AFIP Wednesday Slide Conference - No. 6
7 October 1998

Conference Moderator:
Dr. John Pletcher, Diplomate, ACVP
Pathology Associates International
15 Worman's Mill Court
Frederick, MD 21701
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Case I - 98-040 (AFIP 2638214)
Signalment: Two-year-old, castrated male, Labrador Retriever mixed breed dog.
History: This dog had a one month history of progressive left hemiparesis. The dog was short-strided in the left foreleg and long-strided in the left rear leg. Neurologic examination was consistent with C6-T2 left-sided myelopathy.
Laboratory Results: A myelogram demonstrated an intradural mass at C7-T1.
Gross Pathology: A 4x4x1cm, irregularly shaped mass was present within the left cranial thoracic cavity. The mass thickened the spinal nerves C7, C8, and T1 leading into it, and thickened the peripheral nerves branching caudal and ventral into the thoracic cavity. On cut section, the spinal cord contained a gray intramedullary mass extending from C7-T1.
Case 6-1. Gross photo. Adjacent to the excised spinal cord there is a hemorrhagic, multinodular mass (5x9cm) attached to and surrounding 3-4 spinal nerves. This mass encompasses another longitudinally oriented structure which may represent another nerve or a large blood vessel (aorta?).
Contributor's Diagnosis and Comments: Primitive neuroectodermal tumor involving the cervical spinal cord and left brachial plexus.
Primitive neuroectodermal tumors (PNETs) are embryonal tumors of the nervous system that have been extensively described in children and less so in domestic animals. Medulloblastomas have been reported in the baboon, the dog, the mouse, and the cow. A PNET has been recently reported in a colobus monkey. Historically, PNETs occurring in the cerebellum have been referred to as medulloblastomas. The term medulloblastoma continues to be used specifically for PNETs that occur in the cerebellum; however, the term PNET emphasizes the primitive, undifferentiated phenotype of the cells in these tumors. PNETs are composed of sheets of round to oval to short spindle cells which, in the dog, rarely form rosettes. By immunohistochemical methods, the cells may have evidence of differentiation along neuronal, ependymal, or glial cell lines. In our experience, these tumors in the dog are often very primitive, and in formalin-fixed specimens may only have rare cells positive for GFAP, neurofilament, or synaptophysin.
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Case 6-1. Spinal cord. This section (2x obj) of spinal cord is expanded and replaced by an infiltrative mass supported by a prominant fibrovascular stroma. Tumor cells are quite pleomorphic, and characterized by oval to round cells which may be closely associated or individualized, and include scattered elongated cells separating the neuropil. Nuclei are hyperchromatic.
AFIP Diagnosis: Spinal cord: Primitive neuroectodermal tumor, Labrador Retriever mixed breed, canine.
Conference Note: The tumor histomorphology is consistent with a primitive neuroectodermal tumor (PNET) and may represent a primary tumor within the spinal cord, extension of a peripheral PNET from spinal nerve roots into the spinal cord, or metastasis from a primary brain tumor. The tumor is composed of a dense, monomorphic population of small polygonal to spindled cells that blend into the overlying meninges, separate and surround spinal nerves, and compress and infiltrate the spinal cord parenchyma. While infiltrative, the tumor is well delineated from the adjacent preexisting neuropil. Neoplastic cells occasionally appear round due to artifactual separation of the tumor and neuropil. Tumor cells have indistinct cell borders with scant to small amounts of eosinophilic cytoplasm and hyperchromatic nuclei that are round to elongate and contain indistinct nucleoli. Occasionally, neoplastic cells contain eccentrically placed nuclei that are rounded on one end and taper at the opposite end with small amounts of trailing eosinophilic cytoplasm ("carrot-shaped" cells). There are rare rosettes.
