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Brucellosis is a serious bacterial disease that causes fever, joint pain and fatigue. Brucellosis bacteria spread from animals to people, often in contaminated milk, cheese and other dairy products.

There's evidence that brucellosis affected the people of ancient Rome, and it continues to afflict people and animals in the Mediterranean Basin and elsewhere today. And although the bacteria can spread through the air or through direct contact with infected animals, most people get brucellosis from unpasteurized dairy products.

Brucellosis can usually be treated successfully with antibiotics, but the regimen is lengthy and relapses are common, even after treatment. There is no vaccine for brucellosis in humans, though animals can be vaccinated against the disease.

Brucellosis often begins three to four weeks after infection with signs and symptoms similar to those of the flu:

• Fever, often rising to 104 F or more in the afternoon — a rising and falling (undulating) fever is one of the hallmarks of the disease
• Chills
• Sweats
• Weakness
• Fatigue
• Joint, muscle and back pain
• Headache

Ultimately, brucellosis can affect almost any part of your body, including your reproductive system, liver, heart and central nervous system. In some people, brucellosis becomes chronic, with signs and symptoms persisting for years, even after treatment. Why certain people develop chronic disease isn't entirely clear, though it may be due, at least in part, to differences in the way the immune system responds to the bacteria.

Brucellosis may also disappear for weeks or months and then return. And it's possible to become infected a second or even a third time because the first bout may not provide immunity against the disease. Long-term symptoms include arthritis, spondylitis — an inflammatory arthritis that affects the spine and nearby joints — and infections that start in one part of the body and then spread to other parts of the body (focal infections).

Brucellosis affects many wild and domestic animals, but camels, wild boar, cattle, goats, sheep, pigs, dogs and reindeer are especially prone to the disease. In the United States, brucellosis is increasingly common in buffalo in Yellowstone Park. A form of brucellosis is also found in harbor seals, porpoises and certain whales. Most often, infected animals have reproductive problems — females abort their young, and males have infections of the testicles.

At least six strains of bacteria cause brucellosis in animals, but not all produce illness in humans. The bacteria spread from animals to people in three main ways:

• Raw dairy products. Most people get brucellosis from contaminated dairy products. The bacteria are found in the milk of infected animals, and can spread to humans in unpasteurized milk, ice cream, butter and soft cheeses. Whether hard cheese, yogurt and cultured milk are just as likely to harbor the bacteria is a matter of some debate. The bacteria can also be transmitted in raw or undercooked meat from infected animals.
• Inhalation. Brucellosis bacteria spread easily in the air — inhaling as few as 10 to 100 organisms can cause disease. Farmers, laboratory technicians and slaughterhouse workers are especially at risk of inhaled infection.
• Direct contact. Bacteria found in the blood, semen or placenta of an infected animal can enter the bloodstream through a cut or other wound. Because normal contact with animals — touching, brushing or playing — doesn't cause infection, people rarely get brucellosis from their pets. Even so, people with weakened immune systems should avoid handling dogs known to have the disease.

Brucellosis normally doesn't spread from person to person, but in a few cases, women have passed the disease to their infants during birth or through their breast milk. Although it's uncommon, brucellosis may also spread through sexual activity and through contaminated blood or bone marrow transfusions.

Brucellosis is rare in the United States, with fewer than 200 reported cases each year, most of which are acquired abroad. Other parts of the world have much higher rates of infection, especially the Mediterranean Basin — Portugal, Spain, Italy, Greece, Turkey, the Near East and North Africa — South and Central America, Mexico, parts of Asia and sub-Saharan Africa.

In addition to geography, these factors may increase your risk of brucellosis:

• Raw dairy foods. This is probably the single greatest risk factor for the disease. Because the United States has a federal brucellosis eradication program, the chance of infection from domestic dairy products is low. Most cases occur in travelers who pick up the disease abroad or in people who eat imported dairy products. Hispanic-Americans who use unpasteurized soft cheeses from Mexico have high rates of infection, for example. In countries without brucellosis eradication programs, unpasteurized dairy products are more likely to be contaminated. The bacteria can survive for days in milk and for weeks or months in ice cream and butter.
• Animal-related occupation. People who routinely work with animals, including veterinarians, farmers and slaughterhouse workers, are at especially high risk. They're also more likely to have repeat infections because of ongoing exposure to the disease.
• Hunting. Hunters may become infected through skin wounds or by eating the undercooked meat of infected animals.
• International travel. Travelers who eat raw dairy products in countries where brucellosis is common are at high risk of infection, even if other foods in the region are considered safe. The soft goat cheeses commonly found in Mediterranean countries are especially likely to contain brucellosis bacteria, but more exotic foods such as raw camel's milk, sheep placenta and reindeer meat also may be contaminated. International travel and trade have aided the spread of brucellosis, as has the dislocation of people and animals due to war, natural disasters and drought.

