Diagnosis of Brucellosis in Livestock and Wildlife

Aim

To describe and discuss the merits of various direct and indirect methods applied in vitro (mainly on blood or milk) or in vivo (allergic test) for the diagnosis of brucellosis in animals.

Methods

The recent literature on brucellosis diagnostic tests was reviewed. These diagnostic tests are applied with different goals, such as national screening, confirmatory diagnosis, certification, and international trade. The validation of such diagnostic tests is still an issue, particularly in wildlife. The choice of the testing strategy depends on the prevailing brucellosis epidemiological situation and the goal of testing.

Results

Measuring the kinetics of antibody production after Brucella spp. infection is essential for analyzing serological results correctly and may help to predict abortion. Indirect ELISAs help to discriminate 1) between false positive serological reactions and true brucellosis and 2) between vaccination and infection. Biotyping of Brucella spp. provides valuable epidemiological information that allows tracing an infection back to the sources in instances where several biotypes of a given Brucella species are circulating. Polymerase chain reaction and new molecular methods are likely to be used as routine typing and fingerprinting methods in the coming years.

Conclusion

The diagnosis of brucellosis in livestock and wildlife is complex and serological results need to be carefully analyzed. The B. abortus S19 and B. melitensis Rev. 1 vaccines are the cornerstones of control programs in cattle and small ruminants, respectively. There is no vaccine available for pigs or for wildlife. In the absence of a human brucellosis vaccine, prevention of human brucellosis depends on the control of the disease in animals.Brucellae are Gram-negative, facultative intracellular bacteria that can infect many species of animals and man. Ten species are recognized within the genus Brucella. There are 6 “classical” species: Brucella abortus, Brucella melitensis, Brucella suis, Brucella ovis, Brucella canis, and Brucella neotomae (1,2). This classification is based mainly on differences in pathogenicity and host preference (3). Distinction between species and between biovars of a given species is currently performed using differential tests based on phenotypic characterization of lipopolysaccharide (LPS) antigens, phage typing, dye sensitivity, requirement for CO₂, H₂S production, and metabolic properties (1,2).The main pathogenic species worldwide are B. abortus, responsible for bovine brucellosis; B. melitensis, the main etiologic agent of ovine and caprine brucellosis; and B. suis, responsible for swine brucellosis. These 3 Brucella species cause abortion (“abortion storm” in naive heifers), and when brucellosis is detected in a herd, flock, region, or country, international veterinary regulations impose restrictions on animal movements and trade, which result in huge economic losses. These are the reasons why programs to control or eradicate brucellosis in cattle, small ruminants, and pigs have been implemented worldwide (4).B. ovis and B. canis are responsible for ram epididymitis and canine brucellosis, respectively. In the case of B. neotomae, only strains isolated from desert wood rat (Neotoma lepida) in North America have been reported. Recently 4 new Brucella species have been described: Brucella pinnipedialis and Brucella ceti, isolated predominantly from seals and cetaceans, respectively (5); Brucella microti, isolated from common voles (Microtus arvalis) (6), soil (7), and foxes (Vulpes vulpes) (8); and Brucella inopinata, isolated from a breast implant (9).There is a general host restriction pattern among the different Brucella species, meaning that different Brucella species infect different preferred hosts. Even within the B. suis species, different biovars preferentially infect different animal host species (1-3). Indeed, B. suis biovars 1 and 3 infect suidae, biovar 2 infects suidae and hare (Lepus europeanus), biovar 4 infects reindeer (Rangifer tarandus tarandus) and caribou (Rangifer tarandus granti), and biovar 5 has been isolated from rodents in Russia.All Brucella species may also infect wildlife species. Classical Brucella species have been isolated from a great variety of wildlife species such as bison, elk, feral swine, wild boar, fox, hare, African buffalo, reindeer, and caribou (10). In order to implement appropriate control measures to address wildlife brucellosis, it is very important to distinguish between a spill-over of infection contracted from domestic animals and a sustainable infection (10). In the latter case, the concern of the livestock industry is to prevent the re-introduction of the infection in livestock (spill-back), particularly in regions or states that are “officially brucellosis-free.” If the status of “officially brucellosis-free” is lost, domestic animals must be tested prior to being traded, which imposes huge costs. This is exemplified by recent episodes of cattle being infected with B. abortus transmitted by elk in the Greater Yellowstone Area in the USA (11) and of outdoor reared pigs infected with B. suis biovar 2 transmitted by wild boar in France (12).Brucellosis is an established zoonosis: infections have been attributed to at least 5 of the 6 classical Brucella species in terrestrial mammals. Studies from around the world indicate that elimination of the animal brucellosis reservoir has resulted in a substantial decline in the incidence of human disease (13). Currently, laboratory workers are among those most frequently infected (14). Marine mammal strains of Brucella have been reported to cause the infection of a laboratory worker in the UK (15), as well as naturally-acquired infections in Peru (16) and New Zealand (17).Brucella species and biovars, preferential hosts, and pathogenicity for humans are depicted in