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Decoding Fever of Unknown Origin in Cats

Understanding Fever of Unknown Origin in Cats

As cat owners, we always want to ensure that our furry friends are happy and healthy. However, sometimes our cats may fall ill with symptoms such as fever, yet we struggle to determine the underlying cause.

This condition is known as a fever of unknown origin (FUO), and it can be a source of great concern for pet parents. In this article, we will explore what fever in cats is, its causes, symptoms, and how to diagnose FUO.

Definition of Fever in Cats

Fever is one of the many ways the body fights infections and illnesses. In cats, fever is defined as a body temperature above 101.5F (38.6C).

It occurs when the body’s thermostat (located in the hypothalamus) is triggered to increase the body temperature in response to an infection or other inflammatory processes. The fever signals the immune system to fight off the infection.

Causes of Fever in Cats

Fever in cats can result from a range of underlying causes. It could be due to an infection, inflammation, cancer, immune-mediated diseases, or even environmental, physical, or chemical exposure.

Infection remains the most common cause of fever in cats, and it can be bacterial, fungal, parasitic, or viral.

Symptoms of Fever of Unknown Origin in Cats

The clinical signs of FUO are similar to those of other febrile diseases and include lethargy, decreased appetite, shivering, coughing, sneezing, and nasal or ocular discharges. In severe cases, the cat may experience dehydration, weight loss, and muscle wasting.

It is vital to note that some cats may not exhibit any clinical signs even with a high fever, making diagnosis more challenging.

Diagnosing Fever of Unknown Origin in Cats

Diagnosing FUO requires a thorough physical examination and advanced diagnostic tests. The vet may use various approaches such as blood work, urine analysis, imaging (

X-rays, ultrasound, CT scan), cytology, and biopsies to determine the underlying cause.

Causes of Fever of Unknown Origin in Cats

There are three categories of fever duration: acute (less than seven days), subacute (between seven days and a month), and chronic (over a month). Persistent fevers, which last more than seven days, usually indicate serious underlying illnesses that the vet may have to diagnose.

Identifiable causes of fevers include bacterial, viral, or fungal infections, cancer, and immune-mediated diseases. These causes can be detected using blood work and imaging tests.

However, often, the vet may not find any identifiable cause, and the fever becomes classified as unexplained.

Unexplained Fevers and Diagnosis

When a fever remains unexplained, the diagnostic process becomes challenging and may need prolonged hospitalization and administration of multiple medications. Treating unexplained fevers involves addressing the symptoms and not merely applying antibiotics or antipyretics.

To diagnose unexplained fevers, the vet may need to perform more advanced tests, such as bone marrow aspirates, biopsies of lymph nodes or organs, and testing for infectious diseases that aren’t routinely checked for in the area.


In conclusion, identifying the underlying cause of FUO in cats requires advanced diagnostic procedures and expertise. It is crucial to observe the cat’s clinical signs for any cues that may indicate illness and take them for routine examinations to detect any underlying problems early.

Seeking help from a reputable veterinarian should be the first step if you suspect your cat has a fever. With proper care and attention, your cat can resume a healthy and happy life.

Diagnosis of Fever of Unknown Origin in Cats

Diagnosing fever of unknown origin (FUO) in cats can be challenging, as it requires a comprehensive approach that includes medical history, physical examination, and advanced diagnostic tests. In this section, we will explore the diagnostic process of FUO in cats, including medical history, physical examination, lab tests and procedures, and x-rays.

Medical History and Physical Examination

The first step in diagnosing FUO in cats is the medical history and physical examination. The vet will ask about the cat’s symptoms, including when they began, how long they have been present, and whether they are intermittent or constant.

The vet will also ask about any medications or treatments the cat might have received to date. A thorough physical examination is then performed to assess the cat’s overall health, body weight, dehydration level, respiratory rate, heart rate, and temperature.

The vet may feel for any abnormal masses or organs during the examination.

Lab Tests and Procedures

After taking the medical history and evaluating the cat’s physical status, the vet will often order laboratory tests to determine possible causes of fever. Blood tests, urine analysis, and skin scrapings may be performed to detect infections, as well as identify any blood disorders or organ dysfunction that may be contributing to the fever.

Blood tests are among the most common diagnostic procedures used in the diagnostic workup of a febrile cat. CBC and biochemical profiles are usually run, and if the values are abnormal, follow-up tests might be requested.

Additional tests might include coagulation, serology, immunology, and a complete analysis of all other body fluids.


If the initial lab tests do not exhibit any causes of the fever, x-rays are necessary to identify the issue. Lung x-rays are commonly used when a cat’s fever is accompanied by respiratory symptoms.


X-rays can aid in detecting infections, heart disorders, or neoplasia.

