Research and practical experience have shown a genetic component to the ability of individual cattle, or sire lines, to resist BRD pathogens. Identifying the genes involved and developing genomic-assisted selection tools for health, however, takes hard work and lots of data.
During the BRD Symposium, several presenters outlined research efforts to identify and quantify those genetic influences.
Washington State University animal scientist Holly Neibergs, PhD., presented on genomics and the relevance of genotype to BRD resistance or susceptibility. She conducts research in this field as part of the BRD Consortium, (). This group of researchers from multiple universities are conducting large-population studies to identify DNA loci associated with BRD, analyze interactions between host genomes and pathogens, establish heritability estimates and ultimately, develop selection panels for BRD resistance.
Neibergs notes that previous research has shown differences in morbidity and mortality between cattle breeds and between sire or family lines, supporting the idea of a genetic component to BRD. Heritability estimates for BRD susceptibility range from 0.02 to 0.29, depending on the population and the definition of the disease.
Commercial genotyping currently is available to predict BRD susceptibility in dairy cattle, and can be used in selection of replacement females, she adds. Wider variation in breed types and populations within breeds complicate the process in beef cattle. Consortium researchers have identified loci associated with BRD susceptibility across several populations of cattle.
“The identification of loci and genes associated with BRD provides the possibility to use genomic selection to reduce disease incidence and to better understand the host mechanisms associated with disease susceptibility,” Neibergs concludes.
Kristen Parker Gaddis, PhD, a geneticist with the Council on Dairy Cattle Breeding, also discussed the challenges and opportunities in genomic selection for BRD resistance in dairy cattle.
Parker Gaddis explained that an animal’s phenotype represents the sum of its genetic makeup and environmental influences. Producers traditionally have selected dairy replacements for milk yield, a phenotype that is relatively easy to measure and compare. Selecting for health traits becomes much more difficult, largely because the multi-factorial nature of BRD, ambiguity in case definitions and the large influence of environmental effects on BRD risk. Also, she notes that selection for one trait can have negative effects on others, such as increased milk yield corresponding with lower daughter pregnancy rates.
Relatively low heritability also complicates selection for health traits, she says. Take for example protein yield, where genetic differences explain about 30% of variation while environment affects 70%. In contrast, genetic differences account for only about 3% of the variation in mastitis susceptibility. On the bright side, heritability estimates for BRD risk run somewhat higher than those for other health traits such as ketosis, mastitis, metritis and retained placenta.
While the effort will take time, Parker Gaddis says with enough genotypic and phenotypic data, researchers can develop genomic selection tools for cattle health. “Availability of genomic data dies not negate the necessity of quality phenotypes, in this case, records of BRD incidence,” she says. “Additional data also could be collected through the expansion of currently utilized termination codes and used in conjunction with records of direct health events. Selection for animals with improved BRD resistance is possible at the national level; however, collection of additional phenotypes remains a significant hurdle.”
Genomic Tools for Understanding BRD
While environmental factors and host genetics heavily influence BRD morbidity rates, understanding the genomic and molecular biology tools also can improve our understanding of BRD pathogens and the communities of other microorganisms that help keep those pathogens at bay.
During the BRD Symposium, several presenters outlined technologies including molecular diagnostics and genomic sequencing for non-pathogenic microbes colonizing the bovine respiratory system.
Trevor Alexander, PhD, a microbiologist at Agriculture and Agri-Food Canada’s Lethbridge Research and Development Centre, outlined several studies using next-generation sequencing to explore how the respiratory microbiota affects resistance or susceptibility to BRD.
“While a direct causal relationship between respiratory microbiota and the development of BRD in feedlot cattle has not been fully elucidated, increasing evidence suggests that the microbiota contributes to respiratory health by providing colonization resistance against pathogens and maintaining homeostasis,” he says. Certain management practices such as weaning, transportation, feed transition and antibiotic application can disrupt the respiratory microbiota, potentially altering pathogen colonization.” Development of bacterial therapeutics that target restoring the normal respiratory microbiota could provide alternatives to antibiotics, he adds, while understanding the microbiota also could enhance diagnostics and predictive ability for BRD.
From the University of Nebraska veterinary Diagnostics Center, John Loy, DVM, PhD discussed the role of molecular diagnostics for more accurately identifying BRD pathogens. Opportunistic pathogens, he says, provide unique diagnostic challenges due to variation in virulence factors, immune pressure, host biology and environmental factors.
Currently, he says, cell cultures serve as the gold standard for diagnosing most bacterial pathogens, but the process takes time, technical expertise and high-quality samples, and results are primarily qualitative. Antibody or antigen-based tests require good-quality tissues and laboratory expertise.
Molecular diagnostic tools such as real-time polymerase chain reaction (PCR) tests require significant investment in equipment, but can provide superior speed and accuracy. Sensitivity and specificity of the tests are equivalent to cultures for most bacterial pathogens, and Loy says PCR does better at detecting Histophilus somni, probably detecting positive samples missed by culture methods. Sensitivity and specificity also are higher for PCR for analyzing samples from nasal swabs, and PCR improves co-detection significantly.
PCR tests also can detect specific viral agents, helping identify trends in respiratory virus populations, and can identify resistance genes for antibiotic sensitivity testing.
For more summaries from the BRD Symposium and Academy of Veterinary Consultants Conference, see these articles from BovineVetOnline: