JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

November 2025

I-V12A

Signalment: Dog, breed and age unspecified.

HISTORY: This tissue is from the footpad and nose of a dog that had anorexia, diarrhea, and pneumonia. Central nervous signs were seen agonally.

SLIDE A (JPC# ML 21474-30)

HISTOPATHOLOGIC DESCRIPTION: Junction of haired skin and footpad: Diffusely the footpad epidermis and the adjacent haired skin is characterized by marked parakeratotic hyperkeratosis (up to 1.5 mm thick), irregular acanthosis, and hyperplasia of the basal layer with frequent mitoses. Keratinocytes frequently contain multiple, round to irregular, 4‑15µm, homogeneous, eosinophilic, intracytoplasmic and rarely similar intranuclear viral inclusions. There are rare multinucleated epithelial cells with up to 6 nuclei (viral syncytial cells). Within primarily the stratum spinosum, there are many swollen keratinocytes with vacuolated cytoplasm (intracellular edema). Within the underlying dermal papillae and superficial dermis, there is occasional free melanin and melanin laden macrophages (pigmentary incontinence).

MORPHOLOGIC DIAGNOSIS: Skin, footpad: Hyperkeratosis, parakeratotic, diffuse, moderate, with epithelial cell viral syncytia and intracytoplasmic and intranuclear viral inclusion bodies, breed unspecified, canine.

SLIDE B (JPC# ML-21474-31)

HISTOPATHOLOGIC DESCRIPTION: Mucocutaneous junction, nasal planum: Epidermal changes are similar to those in the footpad. There are rare intracorneal pustules containing few necrotic neutrophils and eosinophilic homogeneous fluid (serum). There are few superficial cocci.

MORPHOLOGIC DIAGNOSIS: Mucocutaneous junction, nasal planum, epidermis: Hyperkeratosis, parakeratotic, diffuse, moderate, with epithelial viral syncytia, intracytoplasmic and intranuclear eosinophilic viral inclusions, and few intracorneal pustules, breed unspecified, canine.

ETIOLOGIC DIAGNOSIS: Morbilliviral footpad hyperkeratosis

CAUSE: Canine morbillivirus (canine distemper virus [CDV])

CONDITION: Canine distemper

SYNONYMS: Hard pad disease, Carre's Disease

GENERAL DISCUSSION:

• Canine distemper is an important, ubiquitous infectious disease of terrestrial carnivores (Canidae, Mustelidae, Procyonidae), ferrets are particularly susceptible
• Systemic disease often with respiratory, gastrointestinal, and central nervous system (CNS) involvement (see N-V11, P-V01, U-V07)
• CDV is a pantropic, negative-sense, single-stranded, enveloped RNA virus, 150-300 nm diameter, genus Morbillivirus, family Paramyxoviridae
• One recognized serotype; variable strain pathogenicity and tissue tropism
• Vaccination with a modified live vaccine is not recommended in non-domestic carnivores as it can induce a CDV infection (Neupane et al, J Vet Diagn Invest 2023)

PATHOGENESIS:

• Virulence factors: Attachment protein (viral H protein) on viral envelope mediate attachment to host cells; fusion protein (viral F protein) allow penetration of host cells and fusion of infected cells with uninfected cells
• Receptors:
  • Signaling lymphocyte activation molecule (SLAM, CD150): Leukocyte-restricted, mediates entry into lymphocytes, monocytes, and macrophages
    • expression of SLAM is exclusive to activated T and B lymphocytes, mature dendritic cells, macrophages, thymocytes, and platelets in humans and mice
    • serves as a co-stimulatory molecule to initiate signal transduction networks in natural killer cells, T cells, and antigen-presenting cells
  • CD46: Widespread
  • Nectin-4: Epithelial receptor in stomach, small intestine, lung and CNS (excluding astrocytes)
    • contains Ig-like domains similar to SLAM; essential in cell-to-cell adhesions and in the regulation of cell polarization, differentiation, movement, proliferation, and survival
    • virus binds to nectin-4 receptors on the basolateral surface to enter epithelial cells
  • Host CD9 transmembrane protein required for viral syncytial fusion (characteristically in the lungs)
• Inhalation or ingestions > virus infects macrophages in upper respiratory tract or lungs > viral replication in tonsils and lymph nodes in first 24 hours > cell-associated viremia by 2 days PI > spread to all lymphoid tissues and blood lymphocytes by 2-5 days PI > lymphocytolysis and leukopenia > immunosuppression > clinical course depends on host immune response:
  • Adequate humoral / cellular immunity: Neutralize virus by 14 days PI; may not shed virus
  • Delayed / intermediate humoral / cellular immunity: Viral infection / persistence in mucosal epithelium and brain (vascular endothelium) > may develop neurologic disease +/- disease associated with epithelial infection
  • Failure to develop neutralizing antibody by 8- or 9-days PI: Virus disseminates to respiratory, GI, urogenital, and central nervous systems; integumentary, exocrine, and endocrine systems also affected > virus shed in all excretions during the systemic phase of infection > secondary infections common
• Lymphocyte depletion by apoptosis during infection is common in morbillivirus infections (induced by capsasae-3)
• Common secondary infections: Adenovirus, Bordetella sp., Clostridium piliforme, Cryptosporidium sp., E. coli, Encephalitozoon sp., Pneumocystis sp., Sarcocystis sp., and Toxoplasma sp.

