JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

September 2025

I-N25A

SLIDE A

Signalment (JPC# 2017869): Horse

HISTORY: This horse had a dermal mass

HISTOPATHOLOGIC DESCRIPTION (Slide A): Haired skin: Expanding and effacing the deep dermis, infiltrating the subcutis, elevating the epidermis, and extending to cut margins is an unencapsulated, well demarcated, densely cellular neoplasm composed of sheets of round cells on a moderate pre-existing collagenous stroma. Neoplastic cells have indistinct cell borders, a scant amount of eosinophilic cytoplasm, and round to ovoid nuclei with finely stippled chromatin and 1-3 variably distinct nucleoli. Anisocytosis and anisokaryosis are moderate, with less than 1 mitotic figure per 2.37 mm². There is multifocal individual cell necrosis. Admixed with neoplastic cells are many small, reactive lymphocytes and large histiocytes up to 20 µm in diameter. Histiocytes have a moderate amount of eosinophilic cytoplasm and occasionally contain intracytoplasmic, variably sized, up to 1x7 µm, eosinophilic, acicular crystals.

MORPHOLOGIC DIAGNOSIS: Haired skin, site not specified: Lymphoma, nonepitheliotropic, breed not specified, equine.

SLIDE B

Signalment (JPC# 2604555): Ox

HISTORY: This ox had multiple, 1-3 cm diameter, cutaneous bumps

HISTOPATHOLOGIC DESCRIPTION: Haired skin: Infiltrating the dermis, separating and surrounding adnexa and pre-existing collagen bundles, elevating the focally eroded epidermis, and occasionally infiltrating into the epidermis and follicular epithelium is an unencapsulated, poorly circumscribed, densely cellular neoplasm composed of sheets of round cells. Neoplastic cells have indistinct cell borders and scant eosinophilic granular cytoplasm. Nuclei are round to ovoid with coarsely-stippled chromatin and 1-4 distinct nucleoli. The mitotic count is up to 4 per individual HPF (0.237mm²). There is multifocal individual cell necrosis. Multifocally, neoplastic cells infiltrate the follicular epithelium and epidermis forming Pautrier’s microabscesses. Within the superficial dermis, there is multifocal moderate hemorrhage admixed with fibrin and edema. Overlying the epidermis and occasionally extending into follicular lumina is a 500µm thick serocellular crust composed of many degenerate neutrophils admixed with abundant serum, necrotic debris, and multifocal colonies of 1µm diameter cocci. Multifocal apocrine glands adjacent to the neoplasm are mildly ectatic.

MORPHOLOGIC DIAGNOSIS: Haired skin, site not specified: Lymphoma, epitheliotropic, breed not specified, bovine.

GENERAL DISCUSSION:

