Anyone who has struggled with the job of vaccinating cattle will quickly appreciate the value of Pat Shewen's current research. The Canadian veterinarian at the Ontario Veterinary College, University of Guelph, is working with colleagues there to develop an easier way to get vaccines into cattle, namely through alfalfa.

These researchers are among growing numbers who are trying to deliver vaccines — for both humans and animals — through plants. Their creations, called edible vaccines, are the latest high-tech pharmaceuticals. They're the result of genetically introducing specific proteins, called antigens, into plants such as corn, alfalfa and tobacco. Once high enough levels of the antigen are expressed in the plant, it may be fed to livestock, in some form, to build immunity. This no-fuss vaccination method would not only eliminate handling stress on the animal (and the vaccinators), it would reduce producer labor costs.

It's all in the antigen

Shewen, along with microbiologist Reggie Lo and plant geneticist Judy Strommer, have isolated the antigens necessary for prevention of shipping fever in cattle. “Alfalfa seemed like the logical plant to start with,” Shewen explains. “By taking advantage of the fact that cows already eat alfalfa and that it is only gradually broken down during rumination, we are getting good exposure to the antigens.”

The research team has successfully introduced one of the proteins into alfalfa and gotten significant expression in the plant. Shewen says early feeding trials also have been encouraging thus far, showing that unvaccinated calves exhibit response to the antigen. “That's just one antigen, though, and we are working toward developing a blended vaccine that would contain four to six antigens to make it more effective,” she says. “We're just over three years into this project, and to get to that point will likely take a few more years.”

The trio recently signed a collaboration agreement with Dow Agro-Sciences, ensuring their work will continue for at least four more years.

This type of vaccine potentially could be developed for any infectious disease of ruminants, Shewen says. “It wouldn't matter if it was bacterial or viral. You just need to determine which antigens will protect against it,” she says.

Regulatory unknowns

If the right antigen package were combined in alfalfa, the edible shipping fever vaccine would still face stringent and somewhat unknown regulatory hurdles. “This is a new concept for vaccine delivery and it will likely require a somewhat unique approval process, as far as production of the crop is concerned,” Shewen says. “From a vaccine standpoint, we would need to show that the alfalfa had measurable and consistent levels of the antigens and provide the same types of animal data that are required now for injectable vaccines.”

The process of defining how and where these crops will be grown has yet to be determined. But in the meantime, several U.S. companies are collaborating with other university researchers to develop edible vaccines for other species, including corn containing antigens to fight transmissible gastroenteritis virus, which can cause severe diarrhea and death in young pigs, and alfalfa containing antigens that prevent Infectious Bursal Disease in poultry.