Salmonella Dublin - Bulk Tank Serology
Salmonella Dublin outbreaks have been commonly diagnosed again this autumn. Many herds in the Dumfries area are endemically infected with some either currently or historically using vaccination as a control tool. Other herds will be naïve and both types of herd are potentially at risk depending on management and biosecurity factors. Our opinion is that Salmonella Dublin associated disease in dairy herds will be under diagnosed if based on just monitoring for typical clinical signs in cows and calves with our without bacteriology.
We now offer bulk tank Salmonella Dublin serology testing and if used routinely this can help take some of the guess work out of health planning for Salmonella Dublin allowing control measures to be implemented before significant clinical disease becomes apparent.
Some facts about how the test performs and can be used are outlined below:
- Used as a one-off test, the herd level sensitivity is 79% if there are clinical signs of infection in the adult herd, and 31% if clinical signs are only present in the calves
- Therefore, if disease is seen predominantly in the youngstock, it may be appropriate to combine bulk tank testing with serology of calves aged greater than 3 months
- Four samples taken three months apart has a herd level sensitivity of 95% and specificity of 83% to detect a herd level prevalence of 5%. Quarterly sampling can be used to monitor the presence of infection within a herd, and to assess the impact of control measures on disease prevalence
- Four negative results three months apart are required for a herd to be considered to have a herd level prevalence of less than 5%. The average time taken for Danish herds to move from a positive to negative herd status, based on bulk tank sampling was two years
- If the antibody level (percentage positivity) of the most recent sample exceeds the average of the previous three by 20%, this acts as an early warning for increased disease risk, and hygiene and management procedures should be reviewed
- A positive bulk tank result may be due to infection or vaccination, and so this test is not suitable for use in vaccinating herds. False positive results can occur where herds are infected with other B or D group Salmonellae, although in these cases, antibody titres tend to be lower than for herds infected with Salmonella Dublin
Another potentially useful indication for Salmonella Dublin bulk milk testing is for calf rearers, looking to source higher health status calves from the dairy herd. Salmonella Dublin can be a significant cause of disease and mortality in calf rearing units, particularly for units multi-sourcing calves. At the age at which these calves are purchased, screening them individually is not usually possible, because of the presence of maternally derived antibodies. Screening the bulk tank from the herd of origin would enable a more informed decision to be taken, and high bulk tank titres for Salmonella Dublin have been linked to poor calf health.
If you are interested in more details on Salmonella Dublin control, please refer to a recent In Practice article written by vets from the Dumfries centre.
If wish to discuss approaches to Salmonella Dublin control, please get in touch with us at the lab.