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.