Research confirms co-circulation of multiple influenza strains

Influenza A virus in swine (IAV-S) is increasingly challenging for US producers, primarily because the epidemiology of IAV-S involves a highly diverse pool of viruses, said Micah Jansen, DVM, veterinary manager of US Pork at Zoetis.

The diversity of the virus along with its ability to move between and within species continue to make it a management struggle. Recent research sheds light on this ever-changing virus and will ultimately help producers and veterinarians find new ways to control it.

Reassortment is the process by which influenza viruses swap gene segments. This genetic exchange is possible due to the segmented nature of the viral genome and occurs when two differing influenza viruses co-infect a cell.

A study by the University of Minnesota and Zoetis[1] confirmed that reassortment of influenza occurs, and it may be occurring more frequently than researchers realized, Jansen said. The study also showed that appropriate vaccination against influenza can reduce the amount of reassortment that takes place, she told Pig Health Today.

“We see a lot of co-circulation of multiple influenza virus strains within US swine herds, so we know the possibility [of reassortment] is there,” Jansen said.

“Because of its segmented genome, it can easily exchange those segments of genetic material back and forth between different viruses,” she noted. “That allows a new virus to be created, and we can see a big change genetically in a very short period of time. We often refer to that as antigenic shift.”

Recent research

Zoetis worked with the University of Minnesota to develop a project that looked at the occurrence of reassortment in pigs with various vaccination protocols.

“There was reassortment in a pretty short period of time following challenge with two different strains of influenza,” Jansen said.

Different types of vaccination protocols were utilized in the study, including “prime boost.”

“Anytime you give two doses of vaccine, regardless of what you use, that would be considered prime boost,” she explained. “You’re priming the immune system with one vaccine dose, and you’re boosting the immune system with a second dose.”

Homologous prime-boost vaccination is described as giving two doses of the same vaccine.

“What we have been exploring more recently against influenza in pigs is this concept of heterologous prime boost,” she said. “With heterologous prime boost, we prime the immune system with one vaccine and then we boost…with a completely different vaccine. In theory, we are activating the immune system in more than one way, which helps it become better able to recognize pathogens.”

The study incorporated homologous prime-boost vaccinations, heterologous prime-boost vaccination and a live attenuated influenza virus vaccine. The idea was to explore vaccine protocols from a viral shedding and immune response standpoint, as well as to identify how much the virus changed in the pigs, Jansen explained.

Both studies (the reassortment study and the larger study on the efficacy of vaccination protocols) were pilot studies, Jansen said, and will ultimately need to be repeated on a larger scale.

Ten animals per treatment group were used in the larger study, and then a smaller segment of those animals was included in the reassortment study because they needed to meet certain criteria.

Key results

“We did see a benefit to vaccination against influenza in reducing reassortment,” Jansen said. “If we only look at the presence of vaccination, whether homologous prime-boost vaccination, heterologous prime-boost vaccination or even a single dose of vaccine that was a live, attenuated influenza virus, we didn’t see any difference in reassortment between vaccinated animals and non-vaccinated animals.

“However, when we sorted off those that received a prime-boost protocol ⸺ either homologous or heterologous — we saw a statistical reduction in the number of reassortment viruses that were detected in those specific animals,” she added.

Take a holistic management approach

Because IAV-S is constantly changing, Jansen suggested producers and veterinarians approach influenza control holistically.

Appropriate vaccination is one part of the equation, but there are many more factors to consider, including biosecurity and management. How animals are moved within the herd and between herds is an important consideration, whether gilts or piglets.

“All of those things are extremely important in improving our ability to control influenza and the clinical disease that it can manifest,” she said. “We need to make sure that we’re approaching it holistically and not just thinking that we can find a vaccination protocol to implement and that will be a silver bullet.

“We want to…see if [the results from the studies] are repeatable in other situations before we can replicate the results on a commercial basis within the industry,” Jansen added. “We have a lot to explore, and we’re looking at different aspects both from a genetic and an immunology standpoint.

“We want to learn more about the flu virus and how it reassorts and changes in pigs,” she continued. “But we also want to learn more about the pig’s immune system and how different vaccination protocols can affect that as well.”

1 Li C, Culhane M, Cheeran M, Schroeder D, Galina-Pantoja L, Jansen M, Amodie D, Mellencamp M, College of Veterinary Medicine, Univ. of Minnesota, Zoetis, Parsippany, N.J. Effects of vaccination protocols on influenza A virus genetic diversity in co-challenged pigs. 2021 American Association of Swine Veterinarians Annual meeting, March, 2021.

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