• General:
  • Lymphomas are neoplasms of the hematopoietic system; can arise in lymph nodes or extranodal
  • Vast majority of lymphomas in domestic animals closely resemble non-Hodgkin lymphomas in humans; simply referred to as lymphoma or malignant lymphoma
    • “Lymphosarcoma” has also been used in veterinary medicine, but should be avoided especially for comparative reasons
    • Historically, lymphoid leukemias and lymphomas are separate entities
  • Current WHO classification of lymphomas based on cell morphology, cell immunophenotype (e.g. B-cell or T-cell), clinical characteristics, histological pattern, cell size, tissue tropism, and where neoplastic cells invade (e.g. cutaneous T-cell lymphoma [CTCL], further subdivided into Epitheliotropic CTCL [I-N26] vs Nonepitheliotropic CTCL [NECTCL])
• Equine Lymphomas:
  • Lymphoma is the most common malignant neoplasm of the horse. Frequently presents as multicentric (2 or more organs), cutaneous, or gastrointestinal
  • T-cell-rich [large] B-cell lymphoma (TCRLBCL, TCRBCL, or TCRBL) is the most common lymphoma in the horse
    • Subvariant of diffuse large B-cell lymphoma (DLBCL)
    • Often multicentric with abdominal or cutaneous masses that may wax and wane for years before progressing to more aggressive disseminated DLBCLs; typically without lymphadenomegaly
    • SQ masses in the skin often the primary clinical presentation
    • In horses, there is often a large histiocyte component, so this can also be referred to as T-cell/histiocytic-rich large B-cell lymphoma (T/HRLBL)
  • Cutaneous: Different sources cite Peripheral T-cell lymphoma (PTCL) or Epitheliotropic CTCL as the second most common; DLBCLs are also reported; equine epitheliotropic CTCL is similar to human/dog
  • Gastrointestinal: EATL type 2 is the second most common GIT neoplasm
    • • Intestinal tumor of intraepithelial lymphocytes
      • Subdivided into EATL type 1 (large cell neoplasms; often associated with necrosis and an inflammatory background) and EATL type 2 (small to intermediate cell neoplasms; have no inflammatory background and rare necrosis)
• Bovine Lymphomas:
  • Lymphoma categorized by frequency of occurrence (sporadic or endemic/enzootic), age at onset, organ system involved, and etiologic agent; broadly classified into enzootic and sporadic forms:
  • Enzootic Bovine Leukosis (EBL):
    • Secondary to Bovine Leukemia Virus (BLV)
    • B-cell origin; most common lymphoma in cattle
    • Multicentric: Involves multiple organ systems (abomasum, uterus, heart); classic distribution recently challenged (Mendes et al, JVDI, 2025) with greater incidence noted in kidney, lymph node, and intestinal wall versus retrobulbar or spinal cord.
    • High incidence in older (4-8 yrs) dairy cattle
  • Sporadic Bovine Leukosis (SBL):
    • Non-BLV-associated lymphoma; most often T-cell lymphoma; younger cattle (1-3 yrs); less common than EBL
    • 3 forms present:
      • Juvenile Multicentric Form / Calf Form: Generalized marked lymphadenomegaly +/- leukemia +/- other organ involvement that develops within first 6 months of life
      • Cutaneous / Skin Form: Cutaneous lesions in 2-3 year old cattle; least common type of bovine lymphoma
      • Thymic Form: Cattle <2 years of age

PATHOGENESIS:

• General:
  • Spontaneous
  • Retroviral: Feline Leukemia Virus and Bovine Leukemia Virus
• Equine:
  • Spontaneous
  • TCRBCL: Speculated that neoplastic B cells produce cytokines that lead to infiltrates of non-neoplastic T cells and +/- other leukocytes
• Bovine:
  • EBL (BLV-associated):
    • BLV-infected lymphocytes are transmitted horizontally (e.g. blood, milk/colostrum, saliva, breeding; blood-sucking arthropods) or iatrogenically (e.g. rectal sleeves, instruments/equipment)
      • BLV does not persist outside the animal, so contact or environmental contamination is not a source of infection
    • BLV lacks a known oncogene and does not integrate into a preferred site in the bovine genome
    • Multiple genes associated with lymphomagenesis:
      • Tax, an activator gene, is important in viral gene activation and modulates expression of other genes, and leads to immortalized CD5-positive, IgM-positive B cells and polyclonal expansion
      • p53 gene suppression
      • Polymorphism in promoter region of the tumor necrosis factor-α (TNF-α)
      • Bovine leukocyte antigen (BoLA) DRB3 polymorphism and methylation of Bone morphogenetic protein 6 (BMP6) gene play a role in host susceptibility
    • Following BLV infection, virus invades/integrates into the genome of infected B lymphocytes -> results in polyclonal B lymphocyte lymphocytosis (persistent lymphocytosis (PL)) in ~30% of cattle; lymphoma in ~5% of cattle
    • Average incubation is 7 to 8 years
    • BLV microRNAs (miRNAs), identified in pre-leukemic and malignant B lymphocytes, showed repression of structural and regulatory gene expression, suggesting miRNAs may play a key role in tumor progression
  • SBL (non-BLV-associated): Unknown

