Herd types of group-B streptococci. Their prevalence among herds in four Danish mastitis control areas and the relation of type to the spread within herds.

Herd types of B-str. were determined for 159 herds on the basis of serological typing of 336 quarter-milk isolates. The herds were selected at random within an area with some 25,000 herds, and assumably the results reflect the type distribution in that area. In contrast with previous findings in Zealand, no significant difference in the distribution of herd types was found between new-infected herds and herds with infections of known duration. However, compared to herds with infections of presumably long standing, newinfected herds had a relative deficit of Herd Type III. Herd Types III and X showed a significantly higher tendency than the other types to spread among cows within a herd. It is concluded that infections caused by Herd Types III, Ia/III and X require special attention in a control program, while in most cases supervision would seem to be adequate for infections caused by other types. Yet, such a program will leave a hygienic problem unsolved.