Diseases Of Different Organs
Canine kidney diseases can occur for a variety of reasons, including the presence of tumours in the body, heart failure, bladder stones, or shock following a severe accident. Dr Bush explains how the kidneys work, how they can be affected by disease, and discusses the treatments available to dogs.
Understanding the nature of kidney diseases in the dog requires first of all a little knowledge of how the kidneys function. Normal kidneys perform a number of important tasks, espeially,
1) Removing from the body the waste products of metabolic processes (excretion), particularly those resulting from the breakdown of proteins such as urea.
2) regulating the amounts of “salts”, for example, electrolytes such as sodium and potassium, and water in the body.
If there is an excess of any of these, the surplus is excreted; if in short supply, excretion is reduced as far as possible so that the substance is conserved. This excretion of substances is achieved by producing urine, the composition of which can be varied. In addition, although not germane to a general consideration of renal disease, the kidneys also control the degree of acidity or alkalinity within the body, secrete hormones affecting blood pressure and red blood cell production, and convert Vitamin D3 to its active form.
The Structure of the Kidney
Within each kidney there are a large number (approximately 400,000) of identical urine-producing units, the nephrons. There is a group of blood capillaries (minute blood vessels)known as the glomerulus. which receives blood from the artoryren. It lies within and in close contact with, a cup-shaped structure. Bowman’s capsule. latter is hollow and leads into a small window tubule which is surrounded by further capillaries. Evantually the tubule joins a collecting tubule, which collects the urine from a number of nephrons.
Function of the Nephron
Two main events occur within the nephron:
In the region of the glomerulus and Bowman’s capsule, substances which have very small molecules are filtered out of the blood. The capilaries lie close up against the wall of Bowmen’s capsule and there are minute holes, only visible using an, electron microscope, which pass all the way through and connect the two. The pressure in the blood capillaries forces a substantial proportion of the substances having sufficiently small molecules through these tiny connecting holes. They pass into the space between the two layers of Bowman’s capsule and then commence along the tubule. Essentially, all substance in the blood plasma, including glucose, urea, amino acids, sodium and potassium, are small enough to be able to pass through, with the exception of the plasma protiens; in other words, this is not a selective process.
As the filtered substances pass along the tubule towards the collecting tubule, varying amounts are taken back into the blood in the surrounding blood capillaries. Molecules which are not reabsorbed in this manner will be excreted in the urine. It is this process, therefore, which regulates the amounts of the different substances which are taken back into the blood and, conversely, the amounts which will appear in the urine. The functions of the kidneys can be summarised by saying that they filter out all substances which have sufficiently small molecules and then reabsorb only those substances which the body needs at that time. Normally, there are some substances that are totally reabsorbed (for example, glucose and amino acids), some that are never reabsorbed (the waste products such as urea and creatinine) and others (such as water, sodium and potassium) that are reabsorbed in varying amounts depending on the requirements of the body at that time. In normal, healthy kidneys, there are far more nephrons than are actually needed; this is described as having a large functional reserve. In fact. a dog can survive with only 25-30% of its normal number of nephrons (it is clear from renal transplant operations that both the donor and recipient each have only one kidney and therefore, at most, 50% of the normal number of nephrons, bit this is sufficient to maintain them in good health). However, if less than 25-30% of the nephrons are present, the kidneys fail to work efficiently, a situation known as renal failure.
Renal diseases are of three basic types,
1) Primary glomertilar diseases
The initial damage is in the region of the glomerulus when much larger holes than usual are produced in the glomerulus, allowing the larger protein molecules to pass through sometimes in considerable numbers.
2) Acute renal failure
3) Chronic renal failure
In both of these last two conditions, less than 25-30% of the nephrons remain functional so that the removal of wast products and the regulation of the sait and water balance are impaired. Acute renal failure develops rapidly and, with prompt treatment, not only may the patient survive the crisis but normal kidney function may be completely regained that is, the failure may only be temporary. On the other hand, chronic renal failure develops slowly and is irreversible. Here, the majority of nophrons have been destroyed and, since the kidney cannot produce new ones, the loss is permanent. Chronic renal failure is far and away the most commonly encountered of these three disease entities.
