Yersinia enterocolitica serotype O:9 cultured from Swedish sheep showing serologically false-positive reactions for Brucella melitensis

In a herd of 20 sheep in Sweden, a country where brucellosis has never been diagnosed in sheep or goats, a total of six sheep were found serologically positive to Brucella melitensis in two different rounds of sampling. Yersinia enterocolitica serotype O:9 could at the time of the second sampling be isolated from four sheep, one of them at the same time serologically positive for B. melitensis. The article describes the case and gives some background information on brucellosis and Y. enterocolitica in general as well as a more specific description of the Swedish surveillance program for B. melitensis and the test procedures used. The problem with false-positive reactions, in particular its implications for surveillance programs in low prevalence or officially brucellosis-free countries, is discussed.