Reuters reports that the outbreak was detected at a farm in the village of Balgarsko Slivov, near the Danube town of Svishtov. More than 100,000 pigs have already been culled at another four farms in the past two weeks.
European Union member Bulgaria has so far detected more than 20 outbreaks of African swine fever at industrial or backyard farms in the north of the country.
Bulgarian President Rumen Radev was meeting on Wednesday with veterinary and other experts tasked with taking action to tackle the spread of African swine fever.
Analysts said that the price of pork in Bulgaria has increased by up to 30 percent in less than a month because of outbreaks of the disease. The price could rise a further 15 percent in the autumn, they said.
On Monday 20-km sanitary zones were being set up around all 62 registered industrial pig farms in the country and home-breeding of pigs without bio-security measures will not be allowed within the zones.
Authorities have already declared a state of emergency in several districts in northern Bulgaria, where there have been outbreaks among home-raised pigs.
Bulgaria has mobilised military and police forces to help combat the disease and protect its pig breeding industry.