AFIP Wednesday Slide Conference - No. 27
April 21, 1999

Conference Moderator: LTC A. Peter Vogel
Pathology Division
US Army Medical Research Institute of Infectious Disease
Ft. Detrick, Frederick, MD 21702-5011.
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Case I - 961722 (AFIP 2643028)

Signalment: Adult, male, Hartley guinea pig (Cavia porcellus).
History: This guinea pig was used on an experimental protocol designed to assess the acute toxicity of inhaled aflatoxin. This animal died 6 days after intratracheal instillation of 0.2 ml of sonicated aflatoxin B1 (AFB1) suspension (15 mg/ml).
Gross Pathology: All guinea pigs that died due to intratracheal (IT) instillation of AFB1 exhibited gross pulmonary and hepatic changes, and hemorrhage in multiple tissues. Principal pulmonary findings consisted of diffuse, red, lobular, mottled pattern distributed throughout all lung lobes, hemorrhage, and blood-tinged to yellow pleural fluid. All livers exhibited dark red centrilobular foci of necrosis disseminated throughout all lobes. Multifocal to diffuse yellow to pale tan discoloration of the liver was frequently observed. Contents of the gallbladder were often discolored and varied from blood-tinged to yellow or orange. Hemorrhage of the mesenteric or submandibular lymph nodes, gastrointestinal tract, and adrenal gland; epistaxis; hemoperitoneum; and petechiae or ecchymoses of the thoracic and abdominal musculature, subcutis, epicardium, and uterus were observed. Gastrointestinal hemorrhage included hemorrhagic gastric or cecal contents, and petechiae of the small intestine, cecum, gastric mucosa, and colon.
Laboratory Results: None.
Contributor's Diagnosis and Comments: Liver: Necrosis, centrilobular, diffuse, marked, with vacuolar change, biliary hyperplasia, and hemorrhage.

