When Pigs Fly: Pandemic influenza enters the
´dia S. TrovãoID, Martha I. NelsonID*
Fogarty International Center, National Institutes of Health, Bethesda, Maryland, United States of America
Influenza A viruses (IAVs) are one of the most intensively studied pathogens, due to the severe
global mortality and economic disruption associated with influenza pandemics . In addition
to annual epidemics, pandemics sporadically occur when a novel IAV host jumps to humans
from an animal reservoir . Wild waterfowl have long been considered the most important
reservoir host and pandemic risk, but there are indications that mammals are emerging as key
reservoirs. Here, we describe how the modernization of swine production during the last half
century provided new opportunities for IAVs to become established in swine globally [3–5],
resulting in the first influenza pandemic of swine origin in 2009 . A key lesson is that the
landscape of pandemic risk is not static but continuously shifting in response to demographic
changes in host populations. It is therefore vital to track how transformations in the economy
and global trade impact animal contact rates, disease dynamics, and pandemic risk among
livestock and companion animals.
For centuries, domestic pigs (Sus scrofa domesticus) were raised on traditional small-scale
farms that could not sustain IAV transmission. Whereas horses and people traveled frequently
within and between urban areas, causing recurrent influenza outbreaks in both species as far
back as the 13th century , influenza was not maintained in any country’s swine population
until 1918, when the Spanish influenza H1N1 pandemic virus was introduced from humans
into swine in the United States . The H1N1 virus circulated in US swine for most of the
20th century without substantially evolving, causing severe disease, or becoming endemic in
swine in other countries.
In the latter decades of the 20th century, the replacement of small-scale swine farms with
larger, more efficient production systems (Fig 1A) had profound effects on disease dynamics.
Enhanced biosecurity improved control of important pathogens, such as hog cholera. But
modern production systems often require pigs to be transported long distances between multi-
ple locations specialized in different growth stages, facilitating the spread of IAV in swine
(IAV-S; commonly known as swine flu) and other pathogens not specifically targeted for erad-
ication. For example, many large breeding operations located in the southern US find it more
efficient to transport fattening pigs to the Midwest “corn belt” than to transport the large vol-
umes of feed back to the south. By the 2000s, trucks were transporting millions of pigs long
distances across North America, facilitating the spread of IAV-S along established “swineways”
In addition, pigs were flying (Fig 1B and 1C). To meet the needs of expanding middle clas-
ses for animal-sourced protein, many countries imported more productive sows (female
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1008259 March 19, 2020 1 / 8
Citation: Trovão NS, Nelson MI (2020) When Pigs
Fly: Pandemic influenza enters the 21st century.
PLoS Pathog 16(3): e1008259. https://doi.org/
Editor: Katherine R. Spindler, University of
Michigan Medical School, UNITED STATES
Published: March 19, 2020
Copyright: This is an open access article, free of all
copyright, and may be freely reproduced,
distributed, transmitted, modified, built upon, or
otherwise used by anyone for any lawful purpose.
The work is made available under the Creative
Commons CC0 public domain dedication.
Funding: This article was funded by Centers of
Excellence for Influenza Research and Surveillance
(CEIRS), National Institute of Allergy and Infectious
Diseases (NIAID), National Institutes of Health
(NIH), and Department of Health and Human
Services (HHS), under contract number
HHSN272201400008C. The funders had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Fig 1. Trends in swine production. (A) Trends in consolidation of swine production in the US, 1964 to 2012 (data available from the US Department of Agriculture
and the National Agricultural Statistics Service Quick Stats Database). (B) Growth of global trade (US$) of live animals between all countries, 1961 to 2017 (data
available from FAOSTAT). (C) The global distribution and density of swine populations (approximately 1 billion animals) is depicted by points shaded along a
gradient from light red (1 to 5 swine per km
) to black (more than 250 swine per km
). Lines with arrows depict the direction and volume of routes of trade (US$) of
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1008259 March 19, 2020 2 / 8
breeding pigs) with improved genetics from North America and Europe. Imported swine
must be declared free of certain pathogens, such as African swine fever (ASF) or foot-and-
mouth disease (FMD). However, influenza is not routinely tested for, and many countries do
not quarantine, facilitating the long-distance spread of IAV-S [3,5] (Fig 2A). Using genetic
sequences of IAV-S collected in different countries over time, the direction and timing of viral
movements can be inferred from the evolutionary relationships depicted on phylogenetic
trees. Trees reveal how international trade of live swine in the 1990s and 2000s facilitated the
long-distance dispersal of IAV-S between trade partners, shaping the global spatial distribution
of IAV-S lineages that is observed today. Many countries in Asia import swine from different
continents (Fig 1C), introducing multiple divergent lineages and creating nexuses for genetic
diversity. By the end of the millennium, the high genetic and antigenic diversity of IAV-S
made it one of the most intractable diseases for swine producers in the US and present in most
swine-producing countries .
