Problems in the serodiagnosis of canine brucellosis: dog responses to cell wall and internal antigens of Brucella canis.

Three procedures are commonly used for the serodiagnosis of B. canis infection: 1) The rapid slide agglutination test (RSAT), 2) the tube agglutination test (TAT), and 3) the modified 2-mercaptoethanol tube agglutination test (2ME-TAT). Hemocultures are always essential for diagnosis. The RSAT was developed to provide a presumptive diagnosis rapidly. It has been found accurate in identifying non-infected dogs; however false positive reactions are common due to shared determinants between the surface antigens of B. canis and certain other gram-negative bacteria. The RSAT has recently been modified to include brief reaction of test sera with 2ME (0.2M) prior to adding test antigen. The modification has improved specificity, but it has not eliminated false positive reactions. The TAT and 2ME-TAT are widely used. Although there is good agreement between tests, both suffer from lack of specificity but are valuable in kennels where B. canis has been identified by blood cultures. Agarose gel immunodiffusion (AGID) tests using extracted (SDC or hot PBS) cell wall antigenic complexes reveal precipitins in the sera of infected dogs usually 8 to 12 weeks postinfection (PI) that may persist 5 years. The antigenic complexes consist of at least 3 antigens; one (antigen '2R') appears to possess high B. canis specificity. Sera from noninfected dogs also may react nonspecifically in AGID tests that employ crude SDC or PBS antigenic extracts leading to 'false positive' interpretations. Cytoplasmic antigens gave up to four precipitin lines with sera from B. canis infected dogs. The antigens (protein or glycoprotein) were present in both S and R Brucella cells, but not in other gram-negative organisms examined. AGID tests that employed cytoplasmic antigens revealed precipitins against one or more (usually 2-3) antigens from PI months 4 through 64. In some dogs, precipitins were present 12 months after the bacteremia had ceased, a time when other tests were diagnostically insignificant, or equivocal. No 'false positive' field sera reached with the cytoplasmic antigens.