When I arrived at the University of Minnesota in 1998 as a newly minted assistant professor, I had then only just begun to dabble in mastitis research. Now 21 years later, when my Dad, a retired dairy farmer, asks me “What are you working on?” and I reply “Mastitis control.”, he invariably responds incredulously “The industry hasn’t got that figured out yet?!” Humorous though this little exchange may be, it exposes a certain truth: While the global dairy industry has made tremendous strides to improve mastitis control and milk quality over the past several decades, there will always be more to learn and further improvements to make. Additionally, public health concerns continue to mount regarding the potential for antibiotic (Ab) use in food animal systems to promote antimicrobial resistance in pathogens of importance to human health. Because the majority of Ab used on dairy farms is for the treatment or prevention of mastitis, we must strive to adopt mastitis control practices that use Ab in the most judicious manner possible, while still maintaining or improving udder health, productivity and animal wellbeing.
Fortunately, passionate and collaborative researchers from around the world continue to produce valuable new information to assist dairy producers in improving mastitis control and milk quality. Over the years, we at the University of Minnesota (St. Paul, MN, USA) have been very fortunate to work alongside, and learn from, many talented people within the global mastitis research community. The following article describes a few of the mastitis control focus areas that faculty at the University of Minnesota and collaborating research partners at other institutions have been working on in recent years. This includes studies on the field validation of rapid culture systems for the strategic treatment of clinical mastitis, field validation of selective dry cow therapy programs, bedding management, and the importance of cloth udder towel hygiene.
Use of Rapid Culture Systems to Guide Strategic Treatment Decisions for Clinical Mastitis
Milk culturing is a laboratory procedure that allows the identification of disease-causing bacteria present in a milk sample. Culturing of milk samples by a diagnostic laboratory is considered the “gold standard” for identifying the cause of either clinical or subclinical mastitis. Professionally trained personnel in diagnostic laboratories use a variety of culture and other confirmatory techniques to accurately identify the type of bacteria present in a sample. However, some of these culture techniques are simple and inexpensive and can more rapidly be done on-farm or in a local veterinary clinic. The purpose of a rapid culture system, such as the Minnesota Easy® Culture System (Image 1), is to eliminate the need to transport the milk from clinical mastitis cases to a reference laboratory located some distance away. This allows for positive culture results to be available in as little as 18 hours.
Done properly, rapid culture systems can provide dairy producers with a quick, simple, and inexpensive way to identify the likely bacterial cause of clinical mastitis. This information can then be used in guiding clinical mastitis treatment decisions. For example, mastitis researchers generally agree that, while infections caused by Gram-Positive organisms generally benefit from short duration intramammary (IMM) Ab therapy, there is no benefit to treating cases that result in ‘no bacterial growth’ on the plate, presumably because the cow’s immune system has already eliminated the infection. Similarly, most infections cause by Gram-Negative organisms are not likely to respond to, or benefit from, IMM Ab therapy. Consequently, over 50% of mild and moderate clinical mastitis cases on most dairy farms may not require IMM therapy.
With help from collaborators in Ontario and Wisconsin, the University of Minnesota was able to complete a multi-site study in 8 commercial herds showing that the use of a rapid culture system to guide strategic treatment decisions for mild and moderate clinical mastitis cases caused by Gram-Positive infections can result in a 50% or greater reduction in IMM Ab use and reduced associated treatment costs, while still maintaining the same future udder health and production potential for the cow (Lago et al., J Dairy Sci. 2011a,b. Vol 94). This finding has since been replicated in other North American field studies. Training, practice and attention quality control are necessary to get the best results from on-farm culture systems. If individual farms cannot or do not wish to set up their own on-farm culture system, then this testing can instead be conducted at their local veterinary clinic.
Field Validation of Two Selective Dry Cow Therapy Programs
Blanket dry cow therapy (BDCT) has played an important role in mastitis control programs. However, in light of continuing improvements in udder health and reduced prevalence of intramammary infection (IMI) at dry-off, BDCT may no longer be necessary in many herds. Selective dry cow therapy (SDCT) is an approach whereby only those cows or quarters with a known or suspected IMI are treated with IMM Ab at dry off. The results of early SDCT studies were mixed: Although some programs were successful, others failed to preserve future udder health and performance. To be successful, we currently believe that SDCT programs will require both the use of a sensitive diagnostic test at dry-off in order to identify those cows or quarters that require Ab treatment, and use of a teat sealant to protect untreated quarters from new infections during the dry period.
