Introduction
Feline lower urinary tract disease (FLUTD) encompasses a number of clinical disorders that commonly affect cats. The causes of FLUTD are varied and the disease remains poorly understood. The processes involved in the formation of calcium oxalate urolithiasis (CaOx) is an example of this lack of knowledge.
In 1984, a study reported an incidence of CaOx as low as 2-3% of all feline urinary stones (uroliths). The situation today is dramatically different with various studies reporting incidences as high as 54%. Although this shift in incidence is likely due in part to better management of struvite urolithiasis (Struvite stones consist of a mineral complex involving ammonium, magnesium and phosphate and the stones most familiar to cat owners), CaOx is indisputably diagnosed much more frequently.
Common Symptoms of FLUTD
Cats that have this disease usually exhibit frequent squatting and straining to urinate only to produce small amounts of urine or no urine at all. Failure to use the litter box, urinating in inappropriate places (such as a sink, bathtub, or beside the litter box), and blood-tinged urine are also signs that your cat may have FLUTD.
FLUTD can manifest itself in many ways. Some cats may get an inflammation of the urinary bladder (i.e. a cystitis), which can lead to an accumulation of blood, mucus, and cellular debris in the bladder. In a few cases, this may lead to a bladder infection, since the blood, mucus and other debris associated with an inflamed bladder provides an ideal environment for bacteria to grow.
FLUTD can affect cats of all ages. Both male and female cats are susceptible to the formation of crystals and stones in the bladder. In female cats, crystallization of dissolved minerals in the urine (struvite or oxalate) may lead to the formation of bladder stones, or uroliths. In male cats, the result could be the formation of plugs which can block the flow of urine, leading to a urinary tract blockage. If a cat is unable to urinate, its kidneys cannot expel the poisonous wastes from the body. Unless the blockage is promptly relieved, the cat will suffer a painful death. Straining to urinate, depression, weakness, vomiting, and collapse, if left untreated, could rapidly lead to coma and death. These clinical signs should prompt an immediate call to your veterinarian.
Which Cats are at Higher Risk?
In cats, CaOx tends to develop in middle aged cats, 4 years and older. Although found in any breed, the Persian is particularly predisposed with Himalayan and Burmese over-represented to a lesser extent. In some studies, the Siamese has been under-represented. Gender does not appear to be a factor, although a greater incidence occurs in sterilized cats. Cats receiving products that acidify the urine are also predisposed. Additional factors may also include exclusive feeding of a single brand of cat food and maintaining cats in an indoor environment. Indoor cats presumably void less frequently allowing the components of bladder stones a longer period of time to precipitate if in a supersaturated state.
In order for a urolith to form, urine must have a high concentration (supersaturation) of the individual mineral components that can form the stone. With respect to CaOx, two opposing factors interplay the presence of calcium and oxalate in sufficient high urinary concentrations to be supersaturated, and the presence of so-called crystallization inhibitors (i.e. Compounds that help prevent the formation of CaOx stones), such as magnesium, citrate and sodium.
Some cats with a genetic deficiency in an enzyme called D-glycerate dehydrogenase, a liver enzyme responsible for breaking down precursors of oxalic acid, have been found to form CaOx stones much more readily than normal cats. However, the likelihood that this deficiency plays a role in the formation of CaOx in the general cat population is unlikely.
The Role of Feline Diets
During the past 10-15 years, feline diets have undergone significant reformulations by manufacturers with a tendency to acidify urine to help prevent struvite urolithiasis. Formulating cat foods to produce acidic urine also results in more calcium (one of the two components of CaOx) being found in the urine. This increased concentration of calcium resulted from more calcium being absorbed from the bones into the bloodstream and excreted by the kidneys into the urine.
One study, however, has indicated that acidifying the diet alone may not be enough to predispose to CaOx formation. The greatest supersaturation of calcium oxalate (and hence the tendency for CaOx stones to form) was found to occur not simply with the lowest pH urine, but rather when the urine was both acidic and restricted in magnesium concentration. Similarly, numerous studies have shown that feeding magnesium restricted diets in cats enhanced the formation of CaOx. In man, magnesium supplements decrease the formation of CaOx. Hence the restriction of magnesium in urine acidifying cat foods (a common practice by pet food manufacturers to help prevent struvite stones) implies a strong role for magnesium in CaOx formation. The exact means by which magnesium may have this inhibitory role is unknown.
Research in man and other species has indicated that increased amounts of animal protein significantly increase urinary excretion of calcium and oxalate. If the same process occurs in cats, then an emphasis on a diet with less animal protein, yet still nutritionally balanced, might be helpful in avoiding reoccurrence of CaOx.
Several studies have examined the relevance of dietary water content in the formation of uroliths, with inconclusive results. With the intent of enhancing the passage of large amounts of urine and maximizing urine turnover time, it would seem appropriate to encourage highly digestible foods with high moisture content.
Since the processes involved in CaOx formation in cats remain poorly understood, it is difficult for veterinarians to make firm recommendations to their clients to help prevent reoccurrences. Avoiding a single diet should be considered, as should diets that do not acidify urine, nor are restricted in magnesium content. Excessive levels of Vitamin D (which will increase intestinal absorption of calcium) and ascorbic acid (a precursor of oxalate) are also best avoided.
Treating FLUTD
Since the exact causes of FLUTD remain unknown, treatment is not always straightforward or standardized. For example, the crystals that form in the urine and eventually lead to stone formation in the bladder may in fact be the result rather than the cause of this disease. In some cats, treatment with antibiotics may help or minimize the problem. In others, changing the diet to a magnesium-restricted diet and/or a diet that promotes urinary acidification may be necessary. Still other cases may require surgical intervention.
Preventing FLUTD
Based on current knowledge, FLUTD can be prevented, provided certain precautions are taken:
- Water: Fresh water should be made available at all times. Change it at least twice daily and try to encourage drinking.
- Diet: Feed a complete and balanced diet. Be careful about feeding table scraps and treats. Ask your veterinarian about special magnesium-restricted, pH-controlled diets that are currently available.
- Obesity and Exercise: Cats that are physically fit and receive sufficient exercise are less likely to develop FLUTD and other ailments.
- Litter Box: Clean litter boxes often and watch for any unusual deposits, particularly blood-tinged urine.
- Eliminative Behavior: Observe your cat and pay particular attention to its elimination habits. Early signs of FLUTD are similar to those associated with constipation straining (frequent straining).