Clinical signs of brucellosis in pigs vary but are similar to those in cattle and goats. Although the disease is often self-limiting, it remains in some herds for years.
Brucellosis due to Brucella suis may occur in domestic animals other than pigs. The incidence of swine brucellosis among domesticated animals in the US is very low. The prevalence in the US is sometimes high among feral pigs. Currently there are no known infected domestic swine herds.
Brucellosis in humans has been reported among packing-house workers and hunters; the usual source is infected pigs.
Etiology and Transmission of Brucellosis in Pigs
Brucella suis is transmitted mainly via ingestion of infected tissues or fluids. Infected boars may transmit the disease during service; the organism can be recovered from semen. In the US, feral swine are the primary source of transmission for B suis.
Clinical Findings of Brucellosis in Pigs
After exposure to Brucella suis, pigs develop a bacteremia that may persist for up to 90 days and continue shedding for up to 3 years. During and after the bacteremia, localization typically occurs in lymphatics, spleen, liver, and mammary and reproductive tissues.
Clinical signs depend considerably on the site(s) of bacterial colonization and commonly include:
temporary or permanent sterility
metritis and abscess formation (occasionally)
The incidence of abortion may range from 0% to 80%. Abortions may also occur early in gestation and be undetected. Usually, sows or gilts that abort early in gestation return to estrus soon afterward and are rebred.
Infection early in pregnancy commonly results in reproductive failure, including decreased litter size, abortion, stillbirth, and birth of weak piglets.
Infertility in sows, gilts, and boars is common and may be the only clinical sign of brucellosis. Before attempting treatment for other diseases, it is logical to test for brucellosis in herds in which sterility is a problem.
Infertility in sows is usually temporary but may be permanent. In boars, orchitis, usually unilateral, may occur, and fertility appears to decrease.
Diagnosis of Brucellosis in Pigs
The principal means of diagnosis in pigs is serologic testing, such as the brucellosis card (rose bengal) test, other serum agglutination tests, or complement fixation tests. These tests are used in series to confirm seropositivity.
It is generally accepted that the tests have limitations in detecting brucellosis in individual pigs. Thus, entire herds or units of herds, rather than individual pigs, must be tested in any control program. Low agglutinin titers occur in almost any size herd, regardless of infection status, and a few infected pigs may have no detectable titer.
Cross-reactions with other pathogens, such as Yersinia enterocolitica, have been reported. The card test is usually more sensitive than other agglutination tests. Supplemental tests designed for cattle may also be used for pigs.
Prevention and Control of Brucellosis in Pigs
Initial isolation and testing
Test and removal of high-risk swine
Replacement swine should be purchased from herds known to be free of brucellosis, or they should be tested, isolated for 3 months, and retested before being added to the herd. Pigs should be isolated on return from fairs or shows before reentering the herd. Domestic pigs should be kept in a manner to prevent interactions with feral swine.
There is no vaccine for brucellosis in swine, and no practical recommendations can be made for treatment. Control is based on test and removal of infected breeding animals. Brucellosis remains a problem in feral swine and is a potential source of infection for domesticated herds and for humans.
Swine brucellosis is a zoonotic disease. Those working with feral swine and in abattoirs should take necessary precautions to prevent exposure.
Serologic testing is necessary for identifying infected swine.
Testing and removal of seropositive swine is the foundation of the control program.