Epidemiological assessment of bovine brucellosis in Iranian dairy farms: regional Seroprevalence and demographic indicators (2023-2024).

Brucellosis remains a significant challenge in Iran's dairy industry, with varying prevalence across provinces. This retrospective study (March 21, 2023-March 20, 2024) assessed bovine brucellosis seroprevalence across all regions of Iran (32 provinces), following Iranian Veterinary Organization (IVO) ethical guidelines. A total of 1,555,086 blood samples were collected from 612,558 dairy cattle at 4,586 industrial dairy farms, indicating that some animals were sampled more than once during the study period. Blood samples were screened using the Rose Bengal Plate Test, confirmed by Wright and 2-ME tests; milk samples were cultured for Brucella. Additionally, 248 milk samples were collected from seropositive farms. Overall, 7.6% (350 farms) had seropositive cases, with the highest rates in Qom (22.8%), Alborz (20.5%), Isfahan (18.1%), and Yazd (14.7%), which showed the highest farm-level prevalence. By contrast, provinces including South Khorasan (0.6%), West Azerbaijan (1%), and Kurdistan (1.1%) showed the lowest farm-level prevalence with positive cases. Provinces with zero positive cases at the farm level include Sistan and Baluchestan, Gilan, South Kerman, Mazandaran, and Hormozgan. Based on our study, the national average prevalence at the animal level in industrial dairy cattle is 0.4%, and the reactor to test percentage is 0.1%. Our analysis showed the highest animal-level prevalence in Qom (2.1%), Yazd (1.6%), and Fars (1.5%). Moreover, the highest reactor to test percentage was in Yazd (1.0%), followed by Qom and Khuzestan (0.5%). Holstein cattle were most affected (76.1%), followed by crossbreds (23%), with minimal infection in Simmental (0.7%) and Jersey (0.1%). Of the infected cattle, 79.6% were born on the farm, and 20.4% came from outside sources. Moreover, 82.2% of reactor cows were nursing, and 49.7% were pregnant at testing. Reproductive issues among positive reactors were noted: 17.1% had abortions, 1.3% had stillbirths, and 4% had weak calves. Vaccination records showed that 68.7% of reactors had received a full dose of the RB51 vaccine, while 31.3% were unvaccinated. Furthermore, 65% had been vaccinated with reduced doses of the RB51 vaccine, and 35% were unvaccinated. Brucella abortus and Brucella melitensis were found in 7.7% and 7.1% of milk samples, respectively. These results highlight the urgent need for stricter biosecurity, movement controls, and better vaccination to manage brucellosis in Iranian dairy farms.