3.1. Disease history and epidemiological factors
Between February 2019 and March 2021, 26 tilapia disease outbreaks, located in 9 provinces in northern Vietnam, were investigated. In tilapia-farming systems using earth-pond and floating cages in rivers in the delta region (Hai Duong, Thai Binh, Hung Yen, Bac Ninh, and Ha Nam provinces), disease outbreaks often occurred from July to October, while for the cage culture in reservoirs in mountainous provinces, such as Hoa Binh, Yen Bai, Tuyen Quang, and Son La, outbreaks occurred from December to March. Water temperature during disease outbreaks ranged from 23.3–29.1 ºC. The 26 affected farms included 5 earth ponds and 21 floating cage farms, with red tilapia (8 farms) and Nile tilapia (18 farms). Tilapia seeds included both imported (19 farms) and domestic (7 farms). The mortality estimated by farmers during disease outbreaks ranged from 30%–65% (Table 1). In all disease outbreaks, clinically sick fish showed gross signs of pale gills due to anemia; no clear external symptoms were observed except for a darkened body in Nile tilapia and pale color in hybrid red tilapia (at a low frequency). Internally, numerous white spots appeared on the spleen and head kidney and were occasionally observed in the liver. Hemorrhage or congestion in the liver was also recorded at a high frequency.