History and Physical Examination

When a febrile patient does not respond to antibacterial treatment, a course of action must be formulated. A thorough history should be obtained and a complete physical examination performed. The history rarely provides clues to the cause of the fever. However, a history of ticks may indicate a rickettsial or hemoparasitic disorder, previous administration of tetracycline (mainly to cats) may indicate a drug-induced fever, and travel to areas where systemic mycoses are endemic should prompt further investigation consisting of cytologic or serologic studies or fungal cultures.

During a physical examination the lymphoreticular organs should be evaluated because numerous infectious diseases affecting these organs (e.g., ehrlichiosis, Rocky Mountain spotted fever, bartonellosis, leukemia, systemic mycoses) may cause fever. An enlarged lymph node or spleen should be evaluated cytologically by using specimens obtained by fine-needle aspiration (FNA). An FNA sample can also be obtained for bacterial and fungal culture and susceptibility testing if the cytologic studies reveal evidence of infection or inflammation. Any palpable mass or swelling should also be evaluated by using specimens obtained by FNA to rule out granulomatous, pyogranulomatous, and suppurative inflammation as well as neoplasia (see Chapter 75).

The clinician should thoroughly inspect and palpate the oropharynx, searching for signs of pharyngitis, stomatitis, or tooth root abscesses. The bones should also be thoroughly palpated, particularly in young dogs, because metabolic bone disorders such as hypertrophic osteodystrophy can cause fever associated with bone pain. Palpation and passive motion of all joints is also indicated in search of monoarthritis, oligoarthritis, or polyarthritis. A neurologic examination should be conducted to detect signs of meningitis or other central nervous system lesions. In older cats the ventral cervical region should be palpated to detect thyroid enlargement or nodules.

The thorax should be auscultated carefully in search of a murmur, which could indicate bacterial endocarditis. A thorough ocular examination may reveal changes suggestive of a specific cause (e.g., chorioretinitis in cats with feline infectious peritonitis or in dogs with monocytic ehrlichiosis).

Clinicopathologic Evaluation

A minimum database consisting of a CBC, serum biochemistry profile, urinalysis, and urine bacterial culture and susceptibility testing should always be carried out in dogs and cats with persistent fever. The CBC may provide important clues regarding the cause of the fever (Table 90-2). A serum biochemistry profile rarely yields diagnostic information in dogs and cats with FUO, although it can provide indirect information on parenchymal organ function. However, the finding of hyperglobulinemia and hypoalbuminemia may indicate an infectious, immune-mediated, or neoplastic disorder (see Chapter 89). The finding of pyuria or white blood cell casts in a urinalysis may indicate a urinary tract infection, which may be the cause of the FUO (i.e., pyelonephritis). Proteinuria associated with an inactive urine sediment should prompt the clinician to evaluate a urine protein/creatinine ratio to rule out glomerulonephritis or amyloidosis as the cause of the fever.

TABLE 90-2 Hematologic Changes in Dogs and Cats with FUO

FUO, Fever of undetermined origin.

Other diagnostic tests that may be called for in patients with FUO are listed in Box 90-1. Echocardiography is indicated only if the patient has a heart murmur because it rarely detects a valvular lesion in dogs without murmurs. Some of the infectious diseases listed in Table 90-1 can be diagnosed on the basis of serologic findings or polymerase chain reaction testing.

Fluid from several joints should be aspirated for cytologic evaluation and possibly bacterial culture because polyarthritis may be the only manifestation of a widespread immune-mediated disorder. Thoracic radiography and abdominal ultrasonography should be performed to search for a silent septic focus. In dogs and cats with neurologic signs associated with fever, a cerebrospinal fluid tap should be performed; in dogs, immune-mediated vasculitis or meningitis can cause marked temperature elevations. If a diagnosis has still not been reached, bone marrow aspirates for cytologic studies and bacterial and fungal culture should also be obtained. A leukocyte or ciprofloxacin scan may reveal a hidden septic focus. Finally, if a definitive diagnosis is ultimately not obtained, a therapeutic trial of specific antibacterial or antifungal agents or immunosuppressive doses of corticosteroids can be initiated.

Treatment

If a definitive diagnosis is obtained, a specific treatment should be initiated.

The problem arises if the clinician cannot arrive at a definitive diagnosis. In these patients, changes in the CBC usually are the only clinicopathologic abnormality (see Table 90-2). That is, results of bacterial and fungal cultures, serologic tests, PCR, imaging studies, and FNAs are negative or normal. If the patient has already been treated with a broad-spectrum bactericidal antibiotic, a therapeutic trial of immunosuppressive doses of corticosteroids is warranted. However, before instituting immunosuppressive treatment, the owners should be informed of the potential consequences of this approach: primarily that a dog or cat with an undiagnosed infectious disease may die as a result of systemic dissemination of the organism after the start of treatment. Dogs and cats undergoing a therapeutic trial of corticosteroids should be kept in the hospital and monitored frequently for worsening of clinical signs, in which case steroid therapy should be discontinued. In patients with immune-mediated (or steroid-responsive) FUO, the pyrexia and clinical signs usually resolve within 24 to 48 hours of the start of treatment.

If no response to corticosteroids is observed, two courses of action remain. In one, the patient can be released and given antipyretic drugs, such as aspirin (10 to 25mg/kg PO q12h in dogs and 10mg/kg PO q72h in cats) or other nonsteroidal antiinflammatories, and then returned to the clinic for a complete reevaluation in 1 to 2 weeks. Antipyretics should be used with caution, however, because fever is a protective mechanism and lowering the body temperature may be detrimental in an animal with an infectious disease. Moreover, drugs such as dipyrone and flunixin can result in marked hypothermia, which may have adverse effects. Also of note is that some nonsteroidal antiinflammatory drugs have ulcerogenic effects, can cause cytopenias, and may result in tubular nephropathy if the patient becomes dehydrated or receives other nephrotoxic drugs. The second course of action is to continue the trial of antibiotics by using a combination of bactericidal drugs (e.g., ampicillin and enrofloxacin) for a minimum of 5 to 7 days.