Pirimary Glowerular Diseases
Primary glomerular diseases glomerulonephritis and renal amyloidosis result from immune reactions often triggered by the presence of tumours or infectious organisms in the body. They appear from middle age onwards. The glomerular damage causes protein to be filtered from the blood in quantities far greater than can be reabsorbed so that large amounts appear in the urine. In order to replace this excessive Prothin loss, the body may break down its own muscle tissue, resulting in muscle- wasting and weakness, even though this protein will be lost in exactly the same way. If the plasma proteins fall to a very low level, they will be unable to exert a sufficient attraction (osmotic pressure) to retain water in the circulation. As a result, the water will pass into the tissues and body cavities. The abdominal cavity, unlike the chest cavity which is bounded by the ribs, is able to expand considerably to accommodate a large volume of fluid, thereby producing obvions distension (ascites) The combination of ascites with a low blood protein level and a high level of protein in the urine is termed the nephrotic syndrome. These glomerular disorders are best treated with a high protein diet to replace the protein which is continuously being lost via the kidneys. Salt intake should be kept to a minimum to limit fluid retention. Substances which act as substitutes for the blood proteins for example, dextran and gelatine, can be given intravenously and the accumulation of fluid can be controlled by administering diuretic drugs and even, if necessary, by draining the abdomen. Hovvever, at best these treatments slow the steady progression of the disease process : they certainly cannot reverse it.
Acute Renal Failure
In acute renal failure, the majority of nophrons suddenly cease to function. There is rapid development of the signs of renal failure (azotaemic signs) accompanied by a dramatic decline in the production of urine. The azotaemic signs, such as loss of appetite and vomiting, are due to the accumulation of protein waste products in the blood. Accumulation occurs when there are insufficient nephrons still working and they are described more fully under the heading of the more common condition, chronic renal failure. The event leading to the development of acute renal failure may arise anywhere in the urinary tract or even outside it.
1) Pre-renal. If the blood-flow through the kidneys is significantly reduced (eg, due to heart failure or a blockage of the renal artery). they are unable to filter the blood efficiently.
2) Post-renal. If there is an obstruction to the flow of urine from both kidneys after if has been produced (eg, a complete blockage of the urethra by a bladder stone) then the pressure of the urine will gradually increase until it is equal to the pressure causing filtration in the kidney, and at that point filtration will stop.
3) RenaI. Damage to the kidneys themselves may be responsible for the loss of function. This may take the form of acute inflammation, chiefly due to leptospirosis, or damage caused by nephrotoxins, kidney poisons such as ethylene glycol in antifreeze or the drug phenacetin, or by long-term interference with the kidneys blood supply such as might follow shock situations such as severe burns or road accident.
If the cause can be dealt with speedily and successfully, for instance, by treating heart failure or removing an obstruction to the flow of urine, then it is possible that renal failure will not be permanent : most of the nophrons and therefore the kidneys, will regain their function. Consequently, treatment is directed towards removing or reducing the effects of the :particular cause and to keeping the animal alive until kidney function can be re- established The latter involves the use of intravenous fluid therapy, both to correct fluid and electrolyte deficits and to restore the flow of liquid through the nephrons. For an animal to survive the immediate crisis, every effort must be made prevent any increase in the level of protein breakdown products, so protein food stuffs should be with held at this time.
Chronic Renal Failure
As stated earlier, this is the most common type of kidney disease in the dog and in contrast to acute renal failure, its onset occurs gradually. lt can arise as a sequel to either of the other two types of kidney disease mentioned before, but thera are other important causes, including among them
1) Chronic inflammation (nephritis). The nephrons are progressively replaced by fibrous (scar) tissue, possibly as the result of an infectious disease.
2) Tumours (cancer). These can gradually grow in size, destroying the nophrons and replacing them with nonfunctional tissue.
3) Pyclonephritis. Inflammation is associated with pus formation as a result of bacterial infection. All of these causes normally result in chronic renal failure developing in middle-aged or elderly dogs, but sometimes this condition develops at an early age because of congenital or inherited abnormalities:
1) Congenital disorders. Very occasionally animaIs are born with undersized kidneys which cannot develop normally (bilateral hypoplasia), resulting in insufficient naphrons for the body’s needs Alternatively, the kidneys may develop in abnormal sites without adequate drainage and this allows the early onset of progressive damage.
2) Inherited defects. Some lines of certain breeds may inherit kidney disorders in which nephron destruction begins at an early age, loading to the onset of chronic renal failure usually at batvvaen 1-24 months old. These breeds include amongst them Cocker Spaniels, Norwegian Elkhounds, Samoyeds, Dobermann Pinschers, Lhasa Apsos and Shih Tzus.
As in acute renal failure, the lack of functional nophrons results in azotaemia, the increased accumulation of a variety of protein-derived waste products. Collectively, these substances have effects on the brain and other. organs responsible for most of the clinical signs of kidney failure. Of all these products, urea is technically the simplest to measure and therefore diagnosis often involves determining whether the blood urea level is abnormally high. Nevertheless, urea is not the only, or even the most important, protein waste product and its concentration may sometimes increase for reasons unconnected with kidney function.
