Brucellosis is an infectious disease caused by bacteria of the genus Brucella. Different Brucella species can infect a variety of vertebrates, including livestock such as cattle, sheep, goats, and pigs, as well as deer, elk, bison, and dogs. Humans can contract brucellosis through contact with infected animals or animal products. As a result, brucellosis is one of the world's most widespread zoonoses (diseases transmitted to humans from animals), although it has been virtually eliminated in many countries.
Worldwide, brucellosis is transmitted to humans primarily through the consumption of unpasteurized milk and other dairy products, especially unprocessed soft goat cheeses. It is also spread through contact with infected livestock, particularly during slaughtering, dressing, and preparing of foods from infected animals. Therefore, slaughterhouse workers and wild-game hunters can be at risk for brucellosis. Veterinarians and veterinary clinic staff sometimes contract brucellosis through contact with contaminated fluids and tissues, especially while assisting animals in birthing. Eating undercooked infected meat is also a risk factor for brucellosis.
Although dogs can be infected with Brucella canis, this species rarely causes human illness. Veterinarians may face a potential risk from exposure to the blood of infected animals, but pet owners are not considered to be at risk for the disease.
Brucellosis is the most commonly reported laboratory-associated bacterial infection. In part, this may be due to aggressive cattle immunization programs in the United States, which have deprived laboratory personnel of experience working with these infectious organisms and can result failure to take precautions when unknown samples arrive in laboratories for analysis. The low-dose infectivity of these bacteria and the ease with which they aerosolize contribute to the risks associated with handling brucella in the laboratory.
Brucellosis occurs worldwide, and human infections are very common in countries were the disease is not controlled in livestock and where dairy products, especially so-called “village cheeses,” are unpasteurized. The World Health Organization (WHO) receives about 500,000 reports of brucellosis annually from around the world. The Mediterranean Basin, Eastern Europe, the Middle East, Asia, Africa, and Latin America remain relatively high-risk regions for brucellosis, especially from the consumption of unpasteurized dairy products. The annual incidence of brucellosis in the Middle East is 86 per 100,000 inhabitants.
In the United States, where brucella infections in cattle have been almost completely eradicated and where almost all milk products are pasteurized, there are only 100–200 cases of brucellosis reported annually—less than 0.5 cases per 100,000 population. Most cases occur among slaughterhouse workers and laboratory personnel in California, Florida, Texas, and Virginia. In contrast, in 1940, prior to the introduction of a cattle vaccine, there were 6,500 cases of brucellosis reported in the United States annually.
Brucella is a genus of intracellular bacteria that are classified into ten species based, in part, on their host specificity. Brucella abortus, B. melitensis, B. suis, and B. canis are all known to infect humans, and other Brucella species may also be able to cause brucellosis in humans. B. abortus is so-named because it causes abortions in infected cows. B. abortus is more prevalent in the United States and Northern Europe than other Brucella species. However, most infections in the United States are caused by B. melitensis, which is generally more common in Latin America, the Mediterranean Basin, and the developing world. B. melitensis infects goats and sheep, and B. suis is most often found in hogs. B. melitensis and B. suis cause the most severe—potentially fatal—human illness. B. canis infects dogs, and B. rangiferi infects reindeer and caribou.
Brucellosis causes a range of flu-like symptoms in humans. The name “undulant fever” derives from the characteristic wave-like rise and fall of intermittent fever that generally spikes each afternoon. Other symptoms of brucellosis include:
Symptoms of brucellosis usually appear between five days and one month after exposure, beginning with a single bout of high fever accompanied by shivering, drenching sweats, and body aches that last for a few days. Severe infections can affect the central nervous system or the heart lining. Untreated brucellosis can become chronic, with fevers, joint pain, and fatigue recurring over a period of months or years and potentially leading to rare but fatal complications, such as pneumonia or bacterial meningitis. Mental depression from brucellosis can be so severe that patients may become suicidal. Infection with B. melitensis can cause miscarriage, especially during the first three months of pregnancy.