How did the first pandemic virus of swine origin evolve?
It is inherently difficult to predict when and where a pandemic virus will evolve. Pandemic
viruses have a rare combination of properties, including animal-origin surface proteins that
are antigenically divergent from human viruses and thus evade immune detection while
retaining a capacity to replicate and transmit in humans. The evolution of such a variant is
facilitated by a process termed “reassortment,” in which whole segments of the IAV genome
are exchanged between viruses coinfecting a host cell, rapidly repositioning genes into differ-
ent genetic backgrounds (Fig 2B). Reassortant viruses are therefore more likely to evolve in
locations where multiple distinct lineages cocirculate and in hosts with high capacities for reas-
sortment, such as wild birds, poultry, and swine. The pandemic viruses of 1957 (H2N2), 1968
(H3N2), and 2009 (H1N1) were all reassortants with chimeric genomes derived from multiple
lineages. The 2009 pandemic was the first to occur in the genomic era, providing large-scale
sequence data to understand how globalized swine production and long-distance viral migra-
tion contributed to the evolution of a novel reassortant virus.
The 2009 pandemic vividly demonstrated both the promise and persisting limitations of
outbreak investigation in the genomic era. Genetic sequencing of the first H1N1 pandemic
viruses isolated from humans in April 2009 rapidly determined that the reassortant virus was
comprised of three genetic lineages of swine origin (Fig 2B) . However, the country of origin
was unknown, due to geographical gaps in IAV-S surveillance. Many countries did not con-
sider IAV-S an important clinical disease. To their credit, the 2009 pandemic stimulated an
expansion of IAV-S research in many countries, filling gaps in our knowledge of IAV-S diver-
sity and evolution on a global scale [10–15]. Expanded surveillance in Mexico revealed how
IAV-S diversity expanded during the 1990s and 2000s, when swine were imported from
Europe and the US, introducing the three IAV-S lineages that reassorted to generate the pan-
demic virus . Going forward, trade flows can identify other countries that import live
swine (and potentially IAV-S) from multiple regions and are at higher risk for generating
novel reassortant viruses with pandemic potential .
An outstanding question is the specific circumstances by which the 2009 H1N1 virus trans-
mitted from swine to humans. Both IAV-S sequence data and epidemiological data in humans
support swine-to-human transmission occurring in Central Mexico [16,17]. However, by the
live swine, summarized by region and over the time period 1996 to 2012. Trade data available from United Nations Comtrade Database. Digital layers from GLW
(version 2.01)  were downloaded from the publicly available Livestock Geo-Wiki database. FAOSTAT, Food and Agriculture Organization Statistical Database
(United Nations); GLW, Gridded Livestock of the World.
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1008259 March 19, 2020 3 / 8
time the first human cases were detected by surveillance, the virus had already diversified
genetically, evidence of transmission in humans for several months . There are multiple
challenges to understanding the human–animal interface for IAVs: (1) There is a need for
coordination between animal health and public health research, (2) the genomics of host
switches are too complex to accurately predict zoonotic potential from genetic sequence alone,
and (3) spillover events can be rare and difficult to detect via traditional modes of virological
surveillance, particularly in developing countries.
Pigs and humans: Who is infecting whom?