Immunohistochemical studies performed at the AFIP demonstrate that the tumor is diffusely positive for synaptophysin, multifocally positive for glial fibrillary acidic protein (GFAP), and diffusely negative for neurofilament protein (NFP) and neuron specific enolase (NSE). The immunohistochemical results are consistent with those reported for PNET's of humans and animals. PNET's are usually positive for synaptophysin, and may be multifocally GFAP and/or NFP positive, demonstrating bipotential differentiation of these primitive tumors. Astrocytomas, which should be considered in the differential diagnosis for PNET, often have extensive GFAP positivity, but are consistently negative for synaptophysin in humans.
Differential diagnosis discussed by conference attendees based on location and histomorphology included astrocytoma and lymphoma. Astrocytomas often arise in the piriform lobe and brain stem and tend to blend with the adjacent neuropil, while the tumor in this dog is fairly well-demarcated. The neoplastic neuroglial cells in astrocytomas often occur within a background of lightly eosinophilic fibrillar material and are usually more differentiated, though poorly differentiated astrocytomas may be difficult to distinguish from PNET without the aid of immunohistochemical markers. Lymphoma was considered based upon the individualization of some tumor cells, but the compressive nature of the neoplasm and cytomorphology suggest a cohesive rather than a discrete cell neoplasm of hematopoietic origin.
Contributor: University of Pennsylvania, The School of Veterinary Medicine, Department of Pathobiology, 3800 Spruce St., Philadelphia, PA 19104.

1. Molenaar W, Trojanowski J: Primitive neuroectodermal tumors of the central
nervous system in childhood: Tumor biological aspects. Crit Rev Onc Hematol 17:1-25, 1994.
2. Berthe J, Barneon G, Richer G, Mazue G: A medulloblastoma in a baboon (Papio papio). Lab Anim Sci 30:703-705, 1980.
3. Jolly RD, Alley MR: Medulloblastoma in calves. Path Vet 6:463-468, 1969.
4. Long PH, Schulman FY, Koestner A, Fix AS, Campbell MK, Cameron KN: Primitive neuroectodermal tumor in a two-month-old black and white colobus monkey. Vet Pathol 35:64-67, 1998.
5. Summers BS: Tumors of the central nervous system. In: Veterinary Neuropathology, pp.375-379, Mosby-Year Book Inc., St. Louis, MO, 1995.
6. Gould VE, et al.: Primitive neuroectodermal tumors of the central nervous system: Patterns of expression of neuroendocrine markers and all classes of intermediate filament proteins. Lab Invest 62:498-509, 1990.
Case II - 97N172 (AFIP 2638859)
Signalment: Six-year-old, male, New Zealand white rabbit, Oryctolagus cuniculi.
History: This rabbit was used only for breeding on a teratology study. No manipulations were performed on this rabbit.
Gross Pathology: A pedunculated mass (18 x 10 x 10 mm) extended from the buccal mucosa of the left side of the mouth.

Laboratory Results: None.
Contributor's Diagnosis and Comments: Odontogenic neoplasm, buccal mucosa. Differential diagnosis includes an ameloblastic odontoma and an inductive fibroameloblastoma.
Epithelial odontogenic tumors can be classified into two groups: those that lack inductive properties on connective tissue and those that have inductive properties on connective tissue. Ameloblastoma and calcifying epithelial odontogenic tumor are considered noninductive. Ameloblastomas are characterized by cords and islands of stellate reticulum with peripheral palisades of polarized columnar cells. Calcifying epithelial odontogenic tumor contains foci of Congo-red positive material surrounded by pleomorphic polygonal cells.
There are five tumors in which induction of mesenchymal tissue is evident: ameloblastic fibroma (fibroameloblastoma), with characteristics of ameloblastoma plus proliferation of closely associated pulp-like mesenchyme; dentinoma consisting of masses of dentin, often with minimal cellular component; ameloblastic odontoma which contains palisaded epithelium and stellate reticulum as in ameloblastoma, as well as foci of dentin and/or enamel; complex odontoma which is a disorderly array of dentin, enamel, ameloblastic epithelium and odontoblasts; and compound odontoma, containing denticles with well-organized tooth morphology.
These types have been reported in dogs, cats, cows, and a vole, but not rabbits. Also, the location of this particular tumor is unique. It appeared as a pedunculated mass arising from the buccal mucosa, not from the mandible or maxilla.
