Veterinary Fever of Unknown Origin: Cross-Species Diagnostic Planning and Reassessment

Fever of unknown origin (FUO) in veterinary medicine describes persistent pyrexia for which the cause remains unidentified after initial routine diagnostic testing. This article provides a structured, cross-species framework for veterinarians to plan diagnostic investigations, select appropriate imaging modalities, and determine when reassessment is warranted. The approach applies to dogs, cats, horses, cattle, sheep, goats, and pigs, with species-specific considerations noted throughout.

At a Glance: FUO Diagnostic Decision Framework

Defining Fever of Unknown Origin in Veterinary Practice

Fever of unknown origin in veterinary medicine follows the general definition established in human medicine: a documented fever of 38.3 degrees Celsius (101 degrees Fahrenheit) or higher on multiple occasions, persisting for at least three weeks, with no diagnosis established after one week of inpatient investigation or three outpatient visits. In veterinary patients, this definition requires adaptation based on species, housing, and clinical context.

The diagnostic approach to fever of unknown origin in dogs has been described in the veterinary literature, emphasizing the need for systematic investigation instead of empirical therapy. The diagnostic approach to fever of unknown origin in cats follows parallel principles but accounts for species-specific infectious and neoplastic diseases. For large animals, FUO definitions must consider herd-level factors, production stage, and environmental exposures.

A practical working definition for veterinary FUO includes:

• Documented pyrexia on at least three separate occasions over 7 to 14 days
• Absence of localizing clinical signs after thorough history and physical examination
• Negative initial diagnostic testing (minimum database)
• No response to appropriate empirical therapy if administered

Core Principles of Cross-Species FUO Investigation

Pathophysiologic Categories

Fever results from endogenous pyrogen release, primarily interleukin-1, interleukin-6, and tumor necrosis factor-alpha, acting on the hypothalamic thermoregulatory center. The underlying causes fall into four broad categories that apply across species:

1. Infectious causes: Bacterial, viral, fungal, protozoal, or rickettsial infections that may be localized or systemic
2. Inflammatory causes: Immune-mediated diseases, vasculitis, sterile abscesses, or granulomatous disease
3. Neoplastic causes: Lymphoma, leukemia, metastatic carcinoma, or other tumors producing paraneoplastic fever
4. Miscellaneous causes: Drug reactions, thromboembolic disease, endocrine disorders, or factitious fever

Paraneoplastic disorders in dogs with hematopoietic tumors have been documented as a cause of fever, highlighting the importance of considering neoplasia in the FUO workup. This principle extends to other species, though the specific tumor types vary.

History and Physical Examination Reassessment

The initial history and physical examination must be repeated and expanded when FUO is suspected. Key elements include:

• Environmental history: Travel history, exposure to other animals, tick or insect exposure, access to wildlife, housing conditions, feed sources
• Vaccination and medication history: Recent vaccinations, antibiotic or anti-inflammatory use, topical treatments, supplements
• Production history (food animals): Feed intake, milk production, growth rates, reproductive performance, herd or flock health status
• Physical examination: Careful palpation of all lymph nodes, abdominal palpation or rectal examination, joint evaluation, fundic examination, oral cavity inspection, skin and coat assessment

Document any subtle abnormalities that may have been overlooked initially. Record body temperature at multiple time points to establish fever pattern. Intermittent fevers may suggest different etiologies than persistent fevers.

Initial Minimum Database

Complete Blood Count

The complete blood count (CBC) provides essential information about the patient's inflammatory and hematopoietic status. Key findings and their implications include:

• Neutrophilia with left shift: Suggests bacterial infection or severe inflammation
• Neutropenia: May indicate viral infection, sepsis, or bone marrow suppression
• Monocytosis: Chronic inflammation or infection
• Eosinophilia: Parasitic infection, hypersensitivity, or certain neoplasms
• Thrombocytopenia: Immune-mediated disease, vector-borne infection, or disseminated intravascular coagulation
• Anemia: Chronic disease, blood loss, hemolysis, or bone marrow disorder

Blood smear review is mandatory for all FUO cases. Examine for blood parasites, toxic neutrophil changes, nucleated red blood cells, and platelet clumps. In dogs and cats, look for Mycoplasma haemofelis, Babesia species, and Cytauxzoon felis. In horses, examine for Anaplasma phagocytophilum morulae. In ruminants, check for Anaplasma marginale and Theileria species.