It should be noted that the human–animal interface looks quite different from the perspective
of the pig. Only one IAV has successfully transmitted from swine to humans to cause a pan-
demic (2009) . In comparison, swine are continuously experiencing pandemics of human
origin. In the US alone, at least eight genetically distinct IAVs have successfully host jumped
from humans to swine (reverse zoonosis). On a global scale, there have been at least 20 success-
ful human-to-swine transmission events, defined by sustained onward transmission in swine
. This number could be much greater, as many countries do not routinely test for IAV-S.
Tellingly, almost every country’s IAV-S population that has been genetically characterized
includes viruses of human origin. Recently, viruses of human origin were identified in swine
in Australia and Chile that have circulated undetected for decades [21,22].
Notably, as soon as the 2009 H1N1 pandemic virus became established in humans, the
virus disseminated back to swine in at least 30 countries, spanning Africa, Asia, North and
South America, Europe, and Australia . In addition to seeding genetically novel viruses in
swine populations globally, the human-origin viruses frequently reassort with other IAV-S lin-
eages. The proliferation of new reassortants complicates control in swine, introducing new
strains that poorly match those included in commercially available vaccines, and presents new
risks for humans. As a case in point, novel IAV-S reassortants with segments derived from
human-origin pandemic viruses have been associated with over 450 zoonotic infections in the
US since 2011, largely in the context of agricultural fairs .
What happened to bird flu?
All three pandemics of the 20th century (1918, 1957, and 1968) were of avian origin, and birds
have been key sources of novel viruses in a range of other mammalian hosts, including swine,
equines, canines, and phocines. For decades, the large numbers of human infections in Asia
with IAVs of the H5N1 subtype and, more recently, H7N9 subtype were considered indica-
tions of impending pandemics, prompting governments to stockpile antivirals and vaccines
targeting H5 and H7 antigens. H5N1 and H7N9 viruses continue to infect humans, but the
absence of sustained human-to-human transmission to date underscores the difficulty of pre-
dicting pandemics, especially as large numbers of human infections and deaths are only one
measure of multifaceted pandemic risk .
Fig 2. IAV-S evolution. (A) Inferred spatial movements of the major Eurasian lineage of IAV-S (avian-like Eurasian H1N1) between countries,
inferred from a time-scaled MCC tree of the N1 segment. Lines represent general directions of movement inferred from available genetic data, and
actual paths may differ and include unsampled locations. (B) Genomic reassortment events between the three swine lineages that produced the 2009
H1N1 pandemic virus. Horizontal bars represent the eight individual segments of the IAV genome, ordered from longest (PB2, 2,277 nucleotides) to
shortest (NS, 890 nucleotides). HA, hemagglutinin; IAV, Influenza A virus; IAV-S, IAV of swine; MCC, maximum clade credibility; MP, matrix
protein; NA, neuraminidase; NP, nucleoprotein; NS, nonstructural protein; PA, polymerase acidic protein; PB1, polymerase basic protein 1; PB2,
polymerase basic protein 2.
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1008259 March 19, 2020 5 / 8
IAV evolution never follows predictions: What will the next surprise be?
Expanded surveillance in mammalian hosts has yielded several surprises, including divergent
IAVs in bats (H17 and H18) , a new genus of Orthomyxovirus (influenza D) in bovines
, and the establishment of IAV in canines (CIV) during the 2000s: CIV-H3N8 in the US
 and CIV-H3N2 in Asia . An outbreak of H7N2 in felines in New York  suggests
that other mammalian species could potentially be capable hosts for IAV transmission at cer-
tain thresholds of animal density and movement. However, IAVs have relatively low reproduc-
tive numbers (R), just above the threshold of 1 required for transmission, and require high
contact rates between susceptible hosts to maintain transmission. For example, CIV in the US
is maintained only in high-turnover animal shelters and dog daycares . In contrast to
swine, whose movements are tracked as livestock commodities and can be directly linked to
disease spread [5,9], efforts to understand and predict emerging threats in companion animals
are impeded by the low availability of public records and virological surveillance. Counterintu-
itively, shifting global attitudes towards dogs as domestic companions has facilitated the emer-
gence of CIV. In the US, CIV-H3N2 appears to be maintained in dog daycares, which have
proliferated and become mainstream. Furthermore, tThe recent introduction of CIV-H3N2
into the US spatial-temporally coincides with efforts by animal rights groups to rescue hun-
dreds of Asian meat dogs for US adoption, although to date no direct link has been established
between any rescue animals and the appearance of CIV-H3N2 in the US . Dogs entering
the US must provide documentation of rabies vaccination but otherwise require no disease
testing or quarantine. The most pronounced change in attitudes towards dogs has been in
China, where pet dogs are surging in popularity in urban areas after being banned for many
decades. High rates of CIV have been observed in pets in Chinese cities . It remains
unclear how populations of meat dogs, strays, and pets interact and collectively contribute to
CIV emergence and transmission. A diversity of swine-origin , human-origin , and
reassortant IAVs have been isolated from dogs in Asia but with unknown degrees of onward
transmission. High contact rates between canines and humans provides frequent opportunities
for zoonotic transmission, and further research is greatly needed to understand the extent to
which Asia’s rapidly expanding dog populations present a pandemic risk.
