An increase in body temperature above the normal range is referred to as fever. In dogs and cats, this is a body temperature above 40 degrees Celsius (see also normal values for dogs and cats).
Fever and overheating (—> Overheating) are not the same thing.
Fever
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Fever is a natural defense reaction of the body where the body temperature rises above the normal range. In dogs and cats, fever is defined as a body temperature above 39.5°C (103.1°F). We consider high fever in our pets starting from a temperature of 40°C (104°F), while critical fever occurs from 41°C (105.8°F). The normal temperature for dogs is 37.5-39.0°C (99.5-102.2°F), for cats 38.0-39.0°C (100.4-102.2°F). These values can vary slightly individually and are influenced by factors such as time of day, physical activity, and ambient temperature.
The sudden onset of high fever differs fundamentally from a slow rise in temperature. While a gradual increase gives the body time to adapt, a rapid temperature rise puts particular stress on the organism. The quick change can lead to stronger impairments and is often an indicator of acute, potentially life-threatening diseases.
It’s important to distinguish between fever and hyperthermia. Fever occurs due to a resetting of the body’s own temperature regulation center in the hypothalamus, usually in response to pathogens or inflammatory processes. Hyperthermia, on the other hand, is overheating due to external factors such as high ambient temperatures or excessive exertion, where the temperature regulation center functions normally but heat dissipation is impaired.
Causes
Fever is usually a sign of infection. The body’s reaction to increase body temperature is helpful and should not be fought against as a matter of principle. More attention should be paid to the cause of the fever.
- Bacteria, Parasites, Viruses
- Tumors
- Autoimmune Diseases
- Chronic Inflammatory Diseases
- Hormonal Disorders
- Blockage of a Blood Vessel (Thrombosis)
Note: In cats, it is not uncommon for a blood vessel in the pelvic area to become blocked. In these cases, the rectal temperature is not elevated but lowered due to reduced blood flow.
Sudden high fever in dogs and cats can be triggered by various factors, with infectious causes being the most common. The main triggers include:
Acute bacterial infections are a common cause of sudden high fever. Particularly dangerous is septicemia, where bacteria enter the bloodstream and spread throughout the body. This can arise from local infections such as wound infections, abscesses, gum inflammation, or urinary tract infections. Leptospirosis, a bacterial zoonosis, can lead to sudden high fever in dogs and is transmitted through contaminated water.
Viral infections such as parvovirus or distemper in dogs, as well as feline panleukopenia or cat flu, can also lead to rapid fever onset. In cats, FIP (Feline Infectious Peritonitis) and FIV (Feline Immunodeficiency Virus) can be accompanied by bouts of fever. Parasitic diseases such as babesiosis, ehrlichiosis, or anaplasmosis, transmitted by ticks, are other possible causes.
Non-infectious causes include immune-mediated diseases such as hemolytic anemias or polyarthritis, which arise from misdirected immune reactions. Neoplasms (tumor diseases) can also be accompanied by fever, especially lymphomas and leukemias. Poisoning by toxins such as certain plants, medications, or chemicals can lead to a febrile reaction.
Special attention should be paid to the so-called “Fever of Unknown Origin” (FUO), where no clear cause for the fever can be found despite extensive diagnostics. These cases require particularly thorough investigation and monitoring.
In cats, it should be noted that a blockage of a blood vessel in the pelvic area (arterial thromboembolism) can paradoxically lead to a lowered rectal temperature, while the rest of the body shows fever – an important diagnostic indication for veterinarians.
Symptoms
- Lethargy
- Panting
- Thirst
- Pain
- Loss of appetite
- No Water Intake
- Breathing difficulties
- Swelling
- Vomiting
The clinical presentation of an animal with sudden high fever includes characteristic symptoms that are attributable to increased metabolism and immune response. Typical signs include:
General impairments are evident through pronounced lethargy and fatigue. The animals appear apathetic, withdraw, and avoid movement. Loss of appetite up to complete food refusal is often observed, while thirst is often increased. Many animals show an altered body posture with an arched back or hanging head.
Physical signs include hot and dry mucous membranes and an increased respiratory rate. Dogs respond with increased panting, while cats often breathe with an open mouth, which is unusual behavior for them. The heart rate is significantly elevated, and warm ears and paws are noticeable upon examination. A characteristic alternation between chills and feeling hot is also common, with animals shivering despite high body temperature.
