Gary Snowder, PhD, associate director of the National Center for Foreign Animal and Zoonotic Disease Defense, housed at Texas A&M University, outlines how environmental, stress and immunological factors have synergistic effects on BRD occurrence. As comprehension of these multiple factors increases, the likelihood of reducing BRD in livestock improves.
Both genetic and environmental factors influence an animal’s expression for resistance or susceptibility to a complex disease such as BRD, Snowder says. Genetic factors include the genetic makeup of both the host and the pathogens. Environmental factors include climate, management, nutrition, and production system. The interaction of genetic and environmental factors often determines whether or not an animal is infected and expresses the disease.
No single solution either genetically or environmentally should currently be expected to prevent BRD. The most likely solution to reducing the prevalence of BRD is a holistic approach addressing genetic, environmental, pathogenic, and immunological factors.
Researchers have only recently begun exploring the genetic factors influencing resistance to diseases, particularly BRD, Snowder says. The heritability estimates for resistance to BRD in preweaned calves have been reported as low to moderate, but he says when the estimated heritability is adjusted to an underlying continuous scale, it appears response to selection for BRD resistance could be large if the phenotype for BRD resistance was known. Research trials have identified differences in susceptibility between breeds, suggesting genetics plays a significant role.
Snowder notes that passively acquired maternal antibodies play a critical role in calf resistance to BRD, so the genetic potential of a dam to produce a high volume or concentration of maternal antibodies could be a factor in reducing BRD in preweaned calves. But some research has shown the opposite, with dams genetically superior for helping calves resist BRD actually producing more susceptible calves. One explanation, he says, might be that a dam’s increased ability to produce and/or sustain production of immunoglobulins for BRD could delay the development of the calf’s own immune system.
The science of genomics, Snowder says, might lead the way in defeating BRD in cattle. The map of the bovine genome provides researchers with a tool to aid their search to improve animal health, and they are just beginning to identify specific gene markers related to bovine health including resistance to BRD.
Scientists also are learning more about the genetic makeup of pathogens involved in BRD, which could lead to more efficacious vaccines or prophylactic treatment, as well as selection for naturally resistant animals.