Culturing the brucella bacterium from a blood sample is the most definitive diagnosis for brucellosis, but such cultures may yield false negatives. When culturing blood or bone marrow samples, the laboratory must be informed that brucella is suspected, so that the sample is cultured for a longer period of time, and laboratory personnel take appropriate precautions. Blood samples can also be tested for antibodies against brucella, but testing should be performed on two samples collected two weeks apart. Brucella also can be cultured from urine samples or cerebrospinal fluid.
Brucellosis is treated with a prolonged regimen of antibiotics and bed rest. A combination of the antibiotics doxycycline and rifampin are generally used for at least six to eight weeks to prevent chronic or recurring infections. Streptomycin may also be used. If symptoms recur, a second course of antibiotics is required.
In the United States, human cases of brucellosis are a notifiable disease that must be reported to local health authorities. The public health goal is the elimination of the bacteria from domestic and wild animals. The National Brucellosis Eradication Program has achieved significant success in reducing affected cattle herds. However, concerns about natural brucellosis outbreaks remain.
Throughout the world, brucellosis management is a growing problem at the boundaries between livestock grazing areas and wildlife preserves. Infected wildlife that migrate outside of protected areas may pose a potential threat to livestock and humans. For example, brucellosis has been almost completely eradicated in the United States, with the exception of the region surrounding Yellowstone National Park. Yellowstone bison have long been infected with B. abortus. Attempts to contain the disease have led to culling of the bison herd that migrates out of the park each winter. Over the first decade of the twenty-first century, more than 3,600 Yellowstone bison were removed, either by capturing for slaughter or shipping elsewhere or by hunting. However, the culling has not only been ineffective in reducing brucella infection in cattle, it has negatively impacted long-term conservation efforts for Yellowstone bison. Of the tens of millions of bison that formerly roamed North America, only about 20,000 wild bison remain, and the Yellowstone bison are considered to be the most genetically pure. Furthermore, research has shown that the bison pose almost no risk for the transmission of brucellosis to cattle. Nevertheless, conflicts continue to rage between Montana cattle ranchers and both bison conservationists and Native American tribes intent on reestablishing native buffalo herds.
Brucellosis is also a bioterrorism concern. The bacteria aerosolize easily and are resistant to drying out. Furthermore, in aerosolized form, only ten to 100 bacteria are required to produce illness in humans and animals. In 1954, B. suis became the first agent of biological warfare to be weaponized by the United States at its arsenal in Pine Bluff, Arkansas. When the biological weapons program was discontinued in the early 1970s, brucella and all other biological weapons were destroyed. However, because of its potential as a biological weapon, B. abortus has remained on the U.S. Government's “select agent” high-security list. As of 2012, there was a proposal in the U.S. Senate to remove it from the list, in order to make brucella vaccine development simpler and less expensive.
Depending on illness severity and treatment timing, recovery from brucellosis may require a few weeks to several months. Early diagnosis and prompt treatment are essential for preventing chronic infection that may linger for years or reoccur, even after a long period of time. Although treatment for brucellosis can be difficult and prolonged, and the intermittent fevers can be exhausting, the disease is rarely fatal. Mortality is less than 2% and is usually associated with damage to the heart.
There is no human vaccine for brucellosis. The most effective prevention is regular testing of livestock for the bacterium, destruction of infected livestock, and immunization of uninfected animals. Cattle vaccines for brucellosis have been available since 1941. In most developed countries, test-and-slaughter programs, compensation for farmers, immunizations, and financial incentives for disease-free herds have, to a large extent, eliminated brucellosis and prevented all but a few cases of the disease in humans. During the 1990s, B. abortus strain RB51 was developed specifically for immunizing cattle. This strain is less likely to cause severe disease in cattle and, presumably, in humans. Nevertheless, on rare occasions RB51 has caused human disease.
See also Bioterrorism ; Meningitis .
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