Transformations in the movement and care of livestock and companion animal populations
have altered animal contact rates, disease dynamics, and pandemic risk. At the same time,
however, technological developments are underway that could enhance pandemic response. A
commitment of resources has mobilized efforts to develop a universal influenza vaccine that
could broadly protect humans against all animal-origin strains , as well as technologies to
accelerate production of new vaccines during a pandemic. Harnessing new forms of digital,
social, and medical claims data could detect outbreaks faster, potentially limiting spread .
Portable nanopore sequencing technologies are capable of tracking epidemics on location to
guide intervention strategies . Given these advances, it is possible to imagine a future when
pandemic influenza no longer presents a threat to global health and security. However, these
initiatives still face a host of technological, logistical, and market-related challenges. In the race
between human ingenuity and pathogen evolution, influenza can never be underestimated.
1. van Wijhe M, Ingholt MM, Andreasen V, Simonsen L. Loose ends in the epidemiology of the 1918 Pan-
demic: Explaining the extreme mortality risk in young adults. Am J Epidemiol. 2018; 187: 2503–2510.
https://doi.org/10.1093/aje/kwy148 PMID: 30192906
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1008259 March 19, 2020 6 / 8
2. Webster RG, Bean WJ, Gorman OT, Chambers TM, Kawaoka Y. Evolution and ecology of influenza A
viruses. Microbiol Rev. 1992; 56: 152–179. PMID: 1579108
3. Vijaykrishna D, Smith GJD, Pybus OG, Zhu H, Bhatt S, Poon LLM, et al. Long-term evolution and trans-
mission dynamics of swine influenza A virus. Nature. 2011; 473: 519–522. https://doi.org/10.1038/
nature10004 PMID: 21614079
4. Lewis NS, Russell CA, Langat P, Anderson TK, Berger K, Bielejec F, et al. The global antigenic diversity
of swine influenza A viruses. Elife. 2016; 5.
5. Nelson MI, Viboud C, Vincent AL, Culhane MR, Detmer SE, Wentworth DE, et al. Global migration of
influenza A viruses in swine. Nat Commun. 2015; 6.
6. Smith GJD, Vijaykrishna D, Bahl J, Lycett SJ, Worobey M, Pybus OG, et al. Origins and evolutionary
genomics of the 2009 swine-origin H1N1 influenza A epidemic. Nature. 2009; 459: 1122–1125. https://
doi.org/10.1038/nature08182 PMID: 19516283
7. Morens DM, Taubenberger JK. Historical thoughts on influenza viral ecosystems, or behold a pale
horse, dead dogs, failing fowl, and sick swine. Influ Other Respi Viruses. 2010; 4: 327–37.
8. Koen J. A practical method for field diagnosis of swine diseases. Am J Vet Med. 1919; 14: 468–470.
9. Nelson MI, Lemey P, Tan Y, Vincent A, Lam TT-Y, Detmer S, et al. Spatial dynamics of human-origin
H1 influenza A virus in North American swine. PLoS Pathog. 2011; 7: e1002077. https://doi.org/10.
