1. When should a renal disease be suspected in a cat?

Renal diseases in cats are mainly suspected when renal failure occurs. However others syndromes direct the clinician towards the urinary system in search of a renal lesion.

2. Renal diseases in cats

2.1. Hereditary and familial renal diseases

Renal amyloidosis

It has been described in cats as a family-related disease of the renal medullary due to the deposition of an amorphous substance which gradually invades the extra-cellular spaces. However, this lesion may develop in the absence of a hereditary context, but then deposits are located in the cortical interstitium and in the glomerulus. Primary amyloidosis occurs concurrently with multiple myeloma, primary macroglobulinemia (Waldenström disease), or with any disorder of immunoglobulin production by plasma cells.

The two histological forms have different biological and clinical findings: there is either a glomerulopathy (at first there is an isolated high proteinuria, then a nephrotic syndrome and/or a blood hypercoagulation syndrome), or a syndrome of tubular reabsorption defect (no proteinuria or a low one).

There is no specific treatment. Symptomatic treatment should correct disorders caused by renal failure according to the degree of insufficiency, avoid the effects of the nephrotic syndrome by the administration of diuretics (furosemide 2 to 4 mg/kg once to twice a day), correcting hypertension, avoid complications due to thromboembolism: (Aspirin 2 mg/kg every 3 days).

Polycystic kidney disease

Polycystic kidney disease in cats is a disorder characterized by the displacement of differentiated normal structures of renal parenchyma by multiple cysts.

It has been demonstrated as early as 1990 that polycystic kidney disease is hereditary in cats. However, in number of observations too many genealogic elements are lacking to assert the hereditary feature of the disease. This disease associated with neurovisceral mannasidosis due to ?-mannasidase deficiency has been described in Persian cats.

Lesions develop through a progressive compression of the healthy tissue, progressively resulting in irreversible renal failure. Kidneys palpated through the abdominal wall are enlarged and dented. Persian cats and long-haired breeds are the most often affected by polycystic kidney disease. Ultrasonographic examinations are the most effective ways to confirm the diagnosis.A puncture is a way of decompressing renal perenchyma.

2.2. Infectious renal diseases

Acute and chronic interstitial nephritis

1° Acute interstitial nephritis is a disease with renal hypertrophy due to the formation of nodular abscesses which invade the renal cortex.

Acute pyelonephritis is morphologically very similar to this disease. Infection may invade the whole kidney, leading to pyonephritis with capsular thickening (capsulite).

2° In chronic interstitial nephritis, kidneys are small and irregular. At the terminal stage of the development of the disease, they are reduced to the size of a pea, they are dented by the scars of renal parenchyma. Pyelic cavity may be enlarged, which constitutes a bacterial site when the disease evolves into chronic pyelonephritis. Chronic interstitial nephritis is a common disease described as the outcome of many renal diseases.

Pyogranulomatous renal disease

It characterizes Feline Infectious Peritonitis. Both kidneys are considerably enlarged by subcapsular haemorrhages. Beneath this haematoma pyogrnulomatous lesions are distributed on the renal surface. The virus produces glomerular lesions of a membranous.

2.3. Immune renal diseases

Membranous glomerulonephritis

Membranous glomerulonephritis is characterized by a diffuse thickening of the wall of glomerular capillaries and by subepithelial deposits of immune complexes (Nash AS- 1969). The disease is due to immune complex deposits of IgG and C3, sometimes but less often they are deposits of IgA and IgM. All types of glomerulonephritis are described in cats.

Clinical forms are nephrotic syndrome, renal failure, blood hypercoagulation syndrome. If, in the case of nephrotic syndrome, an adequate treatment may allow a long enough survival period (from 2,5 to 6 years), in case of renal failure, the prognosis is very adverse and life expectancy is very limited (under 3 months)). The treatment is function of the syndrome. When a specific antigen is identified, the treatment consists in an attempt to eliminate this antigen if this is possible.

2.4. Toxic renal diseases

In cats toxic renal diseases are more numerous than those diagnosed. Toxics include industrial or domestic products such as glycol ethylene, house plants (Lilium ), weed killers, pesticides, solvents, but mainly drugs such as aminoglucosode sulfamides and above all NSAIDs.

Non-steroid antiinflammatory drugs may induce functional renal failure by inhibiting the vasodilatator action of prostaglandins, tubular necrosis and papillary necrosis

The most frequent clinical form is in all cases renal failure with maintained diuresis., severe proteinuria, hyperkaliaemia and hyponatremia.

