Why Is My Dog Or Cat Running A Fever Or Why is It Cold?

Why Is My Dog Or Cat Running A Fever Or Why is It Cold?

Ron Hines DVM PhD

See What Normal Blood & Urine Values Are

The Causes Of Most Abnormal Blood  & Urine Tests

See How Tests Are Grouped

Ear and other Non-contact thermometers are not accurate for dogs or cats. read here  & here

Up = Hyperthermia = Pyrexia = Fever  Low = Hypothermia

Many dogs and cats run a bit hot from anxiety and excitement when visiting their local veterinarian.

Your dog or cat has a normal body temperatures that is a bit higher than yours. Both your and your pet’s body temperature is set by the brain. That can be due to infections, stress, inflammation, certain cancers and autoimmune diseases and certain drugs. It can also be the result of a hot environment.

When your dog or cat ’s body temperature is higher than it should be, most owners would say their pet was running a fever. Technically, that is true, but many vets would call the pet “hyperthermic” if they suspected a non-infectious cause, and “febrile” (feverish)  if they suspected some type of infection.

Most veterinarians consider a rectal temperature of over 102.5 F/39.2 C to be abnormal in dogs and cats. But other things come into play and vets learn what those factors are through their years of experience. (if you are using a digital thermometer, such as a Vick’s ComfortFlex™, your pet’s armpit temperature will be about 2.5 F degrees lower [1.4 C] than its rectal temperature would be using the same device [ref])

Your vet may be less concerned if your dog or cat is emotional and tense or, perhaps, entering the exam room after a long car ride on a hot, humid day. The vet might be more concerned if your pet is a large calm dog – particularly if there is other evidence that the pet is ill. Looking of its charts, the vet might noticed that your pet always “runs hot” on its visits but has never shown any other signs of ill health.

Sometimes, out of caution, pets with slightly high body temperature will be given antibiotics. Many of my clients are adamant that their pet’s receive a “shot of something” and vets, being an agreeable lot, tend to oblige. But the fact that your pet’s temperature went down is really not enough proof that a bacterial or viral infection was the underlying problem.

Feeling your pets nose is not a good way to tell if it is running a fever (any more than feeling your nose tells you very much about your health).

Dogs with elevated body temperature do breathe faster, but many other things, like excitement and pain can cause that too. A glass (or modern digital) rectal thermometer, left up your dog or cat’s anus for up to 2 minutes (or at least until the fluid in older thermometers ceases rising) is the best way to take your pet’s temperature (its core body temperature). Putting some margarine on the tip allows it to be passed easily. Thermometers that measure at the ear, under the arm (axillary area) or in the toe web always give a lower reading than a rectal thermometer. Avoid them.

 It is quite common for pets at the animal hospital to run up to ~ 0.6 degree Fahrenheit too hot. That is due to the car ride there, excitement and agitation (some pets internalize those fears). So knowing how to confirm your pet’s body temperature at home, while it is relaxed, is quite helpful in deciding if the hospital reading was meaningful (if you can’t do that , consider a house call vet).

In normal situations, your pet regulates its body temperature primarily through thermoregulators in it’s brain. When these sensors detect that your pet is too cold, they direct its body to increase heat production through increased metabolism and shivering (at the same time they re-rout circulation to reduce heat loss).

When those sensors determine that the pet is too hot, they stimulate panting to dissipate heat and reverse the other processes.  

Moderate fever is not all bad. It is a part of the natural “acute-phase” response to infections and often plays a positive role in overcoming infections. Many viruses have difficulty multiplying at higher body temperatures. One successful treatment for herpes infection in puppies is to keep them warmer than they would normally be.

But body temperature over 107 F/41.6 C can not be tolerated for long. Things like DIC (= disseminated intravascular coagulation), internal bleeding and serious damage to the brain (cerebral edema) destructive heart beat irregularities (arrhythmias) , heart muscle damage, liver and intestinal lining soon occur. (ref) These pet’s bodies quickly becomes too acidic (metabolic acidosis) as well. When body temperatures remains over 104 F/40 C for longer periods it will severely dehydrate your dog or cat.

It is normal for dogs engaged in strenuous activities and sports to have body temperatures slightly above normal. That occurs even on cold days. (read here)

In a 2018 review of causes of fever in cats, infectious diseases were found to be the most common reason cats had temperatures over 102.6 F/39.2 C (38.7%). Chronic inflammation in various locations in the body accounted for 17.9% and cancer 12.3%. The most common infectious disease causing persistent elevated fever in cats was FIP. After that came cat fight infections (cellulitis/abscesses), ear infections (otitis), infections in the chest (pyothorax), urinary tract and kidney infections, flea-borne mycoplasma felis (aka: hemotrophic mycoplasma aka haemobartonella aka feline infectious anemia aka Mycoplasma haemofelis) or Triad disease.

