AFIP Wednesday Slide Conference - No. 12
18 December 1996

Conference Moderator: LTC LuAnn McKinney
Diplomate, ACVP
Department of Veterinary Pathology
Armed Forces Institute of Pathology
Washington, D.C. 20306-6000

Return to WSC Case Menu

Case I - 95-87 (AFIP 2547950)

Signalment: 4.5-month-old male C57 BL/6J mouse.

History: The mouse was lethargic and had moderate abdominal swelling. A firm mass could be palpated within the central part of the abdomen.

Gross Pathology: There was a firm, 1.5 x 1.1 x 2 cm mass attached to the cranial mesentery, duodenum, and pancreas. On section, the nodular tissue that replaced the pancreas was firm, while the mesenteric tissue was more homogenous and softer.

Laboratory Results: None.

Contributor's Diagnosis and Comments: Pancreatic acinar cell adenocarcinoma.

The animal was transgenic for the SV40 large T antigen under the elastase promoter. Investigators are using these animals to study the molecular immunology of SV40 T antigen. The histologic findings from several of these mice are thus far similar to those described by Glasner, et al (1). There are a number of reports of pancreatic carcinoma related to SV40 T antigen induction in mice. Mouse models using N-methyl- N-nitrosurea are also used to study pancreatic carcinoma. Spontaneous neoplasia of the exocrine pancreas is infrequent in mice, but adenomas and adenocarcinomas have been reported (2).

AFIP Diagnosis: Pancreas: Carcinoma, acinar cell, C57 BL/6J mouse, rodent.

Conference Note: In this transgenic mouse model of pancreatic cancer, the simian virus 40 (SV40) genome is placed under the control of the regulatory elements of the pancreatic elastase-1 gene. Elastase is one of several pancreatic serine proteases that is synthesized in the exocrine cells and secreted into the gut. The expression of elastase normally starts at about day 14 of development, when the acinar cells begin to differentiate. In this mouse model, the elastase-1 gene is fused to an SV40 large T antigen gene. The transforming proteins produced by the SV40 gene can inhibit both the Rb and P53 genes, both of which are important regulators of cell growth.

Contributor: Department of Comparative Medicine, Pennsylvania State University, The M.S. Hershey Medical Center, P.O. Box 850, Hershey, PA 17033.

1. Glasner S., et al: Characterization of the ELSV transgenic mouse model of pancreatic carcinoma. Am J Path 140:1237-1245, 1992.
2. Squire RA, et al: Chapter 12, "Tumors," Pathology of Laboratory AnimalsBenrischke K, Garner FM, Jones TC (editors). Springer-Verlag, NY. Volume II, pp 1151, 1978.

3. Ornitz DM, et al: Pancreatic neoplasia induced by SV40 T-antigen expression in acinar cells of transgenic mice. Science 238:188-193, 1987.
4. Cotran RS, Kumar V, Robbins SL(eds): Robbins: Pathologic Basis of Disease, Vol. 5, W.B. Saunders Co., pp. 268-270, 1994.

International Veterinary Pathology Slide Bank: None.


Case II - OL7716-36 or OL7716-34 (AFIP 2552621)

Signalment: Five-month-old male MRL/lpr mice.

History: None.

Gross Pathology: Marked generalized lymphadenopathy, splenomegaly, swollen kidneys with raised white foci. The joints appeared normal.

Laboratory Results: Marked proteinuria. Clinical chemistry and hematology were not done.

Contributor's Diagnosis and Comments:

1. Tarsal joint: Synovitis, diffuse, chronic proliferative, with neutrophilic and lymphocytic infiltration, moderate to severe.
2. Foot, plantar surface: Cellulitis, multifocal, chronic, non-suppurative, moderate (present in some sections).
3. Arterioles: Vasculitis, multifocal, acute, fibrinoid.
Vasculitis, multifocal, chronic, proliferative.
Condition: Immune complex mediated synovitis and vasculitis.

Longitudinal sections of hindlimb, including tarsal joints, are submitted. The distribution and severity of lesions varies slightly between sections. The predominant lesion is diffuse chronic proliferative synovitis with cellular infiltrates consisting of lymphocytes and histiocytes in all sections, and also neutrophils in most sections. Foci of coagulative necrosis are within the hyperplastic synovium in many sections. Chronic, non-suppurative cellulitis involves the tendon sheaths, skeletal muscle and subcuticular tissues adjacent to the joints. The vascular lesion is prominent in the medullary area of subcuticular adipose tissue in a few sections.