In cats, the symptoms can be more subtle than in dogs. They hide more often and show less obvious signs of illness. An important warning sign in cats is the complete loss of grooming behavior.
The severity of symptoms does not always correlate with the height of the fever. Some animals show significant impairments at 40°C (104°F), while others hardly show symptoms at the same temperature. At temperatures above 41°C (105.8°F), clouding of consciousness, seizures, and collapse can occur, as extreme hyperthermia can lead to organ damage.
In addition to these general fever symptoms, depending on the underlying disease, other specific signs may occur, such as coughing in respiratory infections, vomiting and diarrhea in gastrointestinal diseases, or lameness in joint inflammations.
First Aid
- Use a digital thermometer. Ear thermometers are often easier to use but less accurate (Fig.).
- Apply some petroleum jelly or oil to a standard fever thermometer and insert it rectally about 2 cm (cat, very small dog) to 5 cm (large dog) and wait until the temperature stops rising. The rectum runs straight, so you don’t need to worry about damaging the intestine.
- Do not let go of the thermometer and remove it if the animal becomes very restless.
- An animal with a fever needs rest.
- Make sure your pet drinks enough.
- You can recognize dehydration, for example, by less urine being produced and it looking more concentrated and darker than usual.
- Do not use fever-reducing medications without veterinary advice.
Diagnosis
The diagnosis of sudden high fever begins with correct temperature measurement. Rectal measurement with a digital thermometer is considered the gold standard. The lightly oiled thermometer is inserted about 2 cm (for cats, small dogs) to 5 cm (for large dogs) into the rectum. Alternative measurement methods such as ear thermometers offer more comfort but provide less precise results.
After detecting fever, a systematic diagnostic process follows to determine the cause. A thorough anamnesis covers pre-existing conditions, vaccination status, medication administration, travel history, and possible exposure risks. During the clinical examination, all organ systems are systematically examined, with particular attention to signs of inflammation, swelling, and pain.
Laboratory diagnostic tests form a central component of the diagnosis. The blood count can provide important clues: Leukocytosis indicates bacterial infections, while leukopenia is more common in viral infections or sepsis. Blood chemistry provides information about organ functions and metabolic disorders. Specific inflammatory markers such as C-reactive protein or serum amyloid A can support the diagnosis.
If the cause is unclear, further examinations are indicated. Imaging techniques such as X-rays can provide indications of pneumonia, foreign bodies, or tumors. Ultrasound is suitable for assessing abdominal organs and detecting fluid accumulations or abscesses. In complex cases, CT or MRI can provide additional information.
Microbiological examinations include blood cultures if bacteremia is suspected, as well as urine, stool, or tissue samples depending on the clinical suspicion. PCR tests allow direct pathogen detection in viral or specific bacterial infections. Serological tests can detect antibodies against specific pathogens.
In cases of therapy-resistant fever of unknown origin, more invasive diagnostic measures such as bone marrow biopsy, lymph node biopsy, or exploratory laparotomy may be necessary.
Further veterinary measures
If the symptoms persist for more than a day or additional symptoms such as drowsiness, skin changes, vomiting, diarrhea, and others occur, don’t wait for the fever to disappear on its own, but present it to a veterinarian.
Fever is not a disease, but a symptom. Accordingly, the veterinarian will determine the cause of the fever through a physical examination, laboratory diagnostics, and possibly further procedures such as X-rays, ultrasound, etc.
The treatment of sudden high fever in dogs and cats follows a multi-step approach that includes both symptomatic therapy and treatment of the underlying cause.
Emergency measures for critical fever (>41 °C) initially aim to lower body temperature. Controlled passive cooling can be done with damp cloths on the paws, armpits, and groin area. Active cooling measures such as cool (not cold) water baths should be applied carefully and only under veterinary supervision, as cooling too quickly can lead to circulatory problems. Fans can support evaporative cooling. The target temperature is about 39 °C, after which cooling should be stopped to avoid hypothermia.
Supportive therapy primarily includes fluid therapy, which is essential for fever patients. Intravenous infusions with isotonic crystalloids compensate for fluid losses and support circulatory function. The infusion rate is individually adjusted to the hydration status and body weight. In severe cases, oxygen therapy may be necessary to support the increased metabolic rate.