TYPICAL CLINICAL FINDINGS:

• Disease most common in 12–16-week-old puppies (waning passive immunity)
• Early: Fever, conjunctivitis -> cough, vomiting, diarrhea, depression, anorexia, serous to mucopurulent oculonasal discharge
• Clinical pathology: Lymphopenia, thrombocytopenia, regenerative anemia, hypoalbuminemia, hypergamma- and alpha-globulinemia
• Later (1-4 weeks): Neurologic disease (seizures, cerebellar or vestibular ataxia, paraparesis, myoclonus); may see minimal or no signs of epithelial infection and only neurologic signs in some dogs
• Late manifestations: Hyperkeratosis of footpads and nose, enamel hypoplasia
• Other forms of neurologic disease include multifocal distemper encephalomyelitis, postvaccinal encephalitis and “old dog encephalitis”

TYPICAL GROSS FINDINGS:

• Integument: Hyperkeratosis of footpads and nose; +/- secondary pyoderma (pustular dermatitis, especially ventral abdomen)
• Lungs: Patchy red-tan, rubbery subpleural and marginal lesions (bronchointerstitial pneumonia); may be edematous and consolidated (secondary bronchopneumonia)
• Eyes/conjunctiva: Conjunctivitis, keratitis, retinitis, optic neuritis
• Lymphoid tissues: Tonsillar enlargement, thymic atrophy
• Teeth: Enamel hypoplasia
• Bone: Impaired osteoclastic resorption of the primary spongiosa (necrotic osteoclast) > growth retardation lattice (sclerotic line parallel to the physis)

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Eosinophilic intracytoplasmic (IC) and/or intranuclear (IN) inclusion bodies:
  • Most numerous 10-14 days PI
  • Most obvious in brain (often INIBs) and epithelium (usually ICIBs), especially in urinary bladder (U-V07); less obvious in lymphoid tissues
• Integument: Orthokeratotic and/or parakeratotic hyperkeratosis of footpad, nose, rarely haired skin
  • +/- epidermal hyperplasia and hydropic degeneration
  • +/- INIBs / ICIBs in epithelial cells and syncytia
  • +/- secondary pustular dermatitis (pyoderma)
• Lungs: Bronchointerstitial pneumonia (P-V01)
  • Patchy bronchiolar necrosis/attenuation
  • IBs are most obvious in bronchial/bronchiolar epithelium (persist longer than in neural tissues)
  • Scant suppurative exudate in airways
  • Alveoli: Filled with edema, fibrin, mononuclear cells, necrotic epithelium; type II pneumocyte hyperplasia; septa expanded by mononuclear cells; alveolar epithelial syncytial cells
• Central nervous system: (N-V11)
  • White matter: Demyelination of white matter tracts (most severe in cerebellum, rostral medullary velum, optic tracts, spinal cord, surrounding the fourth ventricle)
    • LS spinal cord most consistently affected with lesions in the lateral funiculi most common, followed by lesions in dorsal funiculi; demyelination, astrocytosis, microgliosis consistent findings, less often gemistocytes and suppurative inflammation (Areco et al, J Comp Path 2021)
  • Gray matter (lesions less common than white matter lesions): INIBs +/- ICIBs in neurons -> neuronal necrosis -> mononuclear infiltrate surrounding necrotic neurons -> nonsuppurative perivascular encephalitis +/- mild meningitis
  • Old-dog encephalitis: INIBs in astrocytes and neurons; nonsuppurative perivascular encephalitis; neuronal necrosis; viral antigen detectable by IHC, but cannot isolate virus from the brain
• Eyes: Early conjunctivitis +/- ulcerative keratitis -> later retinal edema, ganglion cell degeneration, photoreceptor loss, perivacular mononuclear inflammation, INIBs in ganglion cells and glial cells-> neuronal loss, retinal scarring, retinal pigment epithelium proliferation; optic nerve – early papilledema -> later gliosis and demyelinating neuritis
• Lymphoid tissues: Early lymphoid depletion and lymphocytic necrosis -> syncytia +/- IBs -> histiocytic cell hyperplasia
• Teeth: Cystic degeneration of ameloblastic epithelium, syncytia and ICIBs -> enamel defects / hypoplasia
• Other epithelia (urinary bladder; renal pelvis; gastric surface, chief, and parietal cells; cholangiolar epithelium; pancreatic ductular epithelium; epididymis; testis): Degeneration, mononuclear infiltrates, ICIBs +/- INIBs
• Heart: Myocarditis with myocardial necrosis and mineralization (Kim et al, J Vet Diagn Invest 2021)
• Bone (experimental): Osteoclast necrosis -> persistent primary spongiosa (growth retardation lattice)

ADDITIONAL DIAGNOSTIC TESTS:

