Description

Borna disease is a type of brain infection. Microscopically, the disease is characterized by an inflammation of nerve cells in the brain. BDV-specific antibodies are frequently found in clinically healthy horses leading to the conclusion that horses may be infected, but remain asymptomatic.

The cause of Borna disease is an enveloped, negative-strand RNA virus that has not been fully characterized. It affects the nervous system and is disseminated from the site of infection through the nerves. The Borna virus appears in the brain and cerebral spinal fluid three days after infection by the virus. The virus can also be isolated from salivary and mammary glands and the nasal mucous membrane.

The incubation period is usually two to three months. Changes in behavior and disturbances in motor and sensory functions are the first noticeable signs of the disease. Animals lose coordination, appear to be in a severe depression and fit the "sad horse" description described in veterinary literature from the mid 1700's. Borna disease seems to be more prevalent in the spring and early summer months.

Symptoms

• Lack of coordination
• Yawning
• Chewing movements
• Muscle contractions
• Mild colic
• Loss of appetite
• Hypersensitivity to stimuli
• Severe depression
• Sawhorse stance
• Leaning against objects
• Rapid and involuntary eye movement

Causes

Currently, it is believed that transmission of Borna disease virus in cases of natural infection occurs through direct contact with infective nasal secretions and saliva, or through contaminated food and water. The natural host range of BDV is geographically wide and has been underestimated in the past. At present, it is believed that the host range of BDV most likely includes all warm-blooded animals from birds to primates.

Prevention

Experimental vaccines for immunopathology have had mixed results. In some cases, the immunopathology of the infection caused some vaccines to exacerbate the disease, but recent evidence suggests the possibility of effective inoculation. Although some preliminary research indicates success with protective vaccination of mice, at this writing, no pre-exposure vaccine has been developed for use with horses.

Treatment

The antiviral drug amantadine sulfate may be a potential treatment for Borna disease virus. It has been demonstrated in vitro to inhibit wild-type BDV replication and spread of infection. As with prevention of the disease, much research is needed to develop successful treatment strategies because, currently, the fatality rate ranges between 60 to 95 percent for horses, and animals that survive often remain neurologically impaired permanently.