AFIP Wednesday Slide Conference - No. 12
10 December 1997

Conference Moderator: MAJ Mark Mense
Diplomate, ACVP
Division of Pathology
Walter Reed Army Institute of Research
Washington, D.C. 20307-5100
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Case I - W751-97 (AFIP 2597489)

Signalment: Adult, male, wild rabbit.
History: The animal was found dead adjacent to the clinical staff offices. Deaths in feral rabbits in the vicinity of the Clinical Center had been noted several days previous to this animal being found.
Gross Pathology: There were no external lesions. The cadaver appeared fresh and internal lesions were limited to the lungs which were diffusely edematous and contained multifocal hemorrhages, and the spleen which was enlarged approximately twice normal size and was dark. The liver was enlarged with an accentuated pale lobular pattern.
Laboratory Results: Immunohistochemistry on sections of liver was positive for rabbit calicivirus.
Contributor's Diagnoses and Comments:
1. Hepatitis, acute, zonal, periportal to midzonal, necrotizing, severe.
2. Splenitis, acute, necrotizing, massive, with hemorrhage and thrombosis.
3. Lung, intravascular fibrinous thrombosis, multifocal, acute with minimal interstitial changes, rabbit calicivirus (rabbit hemorrhagic disease).
This case is typical for the disease in susceptible adult rabbits, and is almost uniformly fatal. Many animals are found dead without premonitory signs, and those found alive rapidly become comatose and die quietly. Blood-stained frothy fluid may exude from external nares, and the lungs are edematous. Typically, there is complete destruction of the sinusoidal architecture of the liver and spleen, and replacement by fibrinous coagulum containing erythrocytes. Destruction of white pulp may occur as in this case, but is not uniformly present and may be secondary to stress rather than a direct effect. Acute coagulation necrosis in the liver is typically periportal, extending to midzonal areas as in this case, and is usually accompanied by a minimal inflammatory response.
Viral antigen can be localized to areas of splenic and hepatic necrosis using immunohistochemical methods, and the virus appears to specifically target sinusoidal lining cells in these organs. Although thrombi are often found in the lungs and also occasionally in other organs, these are not associated with localization of viral antigen in affected vessels by immunohistochemistry, although viral antigens can be recovered from tonsils, lymph nodes and kidneys using more sensitive methods such as RT-PCR. Intravascular thrombosis is thought to cause rapid death, but histopathologic evidence of DIC may wane if necropsies are delayed for long after death. DIC is thought to develop largely because of the severe necrosis of the liver.
Rabbit hemorrhagic disease caused devastating losses of farmed rabbits in Europe and Asia beginning in 1984. Baby rabbits do not support viral replication to the degree that is found in adults, and younger rabbits (<4 weeks) do not develop overt signs after calicivirus infection. After extensive research, the virus was released into Australia as a biologic method for control of feral rabbits and although marked reduction in rabbit numbers has occurred in some areas as a result, in other areas rabbit numbers appear to have returned to pre-release levels. Reasons for this remain unclear. Overseas, a non-pathogenic strain of rabbit calicivirus has been identified and infection with this strain may protect against the pathogenic isolate. In addition, antibody is protective and exposure of young kits to the virus may allow the establishment of immunity. Possible spread of the virus by fomites, birds and insect vectors is suspected, but in the unusual circumstances in Australia, where upon release/escape in late 1995, cases were recorded up to hundreds of miles away within days, and deliberate spread by the farming community was suspected.
