By Tony Puglisi, MS, DVM, DACVS and Courtney Otto, RVT, VTS - Torus Pet Veterinary Advisory

Kidney disease is one of the most common chronic conditions in cats and a real risk in dogs. Water intake is one of the few variables an owner controls directly. It matters more than most pet parents realize.

Most owners think about hydration in hot weather and forget about it the rest of the year. Kidneys don't work that way. They're filtering blood every hour of every day, and the water available to them at any given moment shapes how hard they have to work. This piece looks at what that relationship actually involves — what the kidneys need water for, which pets carry the most risk, how vets track kidney function over time, and what owners can realistically do about it.

What the kidneys actually do with water

The kidneys filter blood, remove waste, and manage the balance of water, sodium, and other electrolytes in the body. Every day they filter and reabsorb this fluid many times over. Adequate water intake keeps that process working efficiently. It keeps urine dilute enough to flush waste products out rather than let them concentrate and crystallize.

When intake drops, the kidneys have less fluid to work with. They compensate by concentrating urine further, which raises the workload on nephrons - the kidney's filtering units. Over months and years, that added strain contributes to nephron loss. Nephrons don't regenerate. Chronic kidney disease (CKD) is, in large part, a story of gradual nephron attrition.

Cats and dogs have a built-in buffer for this. Healthy kidneys carry far more filtering capacity than daily life requires, so a dog or cat can lose a meaningful share of nephrons and still show normal bloodwork. That buffer is useful, but it's also why kidney disease is so easy to miss early. Reduced function stays hidden until the buffer runs out.

How vets measure kidney function

Standard bloodwork looks at blood urea nitrogen (BUN) and creatinine, two waste products the kidneys normally clear. Unfortunately, these two by-products are not a very sensitive indicator of impending kidney dysfunction.  In fact, your pet can lose up to 50% of their kidney function BEFORE these two compounds even begin to elevate in the bloodstream.  This means your pet can lose a large share of kidney function before your veterinarian will see it reflected in their blood tests.

SDMA - symmetric dimethylarginine - is a newer marker, added to routine panels over the past decade specifically because it rises earlier - often when kidney function has dropped by only 25 to 40 percent, rather than waiting for the > 50 percent loss that BUN and creatinine typically require.

Kidney function typically still remaining when each marker first becomes abnormal.

Pairing SDMA with a urinalysis, particularly urine specific gravity (USG), gives a clearer early picture. A cat producing dilute urine despite normal creatinine is a pattern worth watching, not ignoring.

Once CKD is diagnosed, the International Renal Interest Society (IRIS) staging system is the standard framework vets use to classify severity and guide management, based on creatinine, SDMA, and other findings. Staging matters because treatment recommendations - diet changes, phosphate binders, fluid therapy - shift meaningfully between stages. This is a conversation to have directly with your vet rather than something to interpret from a lab printout alone.

Why cats carry more risk

Cats evolved as desert animals. Their ancestors met most of their water needs through prey, not standing water, and cats retain a low thirst drive as a result. A cat fed dry kibble and relying on a bowl for the rest of its water can run a persistent, low-grade fluid deficit without ever looking dehydrated.

CKD affects an estimated 30 to 40 percent of cats over 10 years old [citation needed - please verify against current source data]. It's one of the leading causes of death in senior cats. Early CKD has almost no visible signs, because the kidneys can lose up to two-thirds of their function before blood values shift enough to catch on standard bloodwork. By the time a cat shows classic symptoms - weight loss, increased thirst, increased urination — significant kidney function is often already gone.

This is why hydration support works better as prevention than as treatment. It's a variable owners can influence years before disease shows up on a lab report.

Certain breeds carry additional genetic risk on top of the general age-related pattern. Persian and Persian-cross cats are prone to polycystic kidney disease, an inherited condition where fluid-filled cysts gradually replace healthy kidney tissue. Abyssinian and Siamese cats also show higher rates of kidney disease in general population studies. None of this makes hydration irrelevant for these breeds - if anything, it makes it more important, since a cat with less structural reserve has less room to absorb the added strain of chronic low water intake.

