Epidemiological Trends and Strain Distribution of Bovine Brucellosis in Vaccinated Industrial Dairy Cattle (2021-2024)

Brucellosis remains a significant zoonotic disease in Iran, affecting both public health and livestock productivity despite decades of control efforts. This study presents findings from a four-year nationwide active surveillance program (2021-2024) targeting industrial dairy cattle herds vaccinated with the Brucella abortus RB51 vaccine. The program encompassed 33,994 herds and over 6.2 million blood samples across 32 provinces, representing one of the most comprehensive surveillance initiatives conducted in the country. Serological screening was performed using the Rose Bengal plate test (RBPT), followed by Wright and 2-ME agglutination tests for confirmation. Lymph node cultures were obtained from 1261 seropositive animals and subjected to classical biotyping and AMOS-PCR for molecular identification. At the national level, both farm- and animal-level prevalence declined over the study period, highlighting progress in disease management. Farm prevalence peaked in 2022 at 9.7% before decreasing to 6.7% in 2024, while animal prevalence dropped steadily from 0.36% in 2021 to 0.11% in 2024. Similarly, suspected cases declined from 5.8% to 4.2% at the farm level and from 0.16% to 0.07% at the animal level. Provinces such as Isfahan, Tehran, Fars, Yazd, and Qom consistently reported the highest infection burdens, while several regions, including Hormozgan, Mazandaran, Gilan, and Sistan and Baluchestan, showed minimal or no positive detections. Bacteriological culture confirmed Brucella infection in 216 animals (17.1%), leading to the isolation of six strains: B. abortus biovars 1 and 3, B. melitensis biovars 1-3, and the vaccine strain RB51. The predominance of B. melitensis biovar 1, particularly in provinces such as Qom and Alborz, underscores the possible risk of cross-species transmission from small ruminants to cattle. Detection of RB51 in several isolates raises concerns regarding vaccine persistence and its limited protective efficacy against B. melitensis. This study demonstrates the persistence of multiple Brucella biovars and the circulation of vaccine-derived strains in vaccinated animals and highlights the need for refined vaccination strategies, stricter biosecurity, and integrated control programs across livestock sectors.