An alternative serological test for the diagnosis of Johne's disease
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Johne’s disease (JD), also known as paratuberculosis, causes a chronic intestinal infection in ruminants and is caused by Mycobacterium avium subsp. paratuberculosis (MAP). Due to Johne’s disease, there is a decrease in milk yield, which causes significant production losses. Since there is no cure available for animals infected with MAP, the main focus of management programs for Johne’s disease is prevention of infection and detecting infected animals with diagnostic tests. The issue with the current serological tests is that they have a low sensitivity. Lately, a paper has been published by Eda et al. (2005) in which a new test has been suggested: the Flow Cytometry Method (FCM). The objective of this study was to make a comparison between the described FCM and the ELISA (IDEXX) to determine the sensitivity and specificity of the FCM as an alternative diagnostic test for Johne’s disease. The hypothesis was that the FCM would have a sensitivity and specificity similar to the ELISA (IDEXX). FCM was optimized in our lab using samples from an infection trial as well as from herds with a known JD status. The results measured by flow cytometry are displayed in graphs. When a sample is negative, a peak in the histogram can be seen above the zero on the x-axis, and thus the histogram will be on the left side of the graph. When a sample is completely positive, the histogram will shift towards the right side of the graph. A wide variety of positivity can be seen in different samples. It was decided to use a cut-off of 70% for the FCM, since this cut-off produced the highest sensitivity (93%) and the highest specificity (84%) for the FCM. When the FCM was compared to the ELISA (IDEXX), the kappa value had an outcome of 0,7760. The positive predictive value is 85% and the negative predictive value is 93%. The FCM is a good candidate to be developed into a sensitive diagnostic test for detection of MAP infection, but further research still needs to be conducted.