Operational mastitis management: with focus on infection dynamics and evaluation of treatment strategies

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandling

Mastitis is inflammation of the mammary gland predominantly caused by bacterial infections. Mastitis is one of the larges challenges to dairy production. It is associated with considerable economic losses and reduced animal welfare. In addition, mastitis constitute a hazard to human health. Either directed through consumption of unpasteurized dairy products or indirectly as it is responsible for the majority of antibiotic usages in the production and thus promotes the development of resistance. Research on control of mastitis has been extensive for the last century and has resulted in control plans that include both prevention and treatment measures. However, there is a need for studies, which better support the causality of recommended control measures and which provide effect measures for economic evaluations. At the same time, studies are needed to support the ongoing development towards more automation and treatment strategies with reduced antibiotic usages.
The overall aim of the PhD project was to evaluate control measures against mastitis. The project had special focus on control of Staph aureus and Strep. agalactiae, milking event control measures involving automation and treatment strategies with reduced use of antibiotics. The project constituted four studies. Study I and Study II focused on milking event control measures involving automation. These studies included methodological evaluations. Study III and Study IV focused on treatment strategies with reduced use of antibiotics.
Study I evaluated automatic flushing of milking clusters. This was an experimental study in 32 milking cluster in a swing-over milking parlor in a commercial dairy herd. Clusters were pairwise subjected to a simulated milking with Staph. aureus contaminated milk. In each pair, one cluster was subsequently flushed automatically with either water or water containing peracetic acid (PAA), while the other cluster remained an unflushed control. Teat cup liners were sampled by rinse with a buffer and samples were cultured for Staph. aureus. Staph. aureus was not recovered from teat cup liners following automatic flushing with 966 (±32) mL water, while Staph. aureus was recovered from un-flushed clusters with a mean concentration 9,040 (±120) cfu/mL. Technical issues obscured results on flush with peracetic acid.
Study I included an experimental pilot study in the laboratory evaluating the feasibility of swab sampling and quantitative polymerase chain reaction (qPCR) analysis for the main study. Twenty Staph. aureus suspensions were sampled for bacterial culture and by swabs for q PCR analysis. Subsequently each suspension was disinfected with PAA and re-sampled for culture and qPCR analysis. Variation in sample mass related to swab absorbency was limited (8%) but attributed to a considerable proportion (48%) of the variation observed in recovery of Staph. aureus by qPCR from swab samples. Following disinfection with PAA, Staph. aureus was no longer detected at culture, but remained detectable by qPCR with and mean Ct-value of 27.40 (±0.09).
Study II evaluated manual and automatic measures to improve hygiene in automatic milking units. This was an experimental study in two commercial dairy herds (Herd A and Herd B) with automatic milking systems (AMS) and group housing of lactating cows. Groups were randomly assigned as intervention and control group. In Herd A, manual wash with foam was introduced in the intervention group. In Herd B, the brush motor was sprayed with disinfectant and brushes were changed daily in the intervention group. The infection status of all lactating cows was assessed longitudinally by qPCR analysis on composite milk samples. Within-herd transmission probabilities were estimated with a Hidden Markov Model. In Herd B, transmission of Strep. agalactiae and Strep. dysgalactiae was lower in the intervention group than in the control group.
Study II included a methodological evaluation comparing HMM model without contact network to a model with infectious contacts during housing and a model with infectious contacts during milking events. The Widely Applicable Information Criteria (WAIC) was used for model comparison.
Study III evaluated individual quarter dry off as a non-antibiotic management strategy for mild or subclinical mastitis. This was an experimental study in 5 organic commercial dairy herds with AMS. Dairy cows with an udder quarter with mild or subclinical mastitis were identified based on somatic cell count diagnostics. By random allocation, 34 cows were subjected to quarter dry-off (QDO) and compared to 36 cows subjected to continual milking of the affected quarter. Cows were evaluated longitudinally for clinical signs of mastitis, elevated rectal temperature and signs of pain. Short-term production losses were estimated from AMS data. Cows subjected to QDO had increased odds of showing signs of pain and tended towards increased odds of fever. Additionally, QDO was associated with a considerable short-term production loss (4.1 kg/day, 95% Confidence Interval: 3.1-5.0). The production loss depended on yield, parity, lactation stage, so that multiparous cows early in lactation had smaller relative production losses compared to primiparous cows late in lactation.
Study IV evaluated herd level strategies including extent of dry cow treatments, extent of slaughters early in lactation and length of the dry period. This was an observational cohort study on eradication of Strep. agalactiae on herd level. Fifty-two herds that had recovered from Strep. agalactiae in the Danish surveillance program from 2013 to 2016 were compared to 146 herds that had remained positive for Strep. agalactiae. Registry data from the Danish cattle database was used to identify herd characteristics and management strategies. Increased percent of early slaughters and increased length of the dry period at dry cow treatment were associated with increased probability of herd recovery from Strep. agalactiae. The proportion of dry cow treatments was not associated with herd recovery from Strep. agalactiae.
Results from the studies focused on milking event control measures support that measures to enhance hygiene in relation to teat cup liners and automatic pre-milking equipment has great potential as effective control measures against Staph. aureus and Strep. agalactiae. However, these results underline that evaluation of such measures should include effect measures on both hygiene level and transmission level. Results from the evaluation of methods suggest that swab sampling of environmental surfaces induces considerable procedure related variation in analytical results. Furthermore, qPCR is not feasible for analysis of environment samples subjected to PAA due to the uncertainty related to viability of recovered bacteria. Methods to evaluate hygiene level in milking systems could benefit from further development and standardization. Incorporation of a milking contact network did not improve transmission modelling for contagious mastitis, but supported the value of implementing diagnostic test characteristics.
Results from the studies on treatment strategies support that QDO may be a feasible alternative to treatment with antibiotics for mild or subclinical mastitis, but that selection criteria for the treatment need to be specified considerably compared to those used in the study. Specifications should consider yield, parity and lactation stage. Additionally, the feasibility of QDO needs further evaluations into the effect on bacterial cure and transmission to other cows. Furthermore, results support reduced use of antibiotic though selective dry cow treatment in control of Strep. agalactiae as the within herd proportion of dry cow treatments did not facilitate herd recovery. Additionally, results established that a longer dry period at dry cow treatment and slaughter of infected cows early in lactation are effective control strategies as they facilitate eradication on herd level. This offers alternatives to reduce within herd prevalence of Strep. agalactiae without use of antibiotic.
OriginalsprogEngelsk
UdgivelsesstedAarhus
ForlagAarhus University
Antal sider150
StatusUdgivet - 2020

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