Brucellosis can be hard to identify, especially in the early stages, when it often resembles the flu. See your doctor if you develop a rapidly rising fever, muscles aches and unusual weakness and have any risk factors for the disease.

Doctors usually confirm a diagnosis of brucellosis by testing a sample of blood or bone marrow. Most tests for the disease have some drawbacks, but the polymerase chain reaction (PCR) test, which looks for the genetic material of the brucellosis bacteria, has certain advantages. It's quick, it can be performed on any type of tissue, and it can provide results as soon as 10 days after infection. Because PCR is fairly new, however, it's generally used in conjunction with other blood tests.

To help detect complications of brucellosis, you may have additional tests, including:

• Urinalysis or urine culture. These tests look for brucellosis infection in the urinary tract.
• Cerebrospinal fluid culture. This checks a small sample of the fluid that surrounds your brain and spinal cord for infections such as meningitis and encephalitis.
• X-rays. X-rays can reveal arthritic changes in your bones and joints.
• Brain computerized tomography (CT) scan. This imaging test helps locate inflammation or abscesses in the brain.

Chronic brucellosis may cause complications in only one organ or throughout the body (diffuse brucellosis). These complications include:

• Endocarditis. This infection of the heart's inner lining is one of the most serious complications of brucellosis. Untreated endocarditis can damage or destroy the heart valves and is the leading cause of brucellosis-related deaths.
• Osteoarthritis. This form of arthritis is marked by pain, stiffness and swelling in your joints, especially the knees, hips, ankles, wrists and spine. Spondylitis, which causes inflammation of the joints between the vertebrae of the spine or between the spine and pelvis, can be particularly hard to treat and may cause lasting damage.
• Inflammation and infection of the testicles (epididymo-orchitis). The bacteria that cause brucellosis can infect the epididymis, the coiled tube that connects the vas deferens and the testicle. From there, the infection may spread to the testicle itself, causing swelling and pain, which may be severe. Brucellosis can also infect the prostate and kidneys.
• Spontaneous abortion. Brucellosis may cause spontaneous abortion in pregnant women.
• Hepatitis. Brucellosis can cause this serious liver disease directly. Or, it can trigger autoimmune hepatitis, which occurs when your own immune system attacks your liver. In either case, untreated hepatitis can lead to liver scarring (cirrhosis) and ultimately to liver failure.
• Central nervous system infections. These include potentially life-threatening illnesses such as meningitis, an inflammation of the membranes surrounding the brain and spinal cord, and encephalitis, inflammation of the brain itself.

Brucellosis is usually treated with two antibiotics for as long as six weeks. The World Health Organization recommends using doxycycline in combination with streptomycin or rifampin, and though this regimen is followed in many parts of the world, other antibiotics are sometimes used. When brucellosis affects the central nervous system, three antibiotics are given for as long as three months. But even after appropriate antibiotic therapy, the disease can return and may become chronic.

Many doctors recommend bed rest for people with brucellosis. Activity seems to make symptoms worse and to delay healing, whereas people who rest recover more quickly. Even with the best care, however, it may take several months for symptoms to go away completely.

The risk of getting human brucellosis can be reduced or eliminated by taking these precautions:

• Avoid raw dairy foods. In recent years in the United States, few cases of brucellosis have been linked to raw dairy products from domestic herds. Still, it's probably best to avoid unpasteurized milk, cheese and ice cream, no matter what their origin. Travelers to other countries should avoid all raw dairy foods. If you're not sure whether a product is pasteurized, don't take chances.
• Cook meat thoroughly. All meat should be cooked until it reaches an internal temperature of 145 to 165 F. When eating out, order beef and pork at least medium-well. It's unlikely that domestic meat in the United States contains brucellosis bacteria, but proper cooking destroys other harmful bacteria such as salmonella and E. coli. When traveling abroad, avoid buying meat from street vendors and order all meat well-done.
• Wear gloves. Veterinarians, farmers and hunters should wear gloves when handling sick or dead animals or when assisting an animal giving birth.
• Take safety precautions. Laboratory workers should handle all specimens under appropriate biosafety conditions.
• Immunize domestic animals. Although vaccination for brucellosis isn't mandatory, many farmers and ranchers vaccinate their herds, and milk is tested two to four times a year for signs of the bacteria. Because the brucellosis vaccine is live, it can cause disease in people. Anyone who has an accidental needle stick while vaccinating an animal should be treated.