A national veterinary service statement said 19 animals had been infected in the village of Valche pole where 65-year-old Rosa Ivanova fell ill with the rare disease late Sunday.
Veterinarians were examining samples from the animals Monday to see where the infection came from.
The disease, called brucellosis, is caused by contact with animals carrying the bacteria or through consumption of infected meat, milk or cheese.
The symptoms of the disease resemble flu-like conditions, including weakness, fever, sweating, headache, and joint pains, and is difficult to diagnose.
It can be fatal for people but is not transferred from human to human.
Brucellosis is rare in Bulgaria with only isolated cases for the past 50 years, veterinary service chief Zheko Baychev told the 24 Hours newspaper Monday.
"Most probably the infection spread from neighbouring Greece as flocks sometimes mingle during grazing in the border area between the two countries," the veterinary service statement said.
In 2005, two Bulgarian shepherds working in a border Greek farm died of the disease while other workers complained of the extremely bad veterinary hygiene conditions in the same farm and said they had seen many animals die from brucella-like conditions there.
All the infected animals in Valche pole will be culled to prevent a further spread of the disease in Bulgaria, the veterinary service said.