Brucella suis is a bacterium that causes swine brucellosis, a zoonosis that affects pigs. The disease typically causes chronic inflammatory lesions in the reproductive organs of susceptible animals or orchitis, and may even affect joints and other organs. The most common symptom is abortion in pregnant susceptible sows at any stage of gestation. Other manifestations are temporary or permanent sterility, lameness, posterior paralysis, spondylitis, and abscess formation. It is transmitted mainly by ingestion of infected tissues or fluids, semen during breeding, and suckling infected animals.
Since brucellosis threatens the food supply and causes undulant fever, Brucella suis and other Brucella species (B. melitensis, B. abortis, B. ovis, B. canis) are recognized as potential agricultural, civilian, and military bioterrorism agents.
Brucella suis is a Gram negative coccobacillus in the family Brucellaceae (class Alphaproteobacteria) Five biovars with different host preferences are currently recognized. B. suis biovars 1, 2 and 3 are the Brucella species usually found in pigs, although B. abortus and B. melitensis may also be detected occasionally. Biovar 4, the agent of rangiferine brucellosis, circulates in reindeer and caribou. Biovar 5 has only been found in rodents.
Note on taxonomy: At one time, the genus Brucella was reclassified into a single species, B. melitensis, based on the genetic and immunological evidence that all members of this genus are closely related. Under this system, the various species of Brucella were considered to be biovars. This proposal was controversial, and it has fallen out of favor for practical reasons.
- suis is common among domesticated pigs in parts of Latin America and Asia. Control programs have eliminated or nearly eliminated this organism in some other areas, including a number of European nations, the U.S., Canada and Australia. However, B. suis is still maintained in feral pigs or wild boar in many of these regions, resulting in sporadic transmission to domesticated swine. Infected pigs have occasionally been documented in some African nations, but surveillance there is limited. Biovars 1 and 3 of B. suis occur worldwide, but biovar 2 appears to be limited to Europe, where it primarily circulates in wild boar. Biovar 4 (rangiferine brucellosis) circulates in the Arctic regions of North America and Eurasia where its reservoir hosts are found (e.g., Siberia, Canada and Alaska).
Porcine brucellosis: biovars 1, 2 and 3 In pigs,
the most common clinical signs are reproductive losses, which may include abortions, stillbirths, the birth of weak piglets (which may die early in life) and decreased litter size. Although abortions have been reported to occur at any time during gestation, they are noted to be most common in mid- to late gestation. Because fetuses may be cannibalized and vaginal discharge is often minimal, abortions can be mistaken for infertility. Pigs may not abort if some live fetuses are still present. Fetal losses early in gestation usually appear as a return to estrus 30-45 days after mating. Uncomplicated abortions are not usually accompanied by signs of illness; however, some cases may be complicated by retention of the placenta and secondary metritis. Epididymitis and orchitis are sometimes seen in males, and may result in infertility. Pigs that are not pregnant may remain asymptomatic; however, they sometimes become lame from arthritis, develop posterior paralysis from spondylitis, or have various complications related to abscess formation in other tissues and organs. While overt clinical signs occur in some herds, chronically infected herds may only have subtle signs such as nonspecific infertility, a slightly reduced farrowing rate, and irregular estrus cycles. Deaths are rare except in the fetus or newborn. Relatively little is known about the effects of B. suis biovar 2 on wild boar, but it has occasionally been implicated in abortions, metritis and orchitis. In horses B. suis can cause inflammation of the supraspinous or supra-atlantal bursa; these syndromes are known, respectively, as fistulous withers or poll evil. The bursal sac becomes distended by a clear, viscous, strawcolored exudate and develops a thickened wall. It can rupture, leading to secondary infection. In chronic cases, nearby ligaments and the dorsal vertebral spines are also involved and may occasionally become necrotic. Brucellaassociated abortions have been reported in horses, but seem to be uncommon. Clinical signs typical of canine brucellosis, as well as subclinical infections, have been reported in dogs infected with B. suis. The signs and syndromes have included nonspecific signs of illness, such as fever, lethargy and vomiting, as well as discospondylitis, lameness, orchitis, epididymitis, enlargement of the prostate, hematuria and abortion. Systemic signs are not necessarily observed in localized infections. Naturally-acquired and experimental infections in cattle generally suggest that B. suis infections are asymptomatic in this species, even in pregnant animals. One study found that this organism might be associated with an increased incidence of retained placentas in cattle, but further study is needed. Rabbits that were inoculated with a biovar 1 isolate from hares developed nonspecific signs (malaise, anorexia) and conjunctivitis. Biovar 2 was found in a moribund, emaciated roe deer fawn with respiratory lesions and an enlarged spleen.
Rangiferine brucellosis: biovar 4
- suis biovar 4 can cause reproductive losses in caribou and reindeer. Abortions may sometimes be complicated by retained placenta and metritis. Arthritis, tenosynovitis, hygromas, subcutaneous abscesses, mastitis and nephritis have also been reported, and males can develop orchitis, epididymitis and seminal vasculitis. Only a few clinical cases caused by biovar 4 have been described in other species: bone abscesses, joint involvement and testicular lesions were reported in naturally infected muskox, and carpal pathology and osteomyelitis were documented in an emaciated, debilitated moose. An experimentally infected moose developed septicemia, with nonspecific signs of anorexia, fever and depression. Experimental infection of bison with biovar 4 did not result in abortions or other clinical signs.
Antibiotics may mitigate the clinical signs or clear B. suis from an animal, but this organism might persist in treated animals, and recrudescence is possible. For this reason, euthanasia of infected animals is often recommended. Some B. suis-infected dogs have been treated successfully with a combination of antibiotics, such as rifampicin and doxycycline. Isolating dogs is advised during treatment, and neutering is recommended if the animal is intact. Periodic serological monitoring might be able to detect rising antibody titers if organisms persist and begin to replicate again in treated animals. Treatment of livestock is generally discouraged due to the zoonotic risks, and it may also not be cost-effective. One study reported that a combination of oxytetracycline and tildipirosin appeared to clear B. suis from pigs, though further research is needed. Oxytetracycline alone temporarily suppressed clinical signs in an experimental herd, but abortions and transmission of the organism increased once antibiotic use ended.
B suis is usually introduced into a herd in an infected animal. B. suis-free herds should not be allowed to contact potentially infected animals, including wild and feral swine, or contaminated environments, such as those where animals recently aborted. Good biosecurity and double fencing have been used to protect pigs raised outdoors; however, some herds in Europe have apparently been infected from environmental sources despite these measures. If possible, replacement stock should be selected from Brucella-free herds. Herd additions should be quarantined and tested before being released into the herd. Some asymptomatically infected animals may be difficult to detect with the current tests. Semen for artificial insemination should only be collected from Brucella-negative animals that are tested regularly. In an infected herd, the placenta, any abortion products and contaminated bedding should be removed promptly and destroyed, and contaminated fomites should be disinfected. B. suis can be eradicated from infected herds by depopulation or test and removal methods. Programs to eradicate this organism from a country also include movement controls on infected herds, surveillance, and tracing of infected animals. There is no vaccine for swine brucellosis in most countries, and the RB51 and strain 19 B. abortus vaccines do not seem to protect cattle from infection by B. suis. A B. suis strain 2 vaccine has been used to immunize pigs and other animals in China, but its efficacy is unclear and it has not gained acceptance elsewhere. Infections in other species are generally prevented by controlling B. suis in its maintenance hosts and avoiding contact with potentially infected animals. Dogs should not be fed raw tissues from domesticated or wild swine, caribou, reindeer, hares or other wild animals that may carry B. suis.