Renal problems

Renal function in brief

The kidneys are the body’s primary metabolic and detoxification organs and are responsible for removing the body’s metabolic waste products through urine. The kidneys also regulate the body’s fluid and salt balance. They are important for bone formation, because they are involved in the metabolism of calcium and vitamin D, and they also regulate the concentration of phosphorus in the body. The kidneys produce the hormones erythropoietin (EPO) and renin. EPO is involved in the production of red blood cells, and renin is involved in regulating blood pressure. The kidneys excrete urea and creatine, the end products of protein metabolism, into the urine.

The kidneys are paired, bean-shaped structures located in the retroperitoneal space directly beneath the sub lumbar muscles. They consist of the renal cortex, renal medulla, and renal pelvis. The renal functional filtration unit is called the nephron. The structure of the nephron can roughly be divided into two parts: the glomerulus and the renal tubule. The glomerulus is a network of blood vessels inside the glomerular capsule (Bowman’s capsule), and it forms initial urine, the so-called glomerular filtrate. The renal tubule has three sections: in the initial part, the substances are reabsorbed; the middle part concentrates the urine by removing water from the initial urine; and the last part transports the concentrated urine to the connecting tubule. Nephrons are normally destroyed to some extent during the life of the animal, but when the destruction accelerates to such an extent that approximately two thirds of renal function has ceased, the visible clinical signs of kidney disease appear. At this stage, the symptoms may be severe, even though the chronic disease has already been developing over a long period of time.

Cat kidney diseases

Kidney disease is very common in aging cats, and the number of kidney defects in cats over seven years of age increases dramatically. About 35% of cats over the age of ten years have some form of kidney disease. For example, kidney disease can develop from hereditary causes, poisoning, poor diet, urinary tract infections and urinary tract obstruction, other inflammations, or other illnesses. A very important factor in the destruction of kidney tissue in cats is high blood pressure. In cats, renal impairment is suspected if the cat drinks and urinates more than usual. The cat may vomit or be apathetic and tired. However, thirst and increased drinking for no apparent reason are usually the first signs of kidney disease. A cat with kidney disease may lose weight or even become anorexic. Poor coat quality can also indicate kidney problems. Renal failure often becomes chronic and is a lifelong, incurable disease.

Acute renal insufficiency

Acute renal insufficiency is a rapidly progressing disease in which kidney function can deteriorate in a few days or even hours. Acute renal insufficiency interferes with the acid-base and fluid-salt balances, and waste products accumulate rapidly in the body due to a dramatic decrease in renal function and urine production. Acute renal insufficiency is a serious condition with a mortality rate of up to 50%. Without treatment, the prognosis for acute renal insufficiency is poor. Early treatment improves the prognosis of the disease. Acute kidney damage can be caused by, for example, the following:

• Urinary tract obstruction
• Pyelonephritis (bacterial inflammation of the kidneys)
• Poisoning, such as ingesting lilies, car coolant (ethylene glycol)
• Snake bite
• Medicines, such as NSAIDs, certain cancer medicines, or antibiotics
• Sepsis
• Heat stroke or severe dehydration
• Vitamin D overdose

Acute kidney damage can cause physical changes in the appearance of the kidneys that are detectable by ultrasound. Ultrasound can be used to determine whether the renal pelvis has dilated because of urinary tract obstruction or pyelonephritis as well as identify the presence of urinary stones in the upper urinary tract. However, medical imaging does not provide a prognosis for the disease.

There is no specific drug treatment for acute renal insufficiency, and treatment also depends on the cause of the kidney damage. Treatment should be started quickly, and adequate hydration and blood acid-base balance restoration should be a priority. All medicines that are harmful to the kidneys, such as antibiotics, painkillers, and cancer medicines, should be discontinued. In urinary tract obstructions, the blockage is removed as soon as possible, and any infections are treated with antibiotics. If the patient urinates very little (oliguria) or not at all (anuria), hydration is proportional to the amount of urine excreted so that the patient is not overhydrated. Severe electrolyte or acid base imbalance can be treated with a variety of drug therapies. In acute renal damage, antihypertensive medication is also sometimes required since hypertension further accelerates renal damage.

