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.

GD2887/06-23

AHEAD IN

ANIMAL HEALTH

antibody positive cattle are detected removal of antibody positive cattle within 8 weeks after four weeks: bulk milk test or serological screening of 3 contact animals antibody negative antibody positive L. Hardjo Free status Treatment antibody negative antibody positive L. Hardjo Free status Treatment positive bulk milk screening conﬁrmative bulk milk test Monitoring L. Hardjo free status (routinely and risk based) results are test negative, monitoring continues I II III 0% 4% 8% 12% 16% 20% 2017 2018 2019 2020 2021 Percentage of dairy herds Purchase from Dutch dariy herd with lower status Total purchase from herds with lower status and/or import Import

Katrien M.J.A. van den Brink 1*, Marian Aalberts ², Nannet D. Fabri ² , Thomas Dijkstra 1 and Inge M.G.A. Santman-Berends 2 1Department of Cattle Health, 2Department of Research and Development, Royal GD, Deventer, The Netherlands

Figure 1: Control actions following detection of antibody positive

cattle or bulk milk screening in the Leptospira Hardjo Control Programme Figure 2: Percentages of dairy herds with a L. Hardjo-free status that purchased cattle from either herds with a lower L. Hardjo-free status and/or from herds abroad per year from 2017 to 2021.

*

In most cases, the introduction of cattle occurred in the same year as the detection of infection, for infections related to introduction of cattle.

Most plausible introduction route 2017 2018 2019 2020 2021 2017-2021 Cattle purchased from herds not participating in LHCP* Import 0 1 2 6 3 12 Purchase 0 0 0 1 2 3 Cattle purchased from herds participating in the LHCP (regardless of status)* Purchase 0 0 1 6 2 9 Not related to introduction of cattle 0 1 0 1 0 2 Total of new infections 0 2 3 14 7 26

Table 1: Number of new infections of L. Hardjo from 2017 to 2021 in the Netherlands classified by most plausible introduction route.

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