Prototheca spp. are algae that cause incurable acute or chronic mastitis in
dairy cows. The aim of this case-control study was the identification of cow-
and herd-level risk factors for this unusual mastitis pathogen. Aseptically
collected composite milk samples from 2,428 milking cows in 23 case and 23
control herds were collected between January and May 2011. A questionnaire was
administered to the producers, and cow-level production and demographic data
were gathered. In 58 of 64 isolates, Prototheca spp. and Prototheca zopfii
genotypes were differentiated using PCR and matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry. All isolates were
identified as Prototheca zopfii genotype 2. The mean within-herd prevalence
for Prototheca spp. was 5.1% (range 0.0-12.5%). Case herds had a significantly
lower herd-level prevalence of Staphylococcus aureus and a higher prevalence
of yeasts than did control herds. The final logistic regression model for
herd-level risk factors included use of intramammary injections of a non-
intramammary drug [odds ratio (OR) = 136.8], the number of different
injectable antibiotic products being used (OR = 2.82), the use of any dry cow
teat sealant (external OR = 80.0; internal OR = 34.2), and having treated 3 or
more displaced abomasums in the last 12 mo OR = 44.7). The final logistic
regression model for cow-level risk factors included second or greater
lactation (OR = 4.40) and the logarithm of the lactation-average somatic cell
count (OR = 2.99). Unsanitary or repeated intramammary infusions, antibiotic
treatment, and off-label use of injectable drugs in the udder might promote
Prototheca udder infection
