zoonoses and communicable diseases common to ... - PAHO/WHO

BRUCELLOSIS 57lance, and (4) screening tests. A single test might serve as operative, as diagnosticallydefinitive, as a screen, or as complementary, depending on the program employing it.Serum agglutination tests (tube and plate) have been and continue to be widelyused. They contributed greatly to the reduction of infection rates in Europe, Australia,and the Americas. Nevertheless, when the proportion of infected herds and worldwideprevalence is reduced, their limitations become apparent in so-called “problem”herds and it becomes necessary to use other tests to help eradicate the infection. Thetests are internationally standardized, easy to carry out, and allow the examination ofa great many samples. In agglutination tests, the IgM reaction predominates. In animalsclassified as suspect or marginally positive, complementary tests are used toclarify their status. However, it is necessary to keep in mind that low agglutinationtiters could be due to recent infection and it is thus advisable to repeat the test.The rose bengal test (with buffered antigen) is fast, easy, and allows processing ofmany samples per day. It is qualitative and classifies animals as positive or negative.In regions where incidence of infection is low or where systematic vaccination ofcalves is practiced, the rose bengal test gives many “false positives,” and so is unspecificif used as the only and definitive test. In many countries, such as Great Britainand Australia, it is used as a preliminary or screening test. Animals showing a negativetest result are so classified and those testing positive are subjected to other testsfor confirmation. In regions of high incidence, results are very satisfactory. Rosebengal may also be used as a complementary test for those animals classified as suspectby agglutination. Many suspect sera test negative to rose bengal, and since thistest is very sensitive (there are few “false negatives”) and detects the infection early,there is little risk of missing infected animals.The principal complementary tests are complement fixation, 2-mercaptoethanol,and rivanol. Other tests have been developed, such as indirect hemolysis, ELISA fordifferent types of immunoglobulins, and radial immunodiffusion with a polysaccharideantigen. All these tests are used to distinguish antibodies caused by the infectionfrom those left by vaccination or stimulated by heterospecific bacteria.Both the direct and the competitive ELISA tests are appropriate for diagnosis ofbrucellosis in all species according to the consensus of groups of experts that havemet several times in Geneva. WHO, with the collaboration of FAO, the InternationalOrganization of Epizootics, the International Atomic Energy Agency, and theseorganizations’ reference laboratories, is coordinating a project to evaluate and standardizethese assays as well as the antigens and other technical variables.In Australia, the ELISA technique and the complement fixation test have beenvery useful in recent phases in the eradication of bovine brucellosis, when many“problem herds” occur with “latent carrier animals.” In comparison with the CF test,ELISA revealed a significantly higher number of reactive animals in infected herds,both vaccinated (with strain 19) and unvaccinated, but gave negative results in herdsfree of brucellosis, whether vaccinated or not. The specificity of ELISA in the groupof infected herds was less than that of CF, but sensitivity—which is what wasneeded—was greater (Cargill et al., 1985). It costs less to eliminate some false positiveanimals in the final phase of eradication than to allow the infection to reassertitself and spread in the herd because one or more infected animals remained(Sutherland et al., 1986). The competitive enzymatic immunoassay also lends itselfto differentiating the reactions of animals vaccinated with strain 19 and animals naturallyinfected, using the O polysaccharide antigen (Nielsen et al., 1989).