We report a retrospective analysis of 1933 Brucella strains isolated from humans and animals in Latin American countries between 1968 and 1991 and in Argentina between 1994 and 2006. During the first period 50% of strains were from humans, mainly from Argentina, Mexico and Peru but, while B. suis was the main cause of infection in Argentina, B. melitensis was responsible for most infections in the other countries. In Argentina in the later years, B. melitensis and B. suis were observed more frequently than in the first period while isolation of B. abortus decreased. Of 145 B. melitensis human isolates, eight gave susceptibility patterns to dyes and penicillin and two were B. melitensis biovar 3, which has never been reported in animals. Forty-six percent of B. suis isolated were resistant to dyes which is an atypical feature in this species.

The zoonotic risk of Brucella canis has been considered fairly high for persons who handle breeding dogs in kennels or are exposed to infected animals. Transmission to humans in other circumstances has been thought to be rare. We describe an uncommon outbreak of brucellosis caused by B. canis which, to the best of our knowledge, is the first reported in the literature. This outbreak involved six persons (three children and three adults), a bitch and three puppies which had close daily contact with the family. The clinical symptoms of the index case led to an erroneous diagnosis and the infection would have gone undiagnosed if culture had not been positive. This report aims to increase awareness of medical personnel of the need to order screening tests for children, immunodeficient persons or pregnant women presenting with fever of unknown origin, unexplained spleen or liver enlargement or other systemic signs. The emerging zoonotic potential of this disease in urban areas and the need to coordinate canine brucellosis surveillance systems should be evaluated.

The transmission of Brucella canis to man commonly occurs through contact with infected dogs or their secretions, or through direct laboratory exposure. The disease is underdiagnosed due to a general lack of serological testing facilities and misconceptions concerning its prevalence. This report shows the potential use of an indirect ELISA (IELISA) for the diagnosis of human brucellosis caused by B. canis in a population of patients negative by smooth-Brucella antigen tests but positive by rapid slide agglutination test (RSAT). One hundred and ten sera from asymptomatic people found negative by tests using smooth Brucella abortus antigen and by RSAT showed an IELISA specificity of 100 % when a cut-off value of 27 % positivity (%P) was selected. For 17 sera from patients with positive B. canis culture or in close contact with culture-positive dogs, the IELISA sensitivity was 100 % with the same cut-off value. The positive patients presented clinical symptoms similar to brucellosis caused by other species of Brucella and some of them received antibiotic treatment and made good progress. Using this cut-off value, we studied 35 patients with negative blood cultures but positive RSATs, and IELISA detected 18 as positive; of the 17 IELISA-negative, two were RSATpositive at dilution 1 : 2 and 15 were weakly positive with pure serum. These samples were probably from patients at an early stage of infection or indicate false-positive results. No cross-reaction was observed among the sera from nine cases with a diagnosis other than brucellosis, but crossreactivity was evident in sera from patients infected with smooth-Brucella species. Since routine brucellosis diagnosis does not include B. canis investigation, infection with this species may be more widespread than is currently suspected. The RSAT could be a suitable screening test for the diagnosis of B. canis human brucellosis, and a supplementary technique, such as IELISA, performed on all positive RSAT samples that were negative by B. abortus antigen could ensure diagnostic specificity and confirm the diagnosis.

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