Etiology: Aflatoxin B1
AFB1 is the most prevalent and most potently cytotoxic and carcinogenic member of a group of bisfurancoumarin mycotoxins produced mainly by Aspergillus flavis, A. parasiticus, and Penicillium puberulum. Acute hepatic injury, carcinogenesis, teratogenesis, induction of chromosomal aberrations, mitotic inhibition, coagulopathy, and immunosuppression are well-known effects of exposure. The variety and amount of individual toxins produced by different strains of Aspergillus are under genetic control, as well as the influence of environmental factors which include quality and moisture content of the substrate, temperature, and relative humidity. The highest levels of toxin accumulate in stored or unharvested, moisture-damaged, mature grains. Although all species of animals are susceptible to the toxic effects of aflatoxins, ducks, rabbits, pigs, trout, calves, dogs, and poultry are considered highly susceptible, while sheep and adult cattle are most resistant. Young animals are generally more susceptible than are adults.
Microscopic changes associated with acute aflatoxicosis include hepatic megalocytosis, focal hepatocellular necrosis, fatty change, cytosegrosome formation, bile ductule proliferation, and reticulin and collagen deposition throughout the hepatic acinus. Bile pigments may accumulate within canaliculi and hepatocytes. At higher doses there may be diffuse fatty change and loss of periacinar hepatocytes, with replacement by a mix of inflammatory cells, fibroblasts, and primitive vascular channels. In dogs, acute fulminating hepatic necrosis accompanied by widespread hemorrhage may be seen.
This case exhibits the periacinar (centrilobular) necrosis and loss of hepatocytes typical of acute high dose aflatoxin exposure. The hepatocellular necrosis is accompanied by hemorrhage, vacuolar change (fatty degeneration), and biliary hyperplasia. Although there is some variation in nuclear size among hepatocytes, megalocytosis is not a feature. Sinusoids and central veins contain numerous mononuclear cell, granulocyte, and erythrocyte precursors of variable maturity, including rare mitotic figures, suggestive of injury to the bone marrow. Although examined sections of bone marrow from this animal were essentially normal, the bone marrow of other similarly treated guinea pigs exhibited depletion of hematopoietic elements, hemorrhage, and fibroplasia. Widespread hemorrhages, typical of acute high dose exposure, were also evident among these animals.
High doses of AFB1 inhibit protein and RNA synthesis, which is thought to contribute to the necrosis and fatty change seen at high dose levels. AFB1 has also been shown to induce lipid peroxidation in rat liver. Coagulopathy is attributed to diminished hepatic synthesis of coagulation factors V, VII, VIII, and fibrinogen. In acute cases with severe hepatic necrosis, DIC can also contribute to the coagulation defects. Although the liver is the principal target organ for AFB1, there is a growing body of evidence implicating the toxin as a potential pulmonary carcinogen following inhalational exposure to AFB1-laden grain dusts as well as after dietary exposure. Clara cells have been shown to possess a high capacity for activation of AFB1 to its carcinogenic form.
AFB1 is primarily metabolized by the hepatic mixed-function oxidase system to a variety of toxic and nontoxic metabolites. The toxic effects of AFB1 are principally due to the binding of these metabolites to cellular macromolecules, particularly mitochondrial and nuclear nucleic acids and nucleoproteins. The principal bioactivation pathway is the epoxidation of AFB1 to AFB1-2,3-epoxide (also referred to as the 8,9-epoxide), the proximal carcinogen and mutagen. Activation of AFB1 to the epoxide is essential for AFB1 to manifest its mutagenic, carcinogenic, and DNA-binding properties. Cytochrome P450's capable of activating AFB1 in animals include members of the 1A, 2B, 2C, and 3A subfamilies. The extent of in vitro microsomal activation and the susceptibility of animals to AFB1-induced hepatocarcinogenesis are affected by treatment with cytochrome P450 inducers (e.g., phenobarbital, 3-methylcholanthrene) and inhibitors (e.g., piperonyl butoxide, cobalt chloride, carbon monoxide).
Monooxygenase-catalyzed hydroxylation and dealkylation of AFB1 to form the metabolites AFM1, AFQ1, AFP1, and AFB2a are considered detoxification pathways. Though much less potent than AFB1, metabolites such as AFM1, AFQ1, and AFP1 still retain carcinogenic and mutagenic activities. AFM1 may be particularly significant in that it is excreted in the milk of lactating animals fed diets containing AFB1, resulting in the exposure of more susceptible young suckling animals.
An alternate non-P450-dependent mechanism of AFB1 bioactivation is epoxidation via lipid hydroperoxide-dependent mechanisms, catalyzed by microsomal prostaglandin H synthase (PHS) and cytosolic lipoxygenases. PHS and lipoxygenases catalyze the oxidation of arachidonic acid to lipid peroxy radicals, which are known epoxidizing agents for xenobiotics. In this co-oxidative process, the epoxidation of AFB1 occurs concomitantly with the oxidation of arachidonic acid. Co-oxidative xenobiotic bioactivation may be most significant in nonhepatic tissues. Relatively high PHS and lipoxygenase activities occur in kidney, lung, and embryonic tissues, and the overall P450 activity in these tissues is lower than that of liver.
In addition to differences in the levels of activating enzymes, the relative activities of detoxifying biotransformation pathways are also critical determinants of species susceptibility. The most important detoxification system is thought to be the glutathione S-transferase-catalyzed conjugation of activated AFB1. Glutathione S-transferases comprise a family of cytosolic and microsomal enzymes that catalyze the conjugation of reduced glutathione (GSH) with compounds possessing an electrophilic center. Conjugation of the electrophilic AFB1-2,3-epoxide with GSH provides an alternative to binding to nucleophilic sites in cellular macromolecules.
AFIP Diagnosis: Liver: Vacuolar degeneration, necrosis, and loss, centrilobular, diffuse, with hemorrhage, biliary hyperplasia, and intravascular hematopoietic cells, Hartley guinea pig (Cavia porcellus), rodent.
Conference Note: Participants discussed several possible causes of diffuse centrilobular degeneration, necrosis, and hemorrhage, including ischemia and toxic etiologies. Participants observed that extensive fatty change and prominent megalocytosis, typical of classic hepatic aflatoxicosis, were absent in this guinea pig, probably due to the acute insult to the liver and sudden death of the animal.
The histomorphology of liver lesions induced by acute aflatoxicosis is species variable. Hepatocellular necrosis is primarily periportal in turkeys, ducklings, chickens, adult rats, and cats. Midzonal necrosis occurs in the rabbit. In swine, cattle, dogs, and guinea pigs, the lesion is primarily centrilobular. In neonatal rats and trout, diffuse necrosis occurs. Additionally, hemorrhage and edema of the gallbladder wall are consistent lesions caused by aflatoxicosis in pigs and dogs.
Contributor: Pathology Division, US Army Medical Research Institute of Infectious Diseases, Ft. Detrick, Frederick, MD 21702-5011.
1. Baker DC, Green RA: Coagulation defects of aflatoxin intoxicated rabbits. Vet Pathol 24:62-70, 1987.
2. Dvorackova I: In: Aflatoxins and Human Health, pp. 1-19, CRC Press Inc., Boca Raton, FL, 1990.
3. Kelly WR: The liver and biliary system. In: Pathology of Domestic Animals, Jubb KVF, Kennedy PC, Palmer N, eds., 4th ed., vol. 2, pp. 319-406, Academic Press, San Diego, CA, 1993.
4. Massey TE, Stewart RK, Daniels JM, Liu L: Biochemical and molecular aspects of mammalian susceptibility to aflatoxin B1 carcinogenicity. Proc Soc Exp Biol Med 208:213-227, 1995.
5. Shen HM, Shi CY, Lee HP, Ong CN: Aflatoxin B1-induced lipid peroxidation in rat liver. Toxicol Appl Pharmacol 127:145-150, 1994.
6. Silvotti L, Petterino C, Bonomi A, Cabassi E: Immunotoxicological effects on piglets of feeding sows diets containing aflatoxins. Vet Rec 141:469-472, 1997.
7. Jones TC, Hunt RD, King NW: Diseases caused by fungi. In: Veterinary Pathology, 6th ed., pp. 539-542, Williams & Wilkins, Baltimore, MD, 1997.