A related calicivirus, European brown hare syndrome virus, produces a similar disease in hares, but the two diseases are distinct entities and cross infection or protection do not occur. These two viruses are unlike other members of Caliciviridae (San Miguel sea lion virus and feline calicivirus) in that they appear to have little genomic variation between isolates. An effective vaccine is available for use in laboratory, commercial and companion rabbits and vaccination of these rabbit stocks in Australia was possible prior to release of the virus. The use of rabbit calicivirus as a biologic method of controlling a massive population of feral rabbits precipitated intense debate. Because the virus kills rabbits so quickly and moribund animals generally die quietly, objections based on animal ethical grounds were largely dismissed. Controversy remains as to whether the depletion of feral rabbit numbers will have a beneficial effect on threatened native wildlife species, because recovery of degraded habitats may be offset by greater predation of these species by feral cats and foxes that normally rely on rabbits as their major food source.
Case 12-1. Liver. Acute necrosis with intact parenchyma. 10X
AFIP Diagnoses:
1. Liver: Hepatitis, necrotizing, acute, periportal to midzonal, diffuse, moderate, rabbit, lagomorph.
2. Lung: Fibrin thrombi, multifocal, with mild acute interstitial pneumonia.
3. Lung, blood vessels: Medial hypertrophy, multifocal, moderate.
4. Spleen: Congestion, hemorrhage, and fibrin deposition, acute, diffuse, severe, with lymphoid necrosis.
Conference Note: Rabbit hemorrhagic disease (RHD) virus affects rabbits of the species Oryctolagus cuniculus; no other rabbit species have been shown to be susceptible.9 The incubation period is 16-48 hours. Originally, reports from China identified peracute, acute, and subacute forms of the disease. Subsequent descriptions in both natural and experimental infections have been consistent with a peracute form, in which rabbits die suddenly with few or no clinical signs, and an acute form, which is seen in areas where RHD is established and in which rabbits exhibit clinical signs before death.
RHD virus is spread by several routes and vectors. The virus is present in excretory products, and enters susceptible hosts usually by the oral or respiratory routes. The virus is stable in the environment, and can be spread by fomites such as bedding material or feedstuffs, or carried short distances by insects. Foxes and polecats have been shown to seroconvert after ingestion of the virus, though their role in transmission of the disease is unclear. Rabbit products, including pelts and meat, serve as potential sources of spread. An RHD epidemic in Mexico was linked circumstantially to frozen rabbit carcasses imported illegally through another country, though direct confirmation of disease transfer from infected meat products was never demonstrated.9
Comparatively, RHD resembles certain viral hemorrhagic diseases of humans and nonhuman primates, including Lassa fever, simian hemorrhagic fever, and Ebola and Marburg viral infections. The pathogenic mechanisms of DIC in these diseases are not completely understood.
Contributor: School of Veterinary Science, The University of Melbourne, 250 Princes Highway, Werribee, Victoria 3030, Australia
1. Brander P, Boujon CE, Bestetti GE: Infectious hemorrhagic disease of rabbits in the Bern Pathology Institute (1988-1990): season and regional distribution and histopathological findings. Schweiz-Arch-Tierheilkd. 134(8):383-389, 1992.
2. Capucci L, Fusi P, Lavazza A, Pacciarini ML, Rossi C: Detection and preliminary characterization of a new rabbit calicivirus related to rabbit hemorrhagic disease virus but nonpathogenic. J Virol 70(12):8614-8623, 1996.
3. Chasey D, Lucas M, Westcott D, Williams M: European brown hare syndrome in the U.K.; a calicivirus related to but distinct from that of viral hemorrhagic disease in rabbits. Arch Virol 124(3-4):363-370, 1992.
4. Fuchs A, Weissenbock H: Comparative histopathological study of rabbit hemorrhagic disease (RHD) and European brown hare syndrome (EBHS). J Comp Pathol 107(1):103-113, Jul 1992.
5. Guittre C, Ruvoen-Clouet N, Barraud L, Cherel Y, Baginski I, Prave M, Ganiere JP, Trepo C, Cova L: Early stages of rabbit hemorrhagic disease virus infection monitored by polymerase chain reaction. Zentralbl-Veterinarmed-B 43(2):109-118, Apr 1996.
6. Ueda K, Park JH, Ochiai K, Itakura C: Disseminated intravascular coagulation (DIC) in rabbit hemorrhagic disease. Jpn-J-Vet-Res 40(4):133-141, Dec 1992.
7. Wirblich C, Thiel HJ, Meyers G: Genetic map of the calicivirus rabbit hemorrhagic disease virus as deduced from in vitro translation studies. J Virol 70(11):7974-7983, Nov 1996.
8. Park JH, Lee Y-S, Itakura C: Pathogenesis of acute necrotic hepatitis in rabbit hemorrhagic disease. Lab Anim Sci 45(4):445-449, 1995.
9. Chasey D: Rabbit haemorrhagic disease: the new scourge of Oryctolagus cuniculus. Laboratory Animals 31:33-44, 1997.
International Veterinary Pathology Slide Bank:
Laser disc frame #14546

