The Ministry of Agriculture and Rural Development announced Saturday the recurrence of blue ear disease with nearly 40 killed in the northern province of Hai Duong.
The Department of Animal Health under the ministry said the disease had returned "after a long time after it was successfully contained nationwide."
In June last year the department had said that Vietnam has successfully curbed the spread of the foot-and-mouth disease, bird flu and the blue ear pig diseases.
A total of 374 pigs in Tu Ky and Binh Giang districts have been affected by the latest outbreak.
All the dead pigs had been buried with officials' instructions.
The department has asked farmers to keep close watch over their herds to recognize the disease as soon as possible. It has also advised local governments to tighten controls over the slaughter and transport of pigs.
Blue ear is considered a dangerous disease that spreads quickly and causes heavy damage.