogs, cats, and horses include immune-mediated thrombocytopenia, VKH (Vogt-Koyanagi-Harada) ocular disease (dogs), and Evans syndrome (which includes both immune-mediated anemia and immune-mediated thrombocytopenia).

An unresponsive progressive pustular and crusting dermatitis with acantholysis in nine cats

BACKGROUND

Between 2000 and 2012, nine cats were examined with a visually distinctive, progressive crusting dermatitis that was poorly responsive to all attempted therapies.

OBJECTIVES

Documentation of clinical and histopathological findings of this disease.

ANIMALS

Nine privately owned cats.

METHODS

Retrospective study.

RESULTS

Eight neutered males and one (presumably spayed) female ranging in age from two to eight years, presented for a progressive, well-demarcated, crusting dermatitis with variable pruritus of 1.5 months to five years duration. All cats lived in northern California, USA; seven lived within a 30 mile radius. Two males were littermates. Histopathological investigation showed both parakeratotic and orthokeratotic crusts, intraepidermal pustules and superficial folliculitis with rare to frequent acantholytic cells. Bacterial and fungal cultures were performed in six cats: meticillin-susceptible Staphylococcus pseudintermedius was isolated in three cats, two colonies of Trichophyton terrestre and three of Malassezia pachydermatis were isolated from one cat each. Treatment with various antibiotics, antifungal and a variety of immunosuppressive medications did not alter the progressive nature of the skin disease.

CONCLUSIONS AND CLINICAL IMPORTANCE

The described disease shares some clinical and histopathological features with pemphigus foliaceus, but the lack of response to treatment, its progressive nature and the possible relatedness of some of the cats set it apart. The aetiology of this acantholytic dermatitis remains unknown.

Clinical, microscopic and microbial characterization of exfoliative superficial pyoderma-associated epidermal collarettes in dogs

BACKGROUND

The microscopic and microbial features of the spreading epidermal collarettes of canine exfoliative superficial pyodermas are poorly characterized.

OBJECTIVES

To characterize the clinical, cytological, microbial and histopathological features of epidermal collarettes in five dogs.

RESULTS

Cytology from the margins of collarettes identified neutrophils, extracellular and intracellular cocci within neutrophils but no acantholytic keratinocytes. Phenotypic and genotypic analyses identified all bacterial isolates from the centre and margin of five epidermal collarettes as Staphylococcus pseudintermedius. PCRs of collarette-associated Staphylococcus strains did not amplify genes encoding for the known exfoliative toxins expA and expB, whereas the predicted siet and speta amplification products were detected in all isolates. Microscopically, epidermal collarettes consisted of interfollicular, epidermal spongiotic pustules. Advancing edges of lesions consisted of peripheral intracorneal clefts in the deep stratum disjunctum above an intact stratum compactum; they contained lytic neutrophil debris, bacterial cocci and fluid, but no acantholytic keratinocytes. This intracorneal location of bacteria was confirmed using Gram stains and fluorescent in situ hybridization with eubacterial- and Staphylococcus-specific probes. The indirect immunofluorescence staining patterns of desmoglein-1, desmocollin-1, claudin-1, E-cadherin and corneodesmosin were discontinuous and patchy in areas of spongiotic pustules, whereas only that of corneodesmosin was weaker and patchy in advancing collarette edges.

CONCLUSION

Epidermal collarettes represent unique clinical and histological lesions of exfoliative superficial pyodermas that are distinct from those of impetigo and superficial bacterial folliculitis. The characterization of possible causative staphylococcal exfoliatin proteases and the role of corneodesmosin in collarette pathogenesis deserve further investigation.

