Influenza is a highly contagious disease that is responsible for significant morbidity and mortality worldwide. Influenza A viruses (IAVs) have the ability to cross interspecies barriers from avian carriers and then rapidly circulate among and infect crowded livestock creating a breeding ground for the emergence of zoonotic viruses with epidemic and pandemic potential. They are susceptible to antigenic shift and drift and hence are the cause of recurring major epidemics and pandemics. Intensive animal farming creates conditions for the emergence and amplification of epidemics because of the physical and genetic proximity of the billions of animals, often in frail health, raised indoors every year. In particular, because swine and poultry are susceptible to infection with both avian and human influenza viruses, novel influenza viruses can be generated by reassortment of influenza viral segments. These are then transmitted via farm workers into the human population. Increased globalization is a significant factor in the worldwide spread of human influenza viruses that spillover from poultry and swine. The efficacy of influenza vaccination as a public health measure is limited by both the efficacy of the vaccine, which needs to be reformulated biannually, and the degree of public access to the vaccine. The most effective public health prophylaxis would therefore be to encourage less consumption of animal products, thus reducing the need for intensive animal agriculture. This will cut the link in the chain of emergence of influenza viruses into the human population, while at the same time, improving public health more directly.
Influenza is a highly contagious respiratory illness that is responsible for significant morbidity and mortality. Approximately 9% of the world’s population is affected annually, with up to 1 billion infections, 3 to 5 million severe cases, and 300,000 to 500,000 deaths each year [1-3].
In the U.S. alone, nearly 20% of the population is affected annually. On average, 25 to 50 million documented influenza cases, 225,000 hospitalizations, and ultimately more than 20,000 deaths occur every year [1,2,4-7]. The estimated average annual total economic burden of influenza to the American healthcare system and society is $11.2 billion .
These figures only show the impact of influenza on society in a typical year. Since the beginning of the 20th century, zoonotic spillover events have given rise to the generation of multiple, well-documented pandemic influenza viruses [9-11]. Four influenza pandemics, or global epidemics, in particular have affected humanity in the past century. The most devastating of these occurred in 1918, causing influenza in about half the world population, with 30–50 million deaths worldwide, affecting principally the young and otherwise healthy . The subsequent influenza pandemics of 1957 (“Asian flu”), 1968 (“Hong Kong flu”) and 2009 (“Mexican flu”) were milder, each claiming the lives of 0.3–2 million individuals. [13,14] In addition to these major pandemics, the chart below shows sporadic cases/fatalities (Figure 1) .
According to one study, since 1940, agricultural drivers were associated with more than 25% of all, and more than 50% of specifically zoonotic infectious diseases that emerged in humans, proportions that will likely increase as agriculture expands and intensifies .
This article focuses specifically on influenza viruses, since these are the ones that have had the greatest impact on human life to date. Of course, there are many other zoonotic infectious diseases, both viral and bacterial, which have the potential to become pandemics.
Influenza is an acute respiratory infection caused by a
negative-strand RNA virus of the Orthomyxoviridae family .
There are four recognized types of influenza viruses (A, B, C, and
D), with influenza D virus (IDV) the most recently discovered .
Types A, B, and C are defined on the basis of variation in the
nucleoprotein antigen. In types A and B, the hemagglutinin and
neuraminidase antigens undergo genetic variation, which is the
basis for the emergence of new strains; type C is antigenically
Influenza A viruses (IAVs) cause the most morbidity in both
humans and animals. They infect multiple species, including
humans, swine, equines, and birds. They are more susceptible to
antigenic variation and hence, are the cause of major pandemics
Influenza viruses are spherical or filamentous enveloped
particles 80 to 120 nm in diameter. The helically symmetric
nucleocapsid consists of a nucleoprotein and a multipartite
genome of single-stranded antisense RNA in seven or eight
segments. The envelope carries a hemagglutinin attachment protein and a neuraminidase .
There are 18 recognized hemagglutinin (HA) and 11
neuraminidase (NA) glycoproteins by which IAVs are subtyped.
IAVs change through mutation, recombination, and reassortment,
frequently challenging the immune systems of their human and
animal hosts .
As segmented RNA viruses, IAVs can exchange the gene
segments through reassortment during co-infection. Specifically,
when two or more IAVs infect the same cell, a hybrid virus can
be produced by assembling the gene segments of the parental
viruses into a nascent virion . Reassortment and mutation are
both the main driving forces for the evolution of IAVs. However,
the effect caused by mutations needs to be accumulated for a long
time, while the reassortment is often a leapfrog evolution.
