SHIC/AASV Webinar Addresses H5N1 Influenza Risk to US Swine

The Swine Health Information Center, in collaboration with the American Association of Swine Veterinarians, hosted a webinar on H5N1 influenza risk to US swine on November 20, 2024. The goal of the webinar was to provide current information on H5N1 in livestock including updates on the H5N1 outbreak in dairy cattle, the first detection of H5N1 in a pig on a small backyard farm in Oregon, research on H5N1 in swine, and a literature review covering gaps in knowledge for H5N1. Additionally, an update on the use of aspirin in livestock was presented.

The webinar can be accessed here.

Fred Gingrich, II, DVM, executive director of the American Association of Bovine Practitioners, shared information on the H5N1 outbreak in dairy cattle first identified in a Texas dairy herd exhibiting unusual clinical signs of illness. On March 25, 2024, USDA officially announced the first isolation of influenza A H5N1 in milk from an affected cow. The initial introduction of H5N1 into dairy cattle was likely from wild birds, potentially as early as December 2023. But since that time, the source of virus transmission among dairy herds is thought to be from infected cattle.

Dr. Gingrich stated that clinical signs of virus infection and impact on lactating dairy cows are different from infection in poultry and wild birds. In dairy cattle, there is low morbidity of 20% – 30% with a low mortality of 2% – 5% reported. He noted that there is no need for depopulation as cattle recover from the infection. H5N1 affects mostly lactating dairy cows, and clinical signs include decreased appetite and rumination, tacky manure, decreased milk production, and milk turning abnormally thick and yellow. Herd-level production losses can average 20% and the long-term impact on herds is yet unknown. ​

Recent research into the dairy outbreak has shown that bulk milk tank samples can have PCR positive test results 10 to 14 days before peak clinical signs are observed. This offers an opportunity for increased surveillance for early detection and can facilitate implementation of biosecurity measures for preventing transmission. Biosecurity remains a challenge due to the segmented dairy industry structure and specialized production sectors. Transportation of adult and young stock cattle occurs daily, sometimes over long distances. Continued development of biosecurity to decrease disease transmission is critically important. Moving forward, Dr. Gingrich said safe movement will be more manageable than stopping movement in the event of an outbreak. Other needed methods for disease control include the development of safe and effective vaccines.

Dr. Gingrich also noted that the dairy industry has worked closely with FDA and USDA to ensure food safety for consumption of products. FDA has performed testing on bulk tank samples of raw milk to confirm that pasteurized milk is safe to consume while unpasteurized milk carries a significant risk for pathogens. ​In addition, USDA has confirmed the safety of eating properly handled and cooked beef. Dr. Gingrich noted that the USDA plan for national bulk tank surveillance is critically important, and that this surveillance effort continues to inform efforts for effective disease prevention and control in the US dairy industry.

Ryan Scholz, DVM, MPH, State Veterinarian of Oregon, provided an overview of the first detection of H5N1 in a pig co-housed with poultry on a small backyard farm. The call to the Oregon Department of Agriculture on October 22, 2024, was initiated due to sick and dying birds observed on the farm. Birds on the farm included ducks, geese, peafowl, and chickens. There were three kunekune pigs housed with sheep and goats and two teacup pigs housed with chickens. A USDA technician was sent to the farm to sample affected birds. The kunekune pigs were sampled because they had direct contact with dead waterfowl on the farm but none of the pigs showed any clinical signs of illness. Samples were submitted to the Oregon Veterinary Diagnostic Lab for testing. After the initial detection of H5N1 in birds and one pig, all poultry were depopulated, and the decision was made to euthanize all of the pigs for a comprehensive diagnostic case evaluation. Samples from one kunekune pig were PCR positive including nasal and tracheal swabs, brain tissue, heart, and lymph nodes, and one teacup mini pig had PCR positive samples of tracheal tissue and tracheal swab. Virus sequencing from poultry sampled on the farm confirmed detection of the D1.2 genotype of H5N1.

Dr. Scholz noted that transitional and backyard pigs are significantly different from commercial swine production for pork. The handling of backyard animals and sampling techniques present unique challenges and alternative methods to address these challenges should be considered. Lifestyle and backyard farms tend to have limited to no biosecurity due to outdoor exposure and direct contact with multiple species. He noted that the knowledge and implementation of biosecurity for small holdings and lifestyle farms can be minimal and resources to address the gap in knowledge are needed. He also said partnership with public health officials is important, especially when dealing with potential zoonotic diseases, as part of emergency response planning.  

