Mastitis is one of the most frequently occurring and costly diseases in dairy cows. Several preventative strategies have been applied to minimize the incidence of bovine mastitis, including optimization of milking procedures and milking hygiene, antibiotic therapies, vaccinations, segregation, and culling of persistently infected cows. However, mastitis remains an important disease on many dairy farms and, due to the high costs of clinical mastitis, reduction in the severity of the symptoms of mastitis and obtaining a more rapid clearance of established infections is of great value to dairy farmers.
Efficacy of vaccination against Staphylococcus aureus and CNS (coagulase-negative staphylococci) is a very different concept than efficacy of vaccination against E. coli. Whereas, with E. coli the vaccine is mostly expected to reduce severity of infection, with Staph. aureus and CNS vaccine is particularly valuable when vaccination results in a reduction of incidence and duration of infection, the key contributors to within herd infection dynamics. Experimental challenge studies with Staph. aureus have shown an effect of vaccination on the amount of bacterial shedding after challenge; however, such experimental studies were not able to demonstrate a reduction in infection transmission. Therefore it is necessary to determine the overall vaccine efficacy both at herd as linked to certain disease parameters and population.
The aim of this study was to evaluate vaccine efficacy of a commercial vaccine (Startvac®, Hipra Spain) aimed at reducing intramammary infections (IMI) with Staphylococcus aureus and CNS under field conditions.
During the 21-mo duration of the study, 1,156 lactations from 809 cows were enrolled in 2 herds (A and B), with a total of approximately 450 dairy cows milking at any point in time. The herds had a known prevalence of Staph. aureus of at least 5% of cows and a bulk milk SCC/mL between 250,000-400,000. No segregation of cows based on IMI status or SCC level was done on either farm.
Vaccination took place according to label directions in the dry period and early lactation. The first vaccination was at 45 d (±3d) before the expected parturition date, the second vaccination at 35 d thereafter (±3d), corresponding to 10 d before the expected parturition date, and the third vaccination was at 52 DIM (±3d). Cows going through a second dry period during the study were kept in the same treatment group (vaccinated or control). No placebo or sham vaccination was used in this trial.
During the first phase of the trial, all cows that were due to calve were vaccinated until approximately 50% of cows in the milking herd were vaccinated (at ~6mo). At that point, when 50% vaccination coverage was reached, cows that were due to calve were randomly assigned to be vaccinated or left as negative controls. We thereby assume that this was essentially a randomized controlled and single-blinded trial, as the herd staff was not aware of the vaccination. Monthly quarter sampling of all lactating cows in herds was done during the trial period. In addition, quarters were sampled by the farm staff when a case of clinical mastitis occurred, when cows were dried off, upon calving, and at culling. Cure rate, rate of new infection, prevalence, and duration of infections were analyzed. The chosen study design, with comingling of vaccinated and control cows, allowed us to estimate population vaccine efficacy within herd using a within-herd randomization schedule.
Vaccine efficacy was moderate in our field trial in 2 commercial dairy herds. Vaccination was able to reduce the basic reproduction ratio of CNS and Staph. aureus in both herds. The data indicated that vaccination will result in reduction of the basic reproduction ratio of Staph. aureus by approximately 45% and the basic reproduction ratio for CNS by approximately 35%. Efficacy was dependent upon the age group of the animals, particularly for Staph. aureus, where first lactation animals showed a significantly higher value compared with animals in third and higher lactation.
The observed vaccine efficacy may vary depending on farm management practices, as we identified significant differences between farms. Prevalence of Staph. aureus remained the same or slightly increased in farm A but dropped dramatically to a very low prevalence in farm B. For example, on farms with good management practices, the basic reproduction ratio (R0) for Staph. aureus would be reduced from 1.5 to 0.83, whereas vaccination on farms with poor management would reduce R0 from 5 to 2.75. In the latter example, Staph. aureus would show a reduced prevalence but remain endemic despite vaccination, whereas, in the first example, Staph. aureus would eventually be eliminated due to vaccination.
Then, vaccination is a valuable tool in reducing incidence. The utilization of vaccine in combination with other infection-control procedures, such as excellent milking procedures, treatment, segregation, and culling of known infected cattle, will result in an important reduction in incidence and duration of intramammary staphylococcal infections.
Y. H. Schukken, V. Bronzo, C. Locatelli, C. Pollera, N. Rota, A. Casula, F. Testa, L. Scaccabarozzi, Ricard March, Daniel Zalduendo, Roger Guix, and P. Moroni (2014). J. Dairy Sci. 97:1–15