The ability of IAVs to rapidly cross interspecies barriers and
circulate in a variety of avian and mammalian species of wildlife
and livestock creates a breeding ground for zoonotic strains with
Aquatic birds specifically often carry IAVs without getting
sick themselves. They can spread the virus broadly into animal
agriculture, since the transmission route of IAVs in aquatic birds
is fecal-oral, and feces can contain highly concentrated amounts
of the virus [22,23].
Over the past 70 years, food animal production in much of the
world has been transformed from traditional small-scale methods
and entrepreneurial organization to industrial-scale operations
and vertically integrated management, in which most if not all
aspects of production (breeding, supply of young animals, feeds,
animal husbandry) are controlled by a single entity [24,25].
Intensive animal farming creates conditions for the
emergence and amplification of epidemics because of the physical
and genetic proximity of the billions of animals, often in frail
health, that are raised indoors every year . As intensification
of livestock production increases, especially pigs and poultry,
disease transmission is facilitated by the increasing population
size and density of the animals [27-29].
Intensification also requires greater frequency of movement
of people and vehicles on and off farms, which further increases
the risk of pathogen transmission since animal workers and other
animal species may introduce new IAV strains to the flocks or
Swine and poultry are two key reservoirs of IAVs and both
are in rapidly growing livestock industries. As livestock industry farms are often home to several thousand birds or pigs, sometimes
harboring multiple subtypes of IAVs, they are considered highrisk
areas for novel IAV generation . Thousands of animals can
be infected within a few days.
The selection of the most profitable species of farmed animals
in intensive farms leads to a high level of genetic similarity,
facilitating the spread of the pathogens as all animals within the
farms are immunologically naïve hosts. Frequent introductions
of more immunologically naïve animals help maintain IAVs
circulating within a livestock population, increasing the chance of
catastrophic epidemics on the farm [28,31,32].
In particular, because swine are susceptible to infection
with both avian and human influenza viruses, novel reassortant
influenza viruses can be generated in this mammalian species
by reassortment of influenza viral segments, leading to the
“mixing vessel” theory that novel viruses, both infectious to
and transmissible by humans, can be created among intensively
farmed swine . While much attention has been placed on
the role of pigs as “mixing vessels”, the potential importance of
chickens for the evolution of humanized influenza viruses should
not be overlooked and, as such, warrants further study .
In summary, one of the key factors in the pathogenesis of
zoonotic diseases is that there have been profound changes
between natural and man-made ecosystems in recent decades,
resulting in increased risk of novel diseases with pandemic
potential (Table 1) .
Zoonotic influenza viruses that acquire the ability to
transmit efficiently among humans via the air, through mutation,
reassortment or both, are at the origin of emerging influenza
viruses with pandemic potential .
Poultry and pigs are the major sources of human infections
with IAVs. Farmworkers who work with livestock such as swine
and poultry are potentially at risk for exposure to IAVs that
originate in birds, pigs, or other species, are novel to humans, and
may pose a pandemic threat [36-39]. In particular, occupational
exposure to pigs has been shown to increase the risk of swine
influenza virus infection in humans [40,41].
The portal of entry for human infections is mostly through the
conjunctiva (e.g., rubbing the eye), nasal and mucosal membranes
(e.g., inhalation of dust, droplets), or probably swimming in
contaminated pools [42-45].
Many factory farm workers are provided with little or no
protective clothing or opportunities for personal hygiene or
decontamination on-site. A study of poultry-house workers
in Maryland indicate that workers take their clothes home for
washing . Thus, it is not surprising that increased risks of
pathogen exposure and infections, both bacterial and viral, have
been reported among farmers, their families, and farm workers at
poultry and swine operations [41,47-49,50-55]
These same animal workers may serve as a bridging
population in moving IAVs circulating among livestock to other
humans . The primary mode of transmission between
humans is via inhalation of infectious respiratory particles (large
droplet transmission) when an infected person coughs or sneezes.
There is also evidence of airborne (small particles transmitted
by talking or exhalation) and fomite transmission [57,58]. The
typical incubation period is 24 to 48 hours. Patients are infectious
one to two days before symptom onset and for five to seven days
afterward. Children and immunosuppressed people may exhibit
prolonged viral shedding [59,60].
Amplification occurs if the size of the epidemic in humans
is increased due to transmission of the influenza virus into the
intensively farmed animal species which leads to an epidemic
in that species, and subsequent transmission back to the local
human population .
In developing countries, humans often live close to their
livestock, leading to greater likelihood of transmission of influenza
viruses from animals to humans [61-63]. Once these diseases
are transmitted into human populations, they are transmitted to
densely human populated areas in the world, where they can have
a considerable public health impact .
Increased globalization is a significant factor in the spread of
human influenza viruses that spillover from poultry and swine.