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Case 6-2. Oral mucosa. Beneath the mucosal epithelium there is a multinodular mass lined by columnar epithelial cells (arranged in palisades) which surround areas of loose stellate reticulum. Occasional foci of mineralized debris and squamous epithelial cells are found within epithelium lined cystic spaces.
AFIP Diagnosis: Buccal mucosa: Ameloblastoma, New Zealand white rabbit, lagomorph.
Conference Note: The histomorphology of the tumor is characterized by islands and anastomosing cords of epithelium with peripheral palisades of tall columnar cells containing antibasilar polarized nuclei (preameloblasts) that enclose large, central areas of stellate reticulum. The columnar cells are separated from the supportive collagenous stroma by a prominent, brightly eosinophilic limiting membrane. Multifocally, there are nests of keratinizing polygonal cells, occasionally containing small fragments of mineral, which are interpreted as hyperplastic rete ridges with entrapped epithelium.
Conference participants agreed that this tumor represents an odontogenic neoplasm. Additionally, most conference participants interpreted this tumor as a noninductive odontogenic neoplasm whose histomorphologic features are most consistent with ameloblastoma. Although the location within the buccal mucosa is highly unusual and difficult to explain, it may represent an ectopic rest of odontogenic tissue. The delicate cellular stroma that resembles the dental papilla and forms the characteristic mesenchymal component of an ameloblastic fibroma is not identified. While stellate reticulum may be present in ameloblastic fibroma, it usually is not abundant or prominent. Additionally, material consistent with enamel or dentin was not identified by conference participants.
Epithelial odontogenic tumors are uncommon, poorly understood, diagnostically challenging lesions in man and domestic animals. The classification scheme of odontogenic neoplasms is largely based upon the inductive interactions between the odontogenic epithelium and the mesenchyme. Ameloblastoma is the least differentiated of the noninductive odontogenic tumors and may cause confusion and difficulties in diagnosis, especially due to the disparity between veterinary and human pathological nomenclature. Ameloblastic fibroma is the least differentiated of the epithelial odontogenic tumors in which there is inductive mesenchymal change, and the histologic similarities shared by the tumors make differentiation difficult.
Regardless of nomenclature, the biological behavior of ameloblastoma and ameloblastic fibroma is similar. While ameloblastoma may be more locally invasive, both tumors are characterized by slow, expansile growth with virtually no tendency for metastasis. Complete surgical excision carries a good prognosis.

Significant differences in morphology and prevalence of odontogenic tumors occur among the various domestic species and humans. The differences may include novel locations of tumors within the oral cavity as evidenced by the rabbit of this case. The species variations and differences in tumor nomenclature should be kept in mind when characterizing odontogenic tumors in domestic animals.

Contributor: Georgetown University, Division of Comparative Medicine, 3950 Reservoir Road NW, Washington DC 20007.
1. Poulet FM, Valentin BA, Summers BA: A survey of epithelial odontogenic
tumors and cysts in dogs and cats. Vet Path 29:369-380, 1992.
2. Gardner DG, Dubielzig RR: Feline inductive odontogenic tumor (inductive and fibroameloblastoma)-a tumor unique to cats. J Oral Path Med 24:185-90, 1995.
3. Walsh KM, Denholm LJ, Cooper BJ: Epithelial odontogenic tumours in domestic animals. J Comp Path 97:503-521, 1987.
Case III - 98-2048 (AFIP 2641082)
Signalment: Seven-year-old, intact male, Rottweiler dog.
History: This dog presented to the referring veterinarian because of a one month history of polyuria, polydipsia, inappetence and lethargy. Vaccinations were current. Physical examination revealed mild jaundice and a 4 mm diameter, raised nodule on the upper left palpebra. Following laboratory and diagnostic imaging results (see below), the dog was referred to surgery for exploratory laparotomy.
Laboratory Results: Diagnostic work-up including thoracic and abdominal radiographs revealed moderate hepatomegaly. Abdominal ultrasound revealed complex echo-patterned masses involving the liver.
