Most cats are generally healthy and resilient but their Achilles' heel tends to be their kidneys and bladder, says veterinarian David Gething, of Creature Comforts (creaturecomforts.com).

"Cats are prone to kidney issues and bladder problems, especially as they get older. I would estimate that over half of the sick cats we see at our hospital have some type of urinary tract issue," he says.

In a healthy cat, blood is filtered by the kidneys, which produce urine. This urine is stored in the bladder until it passes out through the urethra.

"There are a number of issues that can happen during those stages - from the time the waste products are filtered from the blood to the time the urine is passed - and many of these can be potentially serious," the vet says.

The most common issue Gething sees is chronic kidney (or renal) failure. "Cats do put a lot of strain on their kidneys; they eat a high-protein diet, which requires a lot of metabolism and filtering, and often don't drink enough, which can result in premature ageing and damage to the kidneys," Gething says.

Chronic renal failure is a slow, progressive condition. Initial signs include increased thirst and drinking and urinating large amounts frequently. As the condition worsens, cats often lose weight, lose interest in food and, in severe cases, can become nauseous and vomit.

Gething says owners who suspect that their cat has kidney failure should go to a vet. "The condition is easily diagnosed with a blood test, and in some cases an ultrasound. Early treatment is vital, and often fairly simple changes, such as a special diet and supplement, are enough to control kidney problems, although serious cases do require more intensive therapy."

Other common associated issues are bladder crystals and stones. "When any animal eats food, the minerals are absorbed and used as needed in the body, and excess minerals are passed in the urine. Cats, who often eat a meaty diet that is high in minerals, can form a supersaturated solution of minerals in their urine, especially when they don't stay well hydrated. This solution then begins to form microscopic crystals which roughen and irritate the bladder and urethra, resulting in bladder pain and inflammation," he says.

Signs of bladder inflammation (also called cystitis) include frequent visits to the litter tray, passing small amounts of urine that often contains blood, straining to urinate and licking of the groin due to discomfort. A bladder damaged by crystals is highly vulnerable and secondary bacterial infections are common.

He says that the best plan for a cat showing signs of bladder inflammation is to visit a vet. After test results determine which minerals are present, the cat will usually be given a special diet that will prevent future crystal formation, as well as medication to treat any infection and reduce the inflammation.

"Some cats are prone to developing crystals and often need to stay on the special diet for life, but this is generally a fairly simple solution and most pets don't need medication for the long term. Some people also believe taking a natural glucosamine supplement can help reduce crystal formation," the vet says.

In some cases, crystals can combine to form stones, some of which can grow as big as a golf ball.

Apart from causing further irritation, Gething says that bladder stones can be dangerous. The initial sign of a blockage is usually frequent and prolonged visits to the litter tray without any urine passing. As time progresses and the cat still cannot urinate, it will start to develop blood poisoning, resulting in vomit-ing, lethargy, severe abdominal pain and life-threatening complications.

"A blocked bladder is a critical situation, and if you suspect your cat is blocked you should seek immediate veterinary assistance. The vet will generally use a small tube to help unblock your cat and then keep the cat in for further treatment. After they go home, they will generally be placed on a special diet and treated the same way as a cat with crystals, which will prevent the problem recurring," he says.

Gething says the two most common types of urinary issues for cats - chronic renal failure and bladder crystals or stones - are in a large part due to diet.

"Diets that have excessive levels of protein, calcium, magnesium or phosphorus put cats at high risk of developing problems, and feeding a cat a healthy, balanced diet from a young age is an excellent way to promote health."

The vet adds that early treatment is vital and can make all of the difference to the health of a cat.

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