Emerging Salmonella serogroup C infections in Dutch veal calves
Maarten F. Weber, Judith E. de Jong, Tim A.J. Florax and Annet E. Heuvelink Royal GD, Deventer, the Netherlands.
Introduction
Salmonella enterica subsp. enterica serogroup C infections have emerged in Dutch veal calf herds since 2020. A more severe clinical course of serogroup C infections was observed in comparison to other serogroups. Consequently, the aim of this study was to obtain further insights into the epidemiology of serogroup C infections in Dutch veal calf herds.
Materials and Methods
Risk factors for serogroup C infections: Differences between risk factors for serogroup C infections and infections with other serogroups were identified in a case-case multilevel logistic regression analysis. The analysis was conducted on Salmonella spp. isolates from samples submitted from Dutch veal herds between April 2021 and March 2024. Potential risk factors examined included year, year quarter, herd size, calf owner, and the presence of calves from specific countries at the time of sample submission.
Fourier Transform Infrared Spectroscopy (FTIR): The initial serogroup C isolate per herd obtained between January 2022 and June 2023 in 23 veal calf herds was further typed using Fourier Transform Infrared (FTIR) spectroscopy cluster analysis.
Irish calves data: Based on results of the risk factor analysis, a causal association of serogroup C with the presence of Irish calves was suspected. Therefore, all Irish calves present between April 2021 and March 2024 in the study herds were traced back. For each Irish calf, the arrival date and the Irish herd of origin were retrieved.
1. Final multilevel logistic regression model for the probability of a Salmonella isolate belonging to serogroup C, in comparison with the total of other serogroups. Data from 479 isolates from samples submitted between April 2021 and March 2024 from 242 veal calf herds.
Results
Isolates: Between April 2021 and March 2024, Salmonella spp. were isolated from 479 samples collected from 242 veal calf herds: 285, 76, 117 and 1 isolates belonging to serogroups B, C, D and E, respectively.
Risk factors: After correction for calf owner, the presence of Irish calves in the veal calf herd and the presence of Dutch calves were significantly associated with the probability of isolates belonging to serogroup C (Table 1).
FTIR analyses and Irish calves data: Three FTIR clusters of multiple isolates were identified (Clusters 1, 3 and 4, Fig. 1). The occurrence of these clusters was found to be associated with specific weeks of shipment of Irish calves into the veal calf herds (Fig. 2). In clusters 1 and 3, no single herd of origin transferred calves to each of the veal calf herds in the cluster.
1. Clustering of 23 Salmonella serogroup C isolates cultured between February 2022 and June 2023, from 23 different Dutch veal calf herds, based on FTIR analysis.
Figure 2. Distribution by week of first arrival of Irish calves in Dutch veal calf herds. (A) All Irish calves present at any point during the study period in any of the 242 study herds from which Salmonella spp. was isolated during the study period, irrespective of serogroup (99,665 calves in 85 veal herds). (B) Irish calves present in a herd at the time of submission of a sample from which an isolate in cluster 1 was cultured (5693 calves in 9 veal herds). (C) Irish calves present in a herd at the time of submission of a sample from which an isolate in cluster 3 was cultured (4213 calves in 3 veal herds). (D) Irish calves present in a herd at the time of submission of a sample from which an isolate in cluster 4 was cultured (5030 calves in 6 veal herds). Red dashed lines indicate the start and end of the study period.
Discussion
• In clusters 1 and 3, no single herd of origin transferred calves to each of the veal calf herds within the cluster. Thus, the observed clustering could not be explained by a single herd of origin being the source of infection without transmission during transport.
• The observed effect of the presence of Irish calves on the occurrence of serogroup C infections, coupled with the occurrence of FTIR clusters in specific weeks of shipment of Irish calves, indicates that the calves became infected during transport from farms of origin to veal calf farms, and subsequently introduced the serogroup C infection into the veal calf herds.
• Spread of infection of other Salmonella serogroups and other pathogens during transport should be anticipated, in relation to other countries of origin as well.
• Further work is required to define the critical control points to minimize disease transmission during transport.
• Further work is required to quantify potential benefits of segregation of calves from different countries of origin in separate veal calf herds.
m.weber@gdanimalhealth.com www.gdanimalhealth.com
t.florax@gdanimalhealth.com
www.gdanimalhealth.com
Table
Figure