The MRL-lpr/pr mouse is homozygous for a single gene mutation of the Fasapoptosis gene on mouse chromosome 19. Homozygosity of lpr results in non- malignant lymphoproliferation with autosomal antibodies and immunopathology similar to human systemic lupus erythematosus. Serologic abnormalities in affected mice include 1) hypergammaglobulinemia with monoclonal gammapathy, 2) antinuclear antibodies, 3) anti-ssDNA dsDNA antibodies. Lpr was identified at the Jackson Laboratories and has been bred onto several background strains. The lpr phenotype is influenced by background genes and there are variations in the autoantibodies seen within different inbred lpr/lpr strains.

Synovitis in MRL/lpr mice consists of synovial cell proliferation associated with lymphocytic infiltrates. Lymphocytic pannus formation has been described. Immune complex mediated glomerulonephritis and vasculitis also occurs and may account for the 50% mortality seen in 5-month-old mice in most colonies.

AFIP Diagnosis: Rear leg: Tenosynovitis, proliferative, lymphohistiocytic and neutrophilic, diffuse, moderate, with cellulitis, perineuritis, periostitis, and osteolysis, MRL/lpr mouse, rodent.

Conference Note: There is some variation in the severity and distribution of lesions in different histologic sections. The vasculitis noted by the contributor was not present in the sections we examined.

Mice homozygous for lpr develop lymphadenopathy and suffer from autoimmune disease. These mice have a mutation in a cell-surface protein, fas, that mediates apoptosis. The fas gene has been localized to the mouse chromosome 19. The fasantigen is a 45 kd protein belonging to the tumor necrosis factor (TNF) receptor family. A single point mutation which replaces isoleucine with asparagine abolishes the ability of fas to transduce the apoptotic signal. The fas system is thought to play an important role in the intrathymic development of T cells. In fact, it has been reported that neonatal thymectomy prevents lymphoid organ hyperplasia and early autoimmune disease in lprmice.

Contributor: Searle, 4901 Parkway, Skokie, IL 60077.

1. Andrews BS, Eisenberg RA, et al: Spontaneous murine lupus-like syndromes: Clinical and immunopathologic manifestations in several strains. J Exp Med 148:1198- 1215, 1978..
2. Cohen PL, Eisenberg RA: Lpr and gld: Single gene models of systemic autoimmunity and lymphoproliferative disease. Annu Rev Immunol 9:243-269, 1991.
3. Takahashi T, Tanaka M, et al: Generalized lymphoproliferative disease in mice, caused by a point mutation in the fas ligand. Cell 76:969-976, 1994.
4. Hang L, Theofilopoulos AN, and Dixon FJ: A spontaneous rheumatoid arthritis-like disease in MRL/l mice. J Exp Med 155:1690-1701, 1982.

International Veterinary Pathology Slide Bank: None.


Case III - OL7716-33 (AFIP 2552620)

Signalment: Five-month-old male MRL/lpr mice.

History: None.

Gross Pathology: Both kidneys were slightly enlarged, pale and had multifocal slightly raised grayish-white areas throughout the renal parenchyma. There was marked generalized lymphadenopathy and splenomegaly.

Laboratory Results: Marked urinary excretion of protein. Hematologic and serum chemistry evaluations were not performed.

Contributor's Diagnosis and Comments:

1. Kidney: Glomerulonephritis, membranoproliferative, moderate, generalized, segmental to global.
2. Arterioles: Arteriolitis, proliferative and fibrinoid, mild, multifocal.
3. Arterioles: Peri-arteriolitis, lymphocytic, mild, multifocal.

Condition: Immune complex-mediated glomerulonephritis and vasculitis.

Longitudinal or cross-sections of kidneys are submitted. Severity and distribution of lesions is slightly different among slides. The predominant lesions include generalized segmental to global membranoproliferative glomerulonephritis, epithelial crescents, moderate multifocal interstitial and periglomerular lymphocytic infiltrates. There are small numbers of basophilic tubules. The vascular lesions include mild to moderate multifocal endothelial cell proliferation, endothelial hypertrophy, degeneration, and cytoplasmic vacuolation, fibrinoid change in vessel walls and perivascular lymphocytic infiltrates. The vascular lesions are present in the small and medium-sized arteries within the kidney and in the peri-renal fat.

