A fever of unknown origin in dogs is a fever that lasts more than a few days and has no obvious cause. Fevers in dogs can be caused by infections, inflammation, immune-mediated diseases, or other health conditions. A fever is usually a protective mechanism of the body, but it can be very dangerous if it’s prolonged or the fever is very high.

A fever is also known as pyrexia or hyperthermia. Dogs have a fever when their temperature is over 39.2 degrees Celsius at rest. A fever of over 41°C is a medical emergency, as it can lead to life-threatening complications.

What is a fever of unknown origin in dogs?

A normal temperature for dogs is between 38.3 and 39.2 degrees Celsius when resting. A temperature above this range is considered a fever. A fever of unknown origin describes a fever lasting more than a few days with no obvious cause.

• A fever starts due to the presence of a substance called a pyrogen, which can come from something inside or outside the body.

• This pyrogen causes the white blood cells to react, leading to responses elsewhere in the body that increase the temperature.

• A fever is a protective mechanism of the body, as it improves the immune system and reduces the activity of invaders, like bacteria.

• The problem arises when a fever lasts for more than a few days. This leads to symptoms such as lethargy and reduced appetite.

• An extremely high fever (over 41.1°C) can lead to life-threatening conditions such as swelling of the brain, clotting problems, and organ failure.

• A fever is different from heat stroke.

Common causes of fevers include:

• Infections: bacteria (including abscesses), viruses (Parvovirus), fungi and parasites

• Inflammatory diseases, such as pancreatitis

• Autoimmune disease, such as immune-mediated haemolytic anaemia

• Reactions to medications such as vaccinations or antibiotics

• Reactions to poisons, toxins and stings or insect bites

• Cancer, such as lymphoma

A fever can become very serious, especially if left untreated for more than a few days. If you suspect your dog has a fever, it’s best to speak to a vet as soon as possible. The Joii vets are available 24 hours a day for advice.

Symptoms of a fever of unknown origin in dogs

The symptoms of fever are nonspecific and might go unnoticed initially. Common symptoms of a fever of unknown origin include:

• Lethargy

• Reduced appetite

• Fast breathing or panting

• Shivering or trembling

• Stiffness

• Dehydration: dry, sticky gums and loss of skin elasticity

Are some dogs more at risk of a fever of unknown origin?

A fever of unknown origin can affect dogs of all ages, breeds, and sexes. Certain groups may have a higher risk of infectious causes, including:

• Young dogs

• Dogs that have travelled abroad (at risk for Babesia, Leishmania, Ehrlichia, and Rabies)

• Unvaccinated dogs

• Dogs with increased exposure to ticks

How is a fever of unknown origin diagnosed in dogs?

A thorough physical examination is the best way to start an investigation into a fever. If it’s the first time your dog has a fever, the vet may suggest medication and monitoring. However, if the symptoms are not improving or your dog gets repeated episodes of fever, one or more of the following tests will usually be required:

• Blood and urine tests

• Faecal tests

• Biopsy of any lumps that are present

• Imaging: X-rays, ultrasound

• Heart scan

• Specialised tests for infectious diseases

Determining the cause of a fever in dogs can be challenging due to the many different causes and nonspecific symptoms. Diagnosing the issue often requires extensive testing, and results may take time. To help your vet, follow their advice closely and provide a detailed medical history for your dog.

What is the treatment for a fever of unknown origin in dogs?

Treatment of a fever of unknown origin can vary, depending on what the suspected underlying cause is. It may include:

• Antibiotics: if a primary or secondary bacterial cause is suspected

• Non-steroidal anti-inflammatory medication

• Immunosuppressants

• Pain relief

Severe symptoms

• Hospitalisation for monitoring and treatment

• Oxygen therapy

• Blood transfusion

• Fluids given via a drip

• Fever-reducing medication

How to look after a dog with a fever of unknown origin

You can help your dog with a fever at home:

• Provide plenty of fresh water in an area that is easy for your dog to reach.

• Use a highly palatable food. Making their food moist and heating it slightly can improve its palatability. You can also feed small meals of bland food, such as scrambled eggs or chicken, for a maximum of 48 hours.

• Make sure they have a comfortable and quiet place to rest and recover.

• Monitor their symptoms closely, and if they worsen, make sure to get them rechecked by a vet promptly.

Tips on how to prevent a fever of unknown origin in dogs

There are many ways to protect your dog from developing a fever:

• Make sure your dog is up to date with their vaccinations and parasite preventatives.

• Remove any ticks as soon as you find them.

• Prepare well in advance for taking your dog abroad and make sure to follow your vet's advice.

• Treat any wounds promptly; ensure they don’t get infected, especially dog bite wounds.

• Never give your dog human medication.

• Prevent access to toxins and poisons in the house and outdoors.

Is my family at risk of catching a fever of unknown origin?

Many causes of a fever of unknown origin are not contagious, including auto-immune diseases, cancer and reactions to medications. However, there are some conditions that can spread to other pets or humans, such as Salmonella, Campylobacter, and Toxoplasma. This is why it is important to get your dog checked with a vet if they show signs of fever.

When to worry about a fever of unknown origin in dogs

Seek help from a vet in practice as soon as possible if:

• Your dog has been off their food or lethargic for more than 24 hours.

• Your dog has repeated signs of a fever.

• Your dog has any symptoms such as collapse, breathing problems or dehydration.

Speak to one of our Joii vets if:

• You have any questions about which parasite preventatives you should use.

• Your dog has been off their food or lethargic for less than 24 hours.

• You need advice about removing or identifying a tick.