The signs attributable to azotaemia include depression, reduced appetite, vomiting and, sometimes, diarrhoea. The decomposition of urea to ammonia in the mouth can cause inflammation and reddening of the mucous membrane lining, sometimes oven progressing to ulceration, and there may be an obvious ammoniacal smell to the breath. Damage to the blood capillary walls can result in a tendency to bleed more easily and there may be changes in the blood itself, for instance, mild anaemia may occur. Calcium is removed from bone tissue and is replaced by fibrous tissue so that the bones become softer. This is especially likely to involve the jaws, leading to loss of teeth and, on rare occasions, increased flexibility of the bone, the classic sign known as “rubber jaw” The lack of nephrons in chronic renal failure means that each of those which does continue functioning has to work much harder than usual, oven though all the surviving nophrons together cannot perform the same total amount of work as would be performed by normal kidneys. Each individual nophron removes more waste products than it would normally, thus preventing it from reabsorbing as much water as usual in the tubule. Consequently, more water is lost from the body as more urine is produced (polyuria). This increased water loss loads to increased water consumption (polydipsia) simply to maintain the normal water content of the body. The increased output of urine and the increased thirst are two of the most obvious features of this illness. Animals often cannot retain the larger volume for long periods and overnight so that urine is passed in the house or the owner is regularly awoken to let the dog out. It is worthwhile remembering that the loss of water precedes the increased drinking because this problem cannot be solved simply by reducing the dog’s water intake, If it is not allowed to replace the water which has already been lost from the body, the dog will become dehydrated, thus imposing stress which makes its condition worse. If the protein waste products rise to very high levels, the interference with normal brain activity results in coma, sometimes preceded by convulsions, and ultimately death.
The treatment of chronic ranal failure is directed towards arresting the progress of the underlying disese to prevent any further loss of nephrons, if this is possible, and towards enabling :the dog to cope with the existing situation. The condition cannot be reversed; those nophrons which have been destroyed will not be replaced and so the dog cannot be cured in the sense of being restored to normality. Diet is of prime importance in the treatment of chronic renal failure. It should be low in protein content to reduce the production of the all important protein waste. products to a minimum. The body tissues, however, will require some protein to supply essential amino acids for such vital purposes as the repair of damaged tissues. Ideally, around 1-1.5 grams per kilogram body weight, or less, per day should be supplied (equal to 1/6-1/4 ounce, or less, for every 10 lbs of body weight per day). However, this protein should be of high biological value, that is, it should contain all the essential amino acids in the optimum proportions so that all of the dog’s needs are met but also that no excess amounts of amino acids are provided, as these will only be broken down into waste products. Egg is given the highest biological value (100), followed by lean meats such as beef, veal and chicken. Commercial “nephritis” diets unfortunately tend towards too high a protein content, probably to improve their palatability. Lower biological value proteins from offal and vegetable sources are best avoided. Dairy products and food rich in bone meal are not advised because their high phosphate content aggravates an existing accumulation of phosphate. Low protein diets tend to be unpopular with dogs and, to make thorn more acceptable, the addition of a flavouring agent (eg, “Marmite” or the oil from a can of sardines), feeding at blood heat and in a number of small meals daily, should be tried. Adequate amounts of carbohydrates and fats should be provided to ensure that body protein, for instance, muscle tissue, is not broken down as a source of energy. The bulk of the diet can therefore be made up of foods which are usually regarded as unsuitable for normal dogs; these renal failure cases, however, can no longer be regarded as normal. Appropriate foods with no protein include butter, margarine, vegetable oils, chocolate ice cream, jam, honey and sugar; those with minimal vegetable protein include rice, bread, cake, .biscuits, crisps, chips and paste products. As mentioned previously, water intake should not be restricted, the only exception to this lrule being where the animal is vomiting. Liquids by mouth should then be withheld and fluid therapy given intravenously. Because salt and Vitamin B (a water-soluble vitamin complex) are lost in increased amounts along with the water, the diet should be supplemented with these substances. Hopefully, with special care the dog can be kept in relatively good health and free from discomfort despite its restricted renal function. How long it survives depends on the type of disease-it may be from a few weeks up to 2 years or more although ultimately the dog will reach a paint when it can no longer maintain its rather precarious metabolic balance. Then the concentration of broken-down protein substances in the blood rises rapidly and the animal’s condition deteriorates dramatically. Drugs to reduce protein breakdown, control vomiting and diarrhoea and, if necessary, convulsions, and to provide extra calcium, can be given depending on the needs of the individual case. At a time of crisis, it may be possible to lower the level of waste products in the blood by using the specialised techniques of osmotic diuresis and intermittent peritoneal dialysis. However, these procedures are not designed to keep an animal alive indefinitely but merely to help it overcome a crisis. It must be recognised that if stabilisation can be achieved again, it will only be transient. Renal transplantation is not a feasible proposition in the treatment of canine patients. The possibility of a de-compensation crisis developing will be minimised if care is taken to protect the animal from stress; in the form of exposure to cold, fatigue, other diseases, and by feeding it a low protein diet along the lines indicated above.