Health Problems That Can Cause Your Pet’s Body Temperature To Be Too High (Fever, Hyperthermia, Pyrexia):

As I mentioned, excitement stress and agitation will always increase your pet’s body temperature (that is because those situation increase your pet’s levels of norepinephrine,   epinephrine,   thyroxine and muscle activity – all of which increase heat production).

Bacterial and viral infections are common causes as well. They release pyrogens , chemical agents that cause fever. It doesn’t always take a foreign invader to trigger that – your pet can run a fever due to simple tissue inflammation and muscle damage. In those cases, it is its blood monocytes that release inflammatory cytokines. (read here)

The bacteria need not be alive to cause fever, elevated body temperature is a common side effect of leptospirosis vaccinations (although other ingredients in the vaccines might contribute). (read here)

Invasive fungi (like cryptococcus) , rickettsia , like Ehrlichia and protozoa can also cause sustained or intermittent fever in dogs and cats.  I know personally what those dogs and cats feel like. As a young veterinarian living in the Galilee, I was bitten by a tick while tending to a sick dog. For several years I had intermittent fevers (FUOs) until a tick-transferred rickettsia was found to be the culprit.

Several antibiotics (eg tetracycline) as well as levamisole can also elevate body temperature in dogs and cats. 

Heat Stroke

High air temperature, high humidity, confined surroundings and pet agitation can all contribute to heat stroke. Short-nosed (brachycephalic) dogs and long-coated breeds are the most at risk. Aspirin will not bring these type of fevers down – it will just make the situation worse. Some heat stroke cases came to me during San Antonio, TX and Sarasota, FL summers with body temperatures over 107 F  (41.6 C).

When heat stroke is seen in hunting dogs out in the field or in jogging pets (in veterinary internal medicine texts, it is sometimes called hyperpyrexia syndrome). In those cases, the added heat generated by active muscles adds to the problem.


Inborn genetic defects (eg familial Shar-Pei fever) can also be responsible for fever or make the problem worse. The gene for that problem in all wrinkly skinned dogs is on their chromosome 13. (read here &  here)

Consuming an overdose of certain owners’ antidepressant medications (MAOIs), medicinal stimulants like amphetamine, or even household foods like chocolate can cause your pet to run a substantial fever.

Many cats that suffer from feline infectious peritonitis FIP run chronic, low-grade or intermittent fevers. So do cats with underlying feline immunodeficiency and feline leukemia infections.

Bartonella infection in cats  is a rather common cause of perplexing fevers. Similar fevers sometimes occur in dogs infected with babesia and the other blood parasites carried by ticks and fleas.


Another cause of unexplained fever can be a cancer that has not been located yet (particularly in older pets). Some forms of tumor cells produce cytokines that can cause fever as they move through your pet’s blood stream.


The spaces between your dog’s vertebra can harbor persistent bacterial (often staph but even fungi) infections that have moved from other body locations. The problem is called discospondylitis and chronic fever can be one of its first signs. 

Discospondylitis is generally a problem in larger breed dogs. The initial infection might have been in its urinary tract, prostate or blood stream, or it might have been carried into the body through a burrowing grass seed (grass awns ref 1, ref 2).  It might also indicate a weak immune system.

The underlying cause of some cases of chronic fever in dogs is infection with Brucella canis.

Immune-Mediated and Infective Arthritis

Most cases of arthritis I see in pets are due to the wear and tear of time on the pet’s joints and bones. (ref) Those pets do not run fevers. But there are other, rarer forms of arthritis in which low fevers are common. In those pets, white blood cell counts are often abnormal, as are joint fluid exams (synovial fluid exams).

Occasionally, a bacteria or mycoplasma is found in those arthritic joints. But more commonly, the pet is found to have an underlying autoimmune disorder. Some pet owners associate the beginning of these problems with a specific event – such as a vaccination. Although, perhaps, true; that has not been proven as of yet.

Fevers Of Unknown Origin (FUO)

There are many occasions when your veterinarian will not be able to immediately determine the cause of your dog or cat ‘s  fever. These are called “fevers of unknown origin,” or FUO (read about FUO in humans here).

