Fevers of Unknown Origin
Most febrile illnesses encountered in large animal practice are caused by infectious diseases that are readily diagnosed by careful evaluation of history and physical examination or are of short duration, run their course, and progress to complete recovery within 2 weeks without a specific etiologic diagnosis having been made.
Some febrile conditions, however, continue for weeks or months, accompanied only by nonspecific signs of depression, variable anorexia, and weight loss, while the diagnosis remains obscure. Patients with prolonged febrile episodes of 3 weeks' duration or longer in which a diagnosis has not been made after a week of routine diagnostic efforts or after 3 days of hospitalization and diagnostic tests are considered to have fever of unknown origin (FUO). The majority of cases that meet the definition of FUO have infectious causes. Neoplastic diseases, immune-mediated vasculitides, and autoimmune diseases are the next most common causes. Adverse drug reactions and other miscellaneous diagnoses are the least common documented causes of FUO. Most cases of FUO are caused by common diseases with an unusual presentation; an ordered, problem- oriented approach to diagnosis will render a diagnosis in 90% of cases.60-61 The following steps are suggested (Fig. 4.3).Document Fever
A temperature chart consisting of at least twice-daily determination of rectal temperature should be completed to characterize the fever pattern. Intermittentfevers are characterized by diurnal variation in which a peak elevation of temperature of higher than 1.5° F (0.75° C) occurs, followed by a decline in temperature, which in some patients falls within the normal range. Most intermittent fevers peak in late afternoon or evening, with the lowest temperatures occurring in the morning, but approximately 10% of cases will have a reverse pattern.
Intermittent fever is most commonly associated with pyrogenic infections, although it may occur in neoplasia, especially if tissue necrosis and inflammation are concurrent. Remittent fevers are characterized by a period of days in which elevated temperatures occur, followed by several days of normal temperature, only■ BOX 4.4
Infectious Causes of Fever in Horses
Common Causes
Upper respiratory viral diseases Strangles, Streptococci equi Pneumonia, bacterial or viral Pleuropneumonia
Gastrointestinal parasitic infections Enteritis, Clostridium difficile, Lawsonia, or of unknown causes Salmonellosis
Equine monocytic ehrlichiosis (Potomac horse fever)
Proximal duodenitis-jejunitis Rotavirus diarrhea (foals)
Endotoxemia from gastrointestinal disorders Septicemia, septic arthritis, osteomyelitis (foals) Urachal abscess (foals)
Metritis (mares)
Peritonitis
Tetanus
Traumatic tenosynovitis, cellulitis
Localized occult abscesses (thorax, abdomen, upper respiratory system)
Tumors (see list in Box 4.6)
Less Common Causes
Equine encephalomyelitis (EEE, WEE) West Nile virus
Osteomyelitis (adults) Vesicular stomatitis Tyzzer disease (foals) Malignant edema Bacterial endocarditis Mastitis
Pyelonephritis
Equine infectious anemia
Equine viral arteritis Otitis media and interna
Uncommon Causes
Pericarditis
Systemic or pneumonic aspergillosis, candidiasis Pneumocystis carinii (renamed PJiroveci in humans) Brucellosis
Tularemia
Anthrax
Rabies
Lyme disease (Borrelia burgdorferi)
Nocardiosis
Coccidioidomycosis
Babesiosis, piroplasmosis Toxoplasmosis
EEE, Eastern equine encephalitis; WEE, Western equine encephalitis.
to have the cycle repeat again. Brucellosis in ruminants, equine infectious anemia (EIA) in horses, and blood-borne protozoal diseases such as babesiosis may exhibit this type of pattern. Sustained fevers are characterized by a consistently raised temperature without variation and appear as a “flat line” on a temperature chart. Fevers caused by drug administration and certain toxins may be of this type, especially if the patient does not exhibit any other signs of illness.3 Any pharmacologic agents being administered to the patient should be discontinued. Defervescence of fever from drug administration should occur in 48 hours.