This section is a summary of Chapter 2.6.2, on Identification, Serological and other Tests, Vaccines and Diagnostic Biologicals, in OIE Manual of Standards for Diagnostic Tests and Vaccines (2000).
Brucellosis in pigs is a bacterial infection that, after an initial bacteraemia, causes chronic inflammatory lesions localized in the reproductive organs of both sexes, and in the bones. The infection in pigs is caused by Brucella suis biovar 1, 2 or 3. The disease caused by biovars 1 and 3 is similar, while that caused by biovar 2 differs from 1 and 3 in its host range, its limited geographical distribution and its pathology. Biovar 2 is rarely pathogenic for humans, whereas biovars 1 and 3 are highly pathogenic for humans. Porcine brucellosis is of widespread occurrence; generally, however, the prevalence is low, with the exception of South America and Southeast Asia, where the prevalence is higher. In some areas, B. suis infection has become established in wild or feral pigs. Diagnostic methods recommended for wild and feral pigs are the same as for domestic pigs. Various biovars of B. suis cause infections in animals other than pigs, such as reindeer, caribou, hares and various murine species. These are dealt with in an appendix at the end of this chapter.
Signs in sows include abortion at any stage of gestation and birth of dead or weakling piglets. In boars, the most prominent symptom is orchitis, and the secondary sex organs may be affected. There may be B. suis in the semen, sometimes in the absence of clinical signs. Transmission during copulation is more common than is the case with brucellosis in ruminants. In both sexes, bones, and especially joints and tendon sheaths, may be affected, causing lameness and sometimes paralysis. Pigs are susceptible to artificial infection with B. abortus and B. melitensis, but there are no reports of natural disease in pigs being caused by either of these organisms. In humans, the infection is usually confined to those who are occupationally exposed to pigs, and to laboratory workers. However, B. suis is capable of colonizing the bovine udder, causing serious human epidemics.
B. suis is readily isolated from live pigs by culture of birth products, and from carcasses by culture of lymph nodes and organs. Selective media are available for culture of contaminated specimens. In nature, B. suis occurs invariably in the smooth phase - the appearance on solid medium is typical of smooth Brucellae. Biovars of porcine origin agglutinate with monospecific anti-A serum, and not with anti-M serum. Definite identification of species and biovars may be effected by phage lysis and biochemical tests, preferably carried out in specialized laboratories.
To date, no serological test has been shown to be reliable in routine diagnosis in individual pigs. For the identification of infected herds, the buffered Brucella antigen tests (BBAT), i.e. the buffered plate agglutination test and Rose Bengal plate agglutination test are more reliable in practice than are other conventional tests, such as complement fixation and standard agglutination tests, and are therefore recommended. The procedures for the BBAT are the same as those described in Chapter 2.3.1 of the OIE Manual, for bovine brucellosis. Other serological tests have also been developed, including indirect ELISA and competitive ELISA. The allergic skin test is also useful for identifying infected herds. In summary, it has been difficult to eliminate brucellosis in swine by test and removal, and a policy of slaughtering infected herds is preferred.
B. suis (biovar 2) vaccine has been used in China, with apparent good results in pigs and also in small ruminants and cattle for the prophylaxis of brucellosis. It is considered that confirmation of the results obtained in China is required before strain 2 vaccine can be recommended for general use.
During this phase, the magnitude and distribution of the problem should be determined, as described earlier.
Voluntary investigation of abortion and weak piglets incidents, and submission to a diagnostic laboratory for culture (passive).
Inspection of pigs for clinical signs, including especially orchitis (passive).
Sero-surveillance using buffered Brucella antigen tests as herd tests only (active).
Brucellin tests also used to identify infected herds (active).
Sampling of in-contact feral swine (active).
Percentage of abortion and other tissues from which B. suis was isolated (passive).
Bacteriological examination of tissues (mandibular, gastrohepatic, internal iliac and inguinal lymph nodes) and blood for serological examination from breeding age swine at slaughter (active).
No data is available from countries using vaccines against swine brucellosis to support any specific sero-surveillance programme. Off-farm surveillance as in Phase I. It could be that vaccines have demonstrated good efficacy, but they have never been used extensively in swine.
Because none of the existing serological tests are reliable in individual pigs, buffered Brucella antigen tests (including the Card Test) are used to diagnose herd infections. Nevertheless, some countries do attempt herd eradication by testing all eligible animals (usually >6 months of age) every 30 days, and remove reactors for slaughter, continuing until the entire swine herd is negative. If this option fails, then depopulation (sell to slaughter) followed by repopulation with animals from brucellosis-free herds is carried out 30 days after the buildings have been cleaned and disinfected. An alternative option is to carry out offspring segregation, where female pigs (gilts) are separated from dams at approximately one month of age and reared separately. These animals should be tested 30 days prior to breeding.
Abortion incidents, movement tests, trace backs, adjacent herds, and epidemiological investigation of infected herds can be monitored as in bovine brucellosis.
Providing it is possible to trace back to the herd of origin from markets or slaughter using temporary or permanent identifiers, all swine of breeding age should be routinely tested.
If feral pigs are in contact, then continue random testing.
Periodic bacteriological surveillance of reactor pigs from infected herds or randomly selected pigs or herds at slaughter monitored for isolation of B. suis as in Phase 1.
The OIE International Animal Health Code does not prescribe conditions for country freedom for swine brucellosis, but a number of countries are free or are in the process of attaining freedom.
OIE defines a herd as free from porcine brucellosis if it can satisfy the following requirements:
Although not stated, these herds should not have any direct contact with feral swine.
Breeding swine herds (all animals >6 months) can be validated as brucellosis free if:
Randomly test 25% of the swine every 3 months or 10% every month and all are sero-negative.
No swine may be tested twice in a year.
To maintain freedom status, herds should be retested every 12 months.
Surveillance should be maintained for clinical signs.
All movements into Brucella-free herds should either be from Brucella-free herds or, if not, the animals should be sero-negative 30 days prior to movement, isolated on arrival and retested 30-60 days later. If artificial breeding is used, all semen should come from boars in Brucella-free herds.
Periodic bacteriological and serological testing of any reactor or suspect swine sent for slaughter.