Case II - PM91-067 (AFIP 2642601)

Signalment: Two-year-old, male, Cavalier King Charles Spaniel, canine.
History: There was tachypnea of two months duration and occasional diarrhea. The referring veterinarian treated the dog with antibiotics and steroids, but there was no response. The dog was referred to the Small Animal Hospital, University of Liverpool. On clinical examination, the dog was non-febrile, cyanotic, tachypneic, tachycardic, and had prominent mesenteric lymph nodes. Radiographs demonstrated marked interstitial pattern in lungs, an enlarged liver, and possibly enlarged sublumbar lymph nodes. The other lymph nodes were unremarkable.
Gross Pathology: Respiratory System: The external nares, frontal sinuses and pharynx were unremarkable. The larynx, and especially trachea and bronchi, contained pale, stable foam and were lined by pale epithelium. The lungs collapsed partially on opening the chest. The cut surfaces of all lobes were grey-pink, firm, poorly-aerated, and exuded abundant fluid. The pleural surfaces were smooth and glistening without excess fluid. The macroscopic diagnosis was pulmonary consolidation.

Laboratory Results: Microbiology (lung and bronchial swab): A pure culture of Escherichia coli was isolated from both specimens.
Contributor's Diagnosis and Comments: Lung: Chronic active interstitial pneumonia with myriad ring-shaped organisms typical of cyst forms of Pneumocystis carinii, Cavalier King Charles Spaniel, canine.
Autolytic changes include detachment of airway epithelial cells. There is abundant eosinophilic, foamy, granular contents in airway lumina in which a few cells and faintly stained ring structures are also present. There are inflammatory cells (mostly lymphocytes and plasma cells) and patchy fibroplasia in alveolar septa. There is variable hyperplasia of type II pneumocytes. Methenamine silver staining reveals myriad ring-shaped organisms typical of cyst forms of Pneumocystis carinii in airway and alveolar lumina.
This Cavalier King Charles Spaniel had advanced chronic active interstitial pneumonia in which organisms with morphological and staining characteristics of cyst forms of Pneumocystis carinii are identified. This case is unusual. Pneumocystis pneumonia is rare in the dog and is invariably found in immunosuppressed or immunodeficient animals (Lobetti et al, 1996). Splenic extramedullary hematopoiesis and hepatocyte degeneration were observed (sections not submitted), and were presumably a result of chronic hypoxemia.
AFIP Diagnosis: Lung: Pneumonia, interstitial, chronic, diffuse, mild, with abundant alveolar and intra-airway eosinophilic flocculent material (atypical fungi), Cavalier King Charles Spaniel, etiology consistent with Pneumocystis carinii.
Conference Note: Pneumocystis carinii is a unique fungal microbe with worldwide distribution known to inhabit the pulmonary alveoli of humans and animals. An important disease in immunocompromised humans, pneumocystic pneumonia has also been recognized in immunodeficient dogs, horses, swine, goats, rats, mice and monkeys. Organisms have been found in the lungs of other species without evidence of clinical disease.
Based on recent results of nucleotide sequence analyses, P. carinii has been classified as an ascomycetous fungus in the group Archiascomycetes which contains several saprophytic and parasitic plant pathogens. The organism was previously classified as a protozoan, and protozoan terminology is still used in describing the morphologic forms, leading to some confusion. While morphologically the organisms infecting various hosts are indistinguishable, P. carinii organisms isolated from one mammalian host do not cause infection in another host species. Thus, the organisms found in different mammals likely represent different species of fungi with host specificity. This is further supported by the lack of common complement fixing antigens in human and rat strains.
Morphologically, two dominant organism life cycle stages are present in the mammalian lung: the polymorphic trophozoites and the mature thick-walled cysts (asci) that contain up to eight intracystic bodies (ascospores). Thin-walled cysts (containing intracystic bodies resembling trophozoites) are also present in the lung, and represent intermediate stages. The organisms are adherent to each other and type I pneumocytes, but not type II pneumocytes.
Infection causes minimal inflammatory reaction, although type II pneumocytes may become hypertrophied and hyperplastic. The alveoli are filled with an eosinophilic, frothy material representing accumulation of fungal organisms. The Gomori's methenamine silver method demonstrates the thick-walled asci. During the conference, the moderator showed several photomicrographs of immunohistochemical studies performed in his laboratory on lung tissue of monkeys with pneumocystosis. The studies showed that trophozoites comprise the eosinophilic flocculent material within alveoli, and the "foamy" appearance is due to vacuoles within the organisms.
Four cases of pneumocystic pneumonia were recently described affecting young (one-year-old or less), female miniature dachshunds. All dogs had clinical signs of immune incompetence, low globulin levels on serum electrophoresis, and deficiency of immunoglobulins A, G, and M. Three of four dogs responded to therapy and recovered. The immunologic disorder does not appear to represent classic primary severe combined immunodeficiency syndrome. Cases of pneumocystic pneumonia have also been reported in Cavalier King Charles Spaniels in Europe.
Contributor: Department of Veterinary Pathology, University of Liverpool, Liverpool, L69 3BX, United Kingdom.
1. Lobetti RG, Leisewitz AL, Spencer JA: Pneumocystis carinii in the miniature dachshund: Case report and literature review. J Small Anim Pract 37:280-285, 1996.
2. Jones TC, Hunt RD, King NW: Diseases due to protozoa. In: Veterinary Pathology, 6th ed., pp. 581-582, Williams & Wilkins, Baltimore, MD, 1997.
3. Kaneshiro ES: The lipids of Pneumocystis carinii. Clin Microbiol Rev 11:27-41, 1998.
4. Sukura A, et al.: Pneumocystic carinii pneumonia in dogs - a diagnostic challenge. J Vet Diag Invest 8:124-130, 1996.
5. Ramsey IK, et al.: Pneumocystic carinii pneumonia in two Cavalier King Charles spaniels. Vet Rec 140:372-373, 1997.

Case III - 172/98 (AFIP 2643938)