Case II - 95051363 (AFIP 2596338)

Signalment: 23-day-old, female, Quarter Horse.
History: This foal appeared depressed and icteric at 6:30 a.m. and was found dead at 12:30 p.m.
Gross Pathology: The foal was moderately icteric. The stomach had multiple small foci of ulceration in the non-glandular region. The liver was moderately enlarged with rounded edges. On cut surface, the liver bulged, the lobular pattern was accentuated, and the liver was yellow.
Laboratory Results: Bacteriology results: Only a few contaminating bacteria (E. coli and Enterobacter sp.) were isolated from the liver.
Contributor's Diagnosis and Comments: Liver, hepatitis, suppurative, with necrosis, multifocal, coalescing, severe, compatible with Tyzzer's disease of neonatal foals.
Intracellular bacteria compatible with Clostridium piliforme (Bacillus piliformis) were demonstrated in histopathological sections. Lesions and organisms are compatible with Tyzzer's disease in foals.
Case 12-2a. Liver. Locally extensive hepatic necrosis with an influx of neutrophils. 20X
Case 12-2b. Liver. Note the intracellular cluster of long slender bacilli (Clostridium piliformis). Warthin-Starry 40X

AFIP Diagnoses
1. Liver: Hepatitis, necrotizing, neutrophilic, acute, multifocal to coalescing, random, severe, Quarter Horse, equine.
2. Liver: Cholestasis, canalicular, multifocal, moderate.
Conference Note: Tyzzer's disease is an acute, highly fatal, epizootic enterohepatic disease of neonatal or weanling animals. It has been reported in numerous animal species, including horses, cattle, mice, rats, hamsters, guinea pigs, rabbits, foxes, and coyotes. The causative agent, Clostridium piliforme, is a gram-negative, spore-forming, motile, obligate intracellular bacterium with peritrichous flagella. The vegetative form causes the disease state, and appears as bundles or "haystacks" within its target cells, i.e. enterocytes and hepatocytes. Visualization of the bacteria in histologic section is enhanced with silver stains such as the Warthin-Starry procedure.
In horses, Tyzzer's disease usually affects one- to six-week-old foals. Clinical signs include weakness, rapid respiratory rate and pulse, cold extremities, icterus, blindness, and severe depression before death, which occurs within 2 to 48 hours. Diarrhea may also be observed. Outbreaks of the disease are sporadic, typically affecting very few animals in a herd. This suggests that the disease is not highly contagious, and may be limited to immunocompromised foals.1
The pathogenesis of equine Tyzzer's disease is incompletely understood. Clostridium piliforme is transmitted by the fecal-oral route. The initial site of infection is the intestinal epithelium, from which the organism is transported hematogenously to the liver. In rodents, transplacental transmission also occurs, but this has not been demonstrated in the horse.
Gross lesions include icterus, hepatomegaly, and multifocal 1- to 5-mm white foci scattered throughout the hepatic parenchyma. In guinea pigs, mesenteric and colonic lymph nodes are swollen, and the cecum and colon are reddened, thickened, distended, and contain gray pinpoint necrotic foci. Similar lesions can be found in hamsters, but in this species the most striking gross lesion reported is multiple white elevated nodules in the heart, which are necrotic foci with massive infiltrates of large macrophages, neutrophils, plasma cells, lymphocytes, and fibroblasts interspersed with the organisms.
Contributor: Department of Anatomy, Pathology and Pharmacology, 250 Veterinary Medical Bldg., Oklahoma State University, Stillwater, OK 74078-2007.
1. Hook RR, Riley LK, Franklin CL, Besch-Williford CL: Seroanalysis of Tyzzer's disease in horses: implications that multiple strains can infect Equidae. Equine Vet J 1995 Jan;27(1):8-12.
2. Franklin CL, Motzel SL, Besch-Williford CL, Hook RR Jr, Riley LK: Tyzzer's infection: host specificity of Clostridium piliforme isolates. Lab Anim Sci 1994 Dec;44(6):568-72.
3. Humber KA, Sweeney RW, Saik JE, Hansen TO, Morris CF: Clinical and clinicopathologic findings in two foals infected with Bacillus piliformis. J Am Vet Med Assoc 1988 Dec 1;193(11):1425-8.
4. Frisk CS: Bacterial and Mycotic Diseases. In: Laboratory Hamsters, Van Hoosier GL, McPherson CW, editors, Academic Press, Inc., pp. 121-125, 1987.
International Veterinary Pathology Slide Bank:
Laser disc frame #2456, 2934, 4195-9, 5406, 5463, 9305, 10173-4, 10927, 17147-50.

