Salmonella Enteritidis colonization in laying hens : diagnosis, population dynamics and surveillance

Salmonella is one of the major causes of bacterial gastro-enteritis of humans, and the consumption of eggs contaminated with Salmonella Enteritidis (S. Enteritidis) is known as an important source of these infections. Because consumers want to use raw eggs, public health organizations and retailers want to reduce the risk of human salmonellosis without treatment of the raw product. Consequently, control measures should be taken on primary farm level. Total eradication of Salmonella spp. from the environment is not to be expected, however, and the risk for introduction of the bacteria on laying hen farms most likely cannot be reduced to zero. Therefore, it is important to detect a colonized laying hen flock as soon as possible by adequate surveillance. The probability of detection of a colonized flock depends, amongst others, on the characteristics of the tests used for detection of S. Enteritidis in samples from laying hens. In this thesis, the quality of an immunological test using egg was examined. This diagnostic test offers the opportunity to perform multi-analyte analysis, i.e. to combine Salmonella surveillance with other surveillance objectives, like avian influenza, in a single run. Moreover, because sampling can be done automatically at egg-packing stations semi-continuous surveillance of laying hen farms could be performed. The efficacy of a surveillance programme also depends on the within-flock prevalence, which changes over time since introduction of the bacteria into the flock. For development or optimization of surveillance knowledge is required of how long it takes to reach a certain prevalence which in turn depends on the within-flock dynamics of S. Enteritidis. Transmission dynamics were studied in experiments with pairs and groups of laying hens, the latter being more representative for the practical situation. Control measures on primary farm level, like vaccination and hygiene measures, in combination with targeted testing have been associated with a lower reported incidence of S. Enteritidis in egg-laying flocks as well as in humans. By combining knowledge of transmission within flocks and characteristics of detection in surveillance, a further reduction of laying hen-related human exposure to S. Enteritidis may be obtained.