GENETIC RESISTANCE TO NEWCASTLE DISEASE
The existence of a genetic basis for innate resistance to NDV has been known for over a half century, making it one of the earliest known associations between animal genetics and disease resistance. However, it is not until today, with advanced knowledge of the chicken genome sequence, and appropriate technical platforms, that the actual basis of that genetic resistance can be accurately and comprehensively defined and efficiently utilized to improve health in poultry. Past studies have amply documented that genetic variation exists in the response of both chickens and turkeys to NDV.
These studies have primarily documented that different genetic lines, including lines derived from commercial stock and indigenous lines, have different responses to the virus. However, the effects of specific genes such as the major histocompatibility complex and parental effects have also been examined. Pitcovski et al. (1987) clearly demonstrated that divergent genetic selection for antibody response to two vaccines (NDV and E. coli) from one base population was successful, and estimated the very high realized family heritability of 0.70 after four generations of selection for the combined response to the two vaccines.
The exact relationships of response to other antigens and production traits such as growth, however, have presented a mixed picture in regards to whether they are positively or negatively correlated with response to NDV and therefore require additional study. Thus, prior studies have provided clear evidence for the feasibility of identifying the underlying genetics in chickens that determines response and resistance to NDV. The proposed studies not only will define the genetic basis of innate resistance to NDV, but will also provide essential knowledge of value to existing projects that are developing new vaccine approaches to control the disease. For example, Deltamune of South Africa and MCI Santé Animale of Morocco have been funded by GALVmed to manufacture the I2 strain of ND vaccine for use in Africa. Because host genetic resistance is synergistic with the protection provided by vaccines, having both disease control methods (host genetic resistance and vaccination) available will ensure enhanced protection of poultry in Africa against NDV.
The magnitude of improvement in resistance to NDV that can be achieved through genetic selection is not known at this time, because the genetic lines that this project will utilize have not yet been thoroughly analyzed and the advanced genetic technologies (high-density SNP chips and RNA-seq) that will be used have only recently become available. However, based upon the proven genetic variance of host resistance to NDV (described in the previous paragraph), and results with other poultry diseases, we expect that the improvement in NDV resistance can be substantial.
The predominant successful example of genetic selection for resistance to disease in poultry is that of Marek’s Disease (MD), caused by a herpes virus. This case also shows the synergy of genetic improvement and vaccine use, because the use of the MD vaccine is nearly ubiquitous in commercial chicken production. One predominant gene complex, the major histocompatibility complex (MHC), has a major impact on outcome of infection with MDV and it became the target of genetic improvement in commercial lines for decades. However, like most complex traits, many genes influence the outcome of MDV infection; the current availability of genomic technologies is enabling the identification of the other important genomic regions and specific genes, and their use in genomic improvement programs. We anticipate similar successes with our work on NDV host resistance genetics.