Brucella
Brucella refers to a genus of small, gram-negative bacteria that are primarily pathogenic in animals and can also infect humans. These nonmotile, non-spore-forming coccobacilli are found globally and are known to cause brucellosis, a disease that presents with flu-like symptoms such as fever, malaise, and joint pain. Brucella species, such as B. abortus and B. melitensis, primarily infect various mammals, including cattle, sheep, and dogs. Transmission to humans typically occurs through the consumption of unpasteurized dairy products, undercooked meat, or direct contact with infected animal fluids.
The genus Brucella consists of several species, with ongoing research continuously refining their taxonomy based on genetic studies. Treatment for brucellosis generally involves a combination of antibiotics to reduce the risk of relapse, though there is currently no vaccine available for humans. While brucellosis is relatively rare in industrialized nations, it remains more prevalent in regions bordering the Mediterranean and in parts of Latin America and Asia. Understanding Brucella and its implications is crucial for preventing transmission and managing the disease, especially in at-risk populations like veterinarians and those in slaughterhouse settings.
Brucella
- TRANSMISSION ROUTE: Direct contact, ingestion, inhalation
Definition
Brucella are gram-negative, nonmotile, non-spore-forming, nonencapsulated, small coccobacilli of worldwide distribution. They are pathogenic in animals, including humans.
Natural Habitat and Features
The taxonomy of the genus Brucella has gone through several changes. By 1986, there were six recognized species, but scientists believed that not enough genetic difference existed among them and thus suggested that all be combined as B. melitensis, with the old species names listed as subspecies. In 2010, the International Committee on Systematics of Prokaryotes, a subcommittee on the taxonomy of Brucella, unanimously agreed to reinstate the original six species and to add four additional species. The number of species and taxonomy of Brucella continues to undergo changes as new discoveries are made.
Each of the species has a different host range and some metabolic differences. Two species that have been most studied and whose DNA (deoxyribonucleic acid) has been sequenced, melitensis and abortus, have two circular chromosomes, one with just more than two million base pairs and one with just more than one million base pairs.
Brucella spp. are small, gram-negative coccobacilli, approximately 0.5 micrometers (m) in diameter and 0.6 to 1.5 m in length. With the exception of microti, they are slow-growing in culture, often taking several days to weeks to show growth. Nutritionally, they are considered to be fastidious and require several vitamins and amino acids.
Both abortus and melitensis grow best when erythritol is added to the medium as a carbon source. They are considered facultative intracellular parasites and can survive for long periods in soil and water. They have worldwide distribution and reservoirs in several domestic and non-domestic mammal species. Studies with microti suggest that, in addition to its normal vole reservoir, soil could serve as a reservoir for this species. In their animal reservoirs, there is often a high concentration of bacteria in the reproductive organs; these bacteria are transferred during sex, by licking the external genitalia, and by contact with the placenta and fluids released during birth. Carnivores can sometimes become infected when they eat an infected animal.
Pathogenicity and Clinical Significance
Each Brucella species has a small group of mammals that can serve as hosts. For example, abortus affects cattle, bison, buffalo, elk, camels, and yaks; melitensis affects sheep, goats, and camels; suis affects swine; and canis infects dogs. Immunizations and the slaughter of infected animals have eradicated or nearly eradicated brucellosis in many parts of the industrialized world. In the United States, abortus infection in domestic cattle has been almost extirpated; however, a reservoir of the bacteria exists in bison and elk in and around Yellowstone National Park. Humans are accidental hosts when they ingest infected meat or unpasteurized dairy products, or when they come in contact with infected body fluids from one of the usual host animals.
In the United States, human brucellosis is rare, causing between one hundred and two hundred cases per year. It is much more common in the countries bordering the Mediterranean, in countries of the Arabian Peninsula, in India, and in Latin America. In humans, abortus, melitensis, canis, and suis can all cause classical brucellosis, while ceti and pinnipedialis have been associated with neural brucellosis.
Brucellosis, also known as undulant fever, Mediterranean fever, Malta fever, and Crimean fever, has varied, nonspecific, flu-like symptoms such as fever, malaise, joint pain, headache, and fatigue. In humans, melitensis infections are the most common and cause the most severe symptoms, which can, on rare occasions, lead to death. Suis infections are also quite severe and can lead to prolonged illness, often with pus-forming lesions. Abortus and canis infections are often mild and self-limiting.
Brucellosis can be acute or chronic, with chronic infection being associated with severe debility and increased morbidity. In addition, relapsing brucellosis is difficult to distinguish from reinfection. Subclinical infections are also seen in high-risk occupations, such as veterinarians and workers on the kill floor of slaughterhouses. In these subclinical infections, the person is asymptomatic but shows Brucella infection after serologic screening.
Drug Susceptibility
Because of high relapse rates with single drug therapy, most treatment regimens are based on two drugs. For adults, recommended drugs are oral doxycycline or ciprofloxacin for six weeks, with either oral rifampin for six weeks or intramuscular streptomycin or gentamicin for three weeks. The streptomycin or gentamicin treatment leads to fewer relapses, and gentamicin has fewer side effects. For children under eight years of age, doxycycline is not recommended. Instead, a regimen using rifampin and trimethoprim-sulfamethoxazole is the standard. Although immunizations have been developed for several species of domestic animals, there is no human immunization against brucellosis.
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