Serum Biochemistry Profile

The biochemistry profile evaluates organ function and metabolic status. Specific abnormalities to investigate include:

• Elevated liver enzymes: Hepatic inflammation, cholestasis, or infiltrative disease
• Azotemia: Renal disease, prerenal causes, or postrenal obstruction
• Hyperglobulinemia: Chronic infection, immune-mediated disease, or neoplasia
• Hypoalbuminemia: Chronic inflammation, protein-losing enteropathy, or liver disease
• Hypercalcemia: Paraneoplastic syndrome (lymphoma, multiple myeloma), granulomatous disease, or primary hyperparathyroidism
• Hypoglycemia: Sepsis, hepatic disease, or insulinoma

Urinalysis

Urinalysis is a noninvasive test that can reveal occult urinary tract infection, renal disease, or metabolic abnormalities. Perform cystocentesis for culture if pyuria or bacteriuria is present. In horses and ruminants, free-catch samples are acceptable for initial screening, but culture requires aseptic collection.

Baseline Imaging

Thoracic radiographs are indicated in all FUO cases to evaluate for pneumonia, mediastinal masses, lymphadenopathy, or metastatic disease. Abdominal radiographs may reveal organomegaly, masses, or foreign bodies. In horses, thoracic ultrasound provides superior visualization of pleural surfaces and lung parenchyma.

Advanced Laboratory Testing

Serology and Polymerase Chain Reaction

Serologic testing for vector-borne diseases should be guided by geographic location, travel history, and species. Common targets include:

• Dogs: Ehrlichia canis, Anaplasma phagocytophilum, Babesia canis, Bartonella species, Borrelia burgdorferi, Leishmania infantum
• Cats: Bartonella henselae, Mycoplasma haemofelis, Toxoplasma gondii, feline coronavirus
• Horses: Anaplasma phagocytophilum, Leptospira species, equine herpesvirus, equine viral arteritis
• Cattle: Anaplasma marginale, Babesia bovis, Leptospira species, bovine viral diarrhea virus, bovine leukosis virus
• Sheep and goats: Coxiella burnetii, Chlamydia abortus, Toxoplasma gondii, caprine arthritis-encephalitis virus
• Pigs: African swine fever virus, classical swine fever virus, porcine reproductive and respiratory syndrome virus, porcine circovirus type 2

Polymerase chain reaction (PCR) testing offers higher sensitivity than serology for active infections and can detect organisms that are difficult to culture. Real-time PCR panels for vector-borne diseases are commercially available for dogs and cats. For large animals, PCR testing is often available through veterinary diagnostic laboratories.

The laboratory management of Crimean-Congo haemorrhagic fever virus infections provides a model for handling high-consequence pathogens that may present with fever. While this virus primarily affects humans, veterinarians working with livestock in endemic areas should be aware of the zoonotic potential and appropriate biosafety measures. The development of a Luciferase Immunosorbent Assay for detecting Crimean-Congo hemorrhagic fever virus IgG antibodies based on nucleoprotein represents an example of advanced serologic methods that may become available for veterinary use.

Bacterial Culture

Blood culture is indicated in FUO cases when bacterial infection is suspected. Collect three samples from separate venipuncture sites over 24 to 48 hours to maximize yield. Use aseptic technique and appropriate culture media. In horses, blood culture is particularly useful for detecting occult bacteremia. In cattle, blood culture may identify causes of metritis, mastitis, or pneumonia that are not clinically apparent.

Urine culture should be performed even if urinalysis is normal, as occult urinary tract infections can cause fever. Joint fluid culture is indicated if synovitis is present. Cerebrospinal fluid culture is reserved for cases with neurologic signs.

Fungal Testing

Fungal serology and antigen testing are indicated in endemic areas or when fungal infection is suspected based on geographic exposure. Common fungal diseases causing FUO include:

• Blastomycosis: Dogs in North America, particularly along the Mississippi and Ohio River valleys
• Histoplasmosis: Dogs and cats in the Ohio and Mississippi River valleys
• Coccidioidomycosis: Dogs in the southwestern United States and Central America
• Cryptococcosis: Cats worldwide, particularly in association with pigeon droppings
• Aspergillosis: Horses and dogs with sinonasal or disseminated disease

Endocrine Testing

Thyroid and adrenal function testing may be indicated in selected cases. Hypothyroidism in dogs can present with vague signs including lethargy and weight gain, but fever is uncommon. Hyperthyroidism in cats can cause hyperthermia but is rarely a cause of FUO. Hypoadrenocorticism in dogs can present with fever, weakness, and gastrointestinal signs.