Antipyretic (fever-reducing) medications are used in a targeted manner. Non-steroidal anti-inflammatory drugs such as meloxicam or carprofen may be indicated for temperatures above 40 °C or in cases of severe discomfort. It is important that these medications are only administered under veterinary supervision, as they can have side effects on the kidneys and gastrointestinal tract. Paracetamol is absolutely contraindicated for cats and can be fatal.
Causal therapy is directed at the underlying cause. For bacterial infections, antibiotics are used, initially often as a broad-spectrum antibiotic, later targeted based on antibiogram results. Viral infections usually require symptomatic treatment; for certain pathogens, antiviral medications are available. Parasitic diseases are treated with specific antiparasitics. Immune-mediated diseases often respond to corticosteroids or other immunosuppressants.
In severe cases, intensive care with continuous monitoring of vital parameters may be necessary. Monitoring includes regular temperature checks, ECG monitoring, and blood pressure measurements.
Prognosis and aftercare
The prognosis for sudden high fever depends significantly on the underlying condition, the height of the fever, the duration of the condition, and the timing of treatment initiation. Generally, the earlier the treatment begins, the better the chances of success.
For bacterial infections, the prognosis is usually good if adequate antibiotic therapy is started in time. Viral diseases show varying courses – while some, like uncomplicated cat flu infections, have a good prognosis, others like parvovirus or FIP can be more severe. For immune-mediated diseases, the prognosis depends on the responsiveness to immunosuppressive therapies.
It becomes critical at temperatures above 41 °C for several hours, as irreversible organ damage threatens. The central nervous system, liver, kidneys, and gastrointestinal tract are particularly at risk. Prolonged hyperthermia can lead to coagulation disorders and, in the worst case, to multiple organ failure.
Aftercare following a fever episode is crucial for complete recovery. During the convalescence phase, the animal should be kept in a calm, stress-free environment. Physical exertion should be avoided for at least a week after the fever subsides to avoid putting additional strain on the organism.
Nutrition plays an important role in the recovery phase. Easily digestible, high-quality food in small, frequent portions supports regeneration. In case of loss of appetite, food supplements or special nutrition can be offered. Adequate fluid supply remains important even in the aftercare phase.
Follow-up examinations are important to monitor the healing process and detect relapses early. Depending on the underlying condition, laboratory checks, imaging procedures, or specific tests may be necessary. The schedule for these follow-ups is set individually, typically with the first check occurring after 3-7 days.
Special attention is given to recurring fever episodes, which may indicate chronic infections, autoimmune diseases, or neoplasms and require further diagnostic evaluation.
Summary
Sudden high fever in dogs and cats represents a serious condition that requires prompt action. With temperatures above 40 °C, the body reacts to various triggers, with infections being the most common cause. Symptoms include lethargy, loss of appetite, increased respiratory and heart rates, as well as warm mucous membranes, and can be supplemented by specific signs depending on the underlying condition.
Diagnosis begins with correct temperature measurement and includes a systematic assessment through clinical examination, laboratory diagnostics, and imaging procedures. The therapy follows a multi-step approach with emergency measures to reduce temperature in critical fever, supportive treatment through fluid therapy and targeted use of antipyretic medications, as well as causal therapy of the underlying disease.
The prognosis depends significantly on the cause, the height of the fever, and the timing of treatment initiation. Aftercare with adjusted housing, special nutrition, and regular follow-up examinations are crucial for complete recovery and to prevent relapses.
Pet owners should know that fever itself is not a disease, but a symptom and represents an important defense reaction of the body. Nevertheless, sudden high fever requires veterinary attention, as it can indicate serious underlying conditions and, at extreme values, can itself cause organ damage. Early detection and treatment is the key to successful therapy.
Outlook on current research
Veterinary medicine continuously develops new approaches for the diagnosis and treatment of febrile diseases in small animals. Current research focuses on several promising areas.
In the field of diagnostics, point-of-care tests are revolutionizing rapid diagnostics. These allow for the detection of specific pathogens or inflammatory markers directly in the practice within minutes. New molecular biological methods such as multiplex PCR can simultaneously test for multiple pathogens in one sample, accelerating diagnosis. The development of specific biomarkers for various causes of fever could enable more precise differentiation in the future.
In therapy, new antimicrobial substances are being researched that are also effective against multi-resistant pathogens. Targeted immunomodulation through specific biologics represents a promising approach for immune-mediated causes of fever. For viral diseases such as FIP in cats, significant progress has been made in recent years with new antiviral substances.
