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Understanding which farm factors drive Influenza infection

US producers and veterinarians have seen an influx of different types of influenza viruses in the last 10 to 15 years, and that is a major reason why influenza is more difficult to control, said Montserrat Torremorell, DVM, PhD, a professor in veterinary population medicine at the University of Minnesota.

“We had the same virus for many years — an H1N1, which is derived from the classic influenza virus,” Torremorell said. “But in the late 1990s and early 2000s, we had the introduction of triple reassorted viruses. Some of the genes in these viruses originate from people or from avian sources, in addition to swine. They started to create more diversity so multiple viruses were circulating at the farm level.”

The issues with influenza became even more accentuated with the 2009 H1N1 pandemic virus.

“That pandemic virus was introduced into the pigs and confirmed the fact that we were seeing the introduction of more viruses from human origin,” she continued. “Depending on what viruses are circulating in people, we have a higher risk of those viruses being introduced into the pigs, and the dynamics of the infections keep changing.”

Use of vaccine in US sow herds

Veterinarians have put more emphasis on controlling influenza in the past few years. Torremorell said producers commonly employ vaccination and biosecurity measures to reduce influenza A virus (IAV-S) incidence in their herds.

Breed-to-wean farms play an important role in spreading IAV-S because suckling piglets maintain, diversify and transmit IAV-S at weaning to other farms. Understanding the nature and extent of which farm factors drive IAV-S infection in piglets is a prerequisite to reduce the burden of influenza in swine.

A study by Fabian Chamba Pardo, et al., evaluated the association between IAV-S infection in piglets at weaning and farm factors, including farm features, herd management practices and gilt- and piglet-specific management procedures performed at the farm.1 Torremorell was a principal in the study, which started in 2012 with data assembling.

“The  purpose of the study was to look at the effect of vaccination of sows and see how vaccination affected prevalence of IAV-S at weaning,” Torremorell said.

Prior to that time, most of the published studies had evaluated the effects of IAV-S vaccination either experimentally or on very few farms. They were not long-term studies, so it was difficult to determine what strategies were most effective over a large number of farms in a practical setting.

“This was the first attempt to evaluate IAV-S vaccination on sow farms and study its effect on virus prevalence at weaning,” Torremorell said. “We also wanted to look at prevalence because it’s a direct measure of the virus activity.”

The researchers identified farms that were either vaccinating sows pre-farrow or using mass vaccination twice a year, because those were — and still are — the two most common protocols. They also wanted to look at herds that were not being vaccinated at all and see how virus prevalence levels at weaning would change. The long-term goal for producers is to wean an influenza-negative pig, or at least one that is close to being negative, following a similar control approach for porcine reproductive and respiratory syndrome (PRRS), Torremorell said. Researchers were also able to determine whether producers had used an autogenous vaccine or a commercial vaccine.

“It was a prospective longitudinal study,” Torremorell explained. “We enrolled 52 large farms [2,500 to 3,000 sows] and monitored them monthly for 6 months. When we did that study, the protocol was not very common so this study helped us establish baseline surveillance and identify tools that could be used to measure the effect of the vaccine.”
To assess the effects of sow vaccination protocols on influenza infections in pigs at weaning, researchers selected 30 pigs at weaning, one pig per litter, and those pigs were sampled individually with a nasal swab.

“We had representative samples from 30 litters on each of the 52 farms,” Torremorell said. “The samples were collected by farm personnel and then shipped to us at the University of Minnesota.

“The idea of the sample size was to detect at least one positive, which would indicate some degree of infection, assuming there was a prevalence of at least 10%,” she added. “It was a good starting point at the time.”