Advanced Imaging

Thoracic and Abdominal Ultrasound

Ultrasound examination of the thorax and abdomen is a noninvasive method to identify occult lesions. Thoracic ultrasound can detect pleural effusion, pulmonary consolidation, mediastinal masses, and pericardial disease. Abdominal ultrasound evaluates the liver, spleen, kidneys, adrenal glands, gastrointestinal tract, and lymph nodes.

In horses, transabdominal ultrasound is limited by body size and gas-filled bowel, but transrectal ultrasound provides excellent visualization of the caudal abdomen. In cattle, transrectal ultrasound is the primary method for abdominal imaging. In small ruminants and pigs, standard transabdominal ultrasound is feasible.

Computed Tomography

Computed tomography (CT) provides detailed cross-sectional imaging of the entire body and is superior to radiography for detecting small lesions, lymphadenopathy, and bone disease. Whole-body CT is increasingly available in veterinary referral centers and is indicated when ultrasound is negative or equivocal.

CT is particularly useful for:

• Detecting occult neoplasia, particularly lymphoma and metastatic disease
• Evaluating the nasal cavity, sinuses, and middle ear
• Identifying pulmonary metastases
• Characterizing bone lesions
• Guiding biopsy procedures

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) provides superior soft tissue contrast and is indicated for evaluating the brain, spinal cord, and musculoskeletal system. MRI is reserved for cases with neurologic signs or when CT is inconclusive.

Nuclear Scintigraphy

Nuclear scintigraphy using technetium-99m labeled leukocytes or gallium-67 citrate can identify occult inflammatory or infectious foci. This technique is available at some veterinary referral centers and is particularly useful for detecting osteomyelitis, discospondylitis, and deep abscesses.

Tissue Sampling and Biopsy

Fine Needle Aspiration

Fine needle aspiration (FNA) of enlarged lymph nodes, masses, or organs is a minimally invasive technique that can provide a rapid diagnosis. Aspirates should be obtained from any abnormal lymph node, splenic or hepatic lesions, and pulmonary masses. Cytologic evaluation can identify neoplasia, inflammation, or infectious agents.

Core Needle Biopsy

Core needle biopsy provides larger tissue samples for histopathology and culture. Ultrasound or CT guidance improves accuracy and safety. Core biopsy is indicated for:

• Hepatic or splenic lesions
• Renal masses
• Pulmonary nodules
• Bone lesions
• Deep lymph nodes

Surgical Biopsy

Surgical biopsy is indicated when needle biopsy is nondiagnostic or when larger tissue samples are needed. Exploratory laparotomy or thoracotomy may be necessary in cases where imaging fails to identify a lesion but clinical suspicion remains high. Laparoscopy and thoracoscopy offer less invasive alternatives.

Bone Marrow Aspiration and Biopsy

Bone marrow evaluation is indicated when:

• Peripheral blood abnormalities suggest bone marrow disease
• Fever is accompanied by unexplained cytopenias
• Neoplasia such as lymphoma or multiple myeloma is suspected
• Infectious diseases such as ehrlichiosis or leishmaniasis are considered

Species-Specific Diagnostic Considerations

Dogs

The diagnostic approach to fever of unknown origin in dogs has been systematically described. Common causes include:

• Immune-mediated disease: Immune-mediated polyarthritis, immune-mediated hemolytic anemia, immune-mediated thrombocytopenia
• Infectious disease: Ehrlichiosis, anaplasmosis, babesiosis, bartonellosis, leishmaniasis
• Neoplasia: Lymphoma, multiple myeloma, metastatic carcinoma
• Miscellaneous: Sterile panniculitis, steroid-responsive meningitis-arteritis

Dogs with FUO should undergo tick-borne disease testing regardless of travel history, as subclinical infections can persist. Consider testing for Ehrlichia canis, Anaplasma phagocytophilum, Babesia canis, and Bartonella species.

Cats

The diagnostic approach to fever of unknown origin in cats follows similar principles but with species-specific considerations. Common causes include:

• Infectious disease: Feline infectious peritonitis, feline leukemia virus, feline immunodeficiency virus, Mycoplasma haemofelis, Bartonella henselae, Toxoplasma gondii
• Neoplasia: Lymphoma, leukemia, metastatic carcinoma
• Inflammatory disease: Pancreatitis, cholangiohepatitis, inflammatory bowel disease
• Miscellaneous: Drug reactions, sterile abscesses

Cats with FUO should be tested for feline leukemia virus antigen and feline immunodeficiency virus antibody. Feline coronavirus serology may support a diagnosis of feline infectious peritonitis but is not definitive.