Treatment and Management of Fever of Unknown Origin in Cats

The treatment and management of FUO in cats, depend on the underlying cause of the fever. Treatment options may include medications such as antibiotics, antivirals, anti-inflammatory agents, fluids, pain medications, or antiemetics, depending on the cat’s medical history and diagnosis.

The management of the fever includes identifying the source of the fever, addressing any concurrent symptoms, and monitoring the cat’s overall health, recovery, and development.


Severely ill or debilitated cats are often hospitalized for more intensive therapy and monitoring. Infections or neoplasia that cause fever may require hospitalization for the affected area to be monitored and treated.

Hospitalization is also necessary if the cat requires additional diagnostic testing or requires intravenous fluids and medications. Treatment options may also be required for any secondary complications from the fever, such as anemia, dehydration, or decreased appetite.

The vet may also provide cat owners with specific instructions on home care and treatments to be administered to the cat after hospital discharge.

Recovery from Fever

Cats recover from fever when the underlying condition is treated and eliminated. The duration of treatment and rate of recovery depend on the disease suspected to be causing the fever.

Treatment adherence is critical, and follow-up vet visits are vital to monitor the cat’s recovery and identify any new potential issues. In some cases, there might not be an identifiable or treatable underlying condition causing the fever.

In such cases, reducing the fever with medicines and making your cat’s stay comfortable is the best thing to do.

Final Thoughts

Finding out what is recurring fever in cats can be a long and burdensome process in cases when an underlying condition can’t be immediately identified. However, veterinary medicine has made great strides, and with advanced laboratory testing and diagnostic imaging, identifying the cause of fever in cats is more manageable than ever before.

Cat owners must take their furry friends for frequent checkups to prevent rather than cure health-related complications.

FAQs about Fever of Unknown Origin in Cats

Fever in cats can be a cause of concern for many pet owners, as it can be an indicator of underlying illness. In this section, we will answer frequently asked questions about fever of unknown origin (FUO) in cats, including causes of fever, duration of cat fevers, parasites and fever in cats, and recovery from fever.

Causes of Fever in Cats

Fever in cats can result from several underlying causes, including bacterial, fungal, viral, or parasitic infections, inflammation, autoimmune diseases, cancer, and environmental or physical exposure to toxins. Infections are the most common cause of cat fevers, with bacterial infections being the leading cause.

Duration of Cat Fevers

The duration of cat fevers is categorized based on the length of the fever. Acute fevers last less than seven days, subacute fevers last between seven days and a month, and chronic fevers last over a month.

Persistent fevers, where a cat has a fever for more than seven days, are usually an indicator of a severe underlying medical condition.

Parasites and Fever in Cats

Parasites can be a cause of fevers in cats, especially blood parasites such as bartonellosis and cytauxzoonosis. These diseases are caused by bacteria and protists transmitted through insect bites.

Symptoms include a high fever, lethargy, anemia, and in severe cases, seizures and death. Pathogens transmitted via ticks are infamous for causing significant illnesses in mammalian pets, including cats.

Therefore, tick prevention is crucial to avoiding such illnesses.

Recovery from Fever

The recovery from fever depends on the underlying cause of the fever. Once the underlying condition is diagnosed and treated, the cat will start recovering from the fever promptly.

However, recovery time varies in cats and depends on the extent of the illness and the treatment administered. It is essential to ensure that the cat follows the veterinarian’s instructions, including administering the correct medication doses and completing the duration of the prescription treatment.

Follow-up vet visits are also crucial to assess the cat’s recovery progress and ensure no new diseases that bring about fever might be developing. Ensure your cat drinks enough water and eats enough during this period since a fever can decrease appetite and cause dehydration.

Always contact your veterinarian if you observe additional unexpected symptoms or the cat becomes unwell.

Final Thoughts

Cat owners must be aware of the symptoms of a fever of unknown origin in cats and the underlying causes. Cats may not always exhibit clear signs of illness, making diagnosis challenging.

Regular checkups might help identify underlying conditions early. If a cat has a fever, the best course of action is to seek veterinary care.

Healthcare professionals can diagnose the underlying cause of the fever and provide appropriate treatment. Always remember that early detection might be the difference between a rapid recovery and a severe medical condition.

In conclusion, fever of unknown origin (FUO) is a concerning condition in cats that might indicate underlying medical issues. Fever can be caused by infections, inflammation, immune-mediated diseases, cancer, and physical or chemical exposures.

Diagnostic procedures, including medical history, physical examination, x-rays, and lab tests, help identify the underlying causes of fever in cats. Treatment and management of fever vary depending on the underlying cause but may include medications, hospitalization, and follow-up vet visits.

As cat owners, it is crucial to seek veterinary care at the first sign of a fever to ensure early identification and management of any underlying medical issues.

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