Bailey Arruda, DVM, PhD, Research Veterinary Medical Officer, USDA-ARS, National Animal Disease Center, described the new paradigm of the now panzootic H5N1 virus and updated results on H5N1 research in swine. She noted that the number of mammalian species infected with the H5N1 virus since 2021 has significantly increased. The virus naturally changes over time, exhibiting reassortment that results in co-circulating genotypes and potentially different phenotypes. Dr. Arruda noted that in the past, H5N1 viruses contained a Eurasian N1 but some more recent viruses  contain a North American N1.  

Dr. Arruda reviewed the protocol for H5N1 virus challenge studies at the NADC BSL-3 facilities with weaned pigs negative for influenza A virus, PRRSV and Mycoplasma hyopneumoniae. Multiple strains of H5N1 have been used in challenge studies with pigs, but until recently, the viruses were not sourced from the current H5N1 dairy isolates. The goals of the challenge studies are to characterize clinical signs and pathogenesis of each isolate and to generate known positive samples for diagnostic test development and evaluation. 

In summarizing the H5N1 dairy isolate studies, Dr. Arruda noted that three H5N1 strains were utilized from contemporary dairy herd outbreaks, including two cattle origin isolates (cattle/TX/008 and cattle/TX/002) and one from a human case associated with an infected dairy herd (TX/37). In general, pigs exhibited minimal to mild respiratory signs with some showing short-term fever. No pigs exhibited neurologic signs of disease. At necropsy, mild tissue lesions were noted. Virus was found in brain samples for TX/37 and cattle/TX/002 and virus was identified in heart and spleen for all three virus strains. All pigs were positive by nasal swabs across the three viruses studied. Results from additional sample types are pending.

Dr. Arruda concluded by saying pigs are at risk to circulating strains of H5N1, noting reassortment with endemic swine IAV strains is of concern. Clinical signs in the experimentally infected pigs were mild, and there is the potential to overlook infection. For sampling of ill pigs, nasal swabs were shown to be able to detect H5N1. To reduce the risk of introduction into a swine herd, producers should consider biosecurity steps including avoiding the use of untreated surface water, ensuring bird-proofing, restricting scavenger mammals in or around barns, not feeding pigs unpasteurized milk products, and evaluating biosecurity risks posed by area dairy or poultry farms, including shared resources or personnel.

Montse Torremorell, DVM, PhD, Professor, University of Minnesota, shared the results of a recently completed SHIC-funded literature review on H5N1 risks to swine. She stated that the H5N1 2.3.4.4b genotype has demonstrated the ability to spread globally across many different species with clinical presentations ranging from high mortality to subclinical infection. H5N1 genotype 2.3.4.4b can infect pigs and there are concerns that infection with H5N1 will not exhibit similar clinical signs to endemic strains currently seen in commercial pigs.

Dr. Torremorell reviewed that H5N1 viruses exhibit variable clinical presentations, lesions, and transmission risks in pigs, depending on the genotype and animal species origin of the isolate. Knowledge gaps exist in pathogenesis and transmission, accurate methods and appropriate samples for detection, routes of infection, identification of the potential source of introduction, and understanding the virus’s complex epidemiological picture.  

Dr. Torremorell noted outbreak investigations are necessary for gaining needed information for prevention and control of the virus. Investigations can help identify epidemiological links between poultry, swine, and cattle while also elucidating potential pathways of infection and assessing risk of transmission. Understanding the role of nonclinical pigs, identifying the duration of virus infection and shedding on a population level, and quantifying the risk of interactions between other species and shared resources will be important to implement effective intervention strategies.

Defining the applicability of commonly collected swine samples, such as udder wipes, processing fluids, oral fluids and water samples, for accurate and rapid detection of H5N1 is needed. Accurate surveillance can address knowledge gaps around the potential for H5N1 2.3.4.4b reassortment with other endemic swine influenza strains and the impact of those changes for risks to animal and public health. Further knowledge gaps include the impact of partial immunity from endemic viruses, the role of vaccination for protection against H5N1, risk from mammalian adaptation viruses, current and common biosecurity practice effectiveness, aerosol transmission, and effective management practices for virus control.

The final presentation was provided by Locke Karriker, DVM, MS, DACVPM, reviewing the regulatory situation regarding aspirin use in swine. An AASV summary of his presentation can be found here.

The Swine Health Information Center, launched in 2015 with Pork Checkoff funding, protects and enhances the health of the US swine herd by minimizing the impact of emerging disease threats through preparedness, coordinated communications, global disease monitoring, analysis of swine health data, and targeted research investments. As a conduit of information and research, SHIC encourages sharing of its publications and research. Forward, reprint, and quote SHIC material freely. For more information, visit http://www.swinehealth.org or contact Dr. Megan Niederwerder at [email protected] or Dr. Lisa Becton at [email protected].