Author: Veterinarian Mimma Aromaa
I work as a veterinarian at Kaarinan Eläinlääkäriasema Oy (Kaarina, Finland). In particular, I am interested in the internal diseases of cats and dogs, ie. vomiting, diarrhoea, and weight loss - sluggish and tired patients are a daily challenge for me. I specialize in extensive abdominal ultrasound examinations. My two Burmese cats are my daily guide to the interesting soul landscape of cats.
The main function of the kidneys is to filter metabolic waste products from the blood. The kidneys also regulate the body’s fluid balance, acid-base balance and electrolyte balance. In addition, the kidneys take part in the regulation of blood pressure and red blood cell synthesis. The capacity of the kidneys to maintain normal functions is very high. This means that signs of renal insufficiency, or poor function of the kidneys, are seen only when over 70 % of kidney function is lost. Feline kidneys are capable of retaining water very efficiently due to the domestic cat’s ancestors having accustomed to very dry desert conditions. Thus, scarce drinking of normal symptomless cats is not alarming and urine produced by cats is very strong.
A cat with renal insufficiency can no longer retain water as efficiently or produce very concentrated urine typical to the species. This results in thirst, which leads to increased drinking and urinating so that the metabolic waste products are eliminated. Dehydration of the body can also cause constipation. However, increased drinking and urinating might not be the most distinct or first sign of renal insufficiency in a cat. Healthy cats that eat mainly dry kibble drink more than those eating wet or raw food. Some cats are prone to playing with water, and drinking from odd places such as the sink is normal. This means that quantifying limits for abnormal drinking is difficult and it’s important to note changes in your cat’s drinking habits. Is the cat seen drinking more than usual? Are the urine clumps in the litter box bigger than before? Is the change momentary or persistent and not explained by eg. a change in the cat’s diet?
Changes in blood values are seen when metabolic waste products accumulate in the body. Kidney values measured from blood are actually waste products of normal metabolism. The most frequently measured kidney value is creatinine, which is a breakdown product of muscle metabolism. Another common kidney value is urea (blood urea nitrogen or BUN), which is a waste product of protein metabolism. A rise in these values is indicative of impaired renal filtration. Especially the accumulation of urea leads to nausea and anorexia (a sustained loss of appetite) in the cat.
Changes in especially calcium, phosphorus and potassium are seen when the ability of the kidneys to control electrolyte balance fails. The changes in the balance between calcium and phosphorus lead to weakening of bones. Low potassium can be the cause of lethargy and weakness.
High blood pressure and anemia are common findings in the cat with chronic kidney disease, as the kidneys participate also in the regulation of blood pressure and red blood cell production. Chronic illness and poor appetite cause cachexia, or wasting syndrome, which is seen as a decline in body weight and a dry and brittle fur coat.
The most common reason for chronic kidney disease is idiopathic degeneration of the kidneys, which means that the reason for the degeneration is unknown. This disease progresses slowly and the changes are irreversible. Signs of idiopathic kidney degeneration are usually seen in aging cats over 7 years of age. It may be hereditary and is more common in certain breeds. There are also reasons that can lead to damage of kidney tissue in a cat of any age and thus lead to chronic kidney disease. For example, infections in the kidneys or elsewhere in the urinary tract, urinary obstructions, toxins (eg. certain drugs and poisonous plants), tumors, viral diseases such as FIV and FIP and disruptions of renal blood flow can have an effect on kidney function. Some of the aforementioned reasons may at first cause an acute renal injury, which might be reversible if treated efficiently and early on. In degenerative chronic kidney disease the progression of the disease is usually more straightforward. Determining prognosis and progression of the disease may be difficult in chronic kidney disease caused by other factors. In addition, the progression of the degenerative process may quicken or renal insufficiency worsen momentarily due to infections, medications or other chronic illnesses.
It is not always clear if the reason for chronic kidney disease is degeneration or another cause of renal insufficiency. Thorough patient history, signs and symptoms, blood and urine values and other examinations such as ultrasound examination of the kidneys and urinary tract and blood pressure measurement are key when assessing the situation. Kidney biopsies could clarify the situation and prognosis. However, the treatment of chronic kidney disease in cats does not differ much depending on the background factors, so renal biopsies are not routinely taken. The progression of the disease is monitored by keeping a close eye on the condition of the cat and taking blood and urine samples. Kidney values might be momentarily high due to eg. infections, obstructions, medications, dehydration and other factors. Continuously high kidney values or continuous indication of kidney disease in the urine are indicative of chronic kidney failure and the diagnosis cannot be definitively made from just one sample. All other possible reasons for renal insufficiency or the worsening of chronic kidney disease have to be ruled out and treated. When the cat’s condition is stable, staging of chronic kidney disease may be done by controlling blood and urine samples and measuring blood pressure.
The overall situation of the cat needs to be taken into account when interpreting kidney values. We cannot be sure two different cats are in the same stage of chronic kidney disease just by comparing their kidney values. The stage of chronic kidney disease is very different in a cachexic cat with moderate signs, a cat with a healthy weight and slight signs and a cat with a healthy weight and severe signs. The kidney values of the thin cat lacking muscle with moderate signs are probably falsely low due to low muscle mass. The prognosis for this cat is worse than for the cat with a healthy weight and slight signs. On the other hand, the acutely symptomatic cat with a healthy weight probably has acute renal failure, which has to be treated intensively in-hospital to support returning to normal kidney function. After this cat’s condition has stabilized, blood values need to be controlled to check for development of chronic kidney disease.
There is no cure for chronic kidney disease in cats. Life expectancy can be increased with proper diet, dietary supplements and supportive treatment. The cat needs enough water, treatment of possible constipation and it needs to be able to urinate often enough. Keeping a healthy weight is attempted with tasty food and treating nausea with medications if needed.
Control visits are planned individually with the cat’s condition, the owner’s options and the progression speed of the disease in mind. The disease is usually controlled every 3-12 months depending on these variables. A thorough physical examination, blood and urine samples, blood pressure and ultrasound examination are controlled on a case-by-case basis. The diet required for the particular stage of the disease is decided based on these results. The need for dietary supplements for balancing potassium and phosphorus, blood pressure medication and anti-nausea medication is assessed. Other co-occurring illnesses and infections need to be treated.