Pigs can become coinfected with bird and human viruses, allowing genes to swap to form a new, more dangerous virus that can more easily infect humans. This phenomenon, called reassortment, is what gave rise to the 2009 H1N1 pandemic.
On Wednesday, October 30, 2024, USDA reported an avian influenza A(H5N1) virus infection in a pig on a backyard farm in Oregon. This is the first time an H5 bird flu infection has been reported in a pig in the United States.
The pig confirmed to have been infected was one of five on a farm in Crook County in south-central Oregon that was experiencing an H5N1 outbreak in poultry. The farm was put under quarantine and all five pigs on the farm were euthanized and sampled for additional laboratory testing.
Second Pig Confirmed as Positive for H5
The USDA National Veterinary Services Laboratories (NVSL) this week completed the testing on the remaining pigs at the farm and has confirmed that a second pig tested positive for HPAI H5N1. Because the amount of virus from the infected pigs was very low, only partial genomic sequences could be extracted from one of the two samples and these sequences indicate infection with the D1.2 genotype of H5N1, the same as found in the samples from poultry on the farm.
Sequencing Points to Migratory Birds as Infection Source
Genomic sequencing of samples from migratory birds in the area showed very similar sequences, which increases the probability that the pigs and poultry on this farm became infected after coming into contact with infected migratory birds, not dairy cattle or other livestock.
Swine as a Virus “Mixing Vessel”
The discovery that an avian influenza A virus has infected a new mammal species is always concerning, especially when the virus is detected in pigs, which are susceptible to influenza viruses circulating in pigs, humans, birds, and other species.
These flu viruses can swap genes through a process called genetic reassortment, which can occur when two (or more) influenza viruses infect a single host. Reassortment can result in the emergence of new influenza A viruses with new or different properties, such as the ability to spread more easily among animals or people. Reassortment events have happened in pigs in the past. A series of reassortment events in pigs is believed to have caused the 2009 influenza A(H1N1) pandemic. Based on available information, the risk to the general public remains low; however, CDC is continuing to gather information.
Threat Assessment
This farm is a non-commercial operation, and the animals were not intended for the commercial food supply. Per USDA APHIS, there is no concern about the safety of the nation’s pork supply as a result of this finding.
In addition, sequence data from birds in the avian influenza A(H5) virus outbreak on this backyard farm showed no mutations that caused concerns related to disease severity or adaptability to humans.
There is currently no evidence of any person-to-person spread in the United States.
Last week, as part of USDA’s broader efforts to combat the spread of H5N1, APHIS announced plans to partner with State and animal health officials to enhance testing and monitoring for H5N1, building on measures taken by USDA since the beginning of the avian influenza outbreak. This strategy builds on measure taken by states, as well as national risk mitigation measures including the Federal Order requiring testing of lactating cattle prior to interstate movement, which was announced in April 2024.
USDA continues to emphasize to farmers nationwide that biosecurity is the best weapon against the spread of H5N1, and farms should practice good biosecurity even if the virus has not been detected in their state or vicinity. Data collected over the past seven months has shown that H5N1 can be transmitted on equipment, people, or other items that move from farm to farm, including between dairies and poultry facilities.
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Origins of the 1918 Pandemic: Revisiting the Swine “Mixing Vessel” Hypothesis
How influenza A viruses host-jump from animal reservoir species to humans, which can initiate global pandemics, is a central question in pathogen evolution. The zoonotic and spatial origins of the influenza virus associated with the “Spanish flu” pandemic of 1918 have been debated for decades. Outbreaks of respiratory disease in US swine occurred concurrently with disease in humans, raising the possibility that the 1918 virus originated in pigs. Swine also were proposed as “mixing vessel” intermediary hosts between birds and humans during the 1957 Asian and 1968 Hong Kong pandemics.
Increased surveillance in swine has revealed that human-to-swine transmission actually occurs far more frequently than the reverse, and there is no empirical evidence that swine played a role in the emergence of human influenza in 1918, 1957, or 1968. Swine-to-human transmission occurs periodically and can trigger pandemics, as in 2009. But swine are not necessary to mediate the establishment of avian viruses in humans, which invites new perspectives on the evolutionary processes underlying pandemic emergence. American Journal of Epidemiology
Influenza A Viruses in Swine Population – a Scientific Review
Having a high zoonotic potential, swine play an important role in the ecology and spread of influenza to humans. This study reviews the state of the scientific literature on the zoonotic spread of swine influenza A viruses among humans, their circulation in swine populations worldwide, reverse zoonosis from humans to swine, and their role in interspecies transmission. The analysis covers a long period to trace the ecology and evolutionary history of influenza A viruses in swine. Swine have specific receptors for avian, porcine, and human IAV in the tracheal epithelium, which may contribute to the emergence of new reassortant variants that could potentially pose a public health threat. Viruses
Susceptibility of Swine to H5 And H7 Low Pathogenic Avian Influenza Viruses
Low-pathogenic avian influenza (LPAI) strains from various bird populations within the United States are capable of infecting pigs. Although adaptability and transmission of individual strains seem unlikely, the subclinical nature of the infections demonstrates the need to improve sampling and testing methods to more accurately measure incidence of LPAI virus infection in pigs, and their potential role in human-zoonotic LPAI virus dynamics. Influenza and Other Respiratory Viruses
Are Pigs Overestimated as a Source of Zoonotic Influenza Viruses?
The relationship of porcine and human populations with respect to mutual transmissions of influenza A viruses (IAV) is complex. Swine populations reportedly maintain the circulation of swIAV with zoonotic and rarely (pre)pandemic potential. Thus, the importance of pig populations as a source of zoonotic IAV should not be underestimated. On the other hand, decades of intensive pig rearing have not produced frequent swine-to-human transmissions that resulted in new, sustained human IAV lineages. The emergence of the most recent human IAV pandemic in 2009, however, has clearly demonstrated the principal risk of swine populations in which IAV circulate unimpededly. Therefore, the most important lesson to be learnt is to implement regular and close-meshed IAV surveillance of domestic swine populations to be able to follow the dynamics of swIAV evolution. Porcine Health Management