The basics about the kidneys
So let’s get started by talking a little bit about what the kidneys are and what they do. Like humans, dogs and cats have two kidneys which lie in the abdominal or belly area, one on the left side and the other on the right side. The kidneys are essential for maintaining water and salt balance in the body. They receive a lot of blood which they filter to take out a variety of waste substances that need to be removed from the body. These substances include for example urea, creatinine, potassium and acid, all of which if they accumulate in the body can cause severe and in some cases life-threatening problems. The kidneys also remove or hold onto water as necessary to keep the body hydrated. They produce urine which then flows down to the bladder where it is stored and intermittently excreted by the animal from the body. The process of urine production occurs continuously in a healthy animal.
Now in order to understand problems involving the kidneys better it is important for you to realise that each kidney is in fact made up of millions of little units, each of them working separately and alongside each other to make urine. The kidneys actually contain a lot more of these units than they actually need to keep an animal healthy – in other words, there is a large reserve capacity in each kidney. It requires around two-thirds to three-quarters of the total functioning kidney tissue to be lost before signs of kidney failure will develop. This means that when a kidney becomes abnormal, injured or affected by a disease, what happens next will depend on the speed and severity of the problem. As such there are really three different scenarios to keep in mind:
- The first is where enough of the units are damaged and the function of the kidneys becomes so severely reduced that the animal develops what we call acute kidney injury and failure. When the units in the kidney are damaged, they can recover if the damage is not too severe so some of these animals with acute kidney injury can go on to recover. However if the units are damaged too severely, they essentially die off and the failure is irreversible as new units cannot be formed.
- The second scenario is where the kidney suffers an injury that is not too bad at the outset; you may not notice anything different about your pet. However this sets of a process whereby the units in the kidney slowly die off until eventually, after months or even years, enough of them are lost and your pet starts to show signs of being unwell. We refer to this as chronic kidney failure. In some of these animals, you will notice relatively mild signs of illness that will nevertheless prompt you to see your vet.
- However in a small number, your pet which seemed to be doing ok suddenly deteriorates very quickly. In this our third scenario, the animal has what we refer to as an acute decompensation or crisis of their chronic kidney failure.
So just to summarise the three scenarios. We have acute kidney injury and failure; chronic kidney failure with relatively mild signs; and, chronic kidney failure with an acute decompensation or crisis. Hope that makes sense and you will see why we have spent time trying to explain this as the blog proceeds. The final point to make here is that it is possible for one kidney to be affected by a problem while the other one remains healthy.
Okay so now that we have discussed the basics about the kidneys, let’s think about the causes of kidney disease; what can injure the kidney and cause the units we described to be damaged? There are a variety of such causes, although it is also true to say that in a number of patients, we are not sure what the cause is.
- Drugs are one possible cause. Some of the drugs that are used very commonly in dogs and cats can injure the kidneys, either if too much is given or if they are used in the wrong circumstances. Non-steroidal anti-inflammatory drugs, or NSAIDs, such as carprofen or meloxicam, are one group of drugs that are known to potentially injure the kidneys. It is important to realise that we are not saying that animals should not receive these drugs, just that we need to be aware of their potential to injure the kidneys if used at doses that are too high or under the wrong circumstances.
- Toxins or poisons are something else that can injure the kidneys and cause kidney failure and these animals often go from being completely healthy to quickly developing acute kidney failure that makes them very sick. Examples of poisons that can do this include ethylene glycol which is contained in anti-freeze, as well as grapes, raisins, currants or sultanas in dogs, and lily plants in cats. We discussed these poisons in episodes 2 and 3 of these blogs so please have a read of those if you haven’t already.
- Infections are another potential cause of kidney disease. In particular dogs can be affected by a bacterial infection known as leptospirosis. There are a number of different strains of this bacterium but many of them cause kidney injury. In particular the strain that is most common in the United Kingdom as well as in other parts of the world tends to cause kidney injury as well as damage to the liver so the patient develops both kidney failure and jaundice. We also see so-called pyelonephritis in dogs and cats where a bacterial infection affects the kidneys only and can cause their function to be compromised.