The study provided important primary findings:

  • Vaccination is helpful. That finding on its own was valuable, Torremorell said, because the industry wanted to know if sow vaccination was helpful in terms of the virus level of the pigs.
  • No difference in the protocols of vaccinating pre-farrow or vaccinating twice a year when results were shown throughout a 6-month period. “Although in this study there was no difference between both protocols, depending on how you measure and how long you measure the effect of vaccination, the whole-herd protocol (mass vaccination) may have a small advantage compared to the pre-farrow vaccination,” Torremorell said. “For the purpose of this study we did not see a difference, and I think it’s helpful to know different protocols can be used to control influenza.”
  • Influenza is seasonal. Both the vaccinated and non-vaccinated farms showed the most influenza in piglets during the winter and spring, and incidence went down in the summer. “From a design perspective, that’s important,” Torremorell said. “We had farms representing 12 months, which was a strength of the study.”
  • Little difference between traditional, conventional, inactivated or killed vaccines versus the autogenous vaccines. “The autogenous vaccines are supposed to include the strains that are circulating on your farm, so they should have an advantage compared to commercial ones, but in this study we did not see a difference,” Torremorell said. “It’s an interesting observation, but as part of the study it could be improved or repeated in a follow-up study with better information about what is being used in the vaccine and what is circulating at the farm level.” In a follow-up study that used the samples from this study, researchers did characterize the viruses that were circulating at the herd level. They discovered multiple strains were circulating, which may have explained why the autogenous vaccines did not have an advantage over the commercial vaccines.

“One of the take-home messages from this study and from additional research we’ve done is that if you are serious about trying to control influenza, you need a comprehensive plan,” she said. “You have to have surveillance in place, and you need some basic management strategies, including vaccination. Then you measure and put emphasis in the areas that need it.

“Extrapolate the data cautiously because at the end of the day it’s still a small subset of farms when you look at the total number of farms in the US. You want to make sure that’s taken into account when interpreting the results,” Torremorell explained.

Future research needs

Researchers want to continue looking into protocols to help decrease the prevalence of influenza.

“Our research has focused a lot on factors that affect the piglet, with a goal of trying to wean a negative piglet,” Torremorell said. “That’s important. We also need research on whether gilts should be vaccinated or do they need to be influenza-negative? If they are positive when they arrive to the farm, what sort of protocols do we implement so that they don’t bring new viruses onto the farm? We know they play a role; we just don’t know yet how to manage them or introduce them so that they don’t bring a risk to the rest of the population.”

From a biosecurity perspective, Torremorell said researchers know people play a role, but they don’t yet know how to manage the risks.

“More needs to be done to prevent transmission from people to pigs, and also from pigs to people, but that’s less important than from people to pigs,” she said.

Research is needed in the grow-finish population as well, Torremorell said.

“This study did not even address or consider if we would be vaccinating growing pigs or piglets at weaning,” she said. “We need a better understanding of what happens in grow-finish populations because the cost of influenza is in the growing-pig population.”

The parts are just as important as the whole

Having an influenza program in place is almost as important as the pieces that are part of the program, Torremorell said.

“Because we have this influx of different strains, unless you continue monitoring and tightening your biosecurity measures and managing the pig population, whatever you did that was working will stop working eventually,” she said.

“Monitoring helps you anticipate what you have to do. It’s like a PRRS program: Just because you either did the vaccination or an examination, once it’s done you can just forget. No, you have to make sure the herd continues to be stable. You have to take a comprehensive approach to the control of influenza,” she said. “There are things we’re still learning so we don’t have a specific recipe for controlling influenza yet, but this study is an important piece.”



1 Chamba Pardo F, Allerson M, Culhane M, Morrison R, Davies P, Perez A, Torremorell M. Effect of influenza A virus sow vaccination on infection in pigs at weaning: A prospective longitudinal study. Transbound Emerg Dis. 2020;00:1-11. https://pubmed.ncbi.nlm.nih.gov/32652870/


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US producers and veterinarians have seen an influx of different types of influenza viruses in the last 10 to 15 years, and that is a major reason why influenza is more difficult to control.

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Posted on September 3, 2021

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