Horses

Fever of unknown origin in horses presents unique challenges due to the horse's size, temperament, and susceptibility to certain diseases. Common causes include:

• Infectious disease: Anaplasma phagocytophilum, Leptospira species, equine herpesvirus, equine viral arteritis, Streptococcus equi subsp. equi (strangles)
• Neoplasia: Lymphoma, squamous cell carcinoma, melanoma
• Inflammatory disease: Immune-mediated polysynovitis, vasculitis, sterile abscesses
• Miscellaneous: Drug reactions, thromboembolic disease

Horses with FUO should undergo testing for equine infectious anemia, equine herpesvirus, and Anaplasma phagocytophilum. Thoracic ultrasound is particularly useful for detecting occult pleuropneumonia.

Cattle

Fever of unknown origin in cattle often has a herd-level component. Common causes include:

• Infectious disease: Bovine viral diarrhea virus, bovine leukosis virus, Anaplasma marginale, Babesia bovis, Leptospira species, Mycobacterium avium subsp. paratuberculosis
• Neoplasia: Lymphoma (enzootic bovine leukosis), squamous cell carcinoma
• Inflammatory disease: Metritis, mastitis, pneumonia, peritonitis
• Miscellaneous: Drug reactions, thromboembolic disease

Evaluation of multifactorial digestive disorders in a dairy herd at different stages of lactation demonstrates the importance of considering production stage and management factors in FUO investigation. Cattle with FUO should be evaluated for bovine viral diarrhea virus persistent infection, bovine leukosis virus, and Anaplasma marginale.

Sheep and Goats

Fever of unknown origin in small ruminants requires consideration of flock-level diseases. Common causes include:

• Infectious disease: Coxiella burnetii (Q fever), Chlamydia abortus, Toxoplasma gondii, caprine arthritis-encephalitis virus, Maedi-visna virus
• Neoplasia: Lymphoma, pulmonary adenomatosis
• Inflammatory disease: Pneumonia, mastitis, abscesses
• Miscellaneous: Drug reactions, thromboembolic disease

Small ruminants with FUO should be tested for Coxiella burnetii, Chlamydia abortus, and caprine arthritis-encephalitis virus. Q fever is a zoonotic concern, and appropriate biosafety measures should be implemented.

Pigs

Fever of unknown origin in pigs often has herd-level implications. Common causes include:

• Infectious disease: African swine fever virus, classical swine fever virus, porcine reproductive and respiratory syndrome virus, porcine circovirus type 2, Actinobacillus pleuropneumoniae
• Neoplasia: Lymphoma, melanoma
• Inflammatory disease: Pneumonia, enteritis, polyserositis
• Miscellaneous: Drug reactions, thromboembolic disease

Pigs with FUO should be evaluated for reportable diseases, particularly African swine fever and classical swine fever. Consultation with a veterinary diagnostic laboratory is essential for appropriate testing.

Reassessment Triggers and Diagnostic Algorithm

When to Reassess

Reassessment is indicated when:

1. Fever persists for more than 72 hours after initial minimum database without identified cause
2. New clinical signs develop during the diagnostic workup
3. Initial diagnostic tests are negative or equivocal
4. The patient fails to respond to empirical therapy
5. Serial physical examinations reveal new abnormalities

Structured Reassessment Protocol

1. Repeat history and physical examination: Focus on any new findings or changes in clinical status
2. Review all diagnostic results: Ensure no tests were missed or misinterpreted
3. Consider additional testing: Based on reassessment findings, order advanced laboratory tests or imaging
4. Consult with specialists: Internal medicine, radiology, pathology, or infectious disease specialists
5. Consider empirical therapy: If diagnostic testing is exhausted and clinical suspicion remains high, consider a therapeutic trial with appropriate monitoring

Diagnostic Algorithm

A structured diagnostic algorithm for FUO includes:

Phase 1 (Days 1 to 3)

• Complete history and physical examination
• Complete blood count with blood smear review
• Serum biochemistry profile
• Urinalysis with culture
• Thoracic radiographs
• Abdominal ultrasound or radiographs
• Baseline serology for common infectious diseases

Phase 2 (Days 4 to 7)

• Advanced serology and PCR testing
• Blood culture (three samples)
• Urine culture (if not already performed)
• Joint fluid analysis (if synovitis present)
• Cerebrospinal fluid analysis (if neurologic signs)
• Advanced imaging (CT, MRI, nuclear scintigraphy)

Phase 3 (Days 8 to 14)

• Tissue sampling (FNA, core biopsy, surgical biopsy)
• Bone marrow aspiration and biopsy
• Exploratory surgery or laparoscopy
• Therapeutic trial (if indicated)

