3 Specimen collection for farm-level diagnosis of bacterial infections

The identification of a contagious agent on a farm often triggers infection control measures aimed at mitigating the economic impact of the disease on the whole farm. The need to diagnose contagious diseases at the group- or farm-level and less so at the individual animal-level is, therefore, a distinctive feature of farm animal medicine.

Bacteriological culture may yield a true negative result (an uninfected animal) or a false negative result (no bacterial growth from a specimen collected from an infected animal). False negatives may be due to:

• a non-viable agent
• an imperfect sensitivity of culture
• the bacterium not being present in the specimen (intermittent shedding), or present in numbers that are below the limit of detection (LoD) of culture – common situations in many infectious diseases.

Sampling strategies that can bypass these problems and may increase the testing regime sensitivity at the farm level are:

1. analysis of multiple specimens
2. pooling of specimens
3. aggregate testing.