BVDV-Resistant Calf Created Through Gene Editing
The application of gene editing in cattle has blazed another frontier: bovine viral diarrhea (BVDV) resistance.
Previous gene-editing research has produced polled calves that do not require dehorning, and calves with lighter-colored haircoats that are more tolerant of heat stress. Now, USDA researchers, in cooperation with the University of Nebraska, have completed an extensive study that produced a calf proven to have reduced susceptibility to BVDV.
The BVD virus remains one the most challenging cattle diseases in the world. It affects animal health in many ways because it can impair the gastrointestinal tract, respiratory system, and reproductive functions. It also can be a silent scourge, because persistently infected (PI) animals can quietly spread it with herd mates while appearing perfectly healthy themselves.
BVDV is also a crafty virus. Vaccines against it have been available for decades, but its “extensive antigenic diversity in circulating field strains of BVDV poses a challenge in making these vaccines broadly protective,” according to USDA researchers.
In this project, the researchers used CRISPR/Cas9 gene editing technology to replace 6 amino acids in the CD46 gene. The bovine CD46 gene is the sight within the cell to which the BVD virus cleaves and gains entry to infect and replicate in a new host animal.
Cloned embryos from Gir cattle were used in the experiment. (Gir is a Zebu breed and one of the most prominent cattle breeds in India). The edited cells were transferred to some of the embryos, while the other half were left unedited and served as “wild-type” controls. Eight of each type of embryo were implanted into recipient cows.
Of the pregnancies that resulted, one edited and one non-edited fetus were harvested at 100 days to evaluate the BVDV resistance of cells from several body systems. Ultimately, one full-term pregnancy resulted from an edited embryo, and the calf was delivered by cesarean section at 285 days of gestation.
Because no live births resulted from the non-editing embryos, the edited calf was partnered shortly after birth with a newborn Holstein calf from a commercial dairy herd.
Both calves were evaluated for cell-level BVDV susceptibility via tissue and blood samples. The pair of calves also were directly exposed to BVDV from a live PI calf that was housed in the same room with them for 7 days when the study pair was 10 months old.
Among the study’s results:
- Cells from the kidney, lung, small intestine, esophagus, liver, and heart cells from the edited fetus showed significantly lower susceptibility to BVDV in a laboratory setting, compared to the unedited fetus.
- The live tissue samples showed that the edited calf had a significant reduction in BVDV susceptibility in the three cell types tested – skin fibroblasts, lymphocytes, and monocytes – compared to the unedited control calf.
- When exposed to the BVD virus via the live PI calf, both study calves ran a fever, but the edited calf did not exhibit the additional symptoms displayed by the control calf, which included cough, rhinitis, and redness and chafing around the nostrils. BVDV viremia was detected in the blood of both animals, but lasted 28 days in the control calf and only 3 days in the edited calf, which also displayed a significantly lower level of total infection load.
- At age 20 months, the edited calf was healthy and thriving, and displayed no “off-target” (unintended) gene edits as a result of the on-target edit.
Overall, the study showed that the gene editing measure did not make the calf entirely immune to BVDV, but did significantly improve the calf’s ability to withstand the viral challenge. The commercial application of such technology is yet to evolve, as this is the very first “proof of concept” study evaluating the practice of gene editing to build BVDV resistance.
But the researchers pointed out that the ability to aid cattle in resisting BVDV has potentially far-reaching implications. Animal health and welfare could be improved and production losses minimized. In addition, the procedure could reduce the use of antibiotics in food animal production because BVDV often leads to secondary infections that require antibiotic therapy.
The study also is the first to prove the ability of gene editing reduce the impact of a viral disease in general. As more work is done, the procedure could potentially be replicated to minimize other viral diseases as well.