Telemedicine is also gaining importance in veterinary medicine. Remote monitoring systems enable continuous temperature measurement and vital parameter monitoring at home, which can be particularly helpful for chronically ill animals or after fever episodes. These technologies could enable earlier intervention in the future.
Another research focus is on better understanding thermoregulation in different animal species and breeds. Genetic factors that influence susceptibility to certain febrile diseases are increasingly being identified, which could potentially enable individualized prevention and treatment strategies in the future.
The One Health Initiative, which emphasizes the connection between animal, human, and environmental health, is gaining importance in the research of febrile diseases. Many pathogens that cause fever in pets have zoonotic potential or are transmitted by vectors whose distribution is influenced by climate change.
These research approaches promise improved diagnostics and therapy for febrile diseases in dogs and cats in the coming years, ultimately leading to a better prognosis and quality of life for our animal companions.
Frequently asked questions (FAQs)
- At what temperature should I take my pet to the veterinarian?
A veterinary visit is advisable for temperatures above 39.5°C (103.1°F). Temperatures over 40°C (104°F) are considered an emergency and require immediate veterinary assistance. - Can I give my pet fever-reducing medications myself?
No, human fever medications can be dangerous or even fatal for animals. Only administer medications as prescribed by a veterinarian. - How do I correctly measure my pet’s temperature?
Rectal measurement with a digital thermometer is most accurate. Gently insert the lightly oiled thermometer 2-5 cm (depending on the animal’s size) into the rectum and wait until the reading stabilizes. - How can I help my feverish pet at home?
Offer fresh water, provide a quiet environment, and for very high fevers, carefully cool paws and groin areas with damp cloths. However, always seek veterinary advice. - Can stress cause fever in my pet?
Stress alone rarely causes true fever but can lead to a slight increase in temperature. Persistent elevated temperatures usually have other causes. - How long does it take for antibiotics to work in a feverish animal?
Generally, a decrease in fever should be observed within 24-48 hours after starting effective antibiotic therapy. - Can my pet die from a fever?
Extremely high fever (above 42°C/107.6°F) can be life-threatening and lead to organ damage. However, it’s not usually the fever itself but the underlying condition that poses the real danger. - Why is fever harder to detect in my cat compared to my dog?
As natural prey animals, cats show illness symptoms less obviously and often hide when feeling unwell, which can make early detection more difficult. - Can I prevent fever?
Regular vaccinations, parasite prevention, and good hygiene can prevent many fever-causing diseases. Regular veterinary check-ups help to detect problems early. - Is it normal for my pet to develop a mild fever after vaccination?
A mild increase in temperature (up to about 39.5°C/103.1°F) can occur in the first 24-48 hours after vaccination and is mostly harmless. Higher or prolonged fever should be checked by a veterinarian.
Literature
- Löwe, G and Löwe, O.: Emergencies in Dogs and Cats – A Veterinary Guide. Kynos-Verlag, 2021. 208 p.
- Neiger, R. (Editor): Differential Diagnoses in Internal Medicine for Dogs and Cats. From Leading Symptom to Diagnosis. 2nd Ed., Thieme Publishing Group, 2014
- Ramsey IK, Tasker S. Fever. In: Ettinger SJ, Feldman EC, Côté E, editors. Textbook of Veterinary Internal Medicine. 8th ed. St. Louis: Elsevier; 2022. p. 142-146.
- Lappin MR, Blondeau J, Boothe D, et al. Antimicrobial Use Guidelines for Treatment of Respiratory Tract Disease in Dogs and Cats: Antimicrobial Guidelines Working Group of the International Society for Companion Animal Infectious Diseases. Journal of Veterinary Internal Medicine. 2021;35(1):279-294.
- Sykes JE. Canine and Feline Infectious Diseases. 2nd ed. Philadelphia: Elsevier Saunders; 2023.
- Pedersen NC, Perron M, Bannasch M, et al. Efficacy and Safety of the Nucleoside Analog GS-441524 for Treatment of Cats with Naturally Occurring Feline Infectious Peritonitis. Journal of Feline Medicine and Surgery. 2019;21(4):271-281.
- Goddard A, Leisewitz AL. Canine Parvovirus. Veterinary Clinics of North America: Small Animal Practice. 2020;50(6):1307-1327.