Common Failure Patterns in FUO Diagnosis

Incomplete History

Failure to obtain a thorough history is a common cause of diagnostic delay. Key information that may be overlooked includes:

• Travel history, including recent moves or boarding
• Exposure to wildlife, particularly rodents, birds, and ticks
• Dietary history, including raw meat or unpasteurized milk
• Medication history, including over-the-counter supplements
• Vaccination history, including timing and type of vaccines

Inadequate Physical Examination

A cursory physical examination may miss subtle findings. Common oversights include:

• Failure to palpate all lymph nodes, including retropharyngeal and popliteal nodes
• Incomplete oral examination, including dental disease
• Inadequate fundic examination
• Failure to perform rectal examination in large animals
• Incomplete joint evaluation

Premature Empirical Therapy

Administration of antibiotics or anti-inflammatory drugs before diagnostic testing can mask clinical signs and interfere with test results. Empirical therapy should be reserved for cases where:

• The patient is clinically unstable
• Diagnostic testing is delayed
• A specific diagnosis is strongly suspected
• The risks of withholding therapy outweigh the benefits

Failure to Consider Zoonotic Diseases

Fever of unknown origin may be caused by zoonotic pathogens that pose a risk to veterinary personnel and animal owners. Common zoonotic causes include:

• Coxiella burnetii (Q fever)
• Leptospira species
• Brucella species
• Bartonella species
• Francisella tularensis
• Yersinia pestis

Appropriate biosafety measures should be implemented when handling animals with FUO, particularly in endemic areas. The World Organisation for Animal Health provides guidance on animal health and welfare, including zoonotic disease management.

Overreliance on Serology

Serologic testing has limitations, including:

• False-negative results during the acute phase of infection
• False-positive results due to cross-reactivity
• Inability to distinguish active from past infection
• Variable sensitivity and specificity between laboratories

PCR testing and culture should be used to confirm serologic results when possible.

Records and Measurements

Temperature Monitoring

Accurate temperature monitoring is essential for FUO diagnosis and management. Guidelines include:

• Use a calibrated thermometer appropriate for the species
• Record temperature at the same time each day
• Document fever pattern (intermittent, remittent, sustained, relapsing)
• Note any factors that may affect temperature (exercise, stress, environmental temperature)

Diagnostic Test Records

Maintain a systematic record of all diagnostic tests performed, including:

• Date and time of collection
• Type of sample
• Laboratory performing the test
• Test results and interpretation
• Any factors that may affect test results

Treatment Records

If empirical therapy is administered, document:

• Drug name, dose, route, and frequency
• Duration of therapy
• Response to therapy
• Any adverse effects

Herd Records (Food Animals)

For food animal FUO cases, maintain herd-level records including:

• Number of animals affected
• Production parameters (milk yield, growth rate, feed intake)
• Vaccination and treatment history
• Biosecurity measures
• Recent introductions or movements

Welfare and Safety Context

Animal Welfare Considerations

Fever of unknown origin causes significant animal welfare concerns due to:

• Persistent discomfort and malaise
• Reduced appetite and weight loss
• Decreased production in food animals
• Potential for disease progression if untreated

Veterinarians should prioritize pain management and supportive care while diagnostic testing is ongoing. Nonsteroidal anti-inflammatory drugs may be used to reduce fever and improve comfort, but their use should be balanced against the need for diagnostic accuracy.

Zoonotic Disease Risk

Many causes of FUO are zoonotic, posing a risk to veterinary personnel, animal owners, and laboratory staff. Appropriate biosafety measures include:

• Use of personal protective equipment (gloves, gowns, masks, eye protection)
• Hand hygiene after handling animals
• Proper disposal of biological waste
• Reporting of suspected zoonotic diseases to public health authorities

The World Organisation for Animal Health provides guidance on animal health and welfare, including zoonotic disease surveillance and control.

Reportable Diseases

Some causes of FUO are reportable to animal health authorities. Veterinarians should be aware of reportable diseases in their jurisdiction and report suspected cases promptly. Reportable diseases that may present as FUO include:

• African swine fever
• Classical swine fever
• Bovine spongiform encephalopathy
• Foot and mouth disease
• Highly pathogenic avian influenza
• Rabies

Professional Escalation Criteria

When to Refer

Referral to a specialist is indicated when:

1. FUO persists for more than 14 days without diagnosis
2. Advanced imaging or specialized testing is not available in practice
3. The patient requires intensive care or monitoring
4. Surgical biopsy or exploratory surgery is needed
5. The veterinarian lacks experience with the species or condition

When to Consult Public Health Authorities

Consultation with public health authorities is indicated when:

1. A zoonotic disease is suspected or confirmed
2. Multiple human cases occur in association with animal exposure
3. A reportable disease is suspected
4. Bioterrorism is a concern

When to Consider Euthanasia

Euthanasia may be considered when:

1. The patient's quality of life is severely compromised
2. Diagnostic testing is exhausted and treatment options are limited
3. The prognosis is poor regardless of diagnosis
4. Financial constraints prevent further diagnostic or therapeutic intervention

Structured Reassessment Protocol: The Three-Pass Diagnostic Review System

A systematic reassessment protocol prevents diagnostic drift and ensures that no clinical information is overlooked during the FUO investigation. The three-pass diagnostic review system provides a reproducible framework for reevaluating the case at defined intervals, with specific actions required at each pass. This protocol applies across species and practice settings, from ambulatory large animal practice to referral hospital environments.

Pass One: The 72-Hour Minimum Database Review

The first reassessment occurs 72 hours after completing the initial minimum database. At this point, the veterinarian must review all results from the complete blood count, serum biochemistry profile, urinalysis, blood smear, and baseline imaging. The primary question at this pass is whether any abnormality, however subtle, provides a diagnostic direction.

Required actions at Pass One:

1. Reexamine the blood smear personally, even if a laboratory report is available. Look for organisms that may have been missed on initial review, including Anaplasma marginale in ruminants, Mycoplasma haemofelis in cats, and Babesia species in dogs. The diagnostic approach to fever of unknown origin in dogs emphasizes the importance of blood smear examination for detecting hemoparasites that may be present in low numbers.

2. Compare current biochemistry values with reference intervals for the specific species and age group. Mild elevations in liver enzymes or globulins that fall within the reference range may still be significant if the patient's baseline values are normally lower. Document any trends, such as rising creatinine or decreasing albumin.

3. Review imaging studies a second time. Thoracic radiographs should be examined for subtle interstitial patterns, small pleural effusions, or mediastinal widening that may have been overlooked. Abdominal ultrasound images, if obtained, should be reviewed for lymph node size, splenic echotexture, and adrenal gland dimensions.

4. Recheck the history with the owner or herd manager. Ask specifically about any changes in behavior, appetite, or elimination patterns that may have occurred since the initial presentation. In food animals, inquire about feed bunk attendance, rumen fill, and fecal consistency.

5. Document the fever pattern from the past 72 hours. Intermittent fevers that spike every 48 to 72 hours may suggest abscess formation or certain infectious diseases. Sustained fevers are more typical of immune-mediated disease or neoplasia.

Decision point at Pass One:

• If any abnormality is identified, proceed with targeted testing based on that finding.
• If all results are within normal limits and the fever persists, proceed to Pass Two.
• If the fever has resolved and the patient is clinically normal, consider a diagnosis of transient pyrexia and discontinue the FUO workup, but advise the owner to monitor for recurrence.

Pass Two: The Seven-Day Advanced Testing Review

The second reassessment occurs seven days after initiating the FUO investigation, or sooner if advanced test results become available. This pass focuses on integrating results from serology, PCR panels, bacterial cultures, and advanced imaging.

Required actions at Pass Two:

1. Compile all serology and PCR results into a single table organized by pathogen. Note the type of test performed (IgG, IgM, PCR, culture) and the result. Identify any positive results that require confirmation or further characterization.

2. Review blood culture results. Negative blood cultures do not rule out bacteremia, particularly if antibiotics were administered before collection. The Merck Veterinary Manual notes that blood culture sensitivity decreases significantly after antibiotic therapy. Consider repeating blood cultures if clinical suspicion remains high.

3. Evaluate advanced imaging findings. Computed tomography or magnetic resonance imaging may reveal lesions that were not apparent on radiographs or ultrasound. Compare imaging findings with physical examination findings to ensure no region was overlooked.

4. Reexamine the patient physically. Perform a complete physical examination, paying particular attention to lymph nodes, joints, and the oral cavity. In horses, perform a rectal examination to evaluate the caudal abdomen and pelvic structures. In cattle, perform a transrectal examination of the reproductive tract and caudal abdomen.

5. Review the medication record. Document all drugs administered, including antibiotics, anti-inflammatories, and supportive care medications. Note any temporal relationship between drug administration and changes in fever pattern.

6. Assess the patient's clinical status. Has the patient improved, worsened, or remained stable? Weight loss, decreased appetite, or declining body condition score indicate disease progression and warrant more aggressive investigation.

