2022-23 Rapid Ag:Reducing mastitis in the dairy cow by increasing the prevalence of beneficial polymorphisms in genes associated with mastitis resistance - RENEWAL
Brian A. Crooker, Department of Animal Science
- Sandra Godden, Department of Veterinary Population Medicine
- Luciano Caixeta, Department of Veterinary Population Medicine
- Anthony Seykora, Department of Animal Science
- Michael Schutz, Department of Animal Science
- John Lippolis, USDA-National Animal Disease Center
- Ben Rosen, USDA-Animal Genomics and Improvement Laboratory
- Research Scientist, Department of Animal Science
Keeping cows healthy minimizes the need for therapeutic interventions which can reduce use of antibiotics, reduce the cost of producing milk, decrease food waste, and contribute to a continued supply of safe, wholesome milk for consumers. Mastitis is the most expensive health-cost for the dairy industry ($1.7 to 2 billion/yr; about 11% of annual U.S. milk production) due to milk loss and increased labor and drug costs (1). The contemporary Holstein cow has a less robust immune system and is more susceptible to disease and metabolic disorders than her ancestors (2) due in part, to a reduced prevalence of genetic polymorphisms that support a strong immune response and an increased prevalence of detrimental polymorphisms (3,4). We will use unselected Holsteins that represent the 1964 ancestors of contemporary Holsteins to identify polymorphisms that contribute to a robust immune system. Prevalence of beneficial and detrimental polymorphisms in the DNA of contemporary Holsteins will be determined and the information used to identify polymorphisms to be included in gene-assisted selection strategies designed to strengthen immune function and increase mastitis resistance.
Objectives and Goals
Our premise is that previous selection practices have successfully increased the presence of genetic polymorphisms associated with increased milk yield but have decreased the presences of polymorphisms associated with disease resistance. Our overall goal is to increase the presence of polymorphisms in the Holstein cow to strengthen immune function and increase mastitis resistance. Key objectives include:
- Determining differences in mammary gene expression when unselected and contemporary Holsteins are subjected to intramammary pathogen challenges
- Sequencing the complete genome of these Holsteins to identify potentially beneficial polymorphic differences between the genotypes
- Determining the relative presence of these polymorphisms in contemporary Holsteins to identify polymorphisms that could be increased or decreased to improve resistance to mastitis.
Throughout this effort we will inform the industry and lay and scientific communities of our progress towards achieving these objectives.
The contemporary Holstein (CH) has a less robust immune system and is more susceptible to disease and metabolic disorders than her ancestors (2). Efforts to reverse the unintended negative impacts of selection practices through altered selection priorities (greater emphasis on health and fertility traits) and marker assisted selection programs (CDCB, 2017 https://www.uscdcb.com/eval/summary/trend.cfm) have been beneficial but additional, more-focused gene-assisted selection efforts are needed. A better understanding of genetic polymorphisms beneficial to mastitis resistance would strengthen gene-assisted selection efforts to enhance the prevalence of beneficial genes and quicken the pace towards reducing incidence and severity of mastitis in the dairy cow.
Functional genomic studies can determine which genes are activated or de-activated when animals are exposed to specific treatments and thus identify genes responsible for the observed alterations in animal function. In 1964, Dr. Charles Young established a genetically static herd of Holsteins at the University of Minnesota. These unselected Holsteins (UH) have not been subject to selection pressures since then and represent the U.S. Holstein dairy cow population of 1964 (5). Our studies with this unique animal model (UH vs. CH) document the impact of 50 plus years of selection on genomic alterations and their impact on endocrine, metabolic and immune function in CH cows.
Environmental pathogens cause more mastitis than contagious pathogens and the most common pathogens in the US continue to be the gram negative Escherichia coli and gram positive Streptococcus uberis (6). Immune responses are initiated when pathogenic components, such as lipopolysaccharide (LPS) from E. coli and lipoteichoic acid (LTA) from S. uberis, are detected by host cell receptors; primarily toll-like receptor 4, TLR4, for LPS and TLR2 for LTA. Binding of LPS or LTA to its receptor activates immune signaling pathways that release cytokines and chemokines involved in generating immune responses. Activation of these signaling pathways can be reduced in weakened (less robust) immune systems which increases the opportunity for bacteria to establish infections. Polymorphisms in receptor signaling occur in cattle (7) and innate immune response and function differ among dairy breeds (8). We have documented considerable polymorphic (9), transcript (10) and immune response (11) differences between our UH and CH cows.
Our whole genome, single nucleotide polymorphism (SNP) analysis indicates allele frequency differences were greatest (Fig. 1) between UH and CH (red) and least among CH cows (black). Differences were consistent among chromosomes and indicate more than 40% of the 50,000 analyzed SNPs differed between UH and CH cows while there were far fewer differences within H cows (9). This highlights the greater genetic diversity and unique value of UH cows in efforts to understand the impact of selection practices on polymorphisms in genes associated with and responsible for alterations in physiology and function of CH cows. We evaluated gene expression in liver, adipose and mammary biopsies collected at -14, 3, 14 and 42 of lactation from UH and CH cows and demonstrated that expression of more than 26% of the transcripts in mammary tissue differed between the genotypes (10). Our functional genomic study of the innate immune system demonstrated hepatic expression of genes in the TLR4 signaling pathway is less robust in CH than in UH cows when challenged with LPS (11). Our intramammary E. coli challenge study at NADC clearly indicated E. coli growth, milk somatic cell count and impact on milk yield were less in UH cows (12). These results demonstrate the greater ability of the immune system of the UH cow to neutralize E. coli infections.
Using this unique and powerful genomic model, we expect to identify specific polymorphic forms of genes and regulatory factors responsible for the greater susceptibility and prevalence of mastitis in CH cows and of those responsible for the more robust immune response in UH cows (short-term benefit). Reducing the first and increasing the latter in CH cows can be achieved by respectively selecting against and for these polymorphisms in gene-assisted selection programs. Improved understanding of the molecular control of innate immune response in the cow will contribute to development of breeding strategies to enhance mastitis resistance. These efforts will improve financial returns to the producer, improve well-being of the cow and contribute to a continued supply of safe, wholesome milk for consumers (long term benefit). Linking differences in phenotype to differences in genotype has been used with CH cows to increase the prevalence of beneficial polymorphisms associated with fertility (13) and feed intake (14). Similar efforts for mastitis are underway (3) but our unique UH vs. CH animal model offers a much more powerful opportunity to identify beneficial polymorphisms that are less prevalent in CH cows due to 50+ years of selection.
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- Egger-Danner et al. 2015. Invited review: Overview of new traits and phenotyping strategies in dairy cattle with a focus on functional traits. Animal 9:191-207.
- Siebert et al. 2017. Genetic variation in CXCR1 haplotypes linked to severity of Streptococcus uberis infection in an experimental challenge model. Vet. Immuno. Immunopath. 100:45-52.
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- Ma et al. 2019. Genome changes due to artificial selection and their impact on fertility and immunity genes in U.S. Holstein cattle. BMC Genomics 20:128.
- Crooker. 2014. Regulation and integration of hepatic function with mammary and adipose metabolism in Holstein cows during the periparturient period. NRI-USDA-NIFA grant.
- Cousillas. 2018. Effects of milk yield genotype on immune, endocrine and metabolite interactions in dairy cows. PhD Thesis (08/10/18). University of Minnesota. Advisor - Crooker
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- Lippolis 2011. Treatment of an intramammary bacterial infection with 25-Hydroxyvitamin D3. PLoS One. 6:1-7.