[65,66] Modern methods of transportation lead to a more rapid
spread of such viruses to different parts of the world, through the
transportation of both humans and live animals.
Seasonal influenza occurs primarily in the colder months in
temperate regions. Seasonal influenza A viruses (IAVs) circulate
throughout the year in East and Southeast Asia, which are
considered the source regions of the H3N2 strains that cause
winter epidemics in the northern and southern hemispheres
. Pandemic influenza A viruses circulate around the world in
several waves, eventually replacing an existing seasonal influenza
A virus .
In the US, the Centers for Disease Control and Prevention’s
(CDC’s) Advisory Committee on Immunization Practices (ACIP)
and the American Academy of Family Physicians (AAFP)
recommend annual influenza vaccination for all people six months and older who do not have contraindications, with an emphasis on
those at higher risk of developing complications such as the very
young (<5), older adults (≥65), pregnant women, and individuals
with certain health conditions [68,69]. Multiple formulations of
the influenza vaccine are available .
The composition of influenza strains in the vaccine is updated
biannually based upon recommendations from the World Health
Organization (WHO), which tracks clinical data on current
and emerging strains during both the northern and southern
hemisphere influenza seasons. This allows the production of
a targeted vaccine, although further genetic alterations of the
prevalent viral strains can result in a mismatch between circulating
and vaccine strains, leading to poor vaccine effectiveness by the
time vaccines are deployed. An example would be the 2017/18
trivalent vaccine which had a low effectiveness of ∼25% in the
UK due to mismatching of the predominant influenza A strain and
lacking the circulating Yamagata strain .
Following influenza vaccination, antibody titers to influenza
antigens may persist for months. However, the changing nature
of influenza viruses, particularly the influenza A type (antigenic
drift) , warrants reformulation of vaccine each influenza
season in an attempt to match vaccine with the circulating virus
The immune response to vaccination among elderly persons
is reduced compared with younger adults. [73,74] A review
published in 2017 found the vaccine effectiveness against any
type of influenza to be 51% for working-age adults and 37% for
older adults [75,76].
The US influenza vaccine market was $3.82 billion in 2021
, to which can be added the cost of administering the program.
Yet many Americans don’t get vaccinated. Only 59.0% of children
6 months to 17 years old and 43.3% of adults over 18 years old
were vaccinated in the U.S. in the 2016–17 influenza season .
So, the effectiveness of this program is limited by both the efficacy
of the vaccine and the degree of public access to the vaccine,
resulting in the 25-50 million documented influenza cases per
year, and 20,000 deaths.
Intensive livestock farming is increasing worldwide,
encouraged by market demands including urbanization and
expanding global populations, which have changed the way in
which food is produced and supplied .
Increased human consumption of meat, more efficient animal
husbandry practices and the resulting profit potential have led an
increased number of farmers in the developing world to forego
traditional practices, such as free-range or grazing, in favor of
small-scale intensive models. The Council for Agricultural Science
and Technology (1999) notes that these more intensive farms are replacing traditional models at a rate of 4.3 percent per year,
especially in South America, Africa, and Asia, with poultry and
swine farms outpacing any other livestock subsector [80,81].
This increase is largely related to the expansion of the
integrated or industrial model of production  led by both
national and multinational corporations for expanding markets of
increasingly urban consumer populations within these countries
as well as exports . Concerns have been raised over the
relatively weak veterinary and public health infrastructure in
some of these countries .
In the U.S., this change began in the 1930s and now more than
90% of broiler chickens and turkeys are produced in houses in
which between 15,000 and 50,000 birds are confined throughout
their lifespan. For swine, this transformation occurred more
recently and more rapidly: from 1994 to 2001, the market share
of hogs produced in industrial food animal production increased
from 10% to 72% of total U.S. production .
The occurrence of a zoonotic disease can lead to large
economic losses in the agricultural sector [84-91]. At times of
heightened infectious risk, livestock and wildlife are often culled
as a means of restricting their movements and limiting their
interactions with other animals and humans. Indeed, culling ---
or ‘stamping out’ ---remains the major strategy used to control
emergent disease events in animal populations .
Killing chickens to curb influenza outbreaks has significant
costs. Fifteen years after its emergence, the direct economic costs of the ongoing H5N1 HPAI outbreak ---including destroying
more than 250 million birds ---were estimated by the World Bank
(2010) at more than US$10 billion .
Public health is the science of protecting and improving the
health of people and their communities. This work is achieved
by promoting healthy lifestyles, researching disease and injury
prevention, and detecting, preventing and responding to infectious
diseases. Prevention strategies and interventions can be aimed at
the environment, human behavior, or medical care practices.