Dogs face a different set of risks

Dogs have a stronger thirst drive than cats, so primary CKD from chronic low intake is less common. Dehydration still matters for dogs, through two other pathways.

The first is urinary stone formation. Struvite and calcium oxalate stones both form more readily in concentrated urine. Diluting urine through higher water intake is a standard part of stone prevention and management in dogs.

The second is acute kidney injury. Heatstroke, prolonged vomiting or diarrhea, and intense exercise in hot weather can all cause a rapid drop in kidney perfusion. Unlike CKD, acute kidney injury can sometimes be reversed if caught early - but the margin for error is short, and dehydration is almost always a contributing factor.

Breed does play a role here too. Dalmatians have a genetic quirk in how they process uric acid, making them prone to urate stones specifically. Miniature Schnauzers, Bichon Frises, and Shih Tzus show higher rates of calcium oxalate and struvite stones. Bernese Mountain Dogs and Shar-Peis carry elevated risk for a hereditary kidney condition that behaves more like the slow, progressive pattern seen in cats. A dog with any of these risk factors benefits from the same hydration focus, even though the general dog population is less exposed than cats.

Signs worth watching for

●      Skin tenting: gently lift the skin at the scruff. In a well-hydrated pet it snaps back immediately. A slow return suggests dehydration.

●      Dry or tacky gums, rather than moist and slick.

●      Sunken eyes.

●      Lethargy paired with reduced appetite.

●      Increased thirst or urination - often an early CKD sign in cats, not just a hot-weather response.

None of these are a substitute for bloodwork. Urinalysis and a kidney panel remain the only reliable way to catch early-stage CKD. But a pet showing several of these signs at once warrants a vet visit sooner rather than later.

Three myths worth clearing up

"My cat barely drinks and seems fine." A cat can look fine and still be running a chronic fluid deficit for years. Low visible thirst is normal cat behavior, not a sign of adequate hydration. It's the total water intake across food and drinking that matters, not how often you see the cat at the bowl.

"Adding water to dry food solves it." It helps, but it's a partial fix. A splash of water in kibble adds a small fraction of what switching to wet food adds, and it doesn't change the water content of the food itself once your pet finishes eating. Wet food changes the baseline; added water on top of dry food nudges it slightly.

"Clear urine means everything's fine." Pale, dilute urine is generally a good sign of hydration, but persistently very dilute urine in an older cat can also be an early CKD marker, since damaged kidneys lose the ability to concentrate urine even when the body needs them to. This is exactly why lab work, not visual checks, is what catches early disease.

Practical ways to support hydration

Cats and dogs both drink more from moving or fresh-tasting water than from a bowl that's sat out all day and picked up dust, hair, or debris. Multiple water points around the house also help - a pet is more likely to drink if water is close by when the urge hits, rather than requiring a trip across the house.

Wet food is one of the highest-leverage tools for cats specifically. Canned food runs 70 to 80 percent water, versus roughly 10 percent for dry kibble. Cats on a partial or full wet-food diet reliably show higher total water intake than cats on dry food alone, even accounting for the water they drink separately.

Approximate water content by food type.

A sealed hydration system - one that stores water inside its walls and releases it only at the drinking surface - addresses the freshness half of the equation. Less exposure to dust and debris supports cleaner drinking water and can make a pet more willing to keep coming back to it through the day, which is really what consistent hydration comes down to: not one big drink, but many small ones.

Travel and heat both raise the bar. A pet that drinks reliably at home can fall behind on a road trip, a hike, or a hot afternoon in the yard, simply because water isn't as available or as appealing away from its usual spot. Bringing a hydration source along, rather than assuming a pet will drink enough once you're back home, matters most exactly when the risk of dehydration is highest.

Tracking water intake at home

Owners often don't know how much their pet actually drinks, because water bowls get refilled before anyone notices the level dropping. A simple baseline is worth establishing: measure the water you put out over 24 hours, and measure what's left the next day. General guidance puts healthy daily water needs at roughly 1 ounce per pound of body weight, including water from food, though individual needs vary with diet, activity, and climate.

