Role of RB51 vaccine in cow milk contamination of traditional milk sales centers.

Brucellosis poses a major health and economic challenge in endemic areas, especially in Iran. This study aimed to determine the strains and epidemiological features of Brucella isolated from raw milk at traditional milk sales centers in western Iran. A total of 208 raw milk samples were collected from 104 dairy sales centers in Hamadan province during two sampling periods in 2023. The Milk Ring Test (MRT), bacterial culture, and AMOS-PCR assays were utilised to detect Brucella. MRT yielded no positive results, whereas bacterial culture detected Brucella in 9 samples (4.3%). Of these, one sample contained B. abortus biovar 3; another had both B. melitensis biovar 1 and B. abortus biovar 3, while seven samples had the RB51 vaccine strain. AMOS-PCR detected Brucella in 3.8% (8/208) of the samples. Most positive samples, mainly of the RB51 strain, were found in Hamadan, representing 44.5% of cases. MRT showed perfect specificity but no sensitivity compared to culture, while PCR demonstrated high sensitivity, specificity, and overall agreement, making it a more reliable diagnostic tool. The presence of Brucella RB51 in milk suggests that vaccinated cows may excrete the strain under certain conditions, posing a potential risk to human health if unpasteurized dairy is consumed. This emphasizes the need for better brucellosis prevention and control measures in endemic regions, particularly to protect against the risks related to consuming raw milk from vaccinated cows.