TYPICAL CLINICAL FINDINGS:

• General: CTCL presents as cutaneous or subcutaneous plaques or nodules +/- alopecia, +/- ulceration
• Equine:
  • Lymphoma mostly affects adult animals (10-11 years) with no breed or sex predisposition
    • Quarter Horses and Thoroughbreds most commonly develop multicentric lymphomas or cutaneous lymphoma (PTCL or Epitheliotropic CTCL); Standardbreds have higher incident of GI lymphoma (EATL 2) and rarely develop cutaneous lymphoma
  • General wasting disease with anorexia and depression and ventral edema +/- hypoproteinemia
  • Pyrexia and anemia are common; and diarrhea and colic can occur with any form (more prevalent in lymphomas involving GIT)
  • May present with severe generalized pruritus +/- gross or histologic cutaneous lesions
• Bovine:
  • EBL (BLV-associated):
    • Seen in older cattle usually between 4-8 years of age
    • Higher prevalence in dairy cattle
    • Commonly presents as multiple enlarged lymph nodes anywhere in the body with clinical signs depending on organ involvement:
      • Retrobulbar lymph nodes: Proptosis (unilateral or bilateral)
      • Pharyngeal lymph nodes: Dysphagia, stertorous respiration
      • Sublumbar (to spinal cord) lymph nodes, epidural fat, or perineural fat involvement: CNS signs, including lameness -> weakness and paraplegia
      • Mesenteric lymph nodes: Obstruction
      • Abomasal lymphoma: Vagal indigestion or diarrhea; ulceration -> hemorrhagic anemia
      • Myocardial lymphoma: Congestive heart failure
  • SBL (non-BLV-associated):
    • Younger cattle (1-3 yrs old)
    • Juvenile Multicentric Form: Develops within first 6 months of life (may be present at birth); generalized lymphadenomegaly with marked leukemia; myelophthisis (bone marrow failure) leading to nonresponsive, normochromic normocytic anemia, neutropenia, and thrombocytopenia
    • Cutaneous / Skin form: May be cutaneous nodules or multicentric
    • Thymic Form: Large mediastinal mass that causes presternal swelling; jugular distention and local edema; esophageal compression; lung displacement +/- respiratory distress

TYPICAL GROSS FINDINGS:

• Equine:
  • TCRBCL:
    • Multicentric lymphoma is the most common presentation and usually occurs as abdominal and/or thoracic cavity masses +/- multiple subcutaneous, non-ulcerative nodules on the trunk or limbs
      • Usually, no generalized lymphadenopathy
      • Organs affected may include the pancreas, thyroid gland, spinal cord, mediastinal nodes, liver, spleen, and/or the gastrointestinal tract
    • Subcutaneous nodules: Numerous 1-3 cm subcutaneous masses, especially on the thorax, flanks, perineum, face, neck (spares the limbs); round, flattened, and often appear to follow lymphatics
    • GIT: Large, solitary intestinal masses
  • PTCL: Can occur in lymph nodes, spleen, and extranodal tissues (skin or subcutis)
  • Epitheliotropic CTCL: Generalized exfoliative dermatitis (alopecia, scaling, crusting), +/- nodules and ulceration (especially over pressure points)
  • EATL: Typically a uniform mural thickening of the intestinal wall, but can also present as nodular masses; may affect small intestines, large intestine, or both; +/- mucosal erosion or ulceration and mesenteric lymph node enlargement
• Bovine:
  • EBL (BLV-associated):
    • Generally multiple enlarged lymph nodes (may be identified by rectal palpation) +/- other organ involvement, including abomasum, heart, spinal cord, kidney, or uterus, less commonly liver or spleen
    • May present as diffuse organ thickening, multifocal to coalescing nodules, or large masses
    • Abomasal lymphoma: Diffuse thickening of abomasal submucosa +/- ulceration
    • Perineural lymphoma: Slight fleshy pinkness of perineural fat (distinguishes it from normal fat)
  • SBL (non-BLV-associated):
    • Juvenile Multicentric Form / Calf Form: Generalized lymphadenomegaly +/- diffuse organ thickening; +/- bone marrow involvement (with marrow infarction); kidneys usually diffusely involved, +/- spleen, liver involvement
    • Cutaneous / Skin Form: Plaque-like, round, raised, alopecic lesions on head, sides, and perineum +/- ulceration
      • 12-18 mos later, progresses to deep organ involvement that can be indistinguishable from EBL (aside from age of presentation)
    • Thymic Form: See Clinical Presentation

