What is Borna Disease?
Borna disease is caused by one of just a few bornaviruses that occur in a wide variety of warm-blooded animals. This virus attacks the nerve cells within the brain itself causing dangerous levels of inflammation. This virus is frequently lethal once symptoms begin presenting themselves, however, antibodies specific to the virus have frequently been found in the blood of animals who are clinically unaffected. This suggests that some horses and other animals may become infected and remain asymptomatic.
Borna disease is a rare viral infection that causes swelling of the nerve cells in the brain, leading to unusual behaviors and seizures, and is frequently fatal.
Symptoms of Borna Disease in Horses
The symptoms of Borna disease usually occur in horses two to three months after the initial exposure, although there have been reports of incubation periods exceeding six months.
- Chewing motions
- Colic symptoms
- Lack of coordination
- Loss of appetite
- Muscular tremors
- Rapid and involuntary eye movement
- Sawhorse stance
- Severe depression
The bornavirus that is responsible for Borna disease in horses is one of just seven species of bornavirus.
- Elapid 1 bornavirus - The most recently classified bornavirus the Elapid 1 virus was recognized in a garter snake
- Mammalian 1 bornavirus - This is the variety of bornavirus that causes clinical symptoms in mammals; it is fatal to a large percentage of horses and sheep and has recently been implicated in a few human deaths in Germany
- Passeriform bornaviruses 1 & 2 - These varieties of bornavirus affect canary and finch type birds
- Psittaciforme bornaviruses 1 & 2 - Also known as avian bornavirus, the psittaciforme form of the bornavirus is believed to be the cause of PDD, a devastating disease in parrots that interferes with the proper digestion of food
- Waterbird 1 bornavirus - This variety of bornavirus infects waterfowl like ducks and geese
Causes of Borna Disease in Horses
The virus that causes Borna disease in horses is closely related to the bornavirus that affects birds and is believed to be the cause of Proventricular Dilatation Disease (PDD) in birds. The transmission methods for these viruses are not well understood, but direct contact with either saliva or nasal secretions are believed to be a common vector. Borna disease emerges more frequently on farms with poor rodent control and hygiene, and instances seem to peak during the months between March to June.
Diagnosis of Borna Disease in Horses
Your veterinarian will want to perform a full physical on the horse, including standard blood tests like a biochemical profile and complete blood count. These tests will help rule out some of the other causes of similar neurological symptoms such as lyme disease, equine protozoal myeloencephalitis, and toxic plant poisoning. The veterinarian will also perform a neurological examination to assess the horse’s current functionality. This can include limb placement tests, holding one leg while the horse moves forward, and walking the horse up and down an incline.
These tests help the examiner assess the horse’s awareness of the placement of their legs as well as the overall stability of the animal. The functionality of the cranial nerves that control the ability to swallow, see, and salivate will also be assessed. Samples of the CNS fluid drawn from the spine may show indications of an infection, but they are usually unable to pin down the specific virus.
Treatment of Borna Disease in Horses
Supportive treatments may be offered to ease symptoms, such as IV fluids to prevent dehydration, anti-inflammatory medications to ease pain, and medications like Valium to alleviate tremors and seizures, however, there are no consistent treatments for bornavirus infections. Many advancements regarding the transmission and mechanism of this virus have been made, but it remains fatal in sixty to ninety-five percent of the horses that show clinical signs.
Antivirals such as amantadine sulfate are used to fight equine influenza and have been successful in inhibiting the in-vitro replication of the bornavirus, but research is still ongoing. Although there have been many attempts to create an effective vaccine against Borna disease they have had mixed results. Some have shown evidence of effective inoculations, whereas others have actually exacerbated the disease. Horses with bornavirus infections should be quarantined from both other horses as well as from sheep as both are susceptible to the virus.
Recovery of Borna Disease in Horses
The prognosis for horses that show clinical signs of Borna disease have a poor prognosis. This progressive disease ends up being fatal for a large percentage of equines and most of those that survive have severe neurological damage that can interfere with basic functions. Euthanasia is often recommended for horses that have contracted bornavirus as their quality of life is often severely impacted even after the actual infection is eliminated. Owners of horses that develop this disorder should contact the appropriate regulatory agency as this disease has, on extremely rare occasions, jumped from other animals into human hosts.