Studies by scientists at the University of Minnesota and their USDA-ARS collaborators indicate that a unique herd of unselected Holsteins, maintained at the University of Minnesota since the mid-1960s, has a very strong potential to help reduce mastitis in future generations of Holstein cows. These unselected Holsteins have not been subjected to selection pressure since 1964 and thus represent average U.S. Holsteins from the mid-1960s. Work by these scientists has demonstrated that in addition to increasing milk yield, selection practices since the mid-1960s have also altered nucleotide sequences in the contemporary Holstein genome and decreased immune responses in contemporary Holsteins relative to those of their Holstein ancestors.
Selective breeding of dairy cows since the mid-1960s and improvements in dairy management have substantially increased milk and component yields of contemporary cows. These dramatic improvements have increased efficiency and profit and reduced the environmental footprint of the dairy industry, but a persistent concern has been that selective breeding for greater milk production has reduced overall cow health. Improvements in milking management, cow care and therapies have reduced the incidence and duration of mastitis cases, but mastitis continues to be the most common bacterial infection in dairy cows.
Mastitis is inflammation of the mammary gland and typically occurs as part of the immune defensive mechanisms to combat bacteria that invade and infect the gland. Mastitis is detrimental to animal well-being, costly to the producer and most often caused when common gram-negative or gram-positive bacteria enter the gland. Mastitis decreases yield, quality and stability of milk and the negative impacts of subclinical and clinical cases make mastitis the most costly bacterial infection for the dairy industry. Reducing the incidence of mastitis will also reduce the need to administer antibiotics to the cow. Additional advances are needed to continue to reduce the incidence of mastitis, improve animal well-being and increase dairy profitability.
Comparing unselected and contemporary Holsteins can identify genomic alterations induced by selective breeding and the impact of these alterations on physiological differences between these Holstein genotypes. The polymorphic and the immune response differences between unselected and contemporary Holsteins are greater than the differences among contemporary Holsteins. Thus, functional genomic studies that compare unselected and contemporary Holsteins should have greater power to identify specific nucleotide sequences (markers) that are associated with improved immune function than studies that only use contemporary Holsteins.
Differences in DNA
A study led by Dr. Yang Da at Minnesota and USDA-ARS colleagues at the Animal Genomics and Improvement Laboratory in Beltsville, Maryland, compared the genomes of unselected and contemporary Holsteins. Results demonstrated tremendous nucleotide sequence differences (polymorphisms) have occurred throughout the Holstein genome since the mid-1960s. Several large clusters of polymorphic differences were identified in genomic regions that contained genes with known immune functions, including the major histocompatibility complex (MHC) region that has multiple roles in adaptive immune responses in several species. Polymorphisms were also found in specific genes, such as toll-like receptor-4 (TLR4), which is involved with innate signaling and initiating the innate immune response against bacterial infections. These results demonstrated that selective breeding efforts since the mid-1960s not only increased the presence of polymorphisms associated with increased milk yield, but also generated large nucleotide sequence differences in genes associated with immune function.
Lipopolysaccharide (LPS) is a bacterial cell wall component from gram-negative bacteria including Escherichia coli. Studies from my lab at the University of Minnesota revealed that LPS administration generated greater immune responses in unselected Holstein heifers and cows than in their contemporary Holstein herdmates. Our work also demonstrated that hepatic expression of TLR4 and other immune signaling components were greater when LPS was administered to the unselected cows.
Focus on immune response
Dr. John Lippolis at the USDA-ARS National Animal Disease Center in Ames, Iowa, and myself are conducting immune function studies and collaborating with Dr. Ben Rosen and colleagues at the USDA-ARS Animal Genomics and Improvement Laboratory to identify genome sequence differences between the unselected and contemporary Holsteins used in these studies. A key goal of this work is to identify immune function markers that could be used in marker-assisted selection efforts to improve health and resistance to infection and disease in future generations of Holsteins.
An intramammary challenge study conducted at the National Animal Disease Center demonstrated the unselected Holsteins developed less severe mastitis than contemporary Holsteins when E. coli were infused into the mammary gland. Immune response to E. coli in the unselected Holsteins was primarily contained within the mammary gland while both mammary and systemic indicators of infection were greater in the contemporary Holsteins. The results are consistent with the previous LPS study that demonstrated a greater immune signaling response in the unselected Holsteins. This work is being expanded to evaluate mammary responses to gram-positive bacteria.
Less invasive investigations have examined immune cell signaling when whole blood was incubated with heat-killed bacteria known to cause mastitis. When incubated with LPS or heat-killed gram-negative bacteria (E. coli, Klebsiella pneumoniae or Serratia marcescens), response of the immune signaling protein interleukin-1β (IL-1β) was greater in blood from unselected Holsteins than in blood from contemporary Holsteins. These reduced responses in contemporary Holsteins are consistent with their previous results and support the premise that contemporary Holsteins are more susceptible to infections caused by E. coli and possibly by other gram-negative bacteria.
Lipoteichoic acid (LTA) is a component of gram-positive bacteria and greater IL-1β responses were also obtained when purified LTA was incubated with blood from unselected Holsteins. However, IL-1β response did not differ between unselected and contemporary Holsteins when blood was stimulated with heat-killed gram-positive bacteria (Staphylococcus aureus, Streptococcus dysgalactiae or Streptococcus uberis). These results might mean that contemporary Holsteins are not more susceptible to infections caused by gram-positive bacteria, but additional studies, including intramammary challenge studies, are needed to determine the extent to which the previous 60 years of selective breeding has altered immune response to various infectious bacteria.
These functional genomic studies are designed to identify polymorphisms associated with infection severity so that selection for beneficial (and against detrimental) polymorphisms can be used to produce future generations of Holsteins that are more resistant to mastitis.
This work was supported in part by the Minnesota Agricultural Experiment Station, project MIN-18-125, and by a grant from the Rapid Agricultural Response Fund program of the Minnesota State Legislature and Minnesota Agricultural Experiment Station.
References omitted but are available upon request by sending an email to the editor.
