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This condition is rare in horses, however when seen is often in older animals following chronic infections, inflammatory disease or cancer. Often horses affected show signs such as chronic weight loss.
Amyloidosis occurs following the depositing of an amyloid. This substance is an abnormally folded protein, these proteins then deposit throughout the body. This may become fatal if the amyloids become deposited in organs such as the heart, liver, kidneys or lungs and disrupt normal tissue function. This may also present as tumor-like growths under the skin.
The symptoms of amyloidosis often vary depending on the organs that are affected, in cases where the lungs are affected bleeding from the nasal passages may be seen. Generalised symptoms may be:
This disorder occurs following protein folding leading to the deposits of insoluble fibrils. This can occur due to hereditary disposition in some animals, however, in horses this is often due to infections, neoplasia, inflammation or immune-related diseases. Hepatic amyloidosis may also occur following prolonged inflammatory stimulation in horses used for hyperimmune serum production; in some cases, these horses may suffer hepatic rupture.
There are at least 20 other types of protein that may become malformed and fold, becoming depositing in the body tissues. These include:
Deposits of this protein may be caused by chronic conditions such as long-term inflammation, bacterial infections or cancer. In this form of amyloidosis, the amyloids are often deposited in the spleen and kidneys. In animals where kidney deposits occur the build up may lead to kidney failure.
This type of amyloid is often deposited in the nerve tissues and joints.
Abyssinian cats and the Chinese Shar-Pei have shown to be predisposed to a genetic form of amyloidosis. Amyloidosis in older animals, or senile systemic amyloidosis, may occur following the deposits of amyloid in the meningeal and cortical arteries.
Amyloid deposits in the skin may present as tumor like nodules in horses. Some causes of this disease may be contagious, such as spongiform encephalopathies and scrapie in bovines: cheetahs are also known to shed an infectious form of this disease in their feces.
Your veterinarian will first observe your horse from a distance to check for abnormalities that may not be visible while restrained, such as respiratory rate and character and gait. They will then examine all the major organs and auscultate the lungs, heart and digestive system. If you have areas of particular concern it is important that you discuss these with your veterinarian so they are able to examine these closely. Your veterinarian will discuss your horse’s clinical history with you, including any change of behaviour, diet or symptoms that you may have noticed.
Often this disease is difficult to diagnose, although your veterinarian may suspect this disease due to clinical history, particularly if your horse has developed kidney or liver failure following chronic infection or inflammation. Your horse may have a urine sample taken for a urinalysis, protein in the urine may be seen.
Your veterinarian may choose to take a tissue sample for microscopic examination. This sample will be stained with congo red which is a sodium salt used for biochemistry and histology examinations. Under polarized light tissue that has amyloid deposits will show green birefringence, ruling out other similar presenting conditions such as build up of collagen or fibrin.
Amyloidosis is readily recognized at necropsy and in histologic sections by its affinity for dyes such as congo red. Following diagnosis, your veterinarian may choose to do further testing to determine the underlying cause of the condition.
There is no cure to prevent the development of amyloidosis or promote the amyloids to be broken down and excreted. Often the treatment received will be therapeutic during the diagnostic process. Further treatment may be an attempt to reduce the cause, such as infection or inflammation, and manage the symptoms. In some cases, the masses are able to be surgically debulked to improve life quality and reduce symptoms. In other instances, steroidal treatment may be indicated,though there is very little research to support the efficacy of this.
Surgical excision of masses may cause clinical improvement, but often this is often followed by rapid recurrence. Unfortunately, the prognosis for an animal with severe, systemic disease is often grave. In many cases, it is found that this condition is an incidental finding and may not have adverse effects. Therefore, the prognosis depends heavily on the area of which the amyloidosis has localised.
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