Severe mastitis can result in a range of serious general health compli-
cations for the infected dairy cow, including septicemia, which can ulti-
mately lead to death. This cross-sectional study aimed firstly to identify
the herd-level risk factors associated with severe clinical mastitis (CM)
in the diseased dairy cow. The second aim was to investigate herd-relat-
ed factors associated with the incidence of severe mastitis in the dairy
herd. The study was conducted on dairy farms (n = 58) in Northwestern
Germany. In addition to data from dairy herd improvement (DHI) tests,
possible herd-related predictors were collected on dairy farms. The
most frequently isolated pathogens among all CM cases in this study
were coliform bacteria (32.6 %), followed by Streptococcus (Str.) uberis
(17.2 %). Mastitis cases in clean dairy herds (in > 80.0 % of the exam-
ined udders ≤ 10.0 % of the udder surface was soiled), in dairy herds
with > two milkings per cow and day, and in herds with a new infection
risk (NIR) during the dry period ≤ 28.0 % in the last DHI test prior to CM
were identified as herd-related factors associated with more severe
CM in the diseased dairy cow. The group of causative pathogens in
mastitis cases was another risk factor positively associated with severe
CM in the diseased dairy cow. Mastitis caused by coliform pathogens
was more likely to be severe than mastitis caused by other pathogens.
The mean incidence for severe mastitis in this study was 4.1 severe
cases per 100 cow years at risk. The herd milk protein content based
on the average of all DHI tests was significantly associated with the
incidence of severe mastitis, such that dairy herds with a lower herd
milk protein content < 3.4 % were associated with a higher incidence
of severe mastitis.