1371/journal.ppat.1002077 PMID: 21695237
10. Mathieu C, Moreno V, Retamal P, Gonzalez A, Rivera A, Fuller J, et al. Pandemic (H1N1) 2009 in
Breeding Turkeys, Valparaiso, Chile. Emerg Infect Dis. 2010; 16: 709–711. https://doi.org/10.3201/
eid1604.091402 PMID: 20350395
11. Pereda A, Rimondi A, Cappuccio J, Sanguinetti R, Angel M, Ye J, et al. Evidence of reassortment of
pandemic H1N1 influenza virus in swine in Argentina: are we facing the expansion of potential epicen-
ters of influenza emergence? Influenza Other Respi Viruses. 2011; 5: 409–12.
12. Meseko CA, Odurinde OO, Olaniran BO, Heidari A, Oluwayelu DO. Pandemic influenza A/H1N1 virus
incursion into Africa: countries, hosts and phylogenetic analysis. Niger Vet J. 36: 1251–1261.
13. Holyoake PK, Kirkland PD, Davis RJ, Arzey KE, Watson J, Lunt R a, et al. The first identified case of
pandemic H1N1 influenza in pigs in Australia. Aust Vet J. 2011; 89: 427–431. https://doi.org/10.1111/j.
1751-0813.2011.00844.x PMID: 22008120
14. Poonsuk S, Sangthong P, Petcharat N, Lekcharoensuk P. Genesis and genetic constellations of swine
influenza viruses in Thailand. Vet Microbiol. 2013; 167: 314–26. https://doi.org/10.1016/j.vetmic.2013.
09.007 PMID: 24095146
15. Trevennec K, Leger L, Lyazrhi F, Baudon E, Cheung CY, Roger F, et al. Transmission of pandemic
influenza H1N1 (2009) in Vietnamese swine in 2009–2010. Influenza Other Respi Viruses. 2012; 6:
16. Mena I, Nelson MI, Quezada-Monroy F, Dutta J, Cortes-Ferna
´ndez R, Lara-Puente JH, et al. Origins of
the 2009 H1N1 influenza pandemic in swine in Mexico. Elife. 2016;5.
17. Chowell G, Echevarrı
´a-Zuno S, Viboud C, Simonsen L, Tamerius J, Miller MA, et al. Characterizing the
epidemiology of the 2009 influenza A/H1N1 pandemic in Mexico. PLoS Med. 2011; 8: e1000436.
https://doi.org/10.1371/journal.pmed.1000436 PMID: 21629683
18. Lemey P, Suchard M, Rambaut A. Reconstructing the initial global spread of a human influenza pan-
demic: A Bayesian spatial-temporal model for the global spread of H1N1pdm. PLoS Curr. 2009; 1:
RRN1031. https://doi.org/10.1371/currents.RRN1031 PMID: 20029613
19. Garten RJ, Davis CT, Russell C a, Shu B, Lindstrom S, Balish A, et al. Antigenic and genetic character-
istics of swine-origin 2009 A(H1N1) influenza viruses circulating in humans. Science. 2009; 325: 197–
201. https://doi.org/10.1126/science.1176225 PMID: 19465683
20. Nelson MI, Vincent AL. Reverse zoonosis of influenza to swine: new perspectives on the human-animal
interface. Trends Microbiol. 2015; 23: 142–53. https://doi.org/10.1016/j.tim.2014.12.002 PMID:
21. Nelson M, Culhane MR, Rovira A, Torremorell M, Guerrero P, Norambuena J. Novel human-like influ-
enza A viruses circulate in swine in Mexico and Chile. PLoS Curr. 2015;7.
22. Wong FYK, Donato C, Deng Y-M, Teng D, Komadina N, Baas C, et al. Divergent human-origin influenza
viruses detected in Australian swine populations. J Virol. 2018;92.
23. Nelson MI, Gramer MR, Vincent AL, Holmes EC. Global transmission of influenza viruses from humans
to swine. J Gen Virol. 2012; 93: 2195–2203. https://doi.org/10.1099/vir.0.044974-0 PMID: 22791604
24. Epperson S, Jhung M, Richards S, Quinlisk P, Ball L, Moll M, et al. Human infections with influenza A
(H3N2) variant virus in the United States, 2011–2012. Clin Infect Dis. 2013; 57: S4–S11. https://doi.org/
10.1093/cid/cit272 PMID: 23794729
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1008259 March 19, 2020 7 / 8
25. Trock SC, Burke SA, Cox NJ. Development of an influenza virologic risk assessment tool. Avian Dis.
2012; 56: 1058–61. https://doi.org/10.1637/10204-041412-ResNote.1 PMID: 23402136
26. Tong S, Zhu X, Li Y, Shi M, Zhang J, Bourgeois M, et al. New World bats harbor diverse influenza A
viruses. PLoS Pathog. 2013;9.