CBC abnormalities included: elevated Hct (60.6%, normal 37.0-55.0%); elevated Hb (20.1 g/dl, normal 12.0-18.0 g/dl); leukocytosis (36,000/mL; normal 6,000-16,900/mL) of which 92% (33,120/mL) were granulocytes. A differential count showed 30% neutrophils (10,800/mL), 2% bands (720/mL), 4% lymphocytes (1,440/mL), 4% monocytes (1,440/mL), and 60% eosinophils (21,600/mL). A bone marrow aspirate was not performed.
Serum chemistry abnormalities included markedly elevated alkaline phosphatase (1251 UL, normal 30-400 UL) and alanine aminotransferase (161 UL, normal 8-80 UL). Creatinine (1.98 mg/dl, normal 0.50-1.80 mg/dl), total protein (8.41 g/dl, normal 5.20-8.20 g/dl) and globulin (5.19 g/dl, normal 2.50-4.50 g/dl) were mildly elevated. The dog was occult heartworm-negative.
Gross Pathology: At surgery, multiple masses were noted in all liver lobes, and a mass with similar gross appearance was noted in the right kidney. The specimen submitted to the surgical biopsy service consisted of a 15 x 13 x 9 cm wedge of one liver lobe, within which were multiple 2 to 12 cm diameter raised, firm, tan nodules.

Contributor's Diagnosis and Comments: Liver: T-cell lymphoma with tissue eosinophilia, hepatocellular cholestasis, portal fibrosis, and bile duct hyperplasia.

The histologic differential diagnosis for this dog's malignant, poorly differentiated, discrete cell neoplasm included mastocytoma, granulocytic sarcoma (chloroma), lymphoma, and myeloproliferative disorder. Giemsa-stained sections failed to reveal metachromatic granules within the cytoplasm of the neoplastic cells. Granulocytic sarcoma was considered an unlikely possibility due to the absence of eosinophilic myelocytes or other developmental stages, although it could not be excluded. Immunohistochemical studies validated for the dog and performed at the Texas Veterinary Medical Diagnostic Laboratory showed that approximately 60% of the large mononuclear cells and all of the polymorphonuclear cells within the tumor stained positively for CD-18, a marker for cells of myeloid and lymphoid origin. Approximately 40-50% of the large mononuclear cells also stained positively for CD-3, a marker for T-lymphocytes. These observations suggested that the tumor was a T-cell lymphoma.
Leukemoid reaction with neutrophilia and a left shift are documented to occur in the later stages of many cases of canine and feline lymphoma3 and in a pony with intestinal lymphosarcoma2. In humans, there are individual case reports of T-cell lymphoma with peripheral blood eosinophilia1, and a large-cell lymphoma mimicking granulocytic sarcoma4. In the three human patients with peripheral blood eosinophilia, bone marrow biopsies were hypercellular with eosinophilia and myeloid hyperplasia, but did not contain malignant lymphoid cells. Interestingly, myeloid malignancies were later identified in the three reported cases. The patient described in the report of large-cell lymphoma mimicking granulocytic sarcoma had neither peripheral blood, nor bone marrow abnormalities. In both human and veterinary patients, a peripheral blood eosinophil count above 1500/mL is often correlated with metastasis and signals a poor prognosis
Eosinophilopoiesis may be stimulated by several cytokines including interleukin-5 (IL-5), interleukin-3 (IL-3) and granulocyte-macrophage colony stimulating factor5,6. All of these factors are recognized to be secreted by T-lymphocytes. It is proposed that the mechanism for the circulating eosinophilia and intratumoral infiltrate of numerous mature eosinophils seen in this dog is the release of one or more of these cytokines by the neoplastic T-cells.
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Case 6-3. Liver. Large areas of liver parenchyma are expanded, separated, and replaced by a pleocellular infiltrate. There is preservation of the major vessels and some bile ducts, but in the center of this mass there is effacement of hepatic plates.
