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Southern Pines Equine Associates

Case of the Month Case Report: Equine Rabies

Eleanor Lenher, DVM
Southern Pines Equine Associates
Southern Pines, NC

On 08/01/02, a twelve year-old, Tennessee walking horse, gelding was seen on a field call for the primary complaints of pyrexia, inappetence, and colic. His owner had been out of town for a short period of time until 07/31/02. On 07/31/02, he, along with the other horses on the property, was vaccinated with his first West Nile vaccine. He was noticed that day to be quieter and did not walk out of his stall as readily as usual. The morning of 08/01/02, his owner noted he had not eaten his grain, his rectal temperature was 104 degrees F, and that he was pawing and demonstrating signs of abdominal pain. His vaccination history was unknown prior to purchase and since he was purchased he had only received the recent West Nile vaccination. The horse had been purchased in Florida and brought to North Carolina six months prior to examination.

Upon examination his mentation appeared obtunded, his heart rate was 60 bpm, his respiratory rate was 14 bpm, and his rectal temperature was 102 degrees F. His mucous membranes were dry, petechiated, dark red with a yellow hue, and injected with a toxic line. His capillary refill time was greater than 2 s. Gastrointestinal auscultation revealed decreased to absent borborygmi in all 4 quadrants. Digital pulses were within normal limits (WNL). Rectal palpation was unremarkable. Nasogastric (NG) intubation revealed no nasogastric reflux. He was administered two ounces DSS, one gallon mineral oil and four liters of water with added electrolytes via the NG tube. He was also treated with 500 mg flunixin meglumine and 500 mg prednisolone sodium succinate intravenously. Blood was collected for complete blood cell count and biochemical profile. Hospitalization and supportive therapy were recommended and the owner declined based on financial constraints. The owner was instructed to monitor closely the horse's attitude, comfort, rectal temperature, water consumption, and appetite. If the horse did not improve or deteriorated, the owner was instructed to call immediately for further recommendations. Shortly following treatment, the horse appeared to be more comfortable and less obtunded. Mild hypocalcemia, mild hyperglycemia, and evidence of moderate dehydration and anorexia were the only remarkable findings identified on blood work.

The owner reported that as the day progressed he seemed to deteriorate and he was subsequently admitted to the clinic at 10 PM that night for further treatment. Intravenous fluid, electrolyte, and dimethylsulfoxide therapy was initiated in addition to the previously initiated flunixin meglumine. His cardiovascular and hydration status improved over night, but the following morning he had begun to show signs of urinary incontinence. Through the course of the morning he became severely ataxic, developed unilateral facial nerve paresis on his left side, and had intermittent episodes of facial muscle twitching. His mentation was obtunded although he appeared aware of his surroundings and responded to auditory and visual stimuli. Blood gas and electrolyte analysis revealed mild hypokalemia, hyponatriema, and hypocalcemia. He was treated additionally with dexamthasone sodium phosphate intravenously and started on oral pentoxyfilline, omeprazole, and an oral electrolyte supplement. He was offered a bran mash and hay without much interest displayed. Upon recognition of neurological disease, standard isolation protocol was initiated and serum samples were collected for available diagnostics of suspected etiologies of acute onset neurological diseases.

At this point, initial differential diagnoses included Eastern equine encephalitis (EEE), West Nile virus (WNV), equine protozoal myeloencephalitis (EPM), equine herpes virus, type-1 (EHV-1), Western equine encephalitis (WEE), rabies virus, botulism, and other infectious encephalitides.

Although the WNV vaccine is a killed virus, communication with Fort Dodge was initiated because of the proximity of the WNV Vaccination to the onset of clinical signs. Even though the likelihood of vaccine-induced encephalitis was very low and no vaccine reaction similar to this case had been reported, Ft. Dodge offered complete support for diagnosis and treatment of this case. This enabled hospital care of this horse to continue beyond the owner's financial limitations.

Clinically the horse deteriorated rapidly over the following 48 hours with progression of bruxism, recumbency, and dementia. He continued to demonstrate intermittent pyrexia. Supportive therapy was continued during this period until humane euthanasia was elected on 08/04/02 and the corpse was submitted to Rollins State Laboratory for post-mortem examination. Post-mortem examination was performed that day and definitive diagnosis of rabies was made.

This horse's clinical signs could have fit many neurologic diseases including rabies. Horses infected with rabies may present with any combination of clinical signs. Undulating fevers are not uncommon; colic is very often the first noticed sign. Ataxia, visual deficits, hyperesthesia, hind limb paresis, lameness, extra ocular muscle spasms, hyperactivity, aggressive, and/or erratic behavior are some of the more common complaints. Hydrophobia is not common in equids. Diagnosis is made by postmortem examination of the brain. Fluorescent antibody (FA) testing is used to identify rabies antigen in the brain tissue. If this test is negative, histopathology is performed and can be used to identify pathopneumonic intracytoplasmic inclusions (Negri bodies).

Clinical signs of EEE, WEE, and VEE are similar. Acutely, the horse may have a fever or inappetence. WEE may not progress beyond this acute phase of the disease. In EEE signs become more severe and usually lead to death. These signs are consistent with CNS disease and can include undulating fever, behavioral changes, head pressing, aimless wandering, and blindness. Cranial nerve dysfunction may be seen as well. Antemortem diagnosis is difficult. Acute serum titers and convalescent serum titers can be used, as can complement fixation and cross-serum neutralization assays. Results must be considered with vaccination history. CSF analysis along with virus isolation can be used as well.

WNV has clinical signs similar to the encephalitides as well. Those signs include listlessness, stumbling, incoordination, partial paralysis, weakness or ataxia of the hind limbs and death. Many horses can be exposed without ever developing clinical signs. West Nile virus can be diagnosed through serum or CSF IgM ELISA or PCR.

EPM has variable clinical signs. Acute infections with EPM can lead to recumbency. A more insidious onset of the disease can be lameness, or signs of spinal cord disease. CSF sampling for Western blot analysis is necessary for diagnosis.

EHV-1 usually occurs in outbreaks. Alteration in rear limb gait is the first sign usually noticed. Lesions of the caudal spine are most common, resulting in ataxia, urinary incontinence, and paralysis. Diagnosis is usually made from clinical signs and number of horses involved in an outbreak. Virus isolation is required for antemortem diagnosis. Acute and convalescent serum titers may also be performed.

Prevention is the best treatment for most of the neurological diseases mentioned in this case report. Efficacious vaccines are available for EEE, WEE, VEE, rabies virus, and EHV-1. The WNV vaccine is now available and initial studies are promising for efficacy.

As with management of any horse with neurological disease, this case was very labor intensive. Thank you to Fort Dodge for their generous support in this case. To date there have been no significant reactions to the WNV vaccine but Fort Dodge diligently pursues any cases that could be related to the WNV vaccination. Thank you to Heather Burkhardt, DVM, the attending clinician on this case. Thank you to Laura Kellam, DVM and Ali Morton, DVM for their clinical consultations on this case. Thank you to Sarah Kohn, Cindy Hudson, Michelle Martin and Csaba Foldvari-Nagy for their technical support and help with this case.

Questions or comments regarding this article should be directed to Eleanor Lenher at Southern Pines Equine Associates or via email at appts@spequine.com .

e-mail Dr. Jim Hamilton, DVM
Southern Pines Equine Associates
Phone  910-692-8640      fax 910-692-1142
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