Near infrared spectroscopy as a novel non-invasive tool for the detection of lactococcosis in rainbow trout

Lactococcosis, mainly sustained by the warm-water bacterium Lactococcus garvieae, is a disease associated with evident mortalities and economic lossed. Main disease signs are commonly bilateral exophthalmia, hyperpigmentation in the body, haemorrhages around the eye area, opercula, and mouth region, swollen abdomen, and anal prolapse. An effective monitoring system of fish diseases relies on rapid techniques for the timely management of infection outbreaks. We evaluated the potential of the SCiO sensor, a portable near infrared spectroscope, to assess in field the health status of rainbow trout. The study sample was composed of 20 symptomatic and 20 asymptomatic rainbow trout obtained from an aquaculture farm and submitted to molecular detection of L. garvieae and SCiO examination. In order to detect presence or absence of septicaemia condition, blood smears were prepared from peripheral blood. Part of the symptomatic individuals (9/20) showed septicaemia, whereas no bacteria were found in the asymptomatic individuals. PCR assay of bacterial DNA was positive in spleen and kidney of 19/20 symptomatic and in 1/20 asymptomatic individuals. Bacteriological analysis revealed L. garvieae in 18/20 symptomatic samples and in 2/20 asymptomatic samples. The SCiO sensor was found to be able to discriminate between healthy and sick fish [sensibility 0.95 (0.75–1.00 I.C. 95%), specificity 0.80 (0.56–0.94 I.C. 95%)]. By virtue of rapid acquisition, low cost and use on site, the SCiO was considered as a useful diagnostic tool for monitoring L. garvieae infection in rainbow trout.