Chronic kidney disease

In chronic renal insufficiency, the nephrons are usually damaged over a longer time period. Symptoms of chronic renal insufficiency may be vague, but increased drinking and urination are the first obvious signs. Nausea and vomiting may occur. Weight loss and anorexia are associated with the irritating effect of uremic toxins on the intestinal mucosa. Protein catabolism due to anorexia accelerates the production of uremic toxins, further aggravating the cat’s condition. Exhaustion and lethargy occur often. Clinical symptoms as well as biochemical changes in blood and urine appear at the stage when the kidneys are operating at less than 30% efficiency.

The insufficiency can be detected in a blood and urine sample. Blood tests often have elevated levels of urea and creatine, as the kidneys’ ability to filter these substances into the urine is impaired. The urine sample also often contains protein, and the urine might be dilute. Chronic renal insufficiency can cause anaemia, as the kidneys’ ability to produce the hormone EPO, which is necessary for the production of red blood cells, is reduced. Anaemia causes fatigue and weakness.

There is no treatment for damaged renal tissue other than kidney transplantation. Chronic renal insufficiency symptoms can often be relieved and the end stage delayed with the right diet and medication. When a cat has been diagnosed with renal insufficiency, the diet should be changed to support kidney function and slow the progress of renal tissue damage. Because kidney failure cannot be completely cured, the special diet must be maintained for life. A cat on a kidney-supporting diet should not be given snacks or other food, as the diet is essential for the cat’s health. A supportive diet and care for renal insufficiency can reduce the risk of developing other comorbidities.

Other kidney diseases

Hereditary polycystic kidney disease (PKD) has been reported in cats. Here, the kidneys develop fluid-filled cysts and the kidneys become enlarged. Cysts can be detected by ultrasound. Symptoms of PKD are typical of kidney disease: excessive drinking and urination, loss of appetite, weight loss, fatigue, and haematuria. The disease is found in both sexes and is inherited through both female and male. Polycystic kidney disease is particularly common in Persian cats. Chronic interstitial nephritis (CIN) is a renal degenerative disease, the causes of which are not yet fully understood. It is found in cats of all ages, but its prevalence increases with age. In glomerulonephritis, the glomeruli are damaged as a result of inflammation. The root cause of the disease is often unclear. Amyloidosis is a rare disease in which the cat’s various organs, typically the liver and kidneys, accumulate amyloid protein, which gradually destroys the organ. There is no cure for amyloidosis.

Things to consider in your cat’s diet

Protein

Restriction of protein levels is a key aspect of kidney disease management. Studies have proven that feeding a diet limited in protein can prolong the survival rate of cats with CKD by up to three times compared to diets with normal protein levels. The primary reason a protein-restricted diet is recommended include the reduction of nitrogenous wastes and glomerular proteinuria. Nitrogenous wastes can contribute to clinical signs associated with azotaemia and uraemia, polyuria and polydipsia, and anaemia through gastrointestinal ulceration and blood loss. Nitrogenous wastes may also shorten the life span of red blood cells and cause platelet dysfunction. Many animal protein sources such as chicken and beef contain large quantities of phosphorus; however, because cats with renal insufficiency are prone to muscle atrophy, it is necessary to monitor muscle condition. Protein for cats with kidney failure should come from raw material  with as little phosphorus as possible and a moderate protein content.

