5-year-old Thoroughbred mare horse, Equus caballus.The horse presented to the University of Melbourne referral equine clinic with acute, severe, uncontrolled abdominal pain. On examination, the mare was distressed and had no gut sounds, pale pink mucous membranes and marked tachycardia (heart rate 100). A displacement of the large colon was suspected based on rectal findings and at exploratory laparotomy, volvulus of the left dorsal and ventral colon was corrected. There was no visible compromise of the bowel and the volvulus did not appear to fully explain the severity of pain. Subsequently, the mare exhibited signs of severe pain despite intense multimodal analgesia including nonsteroidal anti-inflammatories, alpha-2 agonists, lignocaine and ketamine constant rate infusions and morphine. The horse was euthanized approximately 24 hours after initial presentation. Euthanasia was based on the severity and refractory nature of the pain, of unknown origin.

Gross Description:  

The animal was in lean, fit body condition, with moderate amounts of internal body fat. There was a surgical incision site on the ventral midline of caudal abdomen with associated subcutaneous edema. A small amount of fibrin was noted on and over the serosa of the ventral cecum. The brain and spinal cord were grossly within normal limits. The CSF collected was watery and colorless (within normal limits).

Histopathologic Description:

Sections of brain and spinal cord showed widespread changes affecting mainly the grey matter, with lesions sparing the cerebral cortex, but involving midbrain, brain stem, and spinal grey matter especially in thoracic and lumbar regions, but without any significant change in the nerve roots, ganglia or peripheral nerves. The cerebellar cortex was also spared.

Lesions consisted of thick perivascular cuffs comprised predominantly of small lymphocytes, with lesser numbers of large lymphocytes, plasma cells, and macrophages. In some areas, inflammatory foci extended into the parenchyma, and were associated with neuronal necrosis, especially in the spinal cord. Lesions were intense in the lumbar grey matter. A careful search for protozoa or inclusion bodies failed to reveal any. Apart from a few small ring hemorrhages perivascularly in the brain stem, the lesions were devoid of hemorrhage.

Morphologic Diagnosis:  

Brain and spinal cord: Severe chronic non-suppurative polioencephalomyelitis consistent with Murray Valley Encephalitis.

Lab Results:  

Fibrinogen: 5.6 g/L (2.0 4.0)
Total bilirubin: 68 μmol/L (0 40)
AST: 457 U/L (150 400)
CK: 3439 U/L (50 400)
Total protein: 58 g/L (58 76)

CSF fluid collected at post mortem was within normal limits, with low cellularity and only a few lymphocytes identified in cytospin preparations.

Formalin fixed sections of brain and spinal cord were positive for Murray Valley Encephalitis (MVE) using PCR, and were negative for EHV-1 using PCR.


Murray valley encephalitis virus

Contributor Comment:  

The horse described had both histological evidence of encephalitis and a positive PCR result for Murray Valley Encephalitis virus. Although the PCR has not been validated for fixed tissue, the histological lesions along with the PCR result are highly suggestive for Murray Valley Encephalitis (MVE). Experience with previously confirmed cases of EHV-1 at this institution has often showed extensive hemorrhagic lesions of brain stem and spinal cord of horses, with minimal inflammation. Based on the clinical presentation, CNS pathology and the negative PCR reaction, this case is not consistent with EHV-1.

There have been two neurological syndromes seen in horses in south-eastern Australia in 2011, peaking in March and April (autumn/fall) associated with arboviral infections: central neurological and musculoskeletal clinical diseases, with some overlap in the lesser affected horses. Neurological signs have most commonly included: ataxia, depression, behavioral changes, tremor, hyperesthesia, muscle fasciculations, hypermetria and colic. (1) Musculoskeletal signs reported in horses that have shown no CNS signs include: listlessness, reluctance to walk, stiff gait, pyrexia and anorexia.(1)

Three viruses- Murray Valley Encephalitis (MVE) virus, Kunjin virus and Ross River Virus (RRV), have been associated with these two syndromes. MVE and Kunjin have been most commonly associated with the neurological syndrome and RRV with the musculoskeletal syndrome. Horse deaths have occurred associated with Murray Valley Encephalitis and Kunjin, but most (≥ 85%) of the horses affected have recovered with supportive treatment.(2) The included table shows serological / viral data from thirteen horses with post mortem evidence of encephalitis / encephalomyelitis (spinal cord often not submitted) seen by the Department of Primary Industries, Victoria:
Number of horses with histological evidence
of encephalitis
MVE KunjinHendra
5* + - -^
6** - + -
1* + + NA
1 - - NA