For cats on wet food, this number will look different, since much of their intake comes from the food itself rather than the bowl. The point isn't to hit an exact figure every day - it's to notice when intake shifts meaningfully from a pet's own baseline, which is often the earliest signal available before bloodwork catches anything.

When to call your vet

Any pet drinking noticeably more or less than usual, for more than a day or two, should be seen. Same for a pet showing two or more of the dehydration signs above. Annual bloodwork including a kidney panel is worth prioritizing for cats from around age seven onward, and for any dog with a history of stones or acute illness.

For pets already diagnosed with CKD, hydration support becomes part of ongoing management rather than prevention. Vets may recommend a renal support diet, phosphate management, or in more advanced stages, subcutaneous fluid therapy at home to directly support hydration status between checkups. These decisions depend on IRIS stage and individual bloodwork, and should be made with your own vet rather than guided by general advice - a diagnosed pet's needs are more specific than a healthy pet's.

Hydration alone won't prevent kidney disease. Genetics, age, and underlying illness all play a role that owners can't control. But consistent water intake is one of the few protective factors within reach, every day, for the life of the pet.

Abstract Chronic Kidney Disease (CKD) is among the most frequent and clinically relevant metabolic disorders in cats, affecting a substantial proportion of the adult and geriatric feline population. The prevalence of CKD increases with age, being estimated at 20–50% in cats over 10 years of age. The etiology of this disorder involves genetic predisposition, dietary factors, as well as exposure to toxins or concomitant diseases that may accelerate the progression of renal lesions.

The present study synthesizes data from the scientific literature together with clinical-epidemiological analyses performed on a cohort of 120 cats over a four-year period. The results highlighted an overall prevalence of CKD in the studied cohort, with a significantly higher frequency in cats over nine years of age, particularly affecting purebred animals. A slightly higher prevalence was observed in males compared to females, while reproductive status indicated an increased risk of CKD in neutered patients. Living environment and nutritional status were also influential, with a higher disease prevalence in indoor-only cats and in those fed nutritionally deficient diets.

The multifactorial nature of CKD pathogenesis, combined with the nonspecific clinical manifestations in its early stages, explains the frequent late diagnosis of most cases. Consequently, an in-depth understanding of the epidemiology and associated risk factors constitutes an essential prerequisite for the development of preventive strategies, early screening protocols, and personalized therapeutic management aimed at optimizing quality of life and longevity in feline patients.

Epidemiology of Chronic Kidney Disease (CKD) in Cats: An Analysis of the Factors Involved Mariana Grecu 1 , Robert Capotă 1,* , Cristina Horhogea 2 , Cristina Rîmbu 2 , Valentin Năstasă 1 and Oana Tănase 2 1 Department of Preclinics, “Ion Ionescu de la Brad” Iasi University of Life Sciences, 8 Sadoveanu Alley, 707027 Iasi, Romania; mariana.grecu@iuls.ro (M.G.); valentin.nastasa@iuls.ro (V.N.) 2 Department of Public Health, “Ion Ionescu de la Brad” Iasi University of Life Sciences, 8 Sadoveanu Alley, 707027 Iasi, Romania; cristina.horhogea@iuls.ro (C.H.); cristina.rimbu@iuls.ro (C.R.); oana.tanase@iuls.ro (O.T.) * Correspondence: robert.capota@iuls.ro

Dr Puglisi is a veterinarian based in Georgia, United States, part of the region's professional veterinary community, providing medical, surgical, and preventive services for companion animals.

Courtney Otto is a Level 4 Certified Veterinary Technician at the University of Georgia Veterinary Teaching Hospital, assisting veterinarians in diagnosis, treatment, and care of companion animals across clinical and emergency settings, and teaching fourth-year veterinary students.

This article is intended as general information for pet owners and does not replace direct guidance from your veterinary team. Always follow the specific post-operative instructions given for your pet.