Case III - 97-5457 S10 (AFIP 2594500)

Signalment: 18-year-old, male, castrated, Arabian horse.
History: Neurologic horse with grade 4 ataxia.
Gross Pathology: In the sacral spinal canal there was a thick, fibrous, extradural mass which formed a collar around the sacral cord and filum terminale.
Contributor's Diagnosis and Comments: Cauda equina neuritis.
The extradural mass seen grossly resulted from severe, diffuse, granulomatous cauda equina neuritis where the inflammatory reaction had become confluent. Individual nerve bundles were inflamed and some were completely necrotic. An intense inflammatory response was present in virtually all extradural nerve bundles, and fibrosis and multinucleated giant cell formation added to its severity.
Case 12-3. Cauda equina. The nerve bundle is replaced by macrophages, multinucleated giant cells and a mass of neutrophils in a necrotic area to the top left corner. The lower right shows a mostly intact smaller nerve. 20X

AFIP Diagnosis: Cauda equina with adjacent adipose tissue: Neuritis and ganglioneuritis, granulomatous, diffuse, severe, with moderate chronic steatitis, Arabian horse, equine.
Conference Note: Cauda equina syndrome is an uncommon condition in horses in which the sacral and coccygeal nerves, and occasionally cranial nerves V and VII, are chronically inflamed. Resulting clinical signs include paralysis of the tail, rectum, and bladder; anesthesia in the sacral dermatomes with a surrounding zone of hyperesthesia; gluteal muscle atrophy; and hind limb ataxia and weakness. In horses with cranial nerve involvement, facial paralysis, head tilt, and masticatory muscle atrophy are also seen.
Marked granulomatous inflammation and proliferation of the epineurial and perineurial sheaths are the major histopathologic changes in the extradural nerve roots and spinal ganglia. Intradural roots are usually less severely affected. Though some nerve fascicles remain intact, many contain dense infiltrates of lymphocytes, plasma cells, and macrophages extending into the fascicle interior. Secondary to interruption of axons, retrograde chromatolysis develops in the somatic motor neurons of the affected spinal segments. Destruction of sensory neurons in the dorsal roots and ganglia leads to orthograde nerve fiber degeneration in the spinal dorsal funiculus.6
The exact etiopathogenesis is unknown. Several studies have demonstrated similarities between this condition and both the Guillain-Barré syndrome in man and experimental allergic neuritis (EAN) in rats. Kadlubowski and Ingram5 showed that affected horses have circulating antibodies to P2 myelin protein, a neuritogenic myelin antigen that on injection causes EAN. It is unclear whether these circulating antibodies play a primary role in demyelination or represent a consequence of antigen released in the course of myelin destruction.6 In another study4, equine adenovirus 1 was isolated from cauda equina nervous tissue in 2 out of 3 horses with cauda equina syndrome. In 6 normal horses of similar age, no viral agents were isolated. Though these findings suggest a causal linkage, it is also plausible that the adenovirus infection was established as an avirulent opportunistic infection in the period of stress.
Contributor: Oregon State University, College of Veterinary Medicine, P.O. Box 429, Corvallis, OR 97339
1. Wright JA, Fordyce P, Edington N: Neuritis of the cauda equina in the horse. J Comp Path 97:667-675, 1987.
2. Jubb KVF, Haxtable CR: The nervous system. In: Pathology of Domestic Animals. Jubb KVF, Kennedy PC, and Palmer N., eds., Academic Press, San Diego, 4th ed., Vol. 1, p. 428, 1993.
3. Fordyce PS, Edington N, Bridges GC, Wright JA, Edwards GB: Use of an ELISA in the differential diagnosis of cauda equina neuritis and other equine neuropathies. Equine Veterinary Journal 19(1):55-59, 1987.
4. Edington N, Wright JA, Patel JR, Edwards GB, Griffiths L: Equine adenovirus 1 isolated from cauda equina neuritis. Res Vet Sci 37:252-254, 1984.
5. Kadlubowski M, Ingram PL: Circulating antibodies to the neuritogenic myelin protein, P2, in neuritis of the cauda equina of the horse. Nature 293:299-300, 24 September 1981.
6. Summers BA, Cummings JF, de Lahunta A: Veterinary Neuropathology, Mosby-Year Book, Inc., St. Louis, MO, pp. 432-434, 1995.

International Veterinary Pathology Slide Bank:
Laser disc frame #1588-9, 1676, 2104, 6980, 10539-43, 14179-81, 17068-72, 17115-8.

