Stop what you’re doing! Go take a drink. Drinking is important to avoid dehydration and keep us functioning at 100%. Of course this applies to pets as well. But why do dogs get dehydrated at all? Is it as simple as not enough water in their bowl?

Dehydration and Hypovolaemia: What’s the Difference?

Before we get to that, let’s discuss what dehydration is and how this differs from hypovolaemia. Like us, dogs store around 66% of their water within cells (intracellular fluid, ICF), and the remaining third outside of cells (extracellular fluid, ECF). Salt (sodium) is important for fluid balance as it helps keep the correct amount of fluid within each space.

Hypovolaemia usually involves the loss of water and salt from blood vessels (part of the ECF) resulting in a smaller volume of blood. This results in reduced blood pressure and delivery of oxygen to our tissues. Hypovolaemia is generally a rapid and severe process. Dehydration is a general loss of just body water from both the ECF and ICF.

This means our blood still carries enough oxygen to our tissues initially. Because of this, dehydration is a slower process and less severe in the early stages. Dehydration can eventually lead to hypovolaemia if enough fluid is lost that there is not enough to maintain blood volume. Both can lead to collapse and death if not treated.

In practice, dehydration and hypovolaemia are treated differently so it is important we know the differences. For you as an owner it is better to just recognise the early signs of dehydration and/or hypovolaemia so you can get your pet to the vets quickly.

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The Signs of Dehydration (and Hypovolaemia)

The easiest way you can test for dehydration is to look for a “skin tent”. Gently pull a fold of skin up away from their body – in a healthy well-hydrated dog the skin should spring back immediately. Dehydration will result in a ‘skin tent’ where the skin returns to normal slower instead. The more dehydrated your dog is, the more prolonged the skin tent.

The best places to check this are on the top of their head, the back of their neck and their back. Repeat it a few times in a few locations as some areas of skin are more elastic than others. However, prolonged skin tenting can also occur in older dogs with reduced skin elasticity, dogs with extra skin folds, or obese or very thin dogs.

You should also assess their mucous membranes. The best place is on your dog’s gums. Healthy gums should be pale pink to salmon pink. Hypovolaemia results in pale white gums. If you press on their gums with a finger, they should be moist and not sticky. Press hard enough to push blood away from the area (it will turn white) then remove the pressure – in a healthy dog the blood will return in less than two seconds. This is called the capillary refill time. A dog with hypovolaemia will have an increased capillary refill time.

Look at your dog’s eyes – if they are sunken in the face there may be a degree of dehydration. You can also assess their demeanour, as dehydrated dogs are often lethargic, quiet and depressed. You may also notice they do not produce much urine, have cold feet and a fast heart rate. 

All of these signs are variable and can be caused by other issues as well. However, if you spot more than a few at the same time you should suspect dehydration/hypovolaemia and contact your vets.

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Causes of Dehydration

Heat Intolerance:

When dogs are hot they try and cool down by panting and sweating from their toes and nose. The water evaporation removes excess heat. This is a normal physiological response to overheating, but if done too much without enough water to make up for losses can cause dehydration. The most obvious cause of overheating is if your dog is in a hot environment.

In summer, your dog will pant and sweat more so lose more water. Unless topped up, they become dehydrated more quickly. This is exacerbated if a dog is exercising, as they get hotter more quickly. Always provide water after exercise but especially in warm weather.

Finally, dogs with a fever due to illness may pant more and thus lose water quicker – they will also be using more water internally to fight infection, so providing plenty of water when your dog is sick is crucial. 

Other Primary Fluid Losses:

We’ve already mentioned the most common primary loss of fluid from panting and sweating, but there are others. Vomit and diarrhoea contain a lot of fluid, so a lot of either in a short time can result in dehydration. You should see your vet if your dog has frequent diarrhoea or vomiting anyway, but especially so if there are signs of dehydration.

Bleeding will also cause hypovolaemia. Again, always visit the vet if your dog is bleeding, but remember that sometimes bleeding can be internal – pale gums are a good way to detect unseen bleeding. Finally, fluid may be lost from the ECF or ICF but remain within your dog which can still cause dehydration. It tends to collect in certain areas, including within the lungs, in the abdomen or around the heart. If your dog has a swollen belly, breathing difficulties and signs of dehydration, contact your vet straight away.

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Primary Polyuria:

Polyuria is excessive urination, and can be caused by many diseases. Often polyuria is accompanied by polydipsia, or increased drinking, to compensate for the increased fluid loss via urination. However, sometimes your dog cannot drink enough to make up for the loss of fluid in urine so becomes dehydrated. Diseases that cause polyuria include kidney disease, liver disease, bladder incontinence, Cushing’s, diabetes, prostatitis, some cancers and more.

Certain drugs can also cause increased urination and may lead to dehydration if not accounted for. The signs of these diseases are hugely varied, and your vet will likely have to perform multiple tests to diagnose them. But increased urination alongside dehydration are good early warning signs of systemic disease, so keep an eye out for them.

Low Water Intake:

We’ve already alluded to this as many of the other causes of dehydration listed above can be masked if your dog drinks enough water – if they don’t, their body is tipped over into dehydration. Sometimes though, your dog may not drink enough for other reasons, leading to dehydration. For example, old age or chronic pain can make it difficult for your dog to reach their water bowl. Nausea or illness can make them reluctant to drink. Or your dog might simply have poor access to water – always ensure your dog has fresh water down 24/7.

Diagnosis and Treatment of Dehydration

If you bring your dog into the vets with suspected dehydration, your vet will confirm this with a clinical exam, including checking the parameters listed above. They may also test your dog’s blood and urine to quantify how dehydrated or hypovolaemic they are. If there is no obvious cause they might perform specific tests for diseases causing dehydration such as blood tests, ultrasound or x-rays.

Generally treatment of dehydration or hypovolaemia involves replacing the fluid lost with intravenous fluids. Your dog will probably require a hospital stay until they are stable and no longer dehydrated. If there is a primary cause, treatment will start for this as well. 

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