Borna disease, also known as contagious encephalomyelitis or head disease of ungulates, is a rare viral disease of the nervous system that mainly affects horses and sheep. However, other animals and humans are also susceptible to the Borna virus. The disease was first described in 1813, when a whole group of cavalry horses fell ill in the town of Borna.
The disease is caused by the Borna Disease Virus (BDV), a pathogen similar to the distemper, rabies, and measles viruses. The virus is present in the saliva and eye and nose secretions of infected animals, but also in their urine.
For a long time, it was assumed that transmission occurred through direct contact between animals. According to the latest scientific studies, small rodents, especially the common shrew, serve as a reservoir for the pathogen. It is therefore assumed that horses do not infect each other through contact, as previously assumed, but by sniffing infected mice. The virus first enters the olfactory region of the brain via the mucous membranes of the nose and throat. This would also explain the low regional spread of the disease, as common shrews always remain in the same area.
Borna viruses primarily attack the limbic system, the area of the brain that controls emotions and instinctive behavior and releases endorphins (the body's own opioids). This is where the pathogen multiplies and inflammatory reactions occur. It is also suspected that the virus causes an imbalance in brain neurotransmitters.
After an incubation period of several weeks to months, the exact duration of which is not known, a wide variety of clinical signs may appear, depending on the region of the brain affected. The most commonly observed symptoms include a wide range of behavioral changes, movement disorders, depression, empty chewing, nervousness, aggression, increased skin sensitivity, and teeth grinding.
Affected horses often push against walls or walk in circles, tremble, and may collapse. As the disease progresses, apathy, convulsions, and increased salivation occur. The final stage of the infection is characterized by recumbency, rowing movements, and fever spikes, eventually leading to death.
Due to the diverse and rather unspecific symptoms, which can also be signs of other infectious diseases such as rabies, herpes or botulism, diagnosis is not always easy for the veterinarian. After a thorough clinical examination and a detailed preliminary report from the owner, the veterinarian will be able to express an initial suspicion. Further blood or cerebrospinal fluid tests can be carried out to confirm the suspicion by providing possible evidence. However, the detection of antibodies in the blood serum alone is not sufficient, as this only indicates that the horse has already had contact with BDV.
A definitive diagnosis can only be made after the animal's death. In the histological examination of the brains of horses with Borna disease, the pathologist can find special inclusion bodies in the cell nuclei of the nerve cells, which are considered evidence of a Borna virus infection and are primarily detected in the olfactory brain.
Based on current knowledge, there is no known treatment that can cure the disease. In the event of an acute infection, the veterinarian can only attempt to alleviate the symptoms. In most cases, the disease leads to the death of the animal. Horses that survive the acute phase of the disease usually suffer from lifelong behavioral disorders.
Once the disease has broken out, there is little chance of recovery and it almost always ends in the death of the animal. Horses that have been tested positive for BDV antibodies but show no symptoms are referred to as seropositive animals. These animals are merely carriers of the virus and have a significantly better prognosis than horses that have fallen ill.
It is not possible to reliably prevent infection with the Borna virus. However, as the significance of the Borna virus for humans is still unclear, strict hygiene measures should be observed when handling animals that are likely to be infected.
However, Borna disease is particularly common in certain areas. These so-called endemic areas include Saxony, Saxony-Anhalt, Thuringia, Hesse, Baden-Württemberg, Bavaria, and Switzerland. For a long time, it was therefore recommended that horses from these areas be tested for the Borna virus before being transported to other areas. However, given that the most likely route of transmission is now believed to be via the field vole, this recommendation is no longer valid.
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