Decision point at Pass Two:

• If a specific diagnosis is confirmed, initiate targeted therapy.
• If serology or PCR results are equivocal, consider repeat testing or alternative testing methods. For example, if PCR is negative but serology is positive, consider whether the infection is chronic or past.
• If all advanced tests are negative and the fever persists, proceed to Pass Three.
• If the patient's condition is deteriorating, consider empirical therapy or referral.

Pass Three: The 14-Day Invasive Diagnostic Review

The third reassessment occurs 14 days after the initial presentation, or sooner if the patient's condition warrants. This pass involves consideration of invasive diagnostic procedures, including tissue sampling, bone marrow aspiration, and exploratory surgery.

Required actions at Pass Three:

1. Review all diagnostic results from Pass One and Pass Two. Create a summary document that lists every test performed, the result, and the interpretation. Identify any gaps in testing that may have been overlooked.

2. Perform a detailed risk-benefit analysis for invasive procedures. Consider the patient's age, species, clinical status, and the likelihood of obtaining a diagnosis. In geriatric patients or those with significant comorbidities, the risks of anesthesia and surgery may outweigh the potential benefits.

3. Consult with a specialist if not already done. Internal medicine specialists can provide guidance on additional testing options, interpretation of complex results, and management of difficult cases. The American College of Veterinary Internal Medicine provides a directory of board-certified specialists.

4. Discuss the case with the owner or herd manager. Explain the results obtained to date, the limitations of current testing, and the options for further investigation. Include a discussion of costs, prognosis, and the possibility of not reaching a diagnosis.

5. Consider empirical therapy if diagnostic testing is exhausted. Empirical therapy should be based on the most likely diagnosis given the patient's signalment, history, and clinical findings. Document the rationale for therapy and establish clear criteria for response assessment.

6. Evaluate the need for referral. If advanced imaging, specialized testing, or surgical expertise is not available in the practice, referral to a specialty hospital should be offered. The American Veterinary Medical Association provides resources for finding specialists and referral centers.

Decision point at Pass Three:

• If invasive procedures are indicated and the patient is a suitable candidate, proceed with tissue sampling.
• If the owner declines further investigation or the patient is not a candidate for invasive procedures, consider empirical therapy or palliative care.
• If the patient's quality of life is severely compromised and diagnostic options are exhausted, discuss euthanasia with the owner.

Record System for the Three-Pass Protocol

A standardized record system supports the three-pass protocol and ensures that no information is lost between reassessments. The following record elements should be maintained for each FUO case:

FUO Diagnostic Log

Fever Pattern Record

Medication Record

Common Failure Patterns in the Reassessment Process

Failure to repeat the physical examination

The most common failure in FUO reassessment is relying on the initial physical examination without repeating it. New findings may develop over time, including lymphadenopathy, joint effusion, or abdominal masses. A complete physical examination should be performed at each reassessment pass.

Failure to review all results before proceeding

Veterinarians may proceed to advanced testing without thoroughly reviewing the results of the minimum database. This can lead to unnecessary testing and delayed diagnosis. The three-pass protocol requires a complete review of all results before moving to the next phase.

Failure to document the fever pattern

Intermittent fevers may be missed if temperature is recorded only once daily. The fever pattern provides important diagnostic clues. For example, relapsing fevers are characteristic of certain infections, while sustained fevers are more typical of immune-mediated disease.

Failure to consider the herd or flock context

In food animal practice, FUO in a single animal may be the first indication of a herd-level problem. The evaluation of multifactorial digestive disorders in a dairy herd at different stages of lactation demonstrates the importance of considering production stage and management factors. A single animal with FUO should prompt investigation of the entire herd or flock.

Failure to communicate with the owner

Owners may become frustrated with the prolonged diagnostic process and the lack of a definitive diagnosis. Regular communication about the plan, progress, and limitations of testing is essential for maintaining trust and compliance.

Practical Implementation of the Three-Pass Protocol

In small animal practice

The three-pass protocol can be implemented in a general small animal practice with access to in-house laboratory testing and referral imaging. The 72-hour pass can be completed with in-house CBC, biochemistry, and urinalysis. The seven-day pass requires sending out serology and PCR samples to a reference laboratory. The 14-day pass may require referral for CT or MRI.

In equine practice

The three-pass protocol in horses requires consideration of the horse's size and temperament. The 72-hour pass includes thoracic ultrasound, which is more sensitive than radiographs for detecting pleural disease. The seven-day pass includes blood culture and serology for equine infectious anemia, equine herpesvirus, and Anaplasma phagocytophilum. The 14-day pass may include nuclear scintigraphy or exploratory laparoscopy.