Case IV - 1648 or 1649 (AFIP 2600782)

Signalment: 4-month-old, 65 kg, male, castrated, German Landrace pig.
History: To study the experimental infection with Oesophagostomum dentatum, pigs were orally inoculated with a single dose of 50,000 infective larvae of Oesophagostomum dentatum. None of the animals developed clinical signs of diarrhea. The tissues are from an animal which was euthanized at day 7 post-inoculation.
Gross Pathology: Macroscopic lesions were predominantly found in the cecum and proximal colon. The mucosa was severely edematous and hyperemic. Numerous red and/or white nodules (1-5 mm in diameter) were visible in the mucosa.
Contributor's Diagnosis and Comments: Moderate granulomatous typhlocolitis with nematode larvae.
Etiology: Oesophagostomum dentatum.
Sections of nematode larvae are present in the lamina propria - sometimes extending to the submucosa. Nematode larvae are surrounded by an amorphous eosinophilic capsule which occasionally contains neutrophils and macrophages, by a thin layer of neutrophils and macrophages and by a thick layer of lymphocytes. Eosinophils and mast cells are quite numerous in some parasitic nodules. They are predominantly located in the periphery of the nodules and in the submucosa. Tangential section through parasitic nodules revealing only mononuclear infiltrates in the lamina propria and submucosa are frequent. Some of them have central areas of necrosis. The submucosa is mildly to moderately infiltrated by lymphocytes, macrophages and eosinophils. These infiltrates extend in some slides into the tunica muscularis and serosa.
Sections through mucosa-associated lymphoid nodules are present in some slides. They can be distinguished from parasitic granulomas by the presence of lymphoid follicles which contain tingible body macrophages and are surrounded by a collagenous capsule.
Oesophagostomum dentatum belongs to the family Stongylidae. It is a large intestinal nematode of pigs. Three species of Oesophagostomum occur in pigs: O. dentatum, O. quadrispinulatum and O. brevicaudum. Pigs become infected by oral uptake of infectious larvae. After 24 hours, these larvae penetrate the mucosa of the cecum and proximal large intestine. They induce the formation of parasitic granulomas in the lamina propria and submucosa. In these granulomas they mature and return as 4th stage larvae to the intestinal lumen between days 7 and 14 post-infection (pi). At day 14 pi, only mild multifocal infiltrates of macrophages, lymphocytes and eosinophils remain in the mucosa. In the intestinal lumen the 4th stage larvae develop to adults (females: 11-15 mm, males: 8-10 mm) and start shedding eggs.
O. dentatum has a high incidence especially in breeding pigs. Severe infections may result in necrotizing enteritis. In most herds, the infection is not recognized since it is frequently subclinical. It causes, however, reduced weight gain, reduced litter size, and may complicate other infections. Thus, it is of economic importance. Infection with O. dentatum in pigs induces inflammatory reactions in the intestinal mucosa, but this does not result in protective immunity.
Case 12-4. Colon. Nematode larvae (Oesophagostomum dentatus) in the mucosa is surrounded by mononuclear leukocytes and bordered by several crypt abscesses. 10X
AFIP Diagnosis: Colon: Colitis, subacute and eosinophilic, multifocal, moderate, with
nematode larvae, German Landrace, porcine, etiology consistent with Oesophagostomum spp.
Conference Note: Oesophagostomum infection is found world-wide. In addition to pigs, Oesophagostomum spp. are important intestinal parasites of various ruminants and primates, including man. In cattle, O. radiatum infection can result in severe weight loss, anorexia, anemia and diarrhea. O. columbianum and O. venulosum affect sheep, goats and certain wild ruminants. These species have a similar life cycle to those affecting pigs, but the larvae of each species infect different sections of the intestine. Adult worms of all species are found in the cecal and colonic lumina.
Oesophagostomiasis has been reported to be the most frequent and pathogenic helminth of laboratory primates, and can represent a serious threat to the health of colonies.2 Gross lesions include dark nodules up to 8 mm in diameter within the submucosa or serosa of the cecum and colon, and occasionally in the mesocolon. Incidences of up to 70% in rhesus monkeys and 62% in cynomolgus monkeys have been reported.

Contributor: Institut für Pathologie, Tierärztliche Hochschule Hannover, Bünteweg 17, 30559 Hannover, Germany.
1. McCracken RM, Ross JG: The histopathology of Oesophagostomum dentatum infections in pigs. J Comp Pathol 80:619-623, 1970.
2. Stewart TB, Gasbarre LC: The veterinary importance of nodular worms (Oesophagostomum spp.). Parasitol. Today 5:209-213, 1989.
3. Stockdale PHG: Necrotic enteritis of pigs caused by infection with Oesophagostomum spp. Br Vet J 126:526-529, 1970.
4. Abbott DP, Majeed SK: A survey of parasitic lesions in wild-caught, laboratory-maintained primates: (rhesus, cynomolgus, and baboon). Vet Pathol 1984 Mar;21(2):198-207.
International Veterinary Pathology Slide Bank:
Laser disc frame #5284, 13612
Terrell W. Blanchard
Major, 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.
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