In the animal world, cats are the species most often affected by chronic kidney disease. But slow deterioration of the kidney can occur in any animal, including horses.
Kidneys are vital organs that have many important bodily functions. This pair of dark red organs sits tucked up along the lower back. They filter out nitrogen wastes created from metabolizing food. They produce erythropoietin, a hormone that stimulates the production of red blood cells. (This hormone is used in illicit doping in human athletes.) Kidneys also play an important role in vitamin D regulation, maintain salt levels and produce urine.
Unlike other domestic animals, horse kidneys have calcium-secreting glands. The presence of this calcium mineral gives horses’ urine a sludgy, cloudy appearance. These glands have a crucial role in regulating calcium levels because horses typically eat high calcium food.
Because bodies have two kidneys, they are able to compensate for damage. It is generally accepted that more than 75 percent of the kidney tissue needs to be damaged before clinical signs of kidney disease develop.
There are many things that can damage the kidneys in horses including non-steroidal anti-inflammatory medications such as phenybutazone, or bute. These medications are often given to older horses to manage pain. They act through the inflammation pathways, which reduces the molecules that create pain. But they also alter blood flow in a variety of organs, including the kidney.
A high dose of bute or similar medication combined with dehydration can cause severe damage to the inner portions of the kidney.
Tying up is another frequent cause of kidney damage. The muscle damage from tying up releases molecules that are filtered by the kidneys, damaging the tissue in the process and creating the characteristic dark red urine.
Sometimes the cause of kidney damage is unknown and may be a function of old age. Often by the time the kidney disease manifests, the inciting cause is unknown.
The appearance of chronically diseased kidneys is similar, no matter what the inciting cause: large portions of the kidney are replaced by scar tissue and there can be large cystic structures, thickening of the filtering part of the tissue and low-grade inflammation.
Clinical signs of kidney disease include increased thirst and increased frequency of urination. Other signs are less obvious. Horses can lose weight even with a good appetite. They may have reduced energy and performance as a result of anemia (low number of red blood cells) that accompanies kidney disease. Another non-specific sign is a rough hair coat.
Kidney disease in horses is diagnosed through blood tests that look for changes to blood urea levels and the muscle molecule, creatinine, which is also excreted by the kidney.
The blood tests are combined with analysis of the urine itself.
The urine can be tested for concentration because healthy kidneys produce concentrated urine while those with severe disease cannot. It can also establish if the kidneys are leaking proteins and if it contains substances it shouldn’t, such as microscopic pus or blood. Additional tests include rectal palpation to feel the bladder and kidneys as well as ultrasound.
When it comes to treating chronic kidney disease in horses, it is important to keep in mind that this is a slowly progressing disease with a poor prognosis. The treatment goal is not a cure but to slow down the damage. Intravenous fluids, diet changes and constant access to water are common supportive treatments. It may also be necessary to stop any medications that may damage the kidney further.
Kidney disease is not that common in horses but likely goes underdiagnosed. It is one of many reasons to talk to a veterinarian about screening blood and urine tests, especially in older animals.
Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.