- Cancer is another potential cause of kidney disease that we see. In particular a type of malignant cancer known as lymphoma can affect the kidneys as well as many other sites in the body.
- And certain breeds of cat, such as Persians or Exotic short hairs, can be affected by a condition known as polycystic kidney disease (PKD) in which cysts form in the kidneys. PKD is an inherited condition and can take a long period to become a problem clinically.
As we mentioned before, there are a number of patients with kidney disease in which we are not sure exactly what the cause is. This can be the case in some animals with acute kidney failure but it is especially common in patients with chronic long-term kidney disease. Chronic kidney disease occurs in dogs but it is especially common in cats with a significant proportion of older cats suffering from some degree of compromise to their kidney function.
Now that we have talked about some of the causes of kidney disease, let’s consider what you may notice in your dog or cat. Animals suffering from acute kidney failure that comes on quickly and is severe often start off with being lethargic and off their food and some also vomit. This may be all you notice and then the kidney failure is diagnosed when you take your pet to your vet and blood tests are performed; however some carers do also notice that their pet stops urinating as much as they usually would and sometimes stops urinating completely. Animals suffering from more long-term kidney disease can cope quite well for quite some time until they start to show signs such as weight loss, urinating and drinking more, loss of appetite, depression and dehydration.
The diagnosis of kidney disease starts with blood tests being performed in a patient that has signs which may suggest this problem exists or that has kidneys that feel abnormal when palpated from the outside. It is important to realise that the signs of kidney disease are not specific and can be due to other diseases so blood tests are essential for the diagnosis to be made. As we mentioned at the beginning, the kidneys are responsible for removing certain substances from the body that can be harmful if they are not removed. It is possible to measure some of these substances such as urea, creatinine and phosphorus, and to detect an increase in their blood levels in patients with kidney disease. Some animals that have been suffering from kidney disease for a long time can also become anaemic. Anaemia means that there is a decrease in cells in the bloodstream known as red blood cells and this is because the kidneys are important in making sure that there are enough of these cells in the circulation.
Alongside blood tests, it is also important to analyse a urine sample as this provides valuable additional information in terms of how well the kidneys are still working and may also help with identifying the cause of the kidney disease. Other tests that may then be performed include ultrasound of the kidneys and taking x-rays.
It is also important for animals suffering from kidney disease to have their blood pressure measured. Animals can have their blood pressure measured in a similar way to a human does. A high blood pressure can be the cause of kidney disease but kidney disease may also lead to a high blood pressure so it is important for your vet to check your pet’s blood pressure.
In terms of the treatment of kidney disease it is helpful for us again to consider the different scenarios we discussed before. An animal that was previously healthy and then suddenly suffers acute injury to their kidneys will usually require to be treated to some extent anyway in a different way to an animal that has had kidney disease for a long-time.
The most important thing for us to establish for an animal suffering from acute kidney injury is how much urine they are making. It is quite common for a severely affected animal to make less urine than they need to, and sometimes even completely stop making urine. Animals that recover usually go from this state to one where they make lots of urine before hopefully completely returning to normal. These acutely affected animals need to be on a fluid drip and they will often also be treated with diuretics. It is important to realise that neither the drip nor the diuretics are cures for kidney disease, they do not specifically treat the injury to the kidneys say like antibiotics specifically treat a bacterial infection. But they are nevertheless important treatments.
Some of the worst affected animals will not start to make enough urine despite the drip and the diuretics and there are then only two options for such patients. One is treat them with dialysis, and the other is to put them to sleep. An in-depth discussion of dialysis is beyond the scope of this blog. However it is important to realise that while it can work very well in some cases, it is not yet widely available for dogs and cats, especially not outside of North America; not all patients are considered suitable; and, it is usually very expensive to have done.
In addition to a drip and diuretics, or potentially dialysis, other specific treatments may also be required if a cause of the kidney injury is identified. These animals may also require other medical treatments such as drugs to help with nausea and vomiting or pain-killers.