27. Hause BM, Collin EA, Liu R, Huang B, Sheng Z, Lu W, et al. Characterization of a novel influenza virus
in cattle and swine: proposal for a new genus in the Orthomyxoviridae family. MBio. 2014; 5: e00031–
14. https://doi.org/10.1128/mBio.00031-14 PMID: 24595369
28. Crawford PC, Dubovi EJ, Castleman WL, Stephenson I, Gibbs EPJ, Chen L, et al. Transmission of
equine influenza virus to dogs. Science. 2005; 310: 482–485. https://doi.org/10.1126/science.1117950
29. Song D, Kang B, Lee C, Jung K, Ha G, Kang D, et al. Transmission of avian influenza virus (H3N2) to
dogs. Emerg Infect Dis. 2008; 14: 741–746. https://doi.org/10.3201/eid1405.071471 PMID: 18439355
30. Hatta M, Zhong G, Gao Y, Nakajima N, Fan S, Chiba S, et al. Characterization of a feline influenza A
(H7N2) virus. Emerg Infect Dis. 2018; 24: 75–86. https://doi.org/10.3201/eid2401.171240 PMID:
31. Dalziel BD, Huang K, Geoghegan JL, Arinaminpathy N, Dubovi EJ, Grenfell BT, et al. Contact heteroge-
neity, rather than transmission efficiency, limits the emergence and spread of canine influenza virus.
PLoS Pathog. 2014; 10: e1004455. https://doi.org/10.1371/journal.ppat.1004455 PMID: 25340642
32. Voorhees IEH, Glaser AL, Toohey-Kurth K, Newbury S, Dalziel BD, Dubovi EJ, et al. Spread of canine
influenza A(H3N2) virus, United States. Emerg Infect Dis. 2017;23.
33. Chen Y, Trovão NS, Wang G, Zhao W, He P, Zhou H, et al. Emergence and evolution of novel reassor-
tant influenza a viruses in canines in southern China. MBio. 2018; 9(3):e00909–18. https://doi.org/10.
1128/mBio.00909-18 PMID: 29871917
34. Song D, Moon H-J, An D-J, Jeoung H-Y, Kim H, Yeom M-J, et al. A novel reassortant canine H3N1 influ-
enza virus between pandemic H1N1 and canine H3N2 influenza viruses in Korea. J Gen Virol. 2012;
93: 551–4. https://doi.org/10.1099/vir.0.037739-0 PMID: 22131311
35. Erbelding EJ, Post DJ, Stemmy EJ, Roberts PC, Augustine AD, Ferguson S, et al. A Universal influenza
vaccine: the strategic plan for the National Institute of Allergy and Infectious Diseases. J Infect Dis.
2018; 218: 347–354. https://doi.org/10.1093/infdis/jiy103 PMID: 29506129
36. Simonsen L, Gog JR, Olson D, Viboud C. Infectious disease surveillance in the Big Data era: towards
faster and locally relevant systems. J Infect Dis. 2016; 214: S380–S385. https://doi.org/10.1093/infdis/
jiw376 PMID: 28830112
37. Quick J, Loman NJ, Duraffour S, Simpson JT, Severi E, Cowley L, et al. Real-time, portable genome
sequencing for Ebola surveillance. Nature. 2016; 530: 228–232. https://doi.org/10.1038/nature16996
38. Robinson TP, Wint GRW, Conchedda G, Van Boeckel TP, Ercoli V, Palamara E, et al. Mapping the
global distribution of livestock. PLoS ONE. 2014; 9: e96084. https://doi.org/10.1371/journal.pone.
0096084 PMID: 24875496
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1008259 March 19, 2020 8 / 8