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Case 6-3. Liver. Infiltrating cells include abundant eosinophils with lesser numbers of large pleomorphic round cells admixed with fewer small lymphocytes. Occasional large atypical cells are associated with numerous 2-3u small basophilic bodies (lymphoglandular bodies?). A residual bile duct is at the lower left and strip of brown pigment bearing hepatocytes are at the upper right.
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Case 6-3 . Immunohistochemical staining for CD3 antigen reveals that positively staining T cells with small to medium lymphocyte morphologies are scattered throughout the tumor and admixed with large atypical cells (which do not usually stain).
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Case 6-3 . Immunohistochemical staining for CD45RA antigen is positive in large atypical cells bearing one or more prominant nucleoli, but staining is generally negative in smaller cells with lymphocyte morphology.
AFIP Diagnosis: Liver: Malignant B-cell lymphoma, T-cell rich, with tissue eosinophilia, Rottweiler, canine.
Conference Note: This case was studied in consultation with the Department of Hematopathology.
Most conference participants favored the diagnosis of malignant lymphoma, with tissue eosinophilia.
Participants considered an inflammatory process, but the presence of atypical round cells, effacement of hepatic architecture, and lack of neutrophils and macrophages support neoplasia. Mast cell tumor was strongly considered, but Giemsa and toluidine blue stains did not demonstrate metachromatic cytoplasmic granules in the atypical cells. The absence of immature cells of the eosinophilic series argues against a neoplasm of eosinophils.
An immunohistochemical stain for CD45RA, an antigen expressed by B-cells (and rarely cutaneous T-cell lymphoma cells), stained many, large, atypical round cells that had small amounts of amphophilic cytoplasm, large nuclei with finely-stippled chromation, and multiple, often prominent, nucleoli. CD3, an antigen expressed on the surface of T-cells (and within the cytoplasm of activated NK cells), was expressed by many lymphoid cells; however, in the procedure performed at the AFIP, the cells that stained for CD3 were small to medium-sized lymphocytes that did not have clearly atypical features. The large, atypical cells did not stain for CD3 and did stain for CD45RA (see immunohistochemistry results on the AFIP Veterinary Pathology website). Both of these immunohistochemical tests have been validated in dogs. In the opinions of the Departments of Veterinary Pathology and Hematolymphatic Pathology, the findings support a malignant B-cell lymphoma with nonneoplastic T-cell and eosinophil infiltrates. Application of lymphoid markers to diagnostic cases is a rapidly evolving field in veterinary pathology. Use of different protocols and panels will inevitably lead to differing interpretations. Hopefully, standardized protocols and panels will be developed and accepted.
Humans also develop T-cell rich, B-cell lymphoma. This variant is classified under the umbrella of diffuse large B-cell lymphoma based on biological behavior and response to treatment. Plasma cells and eosinophils are prominent in some human cases. The morphology of this canine lymphoid tumor has some features in common with Hodgkin's disease, another variant of lymphoma that may include large numbers of eosinophils. The defining histologic feature of Hodgkin's disease in people, the Reed-Sternberg cell, is not present in the examined sections of liver from this dog.
Various hypotheses have been offered to explain the prominent T-cell infiltrate within some B-cell lymphomas. Some have postulated that the T-cell infiltrate is a host immune response to the neoplasm. Others believe that cytokines secreted by neoplastic B-cells may act as a chemotactant for T-cells. Alternatively, some have speculated that the T-cells may be the neoplastic population and subsequently induce an atypical proliferation of blastic B-cells. The latter theory, however, is not supported by genotypic findings. Irrespective of the reason for the presence of the T-cell infiltrate, these activated lymphocytes are the likely cause for the attraction of numerous eosinophils.
Contributor: Angell Memorial Animal Hospital, 350 South Huntington Avenue, Boston, MA 02130.
1. Abruzzo LV, et al.: T-cell lymphoblastic lymphoma and eosinophilia associated with subsequent myeloid malignancy. Am J Surg Path 16:236-245, 1992.