MRL-lpr/lpr mice develop a spontaneous autoimmune disease with immunopathologic manifestations similar to systemic lupus erythematosus. The disease in MRL-lpr/lpr mice is characterized by lymphadenopathy, autoantibody production, and immune complex-mediated nephritis, vasculitis and arthritis. These disease manifestations are the result of both single gene mutation (lpr) of the fas apoptosis gene on mouse chromosome 19, and background genes from the MRL strain. The common serologic abnormalities in the affected mice include a) elevated lg concentrations with associated monoclonal gammopathy, b) antinuclear antibodies, and c) antibodies to ssDNA and dsDNA. The glomerular lesions develop due to localization of circulating non-glomerular Ag-Ab complexes in glomeruli, which are visible by immunofluorescence or electron microscopy as granules within, or on either side of, the glomerular basement membrane.

According to the World Health Organization, lupus nephritis is classified into 5 types: 1) normal by light, electron, and immunofluorescent microscopy (class 1), which is quite rare; 2) mesangial lupus glomerulonephritis (class II); 3) focal proliferative glomerulonephritis (class III); 4) diffuse proliferative glomerulonephritis (class IV): and 5) membranous glomerulonephritis, (class 5). Diffuse proliferative glomerulonephritis is the most serious of the renal lesions in SLE and occurs in 35 to 40% of patients who are biopsied.

AFIP Diagnosis: Kidney: Glomerulonephritis, membrano-proliferative, global, diffuse, moderate, with crescents, marked necrotizing vasculitis, and multifocal, lymphoplasmacytic perivasculitis and interstitial nephritis, MRL/lpr mouse, rodent.

Conference Note: Type III hypersensitivity is induced by the deposition of antigen-antibody complexes and the subsequent activation of the complement system. Exogenous or endogenous antigen is introduced into the circulation and interacts with immunocompetent cells, resulting in the formation of antibodies. Antigen-antibody complexes form in circulation and are deposited in various tissues. The most pathogenic complexes are those of small to intermediate size (formed in slight antigen excess). In addition to glomeruli, favored sites of immune complex deposition are joints, skin, heart, serosal surfaces, and small blood vessels. Central to the inflammatory reaction is activation of the complement cascade which results in release of an opsonin (C3b), leukocyte chemotactic factors (C5 fragments, C5b67), anaphylatoxins (C3a, C5a) and formation of the membrane attack complex (C5-9).

Contributor: Searle, 4901 Searle Parkway, Skikie, IL 60077.

1. Andrews BS, Eisenberg RA, et al: Spontaneous murine lupus-like syndromes: Clinical and immunopathology manifestation in several strains. J. Exp Med. 148:1198- 1215, 1978.
2. Cohen PL, Eisenberg RA. Lpr and gld: Single gene models of systemic autoimmunity and lymphoproliferative disease. Annu Rev Immuno, 9:243-269, 1991.
3. Cotran RS, Kumar V, Robbins SL (eds): Robbins: Pathologic Basis of Disease, 5th ed, W.B. Saunders Co., pp. 184-187, 1994.

International Veterinary Pathology Slide Bank: None.


Case IV - 95-475.2 (AFIP 2548600)

Signalment: Female CD-1 mouse

History: Multiple sick mice were noted during an experimental protocol. This mouse presented with bilateral conjunctivitis, and a small scab on inner aspect of the right front leg.

Gross Pathology: The liver was pale brown. The spleen was enlarged to five times normal size and contained multiple, variably sized and irregularly shaped, pale spots.

Laboratory Results: See contributor's comments.

Contributor's Diagnosis and Comments: Spleen and liver: Necrosis, multifocal, minimal to severe.

Tissues not submitted:
Eyelid: Conjunctivitis, subacute, focally extensive, moderate, with necrosis and intraepithelial intracytoplasmic inclusion bodies.
Lung, colon, uterus, and vagina: Necrosis, multifocal, minimal to severe.