These long-term sufferers of kidney disease will benefit from various management strategies at home. An in-depth discussion is beyond the scope of this blog but we would just like to mention the main ones briefly. The first one is feeding them a suitable diet, this usually being one which is restricted in protein and phosphate, and which contains increased amounts of potassium, vitamin B and general calories. As we mentioned before, the kidneys are responsible for removing certain substances from the body that can be harmful if they accumulate. In animals with kidneys that are not working well, we try to reduce how much work the kidneys have to do by reducing the amount of these substances that the body produces and therefore that the kidneys have to remove. In previous times, people would cook suitable diets at home using recipes provided by their vet but while this is still an option and potentially the cheapest option, nowadays it is possible to buy readily available so-called renal or kidney diets. Although potentially more expensive these diets offer much greater convenience and more accurate control of what is being fed. Kidney diets vary to an extent between dogs and cats and your vet will be able to discuss all this with you at the time your pet is diagnosed.
As we mentioned before, chronic kidney disease is especially common in cats and you can find more reliable and useful information about this on the website of the Feline Advisory Bureau based in the United Kingdom. The Feline Advisory Bureau is a charity dedicated to improving the health and wellbeing of cats and the link to their website is http://www.fabcats.org/.
Okay so let’s now discuss what the outlook is for kidney disease. Let’s start with animals that suffer some form of injury or disease that causes their kidney to fail acutely – that is suddenly and severely. Some of these animals can respond to treatment and go on to make a full recovery. In some cases, although the animal goes home and has a normal quality of life, their kidney function remains compromised for what can be quite a long period of weeks to month; and in some cases the kidney function never fully returns and they need to be managed for chronic kidney disease. One of the most relevant factors when considering the outlook for these patients with acute kidney injury is what level of treatment they need and what level of treatment can be provided. As we mentioned before, some of these animals fail to respond to standard treatment with a drip and diuretics and their only hope for possible recovery is with dialysis if this is considered appropriate for the individual patient. However dialysis may either not be available in the area in which you are based or it may not be something that you feel able to afford. As such a significant proportion of animals with acute kidney failure do regrettably end up being put to sleep when it may have been that with dialysis they could have recovered to the point of going home and having a decent quality of life. However you must also remember that we are speaking in general terms here and this discussion needs to be had with your vet in the context of your individual pet.
If we move on to animals with chronic kidney disease, as we mentioned, some of these animals can live for an extended period of time of months to years with appropriate management and regular monitoring as necessary, for example with weight checks, blood and urine tests, and blood pressure monitoring. In many patients a diagnosis of chronic kidney disease is not an instant death sentence however it is also important to realise that chronic kidney failure cannot be reversed, and in most cases, will progress over time despite appropriate therapy. However the rate of progression of kidney disease varies considerably between individuals and appropriate support and treatment can both increase the quality of life of affected animals and also potentially slow down the progression of the disease.
Before we end this blog we wanted to mention kidney transplants that are a potential consideration in humans with kidneys that have failed completely. Kidney transplants have been performed successfully in cats but to our knowledge not in more than one or two dogs. However as far as we know this has only been done for pets in the United States and Australia, and it certainly has not been done in the United Kingdom. There are some deep ethical considerations when it comes to kidney transplants as clearly the kidneys need to be taken from healthy donor cats basically without their consent as an organ resource. The usual protocol is for stray cats in rescue shelters to be used and for the procedure to only be done on the condition that the owner of the recipient cat homes the donor cat as well for the rest of their life. It is also very important to realise that there are strict criteria when it comes to determining which cats may be suitable to receive a kidney transplant as it is not an appropriate undertaking in all cats with kidney disease. And then of course there are the implications in terms of what both cats will go through and the financial costs to the owner.
As always our intention here is to try and provide you with some honest reliable information but you must discuss this with your vet in terms of whether transplants are available in your area and whether your cat may be assessed as suitable to receive one. But as we say, as far as we know, kidney transplants are only being done in pet cats in a very small number of hospitals in North America and Australia.
