What is considered a high temperature in a dog?

Fever is a term that refers to an elevated body temperature. The normal body temperature range for dogs is between 100.5°F and 102.5°F (38.1°C and 39.2°C). To be classified as a fever of unknown origin (FUO), the body temperature must be above 103.5°F (39.7°C) for longer than a few days in duration, with no obvious underlying cause based on history and physical examination.

Table of Contents Show

What causes a fever?
If this is a natural process, why is it a problem?
What are the clinical signs of fever of unknown origin?
How is FUO diagnosed?
What causes FUO?
How is FUO treated?
What is the prognosis for a dog diagnosed with FUO?
Recognizing Fever in Dogs
Detecting a Fever in Your Dog
Know What’s Normal
Watch for Symptoms
Taking Your Dog’s Temperature
What To Do When Your Dog Has A Fever

What causes a fever?

A fever is initiated by the presence of a pyrogen (a fever-producing substance). The pyrogen can be endogenous (produced from within the body) or exogenous (from the outside). This pyrogen causes the release of substances from the white blood cells (leukocytes) such as interleukin-1, interleukin-6, and tumor necrosis factor. These substances, in turn, reset the body's "thermostat", the temperature-regulating region located in the hypothalamus (located in the brain). This activates physiologic responses within the body to elevate the temperature.

If this is a natural process, why is it a problem?

A fever is beneficial to the body because it hampers the ability of viruses and bacteria to reproduce, and improves the immune system response to foreign invaders. However, if the body temperature remains above 105°F (40.5°C) for more than a day or two, the patient becomes lethargic and anorexic, and can rapidly become dehydrated. If the temperature persists above 106°F (41.1°C), cerebral edema (swelling around the brain), bone marrow suppression, and clotting disorders (disseminated intravascular coagulation, or DIC) may develop.

"A fever is beneficial to the body because it hampers the ability of viruses and bacteria to reproduce and improves the immune system response to foreign invaders."

When a fever persists, several consequences occur within the body. Among these are increased metabolic demands to maintain the higher temperature, which causes increased fluid and caloric requirements and increased breakdown of muscle tissues. A persistent high fever (one that lasts for more than forty-eight hours) is considered serious and potentially life threatening.

What are the clinical signs of fever of unknown origin?

Most dogs with a fever are lethargic, reluctant to move, have a loss of appetite, have increased heart and breathing rates, and are dehydrated. They may also be shivering or showing stiffness. With a fever of unknown origin, these clinical signs are present but there is no obvious cause for them.

How is FUO diagnosed?

If your dog has a fever, your veterinarian will perform a thorough physical examination, including looking for problems with the neurological system, the limbs and joints, and your dog’s eyes. Diagnostic blood tests will usually include a complete blood cell count (CBC), a serum biochemistry panel, and a urinalysis. Urine cultures are often performed to search for an underlying urinary tract infection. Additional tests may be recommended based on your dog's clinical signs such as radiographs (X-rays) or ultrasound of the chest or abdomen, radiographs of swollen or sore limbs and joints, cytology (samples from swellings or lumps to look at cell types and to check for infectious agents), echocardiography of the heart (to look for inflamed heart valves), blood cultures (to look for bacteria in the bloodstream), fecal cultures (such as Salmonella infection), serological tests for various infectious agents, and immunological tests looking for immune-mediated diseases.

What causes FUO?

As the name suggests, this is a fever without a demonstrable cause. As a result, determining the cause of an unknown fever in dogs can sometimes be difficult.

As outlined previously, there are many potential causes of a fever of unknown origin. There may be a viral, bacterial, fungal, or parasitic infection in any number of the body’s systems as the root cause of the problem. There may be inflammation due to an immune-mediated problem in the dog’s body. The dog maybe reacting to a drug or vaccination or may have an underlying cancer.

It is important to tell your veterinarian about any recent travel or boarding, any potential exposure to unknown or infected animals, any exposure to fleas or ticks, if your dog has been bred, had any recent vaccinations, is getting supplements or medications, or any other information that might be helpful to determine what may be causing their fever.

How is FUO treated?

It is important to understand that the diagnostic work-up for FUO may be quite involved. Your veterinarian will search for a specific cause of the fever so that it can be correctly treated and that an accurate prognosis can be given. In cases where a diagnosis is unobtainable, it is reasonable to make a diagnosis of FUO and initiate treatment based on the available information. Antibiotics are often prescribed to treat any underlying bacterial infection or to prevent bacterial infections from occurring as a secondary problem. Ideally, however, if antibiotics are used, it is best to perform diagnostic testing beforehand to prevent interference with, or masking of test results.

"It is important to understand that the diagnostic work-up for FUO may be quite involved."

On some occasions, medications to reduce the fever will be given. In people, a fever is often treated with a non-steroidal anti-inflammatory drug (NSAID). For the most part, such drugs do not treat the fever itself, and should be withheld until diagnostic tests are completed. In some cases however, such as panosteitis or hypertrophic osteodystrophy (inflammatory bone diseases), NSAIDs may be the treatment of choice.