Etiology: Ectromelia virus (Orthopoxvirus)
Disease: Mousepox

Mousepox is caused by ectromelia virus, an orthopoxvirus of mice which has been known since 1930 and is enzootic in Europe. It has been seen sporadically in the United States, with nearly 40 individual mousepox episodes reported since 1950, the most recent published outbreak involved multiple facilities in 1979 and 1980. Mousepox in the United States has usually been traced to importation of infected European mice or infected mouse tissues. Morbidity and mortality patterns vary within an epizootic and are influenced by numerous factors, including the strain of the virus and the strain, genotype, age, and immune status of the mice. Mousepox may present as explosive outbreaks with acute deaths, or with mild clinical signs and a slow, minimal spread, even in large colonies of susceptible mice.

From February through April 1995, mousepox was diagnosed and eradicated in the laboratory mouse colony at the Naval Medical Research Institute in Bethesda, Maryland. The virus entered the facility via injection of mice with a contaminated, commercially prepared, U.S. origin, normal pooled mouse serum. The outbreak began with increased mortality in a single room; subsequently, small numbers of animals in separate cages in other rooms were involved. Clinical signs were often mild and overall mortality was low; BALB/c mice were more severely affected and many of them died spontaneously. Conjunctivitis was the most common clinical sign in addition to occasional small crusty scabs on sparsely haired or non-haired areas of skin. Necropsy findings included conjunctivitis, enlarged spleens, and pale livers; hemorrhage into the pyloric stomach and upper small intestine was observed in experimentally infected animals. In both immunocompetent and immunodeficient mice, the most common histologic finding was multifocal to coalescing splenic necrosis; necrosis was seen less frequently in liver, lymph nodes, and Peyer's patches. Necrosis was rarely observed in ovary, vagina, uterus, colon, and lung. Splenic necrosis often involved over 50% of the examined tissue, including both white and red pulp. Hepatic necrosis presented as either large, well-demarcated areas of coagulative necrosis or as multiple, random, interlacing bands of necrosis. Intracytoplasmic eosinophilic inclusion bodies were seen in conjunctival mucosae and haired palpebra. Ectromelia virus was confirmed as the causative agent of the epizootic by electron microscopy, immunohistochemistry, animal inoculations, serologic testing, virus isolation, and polymerase chain reaction. Serologic testing was of little value in the initial stages of the outbreak, although six weeks later, orthopoxvirus-specific antibody was detected in colony mice by both IFA and ELISA procedures.

AFIP Diagnosis:

1. Liver: Necrosis, hepatocellular, multifocal, random, with bacterial emboli, CD-1 mouse, rodent.
2. Spleen: Necrosis, multifocally extensive, with bacterial emboli.

Conference Note: Without a Gram stain, most of the conference participants failed to note the multifocal bacterial emboli. The bacteria, small gram-positive bacilli, were not mentioned by the contributor. The relationship of these bacteria to the lesions is unclear; however, bacterial septicemia secondary to ectromelia virus infection is suspected.

Ectromelia virus belongs to the genus Orthopoxvirinae and is serologically closely related to vaccinia virus. Voles are thought to be the natural host of mousepox. The disease is enzootic in many mouse colonies in Europe, China, and Japan. Sporadic, devastating outbreaks have occurred in North America and Australia following accidental importation of the virus in tissue, serum, ascitic fluid, tumor material or subclinically infected mice.

Mouse strains vary in resistance to mousepox. AKR and C57BL/6 are resistant strains, which are subclinically infected, while susceptible strains include C3H, A, SWR, DF1, DBA/1, DBA/2, and BALB/c. Athymic mice are particularly susceptible. Generally, transmission occurs via skin abrasions, with primary lesions on the face, snout, feet, tail, or ventral abdomen. Virus enters epithelial cells, replicating in the cytoplasm before entering draining lymph nodes and the blood stream. The primary viremia is difficult to detect since titers are low and virus is rapidly removed by the mononuclear phagocytic system in the liver and spleen (3 - 4 days). The virus replicates extensively in these organs, and death due to acute hepatitis may occur. In survivors, a secondary viremia occurs, with viral spread to visceral organs (salivary glands, lung, pancreas, lymph nodes, Peyer's patches, small intestine, kidney, urinary bladder) and skin (6 -10 days). In addition, immunohistochemistry has demonstrated virus in the ovary, uterus and bone marrow. A generalized skin "rash" develops with papules, ulcers, and scabs. Ischemic necrosis and loss of extremities occurs as a result of viral replication within vascular endothelium and subsequent vasculitis (ectro-loss of a part, melos-limb). The skin, nasal turbinates and oral mucosa are the tissues particularly associated with shedding of the virus.