2. Duckett WM, Matthews HK: Hypereosinophilia in a horse with intestinal lymphosarcoma. Can Vet J 38:719-720, 1997.
3. Moulton, JE (ed): In: Tumors of Domestic Animals, 3rd ed., p. 244, University of California Press Ltd., 1990.
4. Whitcomb CC, Sternheim WL, Borowitz MJ, Davila E, Byrne GE Jr: T-cell lymphoma mimicking granulocytic sarcoma. Am J Clin Path 84:706-763, 1985.
5. Rothenberg ME, et al.: Human eosinophils have prolonged survival, enhanced functional properties and become hypodense when exposed to human Interleukin-3. J Clin Invest 81:1986-1992, 1988.
6. Owen WF, et al.: Interleukin-5 and phenotypically altered eosinophils in the blood of patients with the idiopathic hypereosinophilic syndrome. J Exp Med 170:343-348, 1989.
7. Kelley LC, Mahaffey EA: Equine malignant lymphomas: Morphologic and immunohistochemical classification. Vet Pathol 35:241-252, 1998.
8. Warnke RA, et al.: Tumors of the lymph nodes and spleen. In: Atlas of Tumor Pathology, 3rd Series Fascicle 16, Armed Forces Institute of Pathology, Washington DC, 1995.
Case IV - 556-98 (AFIP 2643249)
Signalment: Three-month-old, female, Domestic Shorthair cat.
History: The cat presented febrile (103°F) with a pneumonia that failed to respond to antibiotic treatment. The cat was subsequently euthanized.
Gross Pathology: The lung was diffusely atelectatic, and the thoracic cavity contained a large volume of purulent material. A lung lobe contained an approximately four-centimeter diameter, multilobulated mass.
Laboratory Results: Feline leukemia virus and feline immunodeficiency virus tests conducted antemortem were negative. Purulent material obtained at necropsy yielded pure culture of Rhodococcus equi.
Contributor's Diagnosis and Comments:
1. Lung, pleura: Pleuritis, pyogranulomatous, chronic, severe, due to Rhodococcus equi.
2. Lung: Pneumonia, necropurulent, chronic, focal.
Sections from the submitted lung vary, yet consistently feature atelectasis accompanied by thickened pleura containing large numbers of macrophages, neutrophils, lymphocytes, and plasma cells. Multifocal fibroblasts and collagen accompany the pleural infiltrates. Sections containing the pleural abscess seen grossly also contain large amounts of necrotic debris surrounded by similar pyogranulomatous infiltrates. Several bronchi and bronchioles contain mucinous material. Several submitted slides contain a fragment of foreign material consistent with plant origin. Many pleural macrophages have an eosinophilic granular cytoplasm. Also submitted is a Brown and Brenn stained section which reveals intracellular, gram-positive, predominately coccoid forms, although a few short rod-shaped bacteria are also seen within macrophages.

Rhodococcus equi, previous known as Corynebacterium equi, is a small, pleomorphic, gram-positive rod found in soil, manure, and litter. It most commonly causes pneumonia in foals 1-3 months of age resulting in multiple, sometimes large, abscesses within lung and lymph nodes. Mortality can be high in younger foals. While Rhodococcus equi has been isolated from lesions in dogs, abscesses in cats have been the most commonly reported manifestation in small animals. Organisms may gain access to the body via a penetrating wound. Fragments of plant material entrapped within exudate in this case would be consistent with introduction of the bacteria from a penetrating wound.
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Case 6-4. Lung. There is marked compression and infiltration of alveoli by a cellular infiltrate. Bronchi in the lower left contains an amphophilic exudate. Pleural connective tissue in the upper right is coated by a heavy, partly detached cellular exudate.
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Case 6-4. Lung, pleura. Exudate covering the pleura is composed of abundant plump epithelioid macrophages which occasionally contain variable numbers of 1x3u eosinophilic bodies interpreted as bacilli. Moderate numbers of neutrophils and fewer lymphocytes and plasma cells are scattered throughout this exudate.
AFIP Diagnosis: Lung: Pleuritis, pyogranulomatous, diffuse, severe, with multifocal pyogranulomatous pneumonia, diffuse atelectasis, and numerous intrahistiocytic gram-positive coccobacilli, Domestic Shorthair, feline.