Signalment: 1½ -year-old, male, blackbuck antelope.
History: The animal was found dead in its enclosure in a zoo in Germany in January.
Gross Pathology: At necropsy, the animal was emaciated, and multiple well-circumscribed red nodules, 0.5 cm in diameter, were found in the lung. The cortex of the left kidney showed a solitary, white, firm, slightly elevated, well-demarcated nodule, 0.3 cm in diameter. On the serosal surface of the large intestine, numerous white nodules, 0.2 cm in diameter, were observed. In the mesentery, similar nodules apparently associated with lymphatic vessels were detected. Other gross findings included moderate congestion and acute, diffuse, alveolar edema of the lung.
Laboratory Results: Bacteriologically, Yersinia pseudotuberculosis was isolated from the intestine and mesenteric lymph nodes. Other organs were not examined. The parasitological examination of the feces revealed a moderate infection with trichostrongyloids.
Contributor's Diagnoses and Comments:
1. Lung: Moderate multifocal necrotizing pneumonia with intralesional coccobacilli, blackbuck antelope (Antilope cervicapra).
2. Liver (not present on all sections): Moderate multifocal necrotizing hepatitis with intralesional coccobacilli.
3. Kidney (not present on all sections): Moderate necrotizing nephritis with intralesional coccobacilli.
4. Intestine (not submitted): Severe fibrinonecrotic transmural jejunitis with myriad of intralesional coccobacilli.
Yersinia pseudotuberculosis, a Gram-negative, pleomorphic coccobacillus, can infect a wide range of vertebrates including humans, birds and reptiles, and causes sporadic outbreaks of fatal disease (Baskin et al., 1977). Rodents, lagomorphs and birds are most susceptible to infection. Although the frequency of disease outbreaks in wild animals is higher in winter months, no such seasonal prevalence has been found in zoo animals (Knapp and Weber, 1982). Six different serotypes have been classified by using the O- and H-antigen. Serotype I is the most important one in naturally infected animals in Europe (Knapp and Weber, 1982).
Direct contact with infected animals or ingestion of contaminated food is the major source of infection, and the intestinal tract represents the most important portal of entry. The pathogen may be introduced in a herd of zoo animals by latently infected animals, or feces of free living birds (Knapp and Weber, 1982; Welsh et al., 1992). The duration of the incubation period is influenced by the virulence of the bacteria, the resistance of the host, and environmental factors (Knapp and Weber, 1982). Stress factors, such as malnutrition or endoparasitism, may facilitate the manifestation of the disease (Knapp and Weber, 1982).
The pathogen's virulence is dependent on several factors which are encoded by a 70 kb virulence plasmid. An adhesion factor (YadA) and an outer membrane surface protein, termed invasin, are required for translocation of the pathogen from the intestinal lumen to the Peyer's patches and for cell penetration. In humans it has been shown that the bacterium is taken up by the M-cells (Marra and Isberg, 1997). The generation of V and W antigens is important for the intracellular survival of the bacteria. The significance of an exotoxin for virulence, especially produced by bacteria of the serotype III, remains to be determined.
Similar to the presented case, a peracute clinical course following Yersinia pseudotuberculosis infection has been reported in herds of antelope in Germany and the USA (Baskin et al., 1977; Pleskar et al., 1990; Welsh et al., 1992). The pathological findings of yersiniosis are similar in all species. In acute fatal disease, no lesions may be found. In acute to subacute cases, emaciation, enteritis with or without mild lesions in the mucosa, lymphadenitis of the mesenteric lymph nodes, and serofibrinous peritonitis may be seen. The chronic form is characterized by miliary to pea-sized nodules in the liver, spleen, kidney, lung, and lymph nodes.

Etiological differential diagnosis includes tuberculosis, listeriosis, Francisella tularensis, Yersinia pestis, and Yersinia enterocolitica infection; therefore, microbiological investigations are needed for definitive diagnosis. As Yersinia pseudotuberculosis has zoonotic potential, people having contact with diseased or dead animals should be warned and use extreme caution and sanitary precautions for their own safety.
AFIP Diagnoses:
1. Lung: Pneumonia, embolic, necrotizing, acute, multifocal, moderate, with hemorrhage and large colonies of coccobacilli, black-buck antelope (Antilope cervicapra), bovine.
2. Liver: Hepatitis, embolic, necrotizing, acute, multifocal, moderate, with large colonies of coccobacilli.
3. Kidney: Nephritis, embolic, necrotizing, acute, focally extensive, with microcolonies of coccobacilli (septic infarct).

