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Threshold infection level is the pathogen load of the test animals measured before the appearance of clinical signs and mortality. This study aims to establish the threshold infection levels of WSSV in different weight ranges of Penaeus vannamei using qPCR. Artificial infection experiments were conducted using four weight ranges (3–5 g, 7–8 g, 15–18 g, and 22–25 g). The LD 50 of the different weight ranges of shrimps were achieved at viral dilution of 10 -6 and 10 -5 after 216–240 hpi, and the viral loads of these inoculums have a range of 10 5 –10 6 WSSV DNA copies/g. The viral loads of the samples in the timecourse infection experiments when the mortalities started was determined at 10 9 WSSV DNA copies/g, while for the survivors was at 10 6 WSSV DNA copies/g. The threshold infection level of WSSV in shrimp was determined at 10 7 to 10 8 WSSV DNA copies/g. It was also found out that the threshold infection level was not weight dependent. ; We are very grateful to the Government of Japan-Trust Fund (study code: FH-07- F2015T) and SEAFDEC/AQD for funding this work. We thank Mr. Ivan Yaptangco of IJY Shrimp Farm for providing the experimental animals and the staff of Fish Health Section Diagnostic Services for the technical support.

Threshold infection level is the pathogen load of the test animals measured before the appearance of clinical signs and mortality. This study aims to establish the threshold infection levels of WSSV in different weight ranges of Penaeus vannamei using qPCR. Artificial infection experiments were conducted using four weight ranges (3–5 g, 7–8 g, 15–18 g, and 22–25 g). The LD 50 of the different weight ranges of shrimps were achieved at viral dilution of 10 -6 and 10 -5 after 216–240 hpi, and the viral loads of these inoculums have a range of 10 5 –10 6 WSSV DNA copies/g. The viral loads of the samples in the timecourse infection experiments when the mortalities started was determined at 10 9 WSSV DNA copies/g, while for the survivors was at 10 6 WSSV DNA copies/g. The threshold infection level of WSSV in shrimp was determined at 10 7 to 10 8 WSSV DNA copies/g. It was also found out that the threshold infection level was not weight dependent. ; We are very grateful to the Government of Japan-Trust Fund (study code: FH-07- F2015T) and SEAFDEC/AQD for funding this work. We thank Mr. Ivan Yaptangco of IJY Shrimp Farm for providing the experimental animals and the staff of Fish Health Section Diagnostic Services for the technical support.