Canine pemphigus foliaceus with concurrent immune-mediated thrombocytopenia

A 3 yr old wirehaired fox terrier was presented to his primary care veterinarian with fever, thrombocytopenia, and generalized crusting dermatitis. The skin lesion had progressed for at least 18 days, and thrombocytopenia had developed 3 days before presentation. Histopathology and direct immunofluorescence studies of the skin were consistent with pemphigus foliaceus (PF). Immunofluorescence revealed immunoglobulin G deposition around the keratinocytes in the stratum spinosum. A diagnosis of immune-mediated thrombocytopenia (IMT) was confirmed by the presence of platelet surface-associated immunoglobulin using flow cytometry. Systemic immunosuppressive therapy with cyclosporine and azathioprine was effective, and the dog survived for >2 years from the initial presentation. IMT is rarely associated with PF. This appears to be the first detailed report of a definitive diagnosis of concurrent PF and IMT in a dog. The authors' findings indicate that canine PF could be complicated by hematologic immune-mediated diseases such as IMT.

Recognizing pyoderma: more difficult than it may seem

Although bacterial pyoderma is among the most commonly encountered dermatologic conditions in dogs, some cases present diagnostic challenges even to experienced clinicians. This article presents several unusual manifestations of pyoderma, including bullous impetigo, superficial spreading pyoderma, mucocutaneous pyoderma, and post-grooming furunculosis. Conditions mimicking pyoderma, including juvenile cellulitis, immunomodulatory-responsive lymphocytic-plasmacytic pododermatitis, and pemphigus foliaceus are also described. Diagnostic techniques used for diagnosing and characterizing pyoderma are also discussed.

Staphylococcal skin disease in livestock

This review covers cutaneous manifestations of staphylococcal infection in livestock species. CATTLE: In cattle, staphylococcal infections may present as folliculitis or as impetigo. Both may present as mild forms of a group of conditions loosely termed udder dermatitis, which has various clinical presentations and does not always involve staphylococci. GOATS: In goats, staphylococcal infection may be secondary to chorioptic mange or contagious pustular dermatitis (parapox virus infection). While Staphylococcus aureus is usually implicated, infection with Staphylococcus chromogenes and Staphylococcus hyicus have also been reported. SHEEP: Ovine staphylococcal dermatitis typically involves the head. Trauma due to the close contact of heads over feeding troughs and abrasive plants at pasture may be predisposing factors. PIGS: In pigs, the most common cause of staphylococcal skin disease is S. hyicus, although other bacteria, including Staphylococcus sciuri, Staphylococcus chromogenes and meticillin-resistant S. aureus (MRSA), have also been isolated from some cases of greasy pig disease (exudative epidermitis).

DIAGNOSIS

Routine culture methods are increasingly supplemented by molecular methods to characterize staphylococci.

MANAGEMENT

As commensal bacteria, staphylococcal skin infection is presumed to develop because of predisposing factors. While topical and systemic therapies can be effective, it is important to control for predisposing factors so that recurrences can be prevented. LIVESTOCK-ASSOCIATED MRSA: In recent years, MRSA strains have emerged, particularly in pigs and cattle. While they rarely cause skin disease, they do pose a significant concern for public health authorities. Studies on livestock-associated MRSA may help to progress our understanding of staphylococci in livestock, especially how they spread between animals and humans.

Deficient plakophilin-1 expression due to a mutation in PKP1 causes ectodermal dysplasia-skin fragility syndrome in Chesapeake Bay retriever dogs