Note: Tissue Gram stains performed at the AFIP demonstrated large colonies of small, Gram-negative coccobacilli within pulmonary, hepatic, and renal lesions. Not all slides contain sections of kidney and liver, nor are lesions present in all sections of the kidney.
Conference Note: The three major pathogens causing yersiniosis in animals are Yersinia pestis, Y. enterocolitica, and Y. pseudotuberculosis. They are genetically similar, and all are zoonotic. Humans infected with Yersinia pseudotuberculosis may develop mesenteric lymphadenitis and septicemia. Dogs and cats are sometimes asymptomatic carriers of Y. pseudotuberculosis and serve as a source of infection for humans. In one report, several young children who drank water from puddles and played in a sandbox in an area frequented by a stray cat became infected with Y. pseudotuberculosis; the organism was isolated from the water, soil, and sand.
Contributor: Institut fur Veterinar-Pathologie, Justus-Liebig-Universitat, Frankfurter Str. 96, 35392 Giessen, Germany.
1. Baskin GB, Montali RJ, Bush M, Quan TJ, Smith E: (1977): Yersiniosis in captive exotic mammals. J Amer Vet Med Assoc 171:908-912, 1977.
2. Knapp W, Weber A: Yersinia pseudotuberculosis. In: Handbuch der bakteriellen Infektionen bei Tieren, Blobel H, Schliesser T, eds., pp. 466-503, Gustav Fischer Verlag, Stuttgart, Germany, 1982.
3. Marra A, Isberg RR: Invasin-dependent and invasin-independent pathways for translocation of Yersinia pseudotuberculosis across the Peyer's patches intestinal epithelium. Infect Immun 65:3412-3421, 1997.
4. Pleskar R, Behlert O, Weiss R: Yersinia pseudotuberculosis-Infektion bei Hirschziegenantilopen (Antilope cervicapra). Verhandlungsberichte des 32. Internationalen Symposiums über die Erkrankungen der Zoo- und Wildtiere, Eskilstuna, Akademie-Verlag, Berlin, Germany, 1990.
5. Welsh RD, Ely RW, Holland RJ: Epizootic of Yersinia pseudotuberculosis in a wildlife park. J Amer Vet Med Assoc 201:142-144, 1992.
6. Greene CE: Enteric bacterial infections: Yersiniosis. In: Infectious Diseases of the Dog and Cat, Greene CE, ed., 2nd ed., pp. 241-242, WB Saunders, Philadelphia, 1998.

Case IV - TAMU-95-2 (AFIP 2507550)

Signalment: Two-year-old, female, quarter horse, equine.
History: Four days prior to necropsy, this horse was judged as being "just not right". The next day, it became ataxic and "dizzy acting", with head pressing. It became recumbent that evening. The horse would occasionally rise until the evening prior to necropsy. There was no positive clinical response to treatment, including dimethyl sulfoxide (DMSO), dexamethasone, flunixin meglumine (BanamineÔ), and replacement fluid therapy. The horse had no previous vaccinations for viral pathogens. The horse was euthanized as a rabies suspect.
Gross Pathology: There was some serous inflammation of the pectoral muscles. No other significant lesions were observed at necropsy.
Laboratory Results: A cerebrospinal fluid (CSF) tap was interpreted as nonseptic, suppurative inflammation.
1. CSF analyses: Pandy 1+ microprotein 86 mg/dl, 8000 WBC.
2. Rabies: Negative.
3. Herpesvirus: 1:120.
4. Eastern Equine Encephalitis: 1:640.
5. Western Equine Encephalitis: 1:120.
6. Venezuelan Equine Encephalitis: 1:20.
7. Eastern equine encephalitis virus was isolated from the brain tissue examined.
Contributor's Diagnosis and Comments: Acute meningoencephalitis, hemorrhage, microgliosis.

