Young woman stares at thermometer. (Bigstockphoto)

No medical mystery is more challenging than a fever of unknown origin. An FUO is defined as an elevated body temperature that lasts for three or more weeks without an explanation, according to Marvin M. Lipman, Consumer Reports’ chief medical adviser.

One famous example — not only because of the belatedly discovered cause but also because of the patient’s identity — harks back to 1962, when a 77-year-old woman was admitted to a prestigious New York hospital with an unexplained fever of about six weeks’ duration.

She had lost a significant amount of weight and over the previous year had received several blood transfusions when her bone marrow failed to make red blood cells. Aside from a chest X-ray that revealed an old scar, all her laboratory tests were negative. Her physicians, including several luminaries of the day, were frustrated and mystified.

Several weeks before the woman, Eleanor Roosevelt, died in her New York apartment, cultures of her bone marrow grew tuberculosis bacteria. An autopsy discovered that the cause of death was miliary tuberculosis, a rare form of the disease in which the TB bacilli spread through the bloodstream and infect multiple organs. The bacteria turned out to be resistant to the antibiotics available at the time and would have killed the former first lady even if her illness had been diagnosed promptly. That may have helped ease the conscience of those involved in her care.

Fevers of unknown origin are always worth investigating because they can be caused by many conditions, including serious ones.

First, think infection

One would think that with our current sophisticated diagnostic techniques, FUOs would be less common than in years gone by. Instead, the number has increased, and no one is sure why.

Infections are the cause of about one in five unexplained fevers. They include endocarditis, or an infection of the heart valves, as well as such infections as cytomegalovirus, Epstein-Barr virus and toxoplasma, a parasite. The early 1980s surge of immunocompromised people with HIV/AIDS was associated with an increase of previously obscure and hard-to-diagnose viral and fungal infections, all capable of causing weeks of fever. Today, of course, we know to look for those conditions, which have become less common in the United States with the steady improvement in treatments for HIV/AIDS.

Cancer accounts for about 20 to 30 percent of FUOs, and that hasn’t changed over the last half- century. Lymphomas, especially of the Hodgkin’s disease type, are the most common cancer-related cause of a prolonged fever. Kidney and colorectal cancers are also frequently to blame, even when they’re small and therefore more difficult to pin down.

Long-lasting fevers can also be due to disorders marked by the inflammation of connective tissue, the glue that binds cells together.

Rheumatic diseases such as polymyalgia, rheumatoid arthritis and lupus can cause temperature elevation long before more recognizable symptoms appear. The same can be true for inflammatory bowel diseases such as ulcerative colitis and regional ileitis. Medication, too, can cause FUOs, even after people have been taking them for a while. Among the more common culprits are sulfa drugs, phenytoin (Dilantin and its generic cousins), allopurinol (Zyloprim and generics), methyldopa (Aldomet and generics) and isoniazid.

Finding normal temperature

Since fever can be a marker for many problems, it’s important to know what constitutes a significant elevation in body temperature. The most accessible site that reflects true internal body temperature is the rectum. Rectal temperature can vary from an early-morning low of 97.9 degrees to a late-afternoon high of 100.4. Oral, armpit and ear temperatures are less accurate and can be affected by environmental factors. Normal armpit temperature can be as low as 94.5.

To determine your “normal” temperature, select a site and check it morning, afternoon and evening for three days in a row when you’re feeling well. Then calculate your average temperature for each of the three times of day. Any measurement above your average is a fever, and if you have a fever for three to five days without obvious accompanying symptoms, it’s time to give your doctor a call.

Copyright 2011. Consumers Union of United States Inc.