AFIP Wednesday Slide Conference - No. 27
April 21, 1999
- Conference Moderator: LTC A. Peter Vogel
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
- 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
- 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
- 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,
- 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
- 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
- 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
- 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
- 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
- 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,
- 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
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
- 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,
- 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,
- 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,
- 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,
- 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
- 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
- Contributor's Diagnosis and Comments: Acute meningoencephalitis,
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
- AFIP Diagnosis: Cerebrum: Meningoencephalitis, lymphoplasmacytic
and neutrophilic, diffuse, mild to moderate, with multifocal
vasculitis and rare neuronal degeneration and necrosis, quarter
- 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
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,
- 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
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