Swine flu (also called swine influenza, or simply, flu) is an acute respiratory
disease of pigs (also called hogs or swine) caused by a tiny spheroid virus
that belongs to the Influenza A virus group. Symptoms of swine flu in swine
herds include fever, inactivity, nasal discharge, labored breathing, mouth
breathing, and paroxysmal coughing when the pigs are moved. All ages are susceptible.
Mortality rates are generally low and pigs recover within 5 to 7 days after
initial symptoms.
Healthy swine herd
Source: http://pasture.ecn.purdue.edu/~epados/swine/images/swine/mmcd041.jpg
Swine influenza A viruses are divided into categories on the basis of approximately
500 distinct surface protein spikes that project from the surface of the virus.
The protein spikes are of two kinds:
1. Neuraminidase (NA) and
2. Hemagglutinin (HA).
Model of the influenza A virus showing HA and NA receptors
projecting from the surface of the virus.
Source: Carolyn Buxton Bridges, MD, Influenza Branch, Division of Viral and
Rickettsial Diseases, Centers for Disease Control and Prevention, Atlanta,
Georgia: “Human influenza viruses and the potential for inter-species transmission” available
at: http://www.xl3.info/pdf/prod13