Is H5N1 a threat to U.S. swine herds?

Understanding the status of influenza in U.S. pig populations through targeted surveillance provides information on virus evolution.

May 3, 2024

5 Min Read
National Pork Board

The Swine Health Information Center, in collaboration with the American Association of Swine Veterinarians, recently hosted a webinar on influenza A viruses, with the goal being to understand the threat highly pathogenic avian influenza H5N1 poses to domestic livestock species and to inform producers of actions that can be taken to prevent infection on-farm.

Amy Baker, research veterinary medical officer at the USDA National Animal Disease Center, kicked off the discussion wiht an overview of influenza A virus in swine. Influenza A has a negative strand RNA genome, is enveloped and contains two major surface glycoproteins – hemagglutinin and neuraminidase. Influenza A is prone to rapid evolution by two main processes – genetic mutation and reassortment – and that the virus escapes population immunity by antigenic drift and/or shift. With reassortment, influenza A evolution can lead to antigenic shift which was seen in the 2009 H1N1 pandemic in humans.

Baker noted influenza is constantly changing and shared that wild waterfowl are a native host for the virus. Influenza from waterfowl has the potential to infect swine and those same viruses can also interact with humans and poultry. During her presentation, Baker highlighted how several species, including pigs, birds, wild mammals and humans, have the potential to serve as mixing vessels and share the influenza A virus among mammalian species.

She also reviewed the ongoing USDA IAV surveillance in swine, noting the system has been active since 2009. Viruses identified at veterinary diagnostic laboratories are initially screened for the presence of IAV through PCR. The current testing methodology can detect the different H and N glycoproteins including H5N1. Cases that meet specific criteria can then be added into the surveillance for further whole genome sequences.

Three sample streams are incorporated into the current surveillance system, including:

  1. Case-compatible swine accessions to the NAHLN laboratories

  2. Swine population samples epidemiologically linked to a human case of IAV

  3. Swine exhibiting influenza-like illness at commingling events such as fairs or exhibition events

Goals of the surveillance system are to monitor the genetic evolution of endemic IAV in swine and make influenza isolates from swine available for research. Further, goals include establishing a data management system for genetic analysis to facilitate the development of relevant diagnostic reagents, updating diagnostic assays, and identifying proper isolates for vaccine seed stock.

The ongoing surveillance system helps to address the human/swine influenza A interface. Baker noted that human influenza A greatly influences the pathogen’s diversity in swine due to interspecies transmission from humans to swine. Surveillance of influenza viruses assists with the One Health approach to understand swine influenza A hemagglutinin and neuramidase diversity in the U.S. and globally. Human variant cases from swine hemagglutinin clades are detected globally and monitored by the World Health Organization Collaborating Centres of the Global Influenza Surveillance and Response System. The USDA Swine Surveillance system provides valuable information regarding the evolution of the virus within the U.S. to benefit swine and human populations. Baker highlights the public health-animal health collaboration within the influenza community as a success story. While the current HPAI H5N1 panzootic is a concern for swine health and human pandemic preparedness, robust surveillance and disease investigation are the foundation for improving intervention strategies for animal and public health, Baker concluded.

H5N1: Awareness, precautions critical

Bailey Arruda, research veterinary medical officer with the USDA Agricultural Research Service at the National Animal Disease Center in Ames, began her presentation by discussing the differences between HPAI H5N1 and low pathogenic avian influenza. The HPAI designation refers to a specific clinical presentation in poultry based on the ability of the virus to replicate outside of the gastrointestinal tract. She said there have been multiple introductions of HPAI from 2021 to present with viruses of European decent being maintained in the U.S.

A change in global HPAI epidemiology due to unprecedented detections of the virus in many different mammalian species has increased research interest and concern regarding the potential for mammalian adaptation of the virus. Arruda shared HPAI detections in wild birds are on the rise and incidence in commercial poultry flocks are continuing as well. Swine adapted influenza A infection in the U.S. is common in commercial production and routinely monitored, per data from the SHIC-funded swine disease reporting system and the USDA surveillance.

To understand the risk of HPAI, it is important to note that multiple adaptations of the influenza A virus are required to overcome species restriction, stated Arruda.

In one study led by Arruda and colleagues to characterize the divergent pathology and transmission among avian and mammalian origin isolates of HPAI H5N1 in swine, they discovered that overt clinical signs were not observed in pigs after experimental challenge. All isolates caused lung lesions consistent with influenza A virus infection, but differences included the variability of lesion severity across strains, the antigen distribution and cells affected, and only mammalian isolates had limited nasal shedding and partial transmission to direct contact pigs.

In another recently completed study with full analysis pending, research observed that HPAI H5N1 strains vary in their clinical presentation post exposure in pigs. Based on results from this study, HPAI may need to be considered for the differential list when pigs present with neurologic clinical signs.

From research outcomes, Arruda stated that pigs are at risk from hemagglutinin H5 circulating strains and reassortment with endemic swine strains is a concern. She also stated that risk is higher for incursion into swine located in backyard multi-species or transitional outdoor pig farms that include the presence of poultry and/or wild waterfowl species. Finally, risk of incursion into conventional confinement swine operations in the U.S. is likely low, but awareness and precautions are critical.

Influenza A viruses are constantly evolving and pose a risk to domestic livestock species. To address and mitigate the impact of emerging influenza strains for producers, SHIC and AASV collaborated to provide this influenza A virus webinar with the latest information on influenza in domestic livestock species. Prevention efforts should focus on actions to reduce biosecurity risks including the use of outbreak investigation tools to identify biosecurity hazards at the farm. Understanding the status of influenza in U.S. pig populations through targeted surveillance provides information on virus evolution, distribution and support of diagnostic tests and vaccines. Influenza viruses present a challenge for swine health and production and tools exist to reduce the impact this disease can have on pork producers.

The webinar recording is available here.

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