The primary concern of infectious disease control in public
health, whether in developing or industrialized countries, should
be the reduction, elimination, or even eradication of infectious
disease. This is accomplished by directing control measures to
the agent, the routes of transmission, or the host. Such control
measures include: (1) identifying and then reducing or eliminating
infectious agents at their sources and reservoirs, (2) breaking or
interfering with the routes of transmission of infectious agents,
and (3) identifying susceptible populations and then reducing or
eliminating their susceptibility .
The chart below shows how primary prevention of influenza
viruses needs to occur at the Pathogen Spillover point to prevent
spillover into humans, which then can result in influenza
pandemics (Figure 2) .
Leaders in public health and many prominent policymakers
have promoted plans that argue that the best ways to address
future pandemic catastrophes should entail “detecting and
containing emerging zoonotic threats.”  In other words,
we should take actions only after humans get sick. Despite
the extraordinary successes generated by immunizations,
pharmaceuticals, and evidence-based public health interventions,
the spread of infectious diseases remains a critical issue, so such
approaches are insufficient.
Much of medical practice is based on a disease/treatment
model rather than a prevention model, where the predominant
focus is on treating existing symptoms and conditions. While few
would argue this approach is necessary for acute conditions, this
has not proved to be an efficient or effective way of delivering
preventive care. Impressive evidence supports the value of
preventive medicine. Primary prevention activities deter the
occurrence of a disease in the first place .
The risk of complications from influenza, including lower
respiratory tract infection, admission to hospital, and death vary
depending on factors such as age and the type of comorbidity that
may be present. Heart disease, hypertension, diabetes mellitus,
obesity, asthma and chronic kidney disease increase the risk of
complications of influenza . A plant-based diet can reduce the
risk of all these pathologies and can be a treatment or adjunct for
them as well [98-101].
The worldwide costs of the circulation of continually changing
influenza viruses is massive, in terms of ill-health and lost work
days of 9 percent of the world’s population annually; the costs of
vaccinating the general public every year in those countries that
can afford to do it, due to the evolving nature of the virus; and the
ultimate death of 300-500,000 people every year that we know of.
Intensive farming of animals is an essential link in the
development of influenza. Without these intensive, or factory
farming, operations influenza could not emerge and spread.
Variants would also be much less likely to develop.
The growth of intensive animal agriculture exacerbates this
risk, due to an increase in the size of the reservoir pools among
swine and chickens, an increase in the number and genetic
variabilities of circulating IAVs, and the resulting risk of novel
viruses causing future pandemics.
From a public health standpoint, preventing a disease from
ever occurring in the first place is primary prevention, whereas
seasonal vaccination, which is only moderately effective, is a form
of secondary prevention. Relying on vaccination as a strategy
to prevent the spread and impact of these diseases, is a costly
strategy that has not proven sufficient to stop these viruses from
spreading and damaging public health.
The only way to reduce the risk of such viruses emerging is
to stop the crowding of billions of animals together in intensive
factory farms, thus limiting the size of the viral reservoirs, the
potential for genetic drift and reassortment, and the number of
farm workers who come into contact with these animals and can
transmit them into the human population.
However, there is insufficient farmland to enable the raising
of this number of animals in any other way, so this reduction will
only happen if there is a significant reduction in the demand for
animal products for human food.
The most effective public health prophylaxis would therefore
be to encourage less consumption of animal products, thus
reducing the demand and therefore the need for intensive animal
agriculture. This will cut the link in the chain of emergence of
influenza viruses into the human population.
While this article focused on influenza, a plant-based diet
can reduce the risk of many other pathologies such as coronary
artery disease, type II diabetes, chronic kidney disease, asthma,
and more. It is often an efficacious treatment for those pathologies
While many years ago a plant-based diet might have been
considered fringe that’s no longer the case. One indication of this
is the soaring sales of meat and dairy substitutes. Since reducing
the consumption of animal products will also hugely benefit
both human health and the health of our planet, public health
authorities should look into ways to encourage and facilitate
plant-based eating among their populations with due haste.
National Research Council (US) Committee on Achieving Sustainable Global Capacity for Surveillance and Response to Emerging Diseases of Zoonotic Origin. (2009) Sustaining Global Surveillance and Response to Emerging Zoonotic Diseases. Washington: National Academies Press, USA.
Vandegrift KJ, Sokolow SH, Daszak P, Kilpatrick AM (2010) Ecology of avian influenza viruses in a changing world in The Year in Ecology and Conservation Biology: Annals of the New York Academy of Sciences
Petrie JG, Ohmit SE, Johnson E, Truscon R, Monto AS (2015) Persistence of antibodies to influenza hemagglutinin and neuraminidase following one or two years of influenza vaccination. J Infect Dis 212(12): 1914-1922.