What is the prognosis for a dog diagnosed with FUO?

An accurate prognosis can only be given when the cause of a condition is known. By definition, with FUO the exact cause is unknown; therefore, an accurate prognosis cannot be given. Most dogs respond well to basic supportive care such as keeping them warm and dry, providing plenty of water and nourishment, and administering antibiotics when indicated.

Dogs that have persistent fever or a fever that waxes and wanes must undergo a thorough work-up so that the cause of fever can be discovered and treated before irreversible damage occurs. In cases where this pattern repeats more than once, it is imperative that a thorough diagnostic work-up is undertaken.

Over the years, our Shar-pei, Ty, spiked a few dangerously high fevers and many that were less serious. Catching it early helped him recover more quickly, and I became quite good at recognizing when his temperature was climbing.

Recognizing Fever in Dogs

Let me begin by stating that I’m not a veterinarian. But I have had a lot of practice detecting fevers in dogs. Blitzen, the first Shar-pei we had, died just before his fourth birthday of kidney failure caused by Shar-pei Fever.

Dr. Linda J. M. Tindle, DVM describes Familial Shar-Pei Fever as “a periodic fever syndrome that is characterized by random inflammatory events with high fever, sometimes with swelling about joint/s or face, that usually last 12-36 hours.”

It’s often accompanied by Amyloidosis, a condition that causes abnormal protein build up in the kidneys and liver, which can lead to early death from organ failure.

Being able to quickly detect an oncoming fever was important during Blitzen’s illness. And those skills have served me well when Ty started having unexplained fevers. Below is what I’ve learned about fevers in dogs from taking care my pets.

Detecting a Fever in Your Dog

The most important step in figuring out whether your dog has a fever is knowing his normal temperature.

Know What’s Normal

Just like people, dogs’ normal body temperatures vary a bit. Ty’s was normally at 100.8, and Buster’s was about 100.5. But a dog’s normal temperature can range anywhere between 100.4 and 102.5.

To figure out what’s “normal” for your dog, you’ll need to take his temperature with a rectal thermometer when he’s feeling well. You can also make a note of it during routine vet visits when your pup isn’t sick.

Keep in mind that temperatures can vary a bit throughout the day. Ty’s temperature naturally went up a bit at night. So understanding your dog’s “healthy temperature” could mean tracking his readings at various times of the day for a few days.

Knowing your dog’s healthy pulse, respiratory rate, and capillary refill time are also handy tools in assessing a potential illness.

Watch for Symptoms

A change in your dog’s behavior will be your first sign that he’s coming down with something. You know what it feels like to have a fever, and your dog feels pretty much the same way – yucky!

My first clue that Ty wasn’t feeling well was that he got mopey. “Chillaxing” was his preferred speed, but when he doesn’t want to go for a walk or come running when we make a move for the kitchen, I know something is amiss.

Glassy-looking eyes and feeling warm to the touch are the next hints. You can also watch for shivering, panting, runny nose, loss of appetite, decreased energy, and depression. Any combination of these symptoms means it’s time to get out the thermometer.

Taking Your Dog’s Temperature

Get a good digital thermometer meant for rectal use, and mark it “Dog Thermometer.” Keep it anywhere except in your human medicine cabinet. You don’t want a sick family member accidentally using it in a feverish haze!

Denise Fleck, pet safety guru, provides the following advice on taking your dog’s temperature:

After lubricating the tip of a digital thermometer with petroleum or water soluble jelly, lift your dog’s tail up and to the side to prevent him from sitting. Then carefully insert the thermometer ½” to 1” into the rectum. Wait for the thermometer to beep, indicating that it’s registered your dog’s temperature.

If your pup’s temperature is higher than normal, it might be time to call your veterinarian.

What To Do When Your Dog Has A Fever

Like in humans, your dog’s body temperature will rise to fight off infection or inflammation. An infected cut, virus, urinary tract infection, and pneumonia are just some of the many conditions can cause a fever. So how do you know when to be really concerned?

My rule is that every fever warrants a call to the vet. It’s a good idea to let your vet know what’s going on with your dog and get their advice. Temperatures under 103 can generally be monitored at home for up to 24 hours. But a fever any higher, or one that lasts longer than a day, requires a trip to the vet.

A temperature of 106 degrees or higher can damage a dog’s internal organs and could be fatal. So, this is a very serious condition that needs to be monitored carefully.

Other than offering small amounts of water, consult your vet before taking action to reduce your dog’s fever. Giving aspirin, for example, might prevent the use of other medications that are more effective in lowering temperature.

For fevers serious enough to require a vet visit, expect your dog to receive IV fluids and anti-inflammatory medication. Your vet is also likely to suggest blood work to try to determine the cause of your pet’s fever.

Unfortunately, because so many things can cause a fever, it can be difficult to nail down the culprit. So, do your best to support your dog by keeping him hydrated and comfortable, and hopefully his immune system fights off whatever bug is causing the problem!