Epidemiology of brucellosis in sheep and goats across different production systems in regions with high human brucellosis prevalence.

Brucellosis remains a major zoonotic threat in Iran, where sheep and goat farming play a crucial role in agricultural production. This study examines the relationship between vaccination coverage and brucellosis prevalence at animal and herd levels across Iran's diverse small ruminant production systems. Our cross-sectional study (2021-2024) included 389,225 animals from 3929 herds across rural, traditional, and nomadic systems. Tests such as the Rose Bengal Plate, Wright, and 2-mercaptoethanol (2-ME) detected brucellosis, and bacteria from clinical samples were isolated. Multivariable mixed-effects logistic regression was used to analyze factors such as vaccination coverage, production system, species, and brucellosis positivity. Results showed low vaccination coverage across all systems (8-19.1%), well below the threshold for herd immunity. Traditional farms had 3.4 times higher odds of animal- level brucellosis (95% CI: 2. 98-3. 93, p < 0. 001) and 2.9 times higher odds of herd- level positivity (95% CI: 2. 21-3. 78, p < 0. 001) compared to rural areas, while nomadic systems exhibited scattered, episodic foci. A 1% increase in vaccination coverage was associated with an odds ratio of 0.94 for herd-level brucellosis positivity (p = 0.02) and an OR of 0.96 for animal-level brucellosis positivity (p = 0.06), indicating a non-significant tendency. Goats had higher vaccination coverage than sheep. Most positive cultures originated from aborted fetuses, which were the main source of Brucella melitensis biovar 1. This study indicates that vaccination coverage in Iran is inadequate to control brucellosis, especially on traditional farms where vaccination's protective effects are most noticeable.