TYPICAL LIGHT MICROSCOPIC FINDINGS

• Equine:
  • TCRBCL: Sheets of small, reactive T-cells admixed with histiocytes and eosinophils and large B-cells that are usually <10% of the cell population
    • Large B-cells resemble centroblasts, are round to stellate, and have a moderate amount of eosinophilic cytoplasm, and have a pale vesiculate nucleus 2-3x the diameter of an equine RBC; cells are often binucleated and have prominent eosinophilic nucleoli; high mitotic rate with atypical mitotic figures
    • Non-neoplastic (reactive) small round, mature lymphocytes (T-cells) with a basophilic granular nucleus and little cytoplasm; nuclei are approximately the size of an equine RBC
    • Variable numbers of histiocytes, epithelioid macrophages and occasionally multinucleated giant cells, and eosinophils
  • PTCL: Sheets of neoplastic cells that efface tissue; cells are large with nuclei ~2x an equine RBC
  • Epitheliotropic CTCL: Lymphocytes with a strong tropism for epidermis and adnexa; intermediate-size with round, hyperchromatic, convoluted nucleus, indistinct nucleolus
    • Mycosis fungoides (patches or plaques of neoplastic cells within the skin) with Pautrier’s microabscesses (T cells invading the epidermis that form cavities filled with lymphocytes); may progress to Sézary syndrome (generalized stage with secondary lymphadenopathy and leukemia)
  • EATL: Lymphocytes infiltrate mucosa, submucosa, or transmural; may be large lymphocytes (EATL 1) or small lymphocytes (EATL 2)
    • Epitheliotropism with nests of intraepithelial lymphocytes is an important feature in the diagnosis of EATL
• Bovine:
  • EBL (BLV-associated):
    • Most are high-grade lymphomas with diffuse large neoplastic B-cell; less common are intermediate cell lymphomas
  • SBL (non-BLV-associated): Sheets of neoplastic T-lymphocytes
    • Cutaneous / Skin Form: Often progresses to mycosis fungoides with Pautrier’s microabscesses; may progress to Sézary syndrome

ADDITIONAL DIAGNOSTIC TESTS:

• Immunohistochemistry:
  • B cells: CD20, CD79a, BLA36
    • BLA36 also stains histiocytes, so use with caution in with neoplasms that have inflammatory cell infiltrates
  • T cells: CD3, 80% of epitheliotropic lymphomas in domestic animals express CD8 and the γδ T-cell receptor
• CBC: Lymphocyte count coupled with age has been used as a way to hematologically screen for lymphocytosis associated with BLV infection
• Antibody testing
  • AGID (agarose gel immunodiffusion) and ELISA tests to detect BLV antibodies to ID latent cattle with BLV infection (used in maintenance of BLV-free herds)
• PCR
  • Nested and/or quantitative PCR can be used to ID BLV genetic material in EBL
  • Droplet digital PCR (ddPCR) canbe used for ID of BLV DNA provirus with higher accuracy and sensitivity than antibody testing
  • PCR and genetic analysis has been used to ID highly pathogenic strains of BLV as well as increased host susceptibility

DIFFERENTIAL DIAGNOSIS:

• Gross:
  • Equine cutaneous lymphoma: Urticaria; drug eruptions; multiple collagenolytic granulomas; multiple hypodermal lesions; cutaneous amyloidosis; cutaneous pseudolymphoma
  • Bovine cutaneous lymphoma: Urticaria; drug eruptions; multiple hypodermal lesions; cutaneous pseudolymphoma
• Microscopic:
  • Inflammation: Differentiation between benign inflammatory lesions and early cutaneous lymphoma can be extremely difficult
  • Cutaneous pseudolymphomas: Benign reactive proliferations of lymphocytes that mimic cutaneous lymphomas histologically and clinically

COMPARATIVE PATHOLOGY:

• TCRBCL
  • Reported in dogs, cats, pigs, and horses
  • The most common nodal lymphoma in cats; also referred to as feline Hodgkin-like lymphoma; causes indolent unilateral neoplasm of the cervical lymph nodes, which spreads slowly to adjacent nodes within the chain -> can become multicentric (called Multicentric lymphoma); often FeLV positive
  • Rarely reported in primates (one report ring-tailed lemur and one report in a Japanese macaque)
  • Recent case report in the brain of a horse that was distinguished from inflammation via PARR (Rissi et al, JVDI, 2023)
• PTCL: “Basket”-diagnosis for all nonspecific mature T-cell lymphomas; causes ~20% of nodal lymphomas in dogs
• CTCL
  • Epitheliotropic CTCL: Seen in dogs, cats, and horses
  • Nonepitheliotropic CTCL: Uncommonly seen in dogs
• EATL
  • EATL 1: Commonly seen in older dogs (transmural); seen in older cats as well
  • EATL 2: Commonly seen in older cats (mucosal, with epitheliotropism)
• Retrovirus-induced lymphoma
  • BLV
    • Cause of most spontaneous lymphomas of sheep and a common cause of lymphoma in goats; similar clinical presentation as is seen in cattle
    • Lymphoma has been induced in situ in an alpaca using BLV
  • FeLV: Feline retroviral lymphoma (H-N02) (see above)
  • Murine Leukemia Virus (MuLV)
    • Can induce T-cell thymic lymphoma in mice within 6-12 months of age; most infections are phenotypically silent or can lead to non-neoplastic diseases such as altered hair coat or CNS disease (e.g. hind-limb paralysis)
  • Avian Leukosis Virus (ALV)
    • Alpharetrovirus in chickens that causes Lymphoid Leukosis (LL) or Myeloid Leukosis; LL is an uncommon neoplasia of the bursa of Fabricius with metastasis to other internal organs (liver, spleen, and kidney)
    • Less common than Marek’s Disease (I-V13), a herpesvirus that can induce multicentric lymphoma in chickens and other fowl
  • Gibbon ape leukemia virus (GALV): Gammaretrovirus; causes latency (up to years) followed by disease and neoplasia (malignant lymphoma, lymphoblastic leukemia, myelogenous (granulocytic) leukemia and “osteopetrosis” of long bones) in gibbons
  • Koala retrovirus A (KoRV A): Gammaretrovirus, widespread in Australia, plays a role in immunomodulation and possibly linked with lymphosarcoma, myeloid leukemia, aplastic anemia, myelodysplasia, and immune dysfunction in Koalas
  • Salmon Leukemia Virus (SLV): Infiltration/proliferation of immature plasma cells into viscera and retrobulbar tissue; occurs in sea-cultured salmon (Chinook, sockeye, and Atlantic salmon after about 1 year of age)
  • Simian T-cell leukemia virus (STLV): Deltaretrovirus; common, usually asymptomatic; has been associated with lymphoproliferative; zoonotic
  • Human T-cell leukemia virus (HTLV): Deltaretrovirus; highly homologous with STLV, suggesting NHP origin
• Other viral
  • Hamster polyomavirus is associated with cutaneous T-cell lymphomas in younger hamsters (in addition to abdominal neoplasms); older animals develop trichoepitheliomas
• Other species
  • Hedgehog: Lymphoma is an uncommon finding overall, but was noted within cutaneous nodules in the neck from several animals in a recent retrospective (Van de Weyer, Vet Path, 2025)

References:

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