Fever of Unknown Origin in Cats and Dogs

Fever of Unknown Origin in Cats and Dogs

Animals Affected

Cats and dogs


Fever of unknown origin is a common syndrome in cats and dogs.  Affected animals experience an elevated body temperature, usually accompanied by lethargy, weakness, and lack of appetite.

As the name implies, the cause of fever of unknown origin is not determined in most cases.  However, most pets with the syndrome are treated for bacterial infections.  This treatment usually is successful.


  • An elevated body temperature (usually above 103º F) is the most consistent symptom of fever of unknown origin.  Pets with fevers sometimes, but not always, will feel warm to the touch.  The normal temperature of cats and dogs ranges from 100.0- 102.5º F.
  • Most pets with fever of unknown origin are weak and lethargic due to fever and dehydration.
  • Lack of appetite is a common symptom of fever of unknown origin.
  • Pets with the syndrome sometimes tremble.

Risk Factors

Due to the enigmatic nature of the syndrome, most cases of fever of unknown origin are not correlated with any known risk factors.  However, pets that are at increased risk of infection are at increased risk of the syndrome.  For instance, cats that go outdoors may develop infected wounds and a fever from fighting with other cats.  These wounds may be too small to detect, which may lead to a diagnosis of fever of unknown origin.  Outdoor cats also may be exposed to other pathogens that can cause fever of unknown origin.

Young animals suffer from fever of unknown origin more often than older animals.

Flea infestation is a risk factor for fever of unknown origin.  Fleas transmit a number of diseases, including Mycoplasma and Bartonella, that may cause fever.


Most animals recover from fever of unknown origin when they are treated for infection and dehydration.  However, some cases do not respond to this therapy.  In these cases extensive testing, hospitalization, and intensive care may be required.  A very small number of pets do not survive fever of unknown origin.


Diagnosis of fever of unknown origin is made when no cause can be found for a pet’s elevated body temperature.   In most cases, radiographs (X-rays) are taken and basic blood and urine tests are performed prior to starting treatment for the syndrome.

Pets that do not respond to initial treatment may require comprehensive, advanced testing including diagnostic imaging (ultrasound), cultures for bacteria and fungus, DNA testing for pathogens, sampling of lymph nodes, joints, and organs, and blood tests called titers for exposure to specific pathogenic organisms.


In general it is considered best to treat animals with fever of unknown oirigin in a hospital setting with intravenous fluids, antibiotics, and, often, pain killers (fever is often correlated with body aches).  Rarely, anti-inflammatory medications such as prednisone or NSAIDs are used in the treatment of the syndrome.  Prednisone must be used cautiously because it can exacerbate some types of infection.

The most serious cases of fever of unknown origin require intensive management and supervision in a 24-hour veterinary facility.


In most cases, the symptoms of fever of unknown origin resolve within 48 hours of starting treatment.  Once treatment is complete, most pets do not suffer further episodes of the syndrome.

Pets that do not respond to treatment within 48 hours, or that suffer from recurring episodes of fever, should undergo more extensive treatment and diagnostics as outlined above.


Fever of unknown origin is by definition a mysterious syndrome.  Because of this, it is often frustrating for pet owners as well as veterinarians.  However, as mentioned above, most animals with the syndrome eventually make complete recoveries.

Although most csases of fever of unknown origin are treated with antibiotics (which treat bacterial infections), other causes of fever exist.  They include viral infections, inflammatory conditions in the body, exposure to certain toxins, and some forms of cancer.

Copyright © Eric Barchas, DVM. All rights reserved.
The contents of this page are provided for general informational purposes only. Under no circumstances should this page be substituted for professional consultation with a veterinarian.