Conference Note: Rhodococcus equi is a gram-positive, variably acid-fast, facultative intracellular, pleomorphic coccobacillus. In addition to horses, the organism causes pneumonia occasionally in cattle, sheep, swine, and infrequently in other species including dogs and cats. R. equi has been associated with arthritis in lambs and pyogranulomatous inflammation in the cervical and mandibular lymph nodes of swine. Cutaneous infections may occur in cats due to penetrating trauma, and lesions are characterized by localized swelling with ulceration, fistulas, and purulent drainage that most commonly affect an extremity. There is a report of the organism causing pneumonia and pleuritis in a monkey.
Reports of infection with R. equi in cats are uncommon, but may be increasing in frequency due to concurrent infection with immunosuppressive viruses, such as feline immunodeficiency virus and feline leukemia virus. Age is another factor that determines immune status, especially in people. Though newborns of domestic species are capable of mounting immune responses at birth, the reactions in neonatal and perinatal animals are characterized as primary immune responses, with a considerable lag period and low concentration of antibodies. Foals between two and four months are most susceptible to infection, while infection in horses older than six months is uncommon unless there is concurrent immunodeficiency. Age may have been a predisposing factor for infection in this relatively young kitten.
Reports of human infection with R. equi are increasing due to immunodeficiency caused by HIV infection. Affected humans often have a history of exposure to horses. The organism, a soil saprophyte commonly recovered from the manure of herbivores, causes pneumonia and pulmonary abscesses in affected individuals. Regardless of the route of infection in animals or people, the organism may spread via the lymphatics to regional lymph nodes, and then may disseminate hematogenously to the liver, spleen and visceral lymph nodes.
In addition to host immunocompetency, several proposed bacterial virulence factors may predispose animals and humans to infection; these virulence factors seem to be strain-dependent. While the organism is susceptible to the bactericidal effects of neutrophils, virulent strains are able to resist macrophage defenses, establish residence within the cytoplasm of the macrophage, and replicate. Proposed virulence factors include the presence of a capsular polysaccharide, the exoenzyme cholesterol oxidase, cell wall mycolic acids, and the products encoded by a virulence-associated plasmid.
Contributor: C. E. Kord Animal Disease Diagnostic Laboratory, P.O. Box 40627, Melrose Station, Nashville, TN 37204.
1. Carter GR and Chengappa MM: Rhodococcus. In: Microbial Diseases; A
Veterinarian's Guide to Laboratory Diagnosis, 1993, Iowa State University Press, Ames, IA, pp. 217-219.
2. Green CE: Rhodococcus equi infection of cats. In: Infectious Diseases of the Dog and Cat, CD Green, ed., 1990, WB Saunders, Philadelphia, pp. 606-607.
3. Green CE: Bacterial diseases. In: Textbook of Internal Medicine, S.J. Ettinger, E.C. Feldman, eds., 4th ed., 1995, WB Saunders, Philadelphia, p 372.
4. Jones PC, Hunt D, and King NW: Diseases caused by bacteria. In: Veterinary Pathology, 6th ed., Williams and Wilkins, 1997, Baltimore, MD, p 486.
5. Tizard IR: Immunity in the fetus and newborn. In: Veterinary Immunology: An Introduction, 5th ed., pp. 237-250, WB Saunders Co., Philadelphia, 1996.
6. Hondalus MK: Pathogenesis and virulence of Rhodococcus equi. Vet Microbiol 56:257-268, 1997.
International Veterinary Pathology Slide Bank:
Laser disc frame #2433, 2497.
Ed Stevens, DVM
Captain, United States Army
Registry of Veterinary Pathology*
Department of Veterinary Pathology
Armed Forces Institute of Pathology
(202)782-2615; DSN: 662-2615
* The American Veterinary Medical Association and the American College of Veterinary Pathologists are co-sponsors of the Registry of Veterinary Pathology. The C.L. Davis Foundation also provides substantial support for the Registry.

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