Effectivity of detection of new infections in the Dutch Leptospira Hardjo Control Program
Objectives
In 2020 and 2021, the number of outbreaks of L. Hardjo in the Netherlands seemed to increase as compared to the preceding years. In the Netherlands, an L. Hardjo-free control program (LHCP) is in place and since 2005 only farms with an L. Hardjo-free status are allowed to deliver milk to Dutch dairy companies. Nowadays, the Dutch dairy population is free of L. Hardjo, with only sporadic cases.
In this study we evaluated the national LHCP in the context of early detection of new infections and prevention of between herd transmission of L. Hardjo during 2017-2021.
Materials and methods
The LHCP is based on 1) testing of bulk milk samples three times a year, 2) testing of post-movement sera from cattle introduced from herds of origin without L. Hardjo-free status, and 3) testing of clinically suspected animals, including aborted cattle.
Cases of new infections in herds with an L. Hardjo-free status in the LHCP were described. The risk factors purchase, import and local transmission were evaluated. In a cluster evaluation, it was investigated whether between-herd transmission occurred between neighbouring herds.
Results
• An indication for an infection was detected 144 times in 120 dairy herds between 2017 and 2021.
• For 21 of these herds, this indication for an infection overlapped in time and space (radius <5 kilometre). However, in all of these cases the infection could be traced back to the purchase of infected cattle, indicating that infections never led to local transmission between dairy herds.
• In 26 of the 120 herds, 0.2% of total number of participants (15,000), the new infection led to withinherd transmission, but local transmission was not observed in these cases.
• Most cases occurred in 2019-2021. Many of these caseherds either imported or purchased cattle from herds with a lower L. Hardjo-free status.
Conclusions
Purchase and import of cattle of non-free herds appeared the main risk factors for new infections. Despite high numbers of imported and purchased cattle, the number of new infections remained low, with an incidence of 0.2% over a five year period (2017-2021). Due to early detection of new infections, the role of local transmission was considered minimal. It is therefore concluded that the national LHCP in the Netherlands is effective in the control of L. Hardjo infections.