Animal health workers have uncovered 44 cases of brucellosis in cows grazing around the Volcán Turrialba.
The infected animals present a danger for persons who drink their unpasteurized milk.
The Servicio Nacional de Salud Animal said its workers checked 348 animals as part of the response to the eruptions of the nearby volcano. The animals were on 19 separate farms.
The area around the volcano in the province of Cartago is the location of many dairy farms.
The infected animals were found on three farms, and workers marked the animals so they can be slaughtered. The farms also were placed under quarantine.
Brucellosis can cause abortions in cows and is highly contagious. It is a bacterial infection.
Humans who become infected face a lengthy treatment with antibiotics and may carry the bacteria for the rest of their life. The type of brucellosis that infects cows also can infect bison and elk. Other strains can infect pigs and even dogs.
The disease is endemic in Costa Rica, said the animal health agency. Brucellosis can be prevented by a vaccination.
Humans also can get the disease by having infected milk spill on a cut or wound, said the agency.