Characteristic microscopic findings include: (1) intracytoplasmic inclusion bodies (ICIBs) most commonly in epidermis, pancreas, and intestine, often multiple inclusions per cell; (2) skin: early-focal epidermal hyperplasia, hypertrophy, and spongiosis with ballooning degeneration and ICIBs. Later, necrosis, ulceration, and dermal lymphocytic infiltrate; (3) liver: multifocal random coagulation necrosis with minimal inflammation, hepatocellular syncytia and ballooning degeneration at margins with ICIBs; (4) spleen: focal necrosis involving both lymphoid follicles and red pulp; (5) intestine: erosions almost to the base of the mucosa, frequently adjacent to lymphoid follicles; and (6) occasional focal necrosis of visceral organs and bone marrow degeneration.

Contributor: Naval Medical Research Institute, ATTN: Pathobiology Division (Code 21), 8901 Wisconsin Ave, Bethesda MD 20889-5607.

1. Fenner F: Mouse-pox (infectious ectromelia of mice): A review. J Immunol, 63:341-73, 1949.
2. Werner RM, Allen AM, J.D. Small, et al: Clinical manifestations of mousepox in an experimental animal holding room. Lab Anim Sci, 31:590-94, 1981.
3. Manning PJ, Frisk CS: Clinical, pathologic, and serologic features of an epizootic of mousepox in Minnesota. Lab Anim Sci 31:574-77, 1981.
4. Smith EK, Hartroft PM, Greider MH: Preliminary report of an outbreak of ectromelia (mousepox) at Washington University in St. Louis. Lab Anim Sci, 31:578-82, 1981.
5. La Regina MC, Doyle RE: Mousepox at St. Louis University - Preliminary Report. Lab Anim Sci, 31:583-84, 1981.
6. Dixon LW: Control of mousepox epizootics in St. Louis and Chicago. Lab Anim Sci 31:585-89, 1981.
7. Fenner F: Poxviruses of laboratory animals. Lab Anim Sci 40:469-80, 1990.
8. Briody BA, Hauschka TS, Mirand EA: The role of genotype in resistance to an epizootic of mouse pox (ectromelia). Amer J Hyg 63:59-68, 1956.
9. Fenner F: Mousepox (infectious ectromelia): Past, present and future. Lab Anim Sci, 31:553-59, 1981.
10. Wallace GD, Werner RM, Golway PL, et al: Epizootology of an outbreak of mousepox at the National Institutes of Health. Lab Anim Sci, 31:609-15, 1981.
11. Sundberg JP, Brown KS, Bedigian R: Ulcerative blepharitis and periorbital abscesses in BALB/cJ and BALB/cByJ mice. Jax notes, The Jackson Laboratory, Bar Harbor, ME. #443, Fall, 1990.
12. Collins MJ, Peters RL, Parker JC: Serological detection of ectromelia virus antibody. Lab Anim Sci, 31:595-98, 1981.
13. Allen AM, Clarke GL, Ganaway JR, et al: Pathology and diagnosis of mousepox. Lab Anim Sci, 31:599-608, 1981.
14. Wallace GD, Buller RML: Kinetics of ectromelia virus (mousepox) transmission and clinical response in C57BL/J, BALB/cByJ, and AKR/J inbred mice. Lab Anim Sci, 35:41-6, 1985.
15. Bhatt PN, Jacoby RO: Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. I. Clinical responses. Lab Anim Sci, 37:11-5, 1987.
16. Fenner F, et al: The orthopoxviruses. Academic Press, Inc., Orlando FL, 1989.
17. Jacoby RO, Bhatt PN: Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. II. Pathogenesis. Lab Anim
Sci, 37:16-22, 1987.
18. Bhatt PN, Jacoby RO: Mousepox in inbred mice innately resistant or susceptible to lethal infection with ectromelia virus. I. Clinical responses. Lab Anim Sci, 37:11-15, 1987.
19. Held JR: Summary. Ectromelia (mousepox) in the United States. Lab Anim Sci, 31:630-32, 1981.
20. Dick EJ, et al: Mousepox outbreak in a laboratory mouse colony. Lab Anim Sci 46(6):602-611, 1996.

International Veterinary Pathology Slide Bank: None.
Lance Batey
Captain, VC, USA
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.

Return to WSC Case Menu