Phosphorus

When the kidneys are deficient, their ability to remove phosphorus from the body is reduced. If phosphorus levels in the blood become too high, calcium metabolism may be disrupted. High levels of phosphorus and low levels of calcium in the blood activate the parathyroid gland to produce parathyroid hormone (PTH), which releases calcium from the bones into the bloodstream. A high level of phosphorus is a negative prognostic factor for the survival of a cat with kidney disease because it accelerates the destruction of functional renal tissue. For a cat with renal insufficiency, the concentration of phosphorus from food should be low. Phosphorus in food is mainly bound to animal protein, so the restriction of protein in the diet in turn leads to reduced levels of phosphorus. Additional reduction of phosphorus absorption can be achieved with the addition of phosphorus binders to food.

Fat metabolism

Chronic renal failure causes disturbances in fat metabolism. The diet should be high in omega-3 fatty acids, which can slow the progression of chronic kidney disease. Studies have found that omega-3 fatty acids have a positive effect on kidney blood circulation and can improve renal filtration while also reducing protein excretion into the urine (1). Fats are an important source of energy for a cat suffering from kidney disease. Moreover, the energy content of veterinary food for cats with kidney disease should be high in order to compensate for poor appetite.

Vitamins and trace elements

Excessive urination can result in the loss of vitamins and trace elements into the urine. The special diet should contain enough vitamins and trace elements to adequately compensate for such losses. Excessive urination may cause hypokalaemia, a loss of potassium in the urine that can lead to high blood pressure. Any renal diet must compensate for vitamin and mineral loss with increased intake.

Probiotics and prebiotics

Almost half of body ammonium is produced by bacterial fermentation in the gut. The probiotic E. faecium and prebiotic fibre promote the growth of beneficial gut bacteria with beneficial detoxification properties that help bind ammonium, which is responsible for the formation of urea. Probiotics and prebiotics reduce serum concentrations of gut-derived uremic toxins in the blood of patients with chronic kidney disease. Another benefit of probiotics and prebiotics in kidney disease is a decrease in the bacterial population producing ammonium and other toxins that the kidneys have to expel from the body. Reduced re-absorption of ammonium and bacterial toxic metabolites reduces stress on the kidneys.

Brit VD Renal

Brit VD Renal is a special grain-free diet for cats with renal insufficiency. It is also suitable for cats with cardiorenal syndrome, also known as concomitant heart and kidney insufficiency. Brit VD Renal is suitable for cats with urate or cystine urinary stones. The protein source for Brit VD Renal is highly digestible egg protein. It naturally contains less phosphorus than many other protein sources, and it helps limit the amount of phosphorus in the body. Egg protein helps control the formation of uremic toxins (e.g. creatine, urea), due to a low protein content (14%) and a high proportion of essential amino acids, which play an important role in decreasing the negative effects of a low-protein diet, such as reduced muscle mass and overall condition.

Chitosan is added to the Brit VD Renal diet to bind phosphorus and reduce its intestinal absorption. The gluten-free and chicken protein-free composition helps reduce the risk of AFR. Moreover, improved amino acid composition supports maintenance of muscle mass, and the egg protein prevents muscle mass catabolism. The high energy and fat content help maintain adequate energy intake in the case of poor appetite. Brit VD Renal is rich in both soluble and insoluble prebiotic fibre. The optimal concentration of fibre increases intestinal motility, slows the absorption of ammonia back into the bloodstream, and relieves nausea in anorexic cats. The live probiotic E. faecium promotes healthy gut flora and helps maintain intestinal wall integrity. Probiotics and prebiotics can reduce gastrointestinal symptoms thanks to their ability to bind uremic toxins and ammonium.

Omega-3 fatty acids have anti-inflammatory properties and can slow the progression of chronic kidney disease. Omega-3 fatty acids help promote the filtration capacity of the glomeruli. The added antioxidants selenium, vitamin E, and vitamin C promote normal immune system function.

1) Hu J, Liu Z, Zhang H. Omega-3 fatty acid supplementation as an adjunctive therapy in the treatment of chronic kidney disease: a meta-analysis. Clinics (Sao Paulo). 2017 Jan 1;72(1):58-64. doi: 10.6061/clinics/2017(01)10