Brucella suis is a Gram-negative coccobacillus bacterium that does not form spores. It is a pathogen of concern for both animal and public health.
The Bacterium and Its Hosts
Brucella suis causes swine brucellosis, a disease predominantly found in pigs. It causes chronic inflammatory lesions, particularly in the reproductive organs. While pigs are the primary hosts for biovars 1, 2, and 3, other animals like cattle, horses, and dogs can also become infected, especially through close contact with infected pigs.
Transmission among pigs often occurs through direct contact with infected tissues, fluids, or a contaminated environment. Infected sows can shed the organism in birth products (fetuses, placentas, vaginal discharges), milk, and urine. Boars, symptomatic or not, can excrete bacteria in their semen, making venereal transmission common in swine. Piglets can acquire the infection during nursing or prenatally.
As a zoonotic pathogen, Brucella suis can spread from animals to humans. People typically become infected through direct contact with infected animals or their tissues, via cuts, inhalation of contaminated air, or consumption of contaminated products like unpasteurized dairy. The bacterium can survive in various environments for several months, particularly in cool, moist conditions, contributing to its persistence and spread.
Signs and Effects of Infection
In pigs, Brucella suis infection primarily manifests as reproductive problems. The most common sign is abortion in pregnant sows, which can occur at any stage of gestation. Other effects include temporary or permanent sterility in both sexes, reduced milk production, and the birth of weak or stillborn offspring. Male pigs may develop orchitis (inflammation of the testicles). Lameness, hind limb paralysis, and spondylitis (joint inflammation) can also occur.
In humans, brucellosis caused by Brucella suis can present with a wide range of symptoms, often appearing two to four weeks after exposure, though sometimes much later. A characteristic symptom is an undulant fever, marked by intermittent or irregular patterns. Other common signs include fatigue, generalized weakness, profuse sweating, chills, and a loss of appetite. Individuals may also experience headaches, muscle aches, and joint pain, particularly in the hips, knees, or lower back.
Beyond general symptoms, Brucella suis can lead to more serious, long-term complications. The infection can affect various organ systems, causing abscesses in the liver or spleen. Neurological complications like meningitis or encephalitis (inflammation of the brain or its coverings) can arise. Cardiovascular issues, like endocarditis (inflammation of the heart lining), and skeletal problems, such as spondylitis (spinal inflammation) or osteomyelitis (bone infection), are also possible. In men, inflammation of the epididymis and testicles (epididymo-orchitis) can occur.
Detecting and Treating Brucella suis
Diagnosing Brucella suis in animals involves several methods. Serological tests, such as the Rose Bengal test and ELISA (Enzyme-Linked Immunosorbent Assay), are commonly used to detect antibodies against the bacteria in blood samples. Bacterial culture, growing samples from tissues, reproductive fluids, or aborted fetuses in a laboratory, can directly identify the organism. Microscopic examination of stained smears from tissues or secretions can also provide a presumptive diagnosis.
For humans, diagnosis typically involves blood tests to check for antibodies, often requiring two samples taken several weeks apart. Bacterial culture from blood or bone marrow samples can confirm the presence of Brucella suis, though bone marrow cultures are more likely to yield positive results, especially in chronic cases. Imaging techniques might be used to identify localized infections or complications in specific organs. Brucella suis cannot always be isolated, particularly in chronic cases.
Treatment approaches for Brucella suis differ significantly between animals and humans. For infected domestic pigs, culling is often the primary strategy due to a lack of widely effective treatments and zoonotic risk. While some experimental vaccines have shown limited protection in swine, there is currently no approved vaccine for swine brucellosis in most countries. This approach prevents further spread within herds and to other animals or people.
In humans, brucellosis is treatable with long-term antibiotic regimens to prevent relapse. A common treatment involves a combination of antibiotics, such as doxycycline and rifampicin, typically administered for six to eight weeks. In some cases, other antibiotics like streptomycin, gentamicin, or sulfamethoxazole/trimethoprim may be used in conjunction with doxycycline. Completing the entire course of antibiotics is crucial, as single or incomplete courses can lead to high relapse rates.
Protecting Against Brucella suis
Preventing Brucella suis spread in animal populations relies on strict biosecurity. This includes preventing contact between uninfected herds and potentially infected animals (e.g., wild or feral swine) or contaminated environments. Testing animals and promptly removing infected individuals from the herd are also important control strategies. While some vaccines exist for other Brucella species, a widely effective vaccine specifically for swine brucellosis is not readily available.
Protecting humans from Brucella suis involves several public health recommendations. Avoiding unpasteurized dairy products (e.g., raw milk, unpasteurized cheeses) is a primary safeguard, as pasteurization effectively kills the bacteria. Individuals in high-risk occupations (veterinarians, slaughterhouse workers, hunters) should use protective equipment (gloves, goggles, masks, respirators) when handling animal tissues or potentially contaminated materials. Proper handling and disposal of animal tissues, especially afterbirths and carcasses, reduces exposure risk.
Due to its zoonotic nature and potential for public health impact, Brucella suis is a reportable disease in many regions, making it subject to surveillance programs. These programs help monitor the prevalence of the disease in animal populations and track human cases, allowing for coordinated control efforts. Implementing these measures can significantly reduce transmission risk to both animals and humans.