Etiology: Alphavirus of eastern equine encephalitis (EEE).
The south central United States is still plagued by cases of EEE. We wonder if the recent boom in the ratite industry may have spread EEE because these birds are exquisitely sensitive to EEE infection.
The clinical history and histologic lesions are typical of EEE. Unlike many viral CNS infections, there are many neutrophils in the lesion. The pattern of microgliosis is often exaggerated around vessels. The titers indicate that this horse has had recent exposure to EEE; however, an assay for anti-EEE IgM was not performed. For an unvaccinated horse, the titer to equine herpesvirus is high. The lack of good vasculitis would also argue against the diagnosis of equine herpesvirus-1 (EHV-1). The alphaviruses do cross react, so the presence of titers to EEE, WEE, and VEE is not surprising.
AFIP Diagnosis: Cerebrum: Meningoencephalitis, lymphoplasmacytic and neutrophilic, diffuse, mild to moderate, with multifocal vasculitis and rare neuronal degeneration and necrosis, quarter horse, equine.
Conference Note: Eastern, Western, and Venezuelan equine encephalomyelitis (EEE, WEE, VEE) are important diseases of horses. These encephalitides are caused by Alphaviruses of the family Togaviridae. Alphaviruses are small (35 nm), enveloped, single-stranded RNA viruses transmitted by insect vectors, primarily mosquitos. Clinical cases of disease occur most often during the summer months when insects are most active and abundant. These arthropod-borne viruses occasionally cause neurologic disease in other vertebrates, including humans.
Horses infected with these viruses present with similar clinical signs. Clinical disease varies from inapparent infection with mild fever, to severe systemic illness characterized by leukopenia, depression, tachycardia, anorexia, and occasionally diarrhea. EEE and VEE tend to have a peracute course, while WEE is frequently subacute.

Susceptible horses become infected after a bite from a mosquito whose saliva contains the virus. Viremia develops following the insect bite, and it is suspected that subsequent hematogenous dissemination to the brain and invasion across the vascular endothelium of the CNS occurs. The conference moderator believes that the virus gains entry to the CNS across the blood-brain barrier by way of fenestrated capillaries located in the olfactory lobes. The viruses subsequently infect neurons, causing functional neuronal disturbances and structural disruption of the neuropil due to the inflammatory reaction.
The lack of gross lesions in the brain is typical of alphaviral encephalitis in horses. Microscopically, lesions occur predominately in the gray matter and are most prominent in the cerebral cortex, thalamus, and hypothalamus. There is prominent perivascular cuffing of inflammatory cells, the endothelium is hypertrophied, and vascular necrosis and thrombosis are seen in some cases. In EEE, the inflammatory response is primarily neutrophilic, while in VEE there is a mixture of neutrophils and lymphocytes. In WEE, which is often characterized by a longer clinical course than EEE and VEE, nonsuppurative encephalomyelitis predominates. Infected neurons undergo degenerative changes, such as swelling and margination of the nuclear chromatin, which may progress to neuronal necrosis. Other microscopic lesions include gliosis, small areas of rarefaction, and replacement by gitter cells.
Other viral diseases considered by conference participants included those caused by the flaviviruses (louping ill, Japanese encephalitis, Powassan virus); Borna disease virus (unclassified RNA virus); rabies virus (rhabdovirus); equine infectious anemia (lentivirus, Retroviridae); and equine herpesvirus type-1.
Contributor: Texas A&M University, Department of Veterinary Pathobiology, College of Veterinary Medicine, College Station, TX 77843-4467.
1. Tully Jr. TN, Shane SM, Poston RP, England JJ, Vice CC, Cho D-Y, Panigraphy B: Eastern equine encephalitis in a flock of emus (Dromais novaehollandiae). Avian Dis 36:808-812, 1992.
2. Brown TP, Roberts W, Page RK: Acute hemorrhagic enterocolitis in ratites: Isolation of eastern equine encephalomyelitis virus and reproduction of the disease in ostriches and turkey poults. Avian Dis 37:602-605, 1993.
3. Summers BA, Cummings JF, de Lahunta A: Inflammatory diseases of the central nervous system. In: Veterinary Neuropathology, eds. Summers BA, Cummings JF, de Lahunta A, pp. 144-146, Mosby Year-Book, St. Louis, MO, 1995.
4. Jones TC, Hunt RD, King NW: Diseases caused by viruses. In: Veterinary Pathology, Jones TC, Hunt RD, King NW, eds., 6th ed., pp. 288-291, 369, Williams & Wilkins, Baltimore, MD, 1997.
5. Fenner FJ, et al.: Togaviridae. In: Veterinary Virology, Fenner FJ, et al., eds., 2nd edition, pp. 431-439, Academic Press, San Diego, CA, 1993.
6. Miller MJ: Viral taxonomy. Clin Infect Dis 25:18-20, 1997.
WSC Coordinator:
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
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