Several Western European countries have already mandated the adoption of SDCT. Anticipating increasing demand for adoption of SDCT in North America in the coming years, and with funding from the USDA-AFRI, we set out with research partners from New York, Iowa and California to develop and validate two different SDCT programs in commercial dairy herds. The objective of this study was to compare 2 SDCT approaches (Culture-based or Algorithm-based) against BDCT in a multi-herd randomized, positively controlled, clinical trial for the following outcomes; reduction in Ab use at dry-off, clinical mastitis (CM) risk, culling risk, milk yield and SCC in the first 120 DIM.
Seven herds were recruited from four study sites (CA, IA, MN and NY) from May to July 2018. At enrollment 2 days prior to dry-off, duplicate aseptic quarter-milk samples were collected and cows were randomly allocated to one of three treatment groups; Blanket DCT (“Blanket”, n=429); Culture-based SDCT (“Culture”, n=432), and Algorithm-based SDCT (“Algorithm”, n=414). On the day of dry-off, all quarters of the Blanket group were treated with IMM Ab. In the Culture group, quarters were treated with Ab only if bacterial growth was observed after 30-40 h incubation on the Minnesota Easy® 4Cast® plate, a rapid culture system developed by the University of Minnesota (Image 2).
Image 2. Minnesota Easy® 4Cast® plate. Used in a selective dry cow therapy program to guide quarter-level antibiotic treatment decisions at dry-off.
Algorithm cows had all quarters treated with Ab if they met any of the criteria for treatment: ≥2 cases of CM during lactation, any CM during the 14 d prior to dry-off, or any test day SCC > 200 x 103 cells/ml during lactation. All quarters of all cows were treated with an internal teat sealant. Cows were followed from enrollment until 120 DIM in the subsequent lactation. Results showed that both the Culture-based and Algorithm-based SDCT programs reduced Ab use at dry-off by 55%. Meanwhile, cow-level CM incidence (1-120 DIM) was similar in Blanket (14.5%), Culture (12.2%), and Algorithm (12.2%) groups. Risk of culling (1-120 DIM) was also similar in Blanket (10.8%), Culture (9.8%), and Algorithm (10.6%) cows. Adjusted geometric mean SCC (103 cells/ml) at all herd tests (1-120 DIM) were similar for Blanket (55), Culture (57), and Algorithm (59) cows. Finally, average daily milk yield (kg/day) during the same period were similar among the three treatment groups: Blanket (48.6), Culture (48.6), and Algorithm (47.8). These findings showed that herds could use either a culture-guided or an algorithm-guided SDCT strategy to reduce Ab use at dry-off, without causing negative effects on early lactation health and performance. However, we stress the importance of appropriate herd selection, and careful implementation and monitoring on any farm adopting a SDCT program.
The Importance of Bedding Management in Mastitis Control
Bedding is an important source of teat end exposure to environmental mastitis pathogens (Image 3).
Furthermore, several early studies have demonstrated a positive association between bedding bacteria counts (BBC) and risk for IMI risk, particularly for infections caused by Coliform bacteria. However, a great many questions remain surrounding the selection, management and monitoring of bedding hygiene to promote udder health and milk quality. The objectives of a recent cross-sectional observational study, supported by Boehringer Ingelheim Animal Health, were to: 1) Describe BBC, bedding characteristics, udder hygiene scores, bulk tank milk (BTM) quality, and herd level measures of udder health (UH) in U.S. dairy herds using one of four different bedding materials; 2) Describe the relationship between BBC and herd measures of UH; and 3) Identify benchmarks for monitoring bedding hygiene.
Local dairy veterinarians or university researchers helped to enroll and sample 168 herds from 17 states. Herds were on a DHIA testing program and used one of four bedding types for lactating cows; new sand (NS), reclaimed sand (RS), manure solids (MS) or organic non-manure materials (ON). Each herd was sampled twice (winter/summer) in 2016. Samples and data collected included unused and used bedding, BTM samples for culture, udder hygiene scores, DHIA test data and descriptions of facilities and herd management practices. Bedding samples were cultured to determine the total bacteria count (TBC) and counts of Bacillus spp., coliforms, Klebsiella spp., non-coliform Gram-negative organisms, streptococci or streptococci-like organisms (SSLO) and Staphylococci spp. (Staph) (cfu/cc of wet bedding). Bedding dry matter, organic matter and pH were also measured. Udder health measures included DHIA test day average linear score (AVLS), the proportion of cows with an intramammary infection (IMI) where infection was defined as LS ≥ 4.0, the proportion of cows with a new IMI (NIMI) where new IMI was defined as LS changing from < 4.0 to ≥ 4.0 in the last two tests, the proportion of cows with a chronic infection (CRON) where chronic was defined as a LS ≥4.0 on the last two tests, and the cumulative incidence of clinical mastitis in the 30-day period preceding sample collection.