2.5. Tumoral renal diseases

Renal lymphoma

Renal lymphoma is an infiltration of one or both kidneys of the cat by tumoral lymphocytes, particularly alimentary lymphomas.

In cats as in dogs, neoplastic onset may be focal or diffuse. If it corresponds to one of the predominant lesions, it is usually bilateral. Over 45% of the cats affected by an abdominal type have a renal location. There are three main anatomopathological types: cortical with a complete invasion of cortex, focal and nodular.

The first lesions are often limited to the renal cortex and are observed particularly between the cortex and the medullary. The clinical symptoms of renal lymphomas are associated with the clinical signs of other locations. Abdominal palpation reveals tumefaction and an abnormal and irregular shape of kidneys. Hydronephrosis may develop, following partial or total occlusion of ureters by tumoral tissue. Only a bilateral attack of the organ results in renal failure. 50% cats affected are FeLV- positive.

Other renal tumours

0,6 to 1,7% tumours observed in this species consist of renal tumours. They include primary tumours, metastases and infiltrations whose main cause is lymphoma. Other infiltrations may occur concurrently to leukaemia or mastocytoma.

2.6. Various renal lesions

Perirenal cysts

Perirenal cysts, renal capsulogenic cysts, capsular cysts, pararenal cysts, capsular hydronephrosis are synonyms used to describe renal hypertrophy due to the accumulation of fluid between the parenchyma and renal capsula

The cause for perirenal cysts is unknown but is associated with renal diseases (chronic interstitial nephritis, renal lymphoma, polycystic kidney).

3. Renal diseases in cats: to diagnose them

3.1. History

The animal may be presented for various signs (anorexia, loss of weight, vomiting, asthenia, polyuria-polydipsia, constipation, haematuria, dysuria, diarrhoea) which lead the clinician to suspect renal failure.

When the symptoms are acute, a recent drug administration may allow to suspect the type of renal disease, such as aminoside and tubular-interstitial lesion, NSAIDs and papillary necrosis.

3.2. Clinical signs

The clinical signs may be the signs, already brought up or detected on clinical examination, of renal failure (dehydration, gain in weight, gingivitis, halitosis, mouth ulcers, hypotermia, hypertermia, retinal lesions, peripheral oedema, effusions, neuromuscular abnormalities of hypokalaemia-related polymyopathy, etc...).

In addition, kidneys of abnormal size or shape (or extra-renal signs such as oedemas, thrombus-induced paraplegia or retinal lesions allow to detect or suspect fortuitously chronic renal diseases.

3.3. Non specific biological signs

Non specific serum or urinary signs-anaemia, increased urea or increased creatinine, hypokaliaemia or hyperkaliaemia, metabolic acidosis, diluted urine-allow to diagnose renal failure whatever the type of disease.

Glucosuria-normoglycaemia.

3.4. Specific biological urinary signs

Include proteinuria (urine protein/urine creatinine ratio) and urine sediment

3.5. Specific serum signs

Serum protein electrophoresis, cholesterol, calcium

Hypoproteinemia, hyperproteinemia may be the sign of polyclonal hypergammaglobulinemia or monoclonal hypergammaglobulinemia.

Hypercholesterolaemia is one of marks of the nephrotic syndrome.

Hypercalcemia it is very exceptional in cats.

Screening tests for specific antigens and antibodies

Screening tests for FeLV, FIV, FIP and NAA.constitute elements of the diagnosis.

3.6. Kidney morphology and structure

Ultrasonographic of the kidney and perirenal areas are significant elements of the diagnosis of renal diseases. In cats the 7.5 mgh transducer seems to be the most adequate: a maximum of information is available with this type of high-frequency transducer.

3.7. Renal needle-biopsy

In cats it is very easy to perform with a mild anaesthetic. It obeys the rules stated in dogs. It is not recommended (and has no significance) in case of polycystic kidney, hydronephrosis and perirenal cysts. Renal hypertrophy must be considered with caution as it may be a compensatory hypertrophy, the controlateral kidney being the injured kidney.

4. Conclusion

The knowledge of the incidence of renal diseases of the cat and the use of a clinical, biological and histological methodology allow the clinician to make the diagnosis without cutting corners and as far as it is possible, to avoid performing a needle biopsy. Urinalysis (urine specific gravity and proteinuria) and the evaluation of the size and the shape of the kidney are prophylactic examinations that the clinician should perform every time the animal is presented and particularly during routine visits.