*, The diagnosis is based on demonstration of viral agent(s).
**, In 2 of the 6 horses the diagnosis was based on serology results; the presence of antibodies against Kunjin virus and absence of antibodies against MVE. Kunjin virus was detected by PCR and/or virus isolation in the other 4 horses.
^, One of the 5 horses was not tested.
NA, Not Assessed

Tests used:
The prevalence of all three arboviruses this year is related to very high rainfall experienced over the preceding spring, summer and autumn and the resultant increase in mosquito vectors. The increase in water may also have affected the distribution of water birds (the main reservoir hosts for MVE and Kunjin). In many parts of Victoria the 2010-2011 rainfall measurements have been more than double the long term mean for each area.(3) This is particularly significant as most of the state has been in drought for up to 14 years (depending on area), with rainfall in these years often being much lower than the mean.

MVEV and Kunjin are Flaviviruses present in northern Australia, Papua New Guinea and Indonesia,(4) with more widespread Australian distribution when the seasonal conditions are conducive. Kunjin is closely related to West Nile Virus.(8) The main vector in Australia is Culex annulirostris and the main hosts appear to be water birds, although antibodies have been found in many bird and mammalian species.(4) Usually humans are subclinically infected, but MVE can produce mild disease featuring fever, headache, nausea and vomiting and with Kunjin, a rash, swollen joints, muscle weakness and fatigue. Rarely, both viruses can cause severe disease of meningitis or encephalitis sometimes resulting in death.

RRV is an arbovirus (Alphavirus) which commonly causes human disease in Australia, with approximately 5000 notifications yearly.(4,6) Symptoms of infection in humans include joint pain, joint effusion, rash and pyrexia.(4,6) In horses, the virus is known to cause in the musculoskeletal system the symptoms described above. Macropods and other marsupials are suspected to be the main reservoir species.

Hendra virus has been included in testing to rule out the possibility of this rare, zoonotic disease, which is not an arbovirus. All tested horses have been negative. Horses with Hendra virus infection can have very similar signs to those discussed previosuly including depression, fever, neurological signs and colic.(5) Another common (but not always present) clinical presentation is respiratory disease.(5)

JPC Diagnosis:  

Spinal cord: Poliomyelitis, lymphohistiocytic, diffuse, severe, with marked neuronal degeneration.

Conference Comment:  

The differential diagnosis for lymphohistiocytic poliomyelitis in a horse should include Murray valley encephalitis virus (MVEV), West Nile virus (WNV), Kunjin virus, the alphaviruses (Eastern, Western, and Venezuelan encephalitis viruses), other Flaviviruses such as Japanese encephalitits virus and Dengue virus, Borna virus, and Rabies virus. The lack of Negri bodies usually seen with rabies, lack of Joest-Degen bodies usually seen with Borna virus, and lack of neutrophilic involvement usually seen with the alphaviruses and other flaviviruses makes these conditions less likely. Of these remaining candidate conditions, MVEV is the only one to produce disease with the severity seen in the present case. WNV produces mild to moderate lesions of nonsuppurative polioencephalomyelitis with multifocal gliosis and occasional neuronal necrosis, and primarily affects the gray matter of lower brainstem and thoracolumbar spinal cord. Kunjin virus is typically even less pathogenic than WNV.(7,9)


1. (30th May, 2011).
2. (30th May, 2011).
3. (30th May, 2011).
4. Carver S, Bestall A, Jardine A, et al. Influence of hosts on the ecology of arboviral transmission: Potential mechanisms influencing Dengue, Murray Valley encephalitis, and Ross River virus in Australia. Vector Borne Zoonotic Dis. 2009;9(1):51-64.
5. Field H, Schaaf K, Kung N, et al. Hendra virus outbreak with novel clinical features. Australia. Emerg Infect Dis. 2010;16(2):338-40.
6. Harley D, Sleigh A, Ritchie S. Ross River Virus Transmission, infection and disease: a cross disciplinary review. Clin Microbiol Rev. 2001;(4): 909-932.
7. Maxie MG, Youssef S. Nervous system. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals. 5th ed. Vol 1. New York, NY: Elsevier Saunders; 2007:421-425.
8. Barthold SW, Bowen RA, Hedrick RP, et al. West nile virus. In: MacLachlan NJ, Dubovi EJ, eds. Fenners Veterinary Virology. 4th Ed. London, UK, Academic Press; 2011:472.
9. Zachary JF. Nervous system. In: McGavin MD, Zachary JF, eds. Pathologic Basis of Veterinary Disease. 5th ed. St. Louis, MO: Mosby; 2011:805-6, 839-40.

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