Although much variation existed within and among bedding types, results showed the use of MS bedding to be generally associated with higher BBC, dirtier udders, increased coliform and SSLO counts in BTM, and poorer DHIA herd-level measures of udder health, as compared to ON, RS or NS bedding materials. While controlling for important farm traits and management practices, models showed that increased counts of coliforms, Klebsiella spp., SSLO and Staph, in both unused and used bedding, were associated with poorer values for one or more herd-level measures of UH. Achievable benchmarks identified for counts of coliforms (unused ≤500; used ≤10,000 cfu/cc), Klebsiella spp. (0 cfu/cc for unused and used), Staph (0 cfu/cc for unused and used) and SSLO (unused = 0; used ≤ 500,000 cfu/cc) were identified to monitor bedding hygiene in most bedding materials, with minor variations suggested for SSLO in unused MS (≤ 1,000 cfu/cc) (Patel et al., J. Dairy Sci. 2019. Vol. 102). Low organic matter levels in sand bedding and reduced moisture levels in all types of bedding materials were associated with reduced BBC.
Even though the use of MS bedding was generally associated with worse herd-level measures of UH as compared to other bedding materials, a great deal of variation in BBC and UH existed within the MS group of herds, with some herds achieving very good results. This finding has prompted us to initiate a new study (ongoing) investigating the potential impact of different methods of processing recycled MS on bedding characteristics (e.g. reduced moisture levels), BBC and udder health.
The Importance of Cloth Udder Towel Hygiene in Mastitis Control
Common sense suggests that cloth udder towels (CUT) may function as a fomite for mastitis causing pathogens (Image 4).
However, studies have been lacking to investigate if a direct association exists between bacteria levels in CUT and udder health outcomes. Furthermore, science-based guidelines for monitoring towel hygiene have been lacking. To address this knowledge gap, we set out to complete a cross-sectional study (Rowe et al., J. Dairy Sci. 2019. Vol. 102) with the following objectives: 1) Describe associations between towel bacteria count (ToBC) and quarter-level IMI status in late lactation cows; 2) Establish pathogen-specific target levels of bacteria in CUT to aid the interpretation of towel culture reports and 3) Identify laundering-related risk factors for high ToBC.
The study, supported by Zoetis, was conducted in 67 herds, from 10 dairy states in the U.S. that used CUT. These 67 herds were originally recruited as part of a larger (80 herd) cross-sectional study of bedding management in late lactation cows. Each herd was visited once during December 2017 to April 2018 and quarter-milk samples (n = 4,656) were collected from late gestation (> 180 d pregnant) cows (n = 1,313). Two recently laundered CUT were collected and a questionnaire was used to collect information about pre-milking teat preparation and CUT management practices. Quarter-level IMI status was determined using standard bacteriologic methods. In addition, colony forming units of all bacteria (total bacteria), Staphylococcus spp., Streptococcus spp. or Strep-like organisms (SSLO), Coliforms, non-coliform Gram-negatives and Bacillus spp. were determined for each pair of CUT (log10 CFU/in2).
Results showed the quarter-level prevalence of IMI was 19.6%, which was predominantly caused by non-aureus Staphylococcus spp. (NAS; 10.2%) and SSLO (5.1%). The predominant bacteria in CUT were Bacillus spp. (median = 210 CFU/in2). Results showed that the total towel bacteria count was not associated with odds of IMI (OR = 1.04), likely due to the predominance of Bacillus spp. in CUT and low number of IMI caused by Bacillus spp. However, counts of Staphylococcus spp. and SSLO from towels were positively associated with odds of IMI caused by NAS (OR = 1.40) and SSLO (OR = 1.45), respectively.
Of several towel management practices evaluated, failure to dry towels was a clear predictor of coliform counts, with undried towels being 8 times more likely to have coliform counts above the target of five cfu/in2 (32 cfu/cm2). Furthermore, all towels that were laundered by a professional, offsite service had coliform counts below the target level, while 15% of towels laundered onsite were above the target levels. We acknowledge that larger studies are needed to investigate laundering practices, as only 67 herds were enrolled in the current study. A previous study by Fox (1997) found that towel bacteria counts could be minimized by practicing at least one of the following: 1) use sanitizer, 2) use a hot-air drier and 3) use hot water during washing.
Text and pictures: Sandra Godden DVM, DVSc and Erin Royster DVM.