Application of the FTA elute card coupled with visual colorimetric loop‐mediated isothermal amplification for the rapid diagnosis of Streptococcus agalactiae in farmed tilapia (Oreochromis niloticus)

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Abstract
A method combining the FTA Elute card and visual colorimetric loop‐mediated isothermal amplification (FTA‐e/LAMP) was tested to diagnose Streptococcus agalactiae infections in vitro and in vivo. FTA‐e/LAMP consists of two main steps: first, the FTA card is used to extract DNA and then a colorimetric loop‐mediated isothermal amplification (LAMP) reaction is carried out on the extracted DNA. In vitro sensitivity was 1.9 x 102 CFU/mL, and regarding specificity, all nine S. agalactiae strains tested positive. All Streptococcus spp. tested negative, except for S. dysgalactiae, thereby indicating the need for another set of primers to distinguish this species from S. agalactiae. To diagnose S. agalactiae infections using FTA‐e/LAMP in vivo, two experimental trials on juvenile Oreochromis niloticus infected with bovine or piscine strains were carried out. Sensitivity in symptomatic fish was 100%, and 50.7% of fish without signs were positive. All negative control fish tested negative (n = 28). No bacteria were detected after 16 days post‐infection (dpi). Accuracy during the first week (1–7 dpi) was 89% and decreased to 44% thereafter (10–22 dpi). FTA‐e/LAMP results suggest that this method is a promising tool for early and fast diagnosis of S. agalactiae on tilapia farms.

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