In food animal practice

The three-pass protocol in food animals must consider herd-level factors and production economics. The 72-hour pass includes herd-level testing for infectious diseases such as bovine viral diarrhea virus and bovine leukosis virus. The seven-day pass includes serology for Coxiella burnetii and Leptospira species. The 14-day pass may include culling and necropsy of affected animals to obtain a diagnosis.

In mixed practice

The three-pass protocol can be adapted to mixed practice by maintaining species-specific testing panels and referral networks. The protocol provides a consistent framework for approaching FUO across species while allowing for species-specific modifications.

When to Abandon the Three-Pass Protocol

The three-pass protocol should be abandoned when:

1. The patient's condition deteriorates rapidly, requiring immediate intervention.
2. A specific diagnosis is made before completing all three passes.
3. The owner declines further diagnostic testing.
4. Financial constraints prevent completion of the protocol.
5. The patient's quality of life is severely compromised.

In these situations, the veterinarian should document the reasons for abandoning the protocol and provide appropriate care based on the available information.

Frequently Asked Questions

What is the minimum database for a veterinary FUO case?

The minimum database includes a complete blood count with blood smear review, serum biochemistry profile, urinalysis with culture, thoracic radiographs, and baseline serology for common infectious diseases in the species. This initial testing should be completed within 72 hours of presentation.

How long should I continue diagnostic testing before considering empirical therapy?

Diagnostic testing should continue for at least 7 to 14 days before considering empirical therapy, unless the patient is clinically unstable. Empirical therapy should be based on the most likely diagnosis and should include appropriate monitoring for response and adverse effects.

What are the most common causes of FUO in dogs?

Common causes include immune-mediated disease (polyarthritis, hemolytic anemia, thrombocytopenia), infectious disease (ehrlichiosis, anaplasmosis, babesiosis, bartonellosis), and neoplasia (lymphoma, multiple myeloma). Tick-borne disease testing is essential regardless of travel history.

What are the most common causes of FUO in cats?

Common causes include infectious disease (feline infectious peritonitis, feline leukemia virus, feline immunodeficiency virus, Mycoplasma haemofelis), neoplasia (lymphoma, leukemia), and inflammatory disease (pancreatitis, cholangiohepatitis). Testing for feline leukemia virus and feline immunodeficiency virus is mandatory.

How do I approach FUO in horses?

Horses with FUO should undergo testing for equine infectious anemia, equine herpesvirus, and Anaplasma phagocytophilum. Thoracic ultrasound is particularly useful for detecting occult pleuropneumonia. Blood culture is indicated when bacterial infection is suspected.

How do I approach FUO in cattle?

Cattle with FUO should be evaluated for bovine viral diarrhea virus persistent infection, bovine leukosis virus, and Anaplasma marginale. Herd-level factors such as production stage, nutrition, and management should be considered. Evaluation of multifactorial digestive disorders may be relevant in dairy herds.

What zoonotic diseases should I consider in FUO cases?

Common zoonotic causes include Coxiella burnetii (Q fever), Leptospira species, Brucella species, Bartonella species, and Francisella tularensis. Appropriate biosafety measures should be implemented, and suspected cases should be reported to public health authorities.

When should I refer a FUO case to a specialist?

Referral is indicated when FUO persists for more than 14 days without diagnosis, advanced imaging or specialized testing is not available in practice, the patient requires intensive care, or surgical biopsy is needed. Consultation with an internal medicine specialist is recommended for complex cases.

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• Livestock Nutrition And Feed Management A Cross Species Decision Framework

References and Further Reading

• www.avma.org
• www.acvaa.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• The diagnostic approach to fever of unknown origin in dogs.. Compendium (Yardley, PA), 2009.
• The diagnostic approach to fever of unknown origin in cats.. Compendium (Yardley, PA), 2009.
• Laboratory management of Crimean-Congo haemorrhagic fever virus infections: perspectives from two European networks.. Euro surveillance : bulletin Europeen sur les maladies transmissibles = European communicable disease bulletin, 2019.
• Paraneoplastic disorders in dogs with hematopoietic tumors.. The Veterinary clinics of North America. Small animal practice, 1985.
• Evaluation of multifactorial digestive disorders in a dairy herd at different stages of lactation.. Tierarztliche Praxis. Ausgabe G, Grosstiere/Nutztiere, 2023.
• First Report of Candidatus Mycoplasma haemohominis Infection in Australia Causing Persistent Fever in an Animal Carer.. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America, 2021.
• Development of a Luciferase Immunosorbent Assay for Detecting Crimean-Congo Hemorrhagic Fever Virus IgG Antibodies Based on Nucleoprotein. Viruses, 2025.

This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.