Latvia's hygiene authority, the Food and Veterinary Service for now has not established the source of the plague, the services's director Maris Balodis said on the morning news program of the LNT commercial television on Monday.
Rukas farm in Krimulda with 5,023 animals is the largest pig farm in Latvia affected by ASF so far.
"This is the biggest crisis that Latvia has had so far as concerns ASF" Balodis said, adding that, until now, the disease had struck only small farms.
This is also the first time in Latvia when domestic pigs have been infected with ASF during winter.
The farm had a high biosafety level which had been attested to by inspectors from both the PVD and the European Commission, Balodis said, adding that the European Commission had even held a training session in Rukas in the past.
For now the source of infection is unclear and the incident is being investigated not only by the veterinary authority but also by the Latvian State Police.
So far this year, African swine fever has been found in 43 wild boars, but this is the first case this year the infection has been established in domestic pigs.
In 2016, ASF was discovered among domestic pigs in three Latvian farms - one in Varkava region and two in Gulbene region - where 50, 142 and 119 pigs respectively had to be culled in order to contain the disease. Last year the plague was also found in 1,146 wild boars across Latvia.
The ASF outbreak started in Latvia in June 2014 close to the border with Belarus.
African swine fever is an extremely dangerous and contagious virus infection affecting pigs. If an infected pig is found in a farm, all pigs in the particular farm have to be culled, which means big losses for farmers.
Prime Minister Maris Kucinskis called a crisis meeting January 16 to discuss the outbreak and how to deal with it.