In humans, congenital and hereditary skin diseases associated with epidermal cell-cell separation (acantholysis) are very rare, and spontaneous animal models of these diseases are exceptional. Our objectives are to report a novel congenital acantholytic dermatosis that developed in Chesapeake Bay retriever dogs. Nine affected puppies in four different litters were born to eight closely related clinically normal dogs. The disease transmission was consistent with an autosomal recessive mode of inheritance. Clinical signs occurred immediately after birth with superficial epidermal layers sloughing upon pressure. At three month of age, dogs exhibited recurrent superficial skin sloughing and erosions at areas of friction and mucocutaneous junctions; their coat was also finer than normal and there were patches of partial hair loss. At birth, histopathology revealed severe suprabasal acantholysis, which became less severe with ageing. Electron microscopy demonstrated a reduced number of partially formed desmosomes with detached and aggregated keratin intermediate filaments. Immunostaining for desmosomal adhesion molecules revealed a complete lack of staining for plakophilin-1 and anomalies in the distribution of desmoplakin and keratins 10 and 14. Sequencing revealed a homozygous splice donor site mutation within the first intron of PKP1 resulting in a premature stop codon, thereby explaining the inability to detect plakophilin-1 in the skin. Altogether, the clinical and pathological findings, along with the PKP1 mutation, were consistent with the diagnosis of ectodermal dysplasia-skin fragility syndrome with plakophilin-1 deficiency. This is the first occurrence of ectodermal dysplasia-skin fragility syndrome in an animal species. Controlled mating of carrier dogs would yield puppies that could, in theory, be tested for gene therapy of this rare but severe skin disease of children.

Acantholytic folliculitis and epidermitis associated with Staphylococcus hyicus in a line of white Leghorn laying chickens

Several mature Leghorn-type hens with the same genetic background experienced skin and feather problems in a breeder flock. There was almost-total feather loss on the head and neck, as well as thickened, scaly skin, and follicular ostia were plugged with keratin debris. Other individuals exhibited prominent subcutaneous nodules multifocally on the head. Histologic examination of the skin revealed a severe hyperplasia of follicular epithelium with hyperkeratosis and cystic dilation. Numerous clefts and vesicles were detected along the epidermis and follicular epithelium, some containing acantholytic keratinocytes. A mild heterophilic inflammation was associated with these lesions, and few gram-positive cocci were present in the keratin plugs. Bacterial culture of the skin yielded a variable amount of Staphylococcus hyicus. Immunochemistry looking for chicken IgY revealed no intercellular staining in the epidermis or follicular epithelium. All these findings supported a diagnosis of Staphylococcus-associated acantholytic epidermitis and folliculitis. This case suggests that S. hyicus could be a significant pathogen in poultry production. The close genetic relationship among affected individuals could indicate a hereditary predisposition in this line of White Leghorn laying chickens.

Staphylococcus pseudintermedius exfoliative toxin EXI selectively digests canine desmoglein 1 and causes subcorneal clefts in canine epidermis

Staphylococcal exfoliative toxins are known to digest desmoglein (Dsg) 1, a desmosomal cell-cell adhesion molecule, thus causing intraepidermal splitting in human bullous impetigo, staphylococcal scalded skin syndrome and swine exudative epidermitis. Recently, a novel exfoliative toxin gene (exi), whose sequence shares significant homology with previously identified exfoliative toxins, was isolated from Staphylococcus pseudintermedius. Little is known about the pathogenic involvement of this toxin in canine pustular diseases such as impetigo. The aim of this study was to determine whether EXI, the product of the exi gene, digests canine Dsg1 and causes intraepidermal splitting in canine skin. An exi gene was isolated from chromosomal DNA of an S. pseudintermedius strain obtained from a pustule of a dog with impetigo, and was used to produce a recombinant EXI by Escherichia coli expression. When purified recombinant EXI was injected intradermally into normal dogs, it caused the development of vesicles or erosions with superficial epidermal splitting. In addition, the EXI abolished immunofluorescence for Dsg1, but not for Dsg3, at the injection sites. Moreover, the EXI directly degraded baculovirus-secreted recombinant extracellular domains of canine Dsg1, but not that of canine Dsg3, in vitro. The EXI also degraded mouse Dsg1α and swine Dsg1, but not human Dsg1, mouse Dsg1β and Dsg1γ. Conversely, recombinant SIET, previously designated as S. intermedius exfoliative toxin, did not cause intraepidermal splitting or degradation of any Dsgs. These findings indicate that EXI has a proteolytic activity that digests canine Dsg1, and this characteristic might be involved in the pathogenesis of intraepidermal splitting in canine impetigo.