Some of these cases in dogs and cats are difficult-to-diagnose infections. Things like heart endocarditis, kidney infections (pyelonephritis) prostate infections, etc. An abnormal white blood cell count might draw your veterinarian’s attention to those causes and supplemental tests.

Malignant Hyperthermia

Malignant hyperthermia is a sudden increase in your pet’s body temperature triggered by exposure to a drug. Often, it is an anesthetic drug used in general anesthesia, but it can also be things like brewery waste (ref), inherited tendency (ref) and probably many more as yet unknown.


During birthing or in the period thereafter, some recently-pregnant dogs have problems mobilizing their calcium reserves . In that situation, they can develop a condition called “milk fever” (= eclampsia, pregnancy tetany, post parturient or postpartum hypocalcemia) because their parathyroid glands are unable to shift bone calcium reserves into their blood stream fast enough. Many of these pets run fevers.(ref). Eclampsia is considerably less common in cats; but does occasionally occur.

Heartworm Treatment

When dogs with a substantial adult heartworm burden (numbers) are treated with the only approved arsenic-containing drug on the market, dead heartworms disintegrate in their circulatory system and lung tissue. It is common for dogs to run fevers during that period.

Hyperthyroidism In Cats

Some cats with hyperthyroidism are said to run low fevers. But fever can also be a side effect of the methimazole used to treat them.

Feline Infectious Peritonitis, =  FIP

FIP is the most common cause of a lingering fever in cats that are not showing any identifiable causes.

In the early stage of this disease, shortly after the common coronavirus of cats has mutated into potentially lethal FIP virus, cats often run fevers. Those cats also tend to have very low blood  lymphocyte counts.   (read here)

Teething Puppies And Kittens

Tooth eruption and a fever have been linked for a long time. The problem is that there are a lot of other possible causes of fever at that stage of the development. Teething often causes drooling, strong breath and lessened appetite in puppies and kittens; but do not assume by yourself that it is the cause of fever in your pet or child. (ref

Health Problems That Can Cause Abnormally Low Body Temperature (Hypothermia) In Your Pet:

Your pet’s body temperature drops in all diseases that are grave. When that occurs, it is often a sign of terminal exhaustion. I is always a very worrisome sign unless it can be tied to exposure to low temperature surroundings or recent general anesthesia. 

Overwhelming Infections

Overwhelming bacterial infections (septicemia) that cause shock, splenic (spleen) and stomach torsions, the coma and collapse that follow any life threatening illness all cause subnormal body temperature.

Sedatives And Narcotics

All sedative medications given in too large a dose, overdoses of certain mood altering medications such as amitriptyline given to anxious cats,   clomipramine for anxiety or acepromazine to dogs to alter behavior.

Narcotic painkiller overdose, ingestion of alcohol, low blood sugar/hypoglycemia and insulin overdose can all lower body temperature.


Cats with hepatic lipidosis, severe starvation in pets or long -term malnutrition are other causes of subnormal body temperature.


Hypothyroidism in dogs can also cause low body temperature – although that is not an accurate way to diagnose the problem.

Late Pregnancy

In mothers, just before the approaching puppy or kitten delivery date, the dog’s body temperature should drop below 100 F (37.8 C) ; the same in cats (If your dog or cat’s temperature goes up near or after delivery call your veterinarian).


The body temperature of puppies and kittens is normally slightly lower than their mother’s. During their infant period, they are very susceptible to dangerous hypothermia. That is because they have greater surface skin area in proportion to their body weight than adults. That allows their body heat to escape easily. The problem can be life-threatening. It can be due to maternal factors, the whelping environment or problems within the infant itself. Read more about that here.

Complementary Tests:

Since fever in your dog or cat can have almost as many causes as there are named diseases, your veterinarian will be looking closely for any possible indications as to what might be underlying the fever – based on the physical examination, your pets age, species, breed, history and lifestyle.

Veterinarians usually begin with the more simple and routine tests, look for suspicious values, and then schedule more sophisticated tests based on those earlier results.

Some of those tests might be:

A standard CBC/WBC and blood chemistry panel , FLV/FIV test in n cats,  urinalysis,   c-reactive protein, presence of stress leukogram or monocytosis, trial response to antibiotics, sedimentation rate,    X-ray and other imaging techniques,   tick panel,   blood culture for pathogens,   lymph node aspiration/biopsy, bone marrow examination/ M:E ratio, Repeat earlier tests; when borderline or negative and diagnosis remains unknown, antibody titers. Blood and urine myoglobin levels when exertional or malignant hyperthermia are suspected.  There are far too many more potential tests for me to list them all. 


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