• Virus isolation, immunohistochemistry (IHC), fluorescent antibody test
• IHC on skin lesions shows CDV antigen at all levels of the epidermis but most abundant in stratum spinosum, granulosum, and in epithelium of eccrine sweat glands; rare in basal layer
• Viral antigen expression most common in sweat glands, followed by epidermis and vascular endothelium (Areco et al, J Comp Path 2022)
• Cytology (blood smear): Viral inclusions can be seen early in infection in erythrocytes and leukocytes; inclusions are irregular to round to ring-shaped, 1-6 um diameter, intracytoplasmic, and stain blue grey on modified Wright-stained blood film and stain eosinophilic/magenta on Romanowsky-type stains (including Diff-Quik)

DIFFERENTIAL DIAGNOSIS (for cutaneous lesions):

• Idiopathic nasodigital hyperkeratosis
• Immune mediated diseases (pemphigus vulgaris, bullous pemphigoid, lupus erythematosus, toxic epidermal necrolysis): Lack many systemic signs, more widespread skin lesions
• Parakeratosis in dogs:
  • Thallium toxicosis (I-T01): Swollen paws may resemble hard pad disease, but pattern of cutaneous lesions is distinct (begin at lip commissures or nasal cleft (or ear margins) > expand over face and head > later involve interdigital skin, footpads, axillae, inguinal areas, perineum, lateral extensor surfaces)
  • Superficial necrolytic dermatitis (I-M16, hepatocutaneous syndrome): Histologic lesions virtually pathognomonic [red (parakeratotic hyperkeratosis), white (superficial epidermal edema and acanthosis), and blue (basal cell hyperplasia)]
  • Zinc-responsive dermatosis (I-M18): Two clinical syndromes [syndrome I = inherited (Alaskan malamutes and Siberian huskies); syndrome II = rapidly growing, large breed dogs with relative dietary Zn deficiency]
  • Generic dog food dermatosis: Deficiency of multiple trace minerals, vitamins, and amino acids; rapid onset; systemic illness is common
  • Lethal acrodermatitis of bull terriers: Autosomal recessive inheritance; pathogenesis of low plasma Zn disease unknown but copper deficiency may play a role; growth retardation, progressive dermatopathy, paronychia, diarrhea, bronchopneumonia, death by 18 months of age; thymic and lymph node hypoplasia and giant cell bronchopneumonia are important findings
  • Vitamin A responsive dermatosis: Lesions primarily in chest and abdomen; histologically there is marked follicular keratosis
  • Sarcoptic mange (I-P06) and flea-hypersensitivity dermatitis may also have diffuse parakeratosis, but usually many more lymphocytes, eosinophils, plasma cells, and/or mast cells are present

COMPARATIVE PATHOLOGY:

• Other species affected:
  • Canidae – dingo, fox, coyote, jackal, wolf
    • Gray foxes with listeriosis often co-infected with CDV (Weyna et al, J Vet Diagn Invest 2022)
    • Vaccine induced co-infection with CDV and Sarcocystis in a gray fox (Neupane et al, J Vet Diagn Invest 2023); CDV infection as primary causing immunosuppression allowing for proliferation of concurrent Sarcocystis CNS infection and Histoplasma lung infection
      • Concurrent S. neurona and CDV infection has been reported in 4 raccoons and a skunk w/ severe severe cavitating necrosis with numerous eosinophils, abundant mineralization, many merozoites and meronts, and ICIBs and INIBs in glial cells
    • Dual infection with America-4 strain CDV and CPV-2b documented in an Arctic wolf- Lymphoid and hematopoietic necrosis, failure of endochondral ossification, and IN and IC inclusions
    • Co-infection with Clostridium piliforme and CDV documented in puppies and a gray fox kit (Jacobson et al, J Vet Diagn Invest 2022)
  • Procyonidae – raccoon, coati
    • Cryptosporidiosis co-infection with CDV common in raccoons (Gonzalez-Astullido et al, J Vet Diagn Invest 2021)
  • Felidae – lions, leopards, cheetahs, tigers, civets
    • Large felids: Fatal neurologic disease due to a distinct CDV variant
    • African lions contract the virus from hyenas and feral dogs
    • Hard pads are rare – CNS signs most common
  • Phocids (e.g. Baikal seals) susceptible to CDV and phocine distemper virus
    • California seal lions might be less susceptible than other phocid species (Gonzales-Viera et al, Vet Pathol 2024)
  • Tayassuidae (peccary), mustelids (black footed ferrets esp. susceptible- Can be fatal following MLV), ursidae (bear/panda), viverridae (civets)
• Other morbilliviruses:
  • Rinderpest (RP, D-V28): Primarily large ruminants – oral mucosal erosions, diarrhea
  • Pest de Petits Ruminants (PPRV, P-V04): Small ruminants – respiratory signs, oral mucosal erosions, diarrhea
  • Cetacean morbillivirus (P-V03, dolphin morbillivirus and porpoise morbilliviruses are two strains of the same species): Closely related to RP and PPRV (Wessels et al J Comp Path 2021)
  • Phocine distemper virus (N-V12, PDV): Closely related to CDV
  • Measles virus: Humans and non-human primates

References:

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