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Tuberculosis (TB) is an infectious disease caused by the acid-fast bacilli of the genus Mycobacterium. Strains of this disease can affect mammals of all species, including humans, horses, cattle, cats, and dogs. TB can take various forms, and although it is often a chronic, debilitating disease, it may also have acute onset.
M tuberculosis has developed multidrug-resistant and extensively drug-resistant strains and therefore this strain should be reported to public health and regulatory officials. Horses are most commonly infected with bovine TB, which may still carry a zoonotic risk; this means it may be passed to humans. Human infection of this form of TB occurs through ingestion of infected meat or milk. Therefore, disease control is critical to prevent infection of stock.
Unfortunately, the prognosis following infection with TB in horses is grave. However, diagnosis is essential. If your horse is unwell, or suffering from the symptoms of TB it is vital that they are isolated from other animals and your veterinarian is contacted.
The symptoms of tuberculosis in horses often include:
Horses are mostly resistant to the multi-drug resistant form of tuberculosis caused by Mycobacterium tuberculosis. Horses are most commonly affected by M bovis which can cause tuberculous lesions to form in the lungs as well as throughout the body, most commonly in the liver and lymph nodes.
M. bovis is very resilient strain which can survive for months in cold, damp conditions, making manure and soil an ideal environment. The bacteria is often killed in sunlight and dry atmosphere.
Although this disease is zoonotic, exposure to the disease due to contact with infected animals is unlikely to cause infection. Instead, humans become infected through drinking unpasteurized milk from infected cows.
Transmission of the disease is most commonly through inhaled bacilli in infected droplets that enter the lung. Other modes of transmission are through the ingestion of infected milk or water; however, this is less common. Once in the lungs, the alveolar macrophages may destroy the antigens, or mycobacterial proliferation may occur. In this case, cell buildup can lead to purulent lesions forming that may become calcified, leading to granulation of tissue and the formulation of tubules typical of the disease. This is known as the primary complex of the disease. This form of the disease rarely heals, and has been seen to progress in slow chronic disease, as well as acutely. In some cases, disease may spread to the lymph nodes, tonsils and intestines and nodular lesions may form throughout the body on all organs and the central nervous system. Risk factors for the disease include:
Upon examination, your vet will discuss your horse’s clinical history with you. It is vital that you inform them of any concerns you have, symptoms you have noticed, or changes to your horse’s behavior.
Your vet will perform a full clinical examination on your horse. Initially, they will observe your horse from a distance to watch behavior and gait. They may notice signs such as lethargy or weakness, or increased respiratory effort.
Your veterinarian will then perform a head to tail examination. They will carefully run their hands over your horse’s body. They may be able to feel enlarged lymph nodes which could be a sign of this disease. They will auscultate the lung, heart, and gastrointestinal tract. Your vet may be able to detect damage to the lungs through auscultation and percussion.
The intradermal tuberculin test can be performed on your horse for diagnosis, although this test is often considered unreliable. This test is performed by injecting tuberculin into the subcutaneous layer of the skin. In positive results a raised, hard area will be seen.
Unfortunately, treatment is seldom given for tuberculosis. Instead, humane euthanasia should be considered to prevent suffering and further spread of the disease. Documentation is limited as to available treatment due to lack of successful attempts to cure an equine of the disease. Effort and expense may be prohibitive, particularly as the condition is often discovered in the late stages.
The prognosis for animals infected with this disease is unfortunately grave. Therefore, prevention and reduction of risk is the most important factor in the control of TB in horses.
Work with your veterinarian to provide your horse with excellent health care and appropriate vaccinations. By supporting your horse’s immune system, you reduce the chance of immune system compromisation which may prevent your horse from fighting the disease if contact occurs.
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