Effectiveness of the Leptospira Hardjo Control Programme and Detection of New Infections in Dairy Cattle in The Netherlands

SIMPLE SUMMARY: Leptospirosis in cattle is a zoonotic bacterial disease for which participation in a control programme is mandatory for dairy herds in the Netherlands. Nearly all dairy herds have an L. Hardjo-free status, and only sporadic cases of this disease occur on dairy farms. In this study, the effectiveness of the programme for early detection of new L. Hardjo infections between 2017 and 2021 were evaluated. During the analysed period, a suspected infection was detected 144 times in 120 dairy herds. Of these 144 introductions, only 26 lead to within-herd transmission of Leptospirosis. The main cause of the introduction of the disease was the purchase of cattle from herds without an L. Hardjo-free status. Additionally, there were no cases where the infection seemed to have spread locally. It was, therefore, concluded that the control programme was highly effective in the early detection and subsequent control of new infections. ABSTRACT: Since 2005, a mandatory L. Hardjo control programme (LHCP) has been in place for Dutch dairy herds. Almost 100 percent of dairy farms participate and have an L. Hardjo-free status. In 2020 and 2021, the number of outbreaks seemed to increase as compared to the previous years. In this study, we evaluated the effectiveness of the national LHCP in the Netherlands during 2017–2021. Cases of new infections in herds with an L. Hardjo-free status in the LHCP were described, including the role of risk factors for the introduction. Both the percentage of dairy herds with an L. Hardjo-free status that purchased cattle from herds without a free status and the number of purchased cattle increased over the years. A between-herd cluster evaluation showed that between 2017 and 2021, a suspected infection was detected 144 times in 120 dairy herds. In 26 cases (26 herds, 0.2%) new infections were identified, including within-herd transmission. No infection clusters were identified, indicating that infections never led to local transmission between dairy herds. The introduction of cattle from non-free herds appeared to be the cause of all L. hardjo infections in herds participating in the LHCP. Therefore, the national LHCP seems to be highly effective in the control of infections in dairy herds.