What Pet Owners Should Know About CKD in Dogs and Cats

Adapted from the IRIS education article by Stephen P. DiBartola, DVM, Diplomate ACVIM (Internal Medicine), July 10, 2019.

Most people know that the main job of the kidneys is to clear waste products from the blood, but it is also important to know a little about how they do it, what else the kidneys do, and what happens when things go wrong.

Normal Functions of the Kidneys

The kidney is made up of hundreds of thousands of functional units called nephrons. Each nephron consists of a tiny filter called the glomerulus (plural, glomeruli) and a long passageway called the tubule. All of the tubules eventually join to form a collecting system that carries the waste products, as urine, to the bladder. As the heart beats, pressure within the small blood vessels of the kidney causes blood to be filtered through the glomeruli. A filtrate of water and small substances, such as sugars, amino acids, electrolytes, small proteins, and waste products, is forced into the tubules, while blood cells and larger proteins remain in the blood. Then, as the fluid flows along the tubules, the kidneys reclaim the water and substances the body needs and let the waste products and any excess water pass into the urine. By volume, more than 99% of the fluid that is filtered is returned to the blood, and less than 1% is excreted.

The famous kidney physiologist Homer W. Smith once said, "Superficially it might be said that the function of the kidneys is to make urine; but in a more considered view, one can say that the kidneys make the stuff of philosophy itself." This was an eloquent way of saying that the kidneys are more concerned with what they keep in the body than with what goes out in the urine. The real job of the kidneys is to regulate and stabilize the volume and composition of the body's internal fluids. The urine they produce is merely a by-product of that work of conservation. The kidneys also have other important functions, such as producing a hormone called erythropoietin, which stimulates the bone marrow to make red blood cells, and converting vitamin D from the diet into its final active form, called calcitriol.

Signs of Kidney Disease

The most common form of kidney disease in dogs and cats develops very slowly over time, and so it is known as chronic kidney disease, or CKD. By the time the disease is advanced, the kidneys can be so altered by scarring and inflammation that it may be nearly impossible to tell what started the disease process. In his classic textbook "Clinical Nephrology," Solomon Papper quoted the American pathologist Simeon Burt Wolbach to make this point: "It is often difficult to ascertain the nature of the edifice that has burnt down from a study of the ashes." Unfortunately, because dogs and cats cannot tell us how they feel, kidney disease is often very advanced by the time pet owners and veterinarians recognize it.

Increased Thirst and Urination

For the observant pet owner, the first clue that a pet may have CKD is that it is producing more urine than before and drinking more water. These changes happen because, as kidney disease progresses and nephrons are lost, the remaining nephrons must handle a much larger load of water and small substances filtered from the blood, which leads to a larger-than-normal volume of dilute urine. The animal drinks more to make up for the water lost in the urine.

These changes, called polyuria (increased urine production) and polydipsia (increased water consumption), are easier for dog owners to recognize than for cat owners. Dogs are typically taken outside to urinate, and if they are producing more urine and are not let out often enough, they will urinate in the house, which gets their owners' attention. The owner may also notice that the water bowl needs refilling more often during the day. It is more challenging with cats, because they normally do not empty the large water bowls they are usually given, and they urinate in a litter box. It takes a very observant owner to recognize that a cat is drinking more, but it is a little easier to notice when the litter box is filling with more and larger wet clumps and needs cleaning more often. Polyuria and polydipsia are the first changes to be noticed, but by the time they occur, the kidneys have already lost about two-thirds of their function, and the disease is actually fairly advanced.

Changes on Blood Tests

The next change in dogs and cats with CKD is often noticed by the veterinarian on blood tests performed as part of routine care for older pets. As the disease advances, the overall filtering function of the kidneys falls to the point where waste products begin to build up in the blood. Tests for two of these waste products, blood urea nitrogen (BUN) and creatinine, are included in the blood chemistry profiles veterinarians commonly run. Unfortunately, by the time these values rise above the laboratory's normal reference range, the kidneys have already lost about 75% of their function. If the veterinarian repeats blood tests regularly as the animal ages, it may be possible to detect a decline in kidney function from consistent increases in BUN and creatinine, even while they are still within the reference range. Yearly veterinary visits as a pet gets older can therefore allow earlier detection of CKD. Later, when the kidneys have lost about 85% of their function, the concentration of the mineral phosphorus in the blood rises.

A newer blood test called symmetric dimethylarginine, or SDMA, may allow veterinarians to recognize kidney disease much earlier, when the kidneys have lost about 40% of their function. The SDMA test could potentially identify kidney disease in dogs and cats even before owners notice changes in water intake and urine production. Veterinary researchers and clinicians continue to gain experience with the SDMA test to determine its full value in identifying early kidney disease.

The International Renal Interest Society (IRIS) has developed guidelines to help veterinarians recognize and classify CKD in dogs and cats. These guidelines divide the progression of CKD into four stages based on clinical findings and laboratory test results, which help veterinarians recognize kidney disease and treat it appropriately as early as possible. Learn more in our article on the IRIS staging system for CKD.

Signs of Advanced Disease

Once kidney disease is advanced and waste products such as BUN and creatinine build up in the blood, more obvious signs of illness, such as lethargy, loss of appetite, weight loss, and a poor hair coat, may become apparent, but by this point the disease is quite advanced. Dogs with advanced kidney disease often also have episodes of vomiting, which is less common in affected cats. When an animal is losing a lot of fluid in the urine, is not eating or drinking, and may be vomiting often, severe dehydration and chemical imbalances, such as abnormal blood mineral concentrations and excessive acidity of the body fluids, can develop. These worsen the animal's condition and can be life-threatening if not treated properly.

The goal is for owners to recognize the early signs of CKD, increased water intake and urine production, and seek veterinary care promptly. If chronic progressive kidney disease is recognized at an earlier stage, it is more likely that it can be managed and its progression slowed by appropriate medical treatment.

Examining the Kidneys

When evaluating a pet with possible CKD, the veterinarian will try to determine the size of the kidneys, because they become smaller and firmer, from scarring, as the disease progresses. In many cats, the veterinarian can easily estimate the size and texture of the kidneys, because they can often be felt (palpated) during an examination of the abdomen. It is more difficult to palpate the kidneys in dogs. In both species, an X-ray of the abdomen gives more information about kidney size. An abdominal ultrasound examination allows the veterinarian to evaluate not only the size and shape of the kidneys but also their internal structure.

Treatment of Kidney Disease

If the animal is well hydrated and has relatively normal electrolyte and acid-base balance on blood tests, the veterinarian can focus on medical management, which mainly involves changes to the diet.

Always Provide Fresh Water

It is crucial that dogs and cats with CKD have fresh water available at all times. They cannot concentrate their urine adequately and lose a lot of water in the large volume of urine they produce, so they must drink large quantities of water to stay hydrated. An owner should never withhold water from a dog or cat with CKD in the hope that it will urinate less. On the contrary, restricting water will only put the pet at risk of becoming dehydrated and more seriously ill.

Kidney Diets

One of the most important dietary changes for a dog or cat with CKD is restricting phosphorus intake. This can reverse a harmful compensatory mechanism that develops in CKD, called renal secondary hyperparathyroidism, and it may also limit some of the progressive structural damage to the kidneys caused by calcium and phosphorus imbalance. A diet limited to a moderate amount of high-quality protein may also be beneficial, although the ideal amount of protein for a dog or cat with CKD has been debated for many years. It is important to provide enough protein to maintain as close to ideal a body weight and hair coat as possible, but not an excessive amount. In addition to body weight, the veterinarian can use tools such as the body condition score and the blood concentration of a protein called albumin to help assess the animal's nutritional status.

Fortunately, several commercial therapeutic foods designed specifically for dogs and cats with CKD are available, and it is usually possible to find a product, or a combination of products, that the pet will accept. Effective diets are typically restricted in phosphorus, protein, and sodium and supplemented with polyunsaturated fats, and, in kidney diets for cats, with potassium. Some owners have to resort to a homemade diet, but this is less than ideal, because it is difficult to make sure such diets are balanced and contain all the necessary vitamins and minerals as well as the right balance of protein, carbohydrate, and fat. Diet changes work best when they are started in IRIS Stage 2 or 3, while the pet still has a reasonably good appetite. The diet should be changed gradually over several weeks, increasing the amount of the new food while decreasing the old until the pet has fully transitioned. In one study of cats with CKD, those eating a commercial kidney diet survived about twice as long as those eating a regular diet.

Potassium and Sodium

Some cats with CKD need additional potassium supplementation. If a potassium source such as potassium gluconate is used, the gluconate is metabolized in a way that helps reduce the acidity of the body fluids, which can be helpful in pets with advanced kidney disease and excessively acidic body fluids.

It is also important to be aware of the sodium content of the diet. The reasoning behind sodium restriction is that it may help control blood pressure and reduce the risk of high blood pressure (hypertension). However, the effect of sodium restriction on blood pressure may be limited, and too much sodium restriction can activate a hormonal system in the body, the renin-angiotensin-aldosterone system, that promotes sodium retention and increases potassium excretion. Increased activity of this system can lead to further loss of potassium in the urine and contribute to potassium depletion.

Phosphorus Binders

If dietary phosphorus restriction alone is not enough to reverse renal secondary hyperparathyroidism, the veterinarian may add a phosphorus binder, a drug that binds phosphorus in the intestinal tract, prevents it from being absorbed from the food, and allows it to be excreted in the feces. These medications should be given around mealtimes for the greatest benefit. Some phosphorus binders are available over the counter, such as calcium carbonate, and others are prescription products. They should always be used under the supervision of a veterinarian, because giving too much can cause adverse effects.

Vomiting and Appetite

Anti-nausea drugs such as maropitant or ondansetron can be used for vomiting and nausea in dogs and cats with CKD, and appetite stimulants such as mirtazapine or capromorelin can help pets that are eating poorly or losing weight. In cats, mirtazapine also reduces how often they vomit. In dogs with CKD that show signs of stomach bleeding, such as black, tarry stools, or irritation of the esophagus from repeated vomiting, the veterinarian may recommend an acid-suppressing medication such as omeprazole. Cats with CKD rarely develop stomach ulcers, so acid-suppressing medications are generally reserved for cats in which stomach bleeding is suspected.

Protecting the Kidneys and Controlling Blood Pressure

Angiotensin II is a hormone in the renin-angiotensin-aldosterone system. It helps maintain blood pressure by constricting blood vessels and promoting sodium retention by the kidneys. However, it can have harmful effects in dogs and cats with CKD, such as increasing protein loss in the urine and promoting scarring and inflammation in the kidneys. Drugs that reduce the production of angiotensin II (angiotensin-converting enzyme, or ACE, inhibitors such as benazepril) or that block it from binding to its receptor (angiotensin II receptor blockers such as telmisartan) are used to limit some of these harmful effects and possibly slow the progression of the disease.

ACE inhibitors may have only a modest effect on blood pressure. Dogs with high blood pressure are usually started on an ACE inhibitor, and many need a second drug that blocks the effect of calcium on blood vessel constriction, such as amlodipine. In cats with high blood pressure, amlodipine or telmisartan is usually the first choice. It can be difficult to tell whether a dog or cat really has high blood pressure that needs treatment, because pets, like many people, are subject to the "white coat effect." This refers to the observation that some people's blood pressure rises when they visit the doctor and are anxious about it. Dogs and cats do not worry about their blood pressure, but they are usually very nervous at the veterinary clinic, so a similar effect can occur. It takes a patient, well-trained technician measuring the pet's blood pressure carefully and repeatedly, often in the calming presence of the owner, to determine whether the animal truly has high blood pressure that needs treatment. It is nevertheless important to find out whether a dog or cat with CKD has high blood pressure, because hypertension is a treatable complication that, if well controlled, may improve the pet's well-being and possibly slow the progression of the kidney disease.

Anemia and Calcium Balance

Some pets with advanced kidney disease develop problems related to the kidney's role in red blood cell production and calcium balance. In these patients, additional drugs such as erythropoietin and calcitriol may be used. Using these medications requires a sophisticated understanding of the abnormalities that can develop in advanced kidney disease, and they are best used by specialists in veterinary internal medicine, together with owners who understand the potential complications of treatment.

Fluids and Feeding Tubes at Home

In some cases, owners can be taught to give electrolyte-containing fluids, such as lactated Ringer's solution, under the skin (subcutaneously) to cats or small dogs with CKD when the animal is not eating and drinking enough to stay hydrated. Doing this successfully requires an owner who is not squeamish about needles and a pet that will tolerate the treatment at home with minimal restraint. In some cats with advanced CKD, a long-term feeding tube can be placed into the stomach (a gastrostomy tube), which allows the owner to give liquid food, water, and medications easily. Many cats with CKD tolerate a gastrostomy tube remarkably well.

Outlook for Dogs and Cats With Kidney Disease

Owners sometimes ask how long their pet will live with CKD. It is impossible to predict with certainty, because the rate at which CKD progresses varies considerably between animals. The veterinarian can get some idea of how quickly an individual animal's disease is progressing by following its blood creatinine concentration over many months and using the trend to estimate the rate of progression. Following the disease this way also helps the veterinarian judge whether changes in treatment are slowing its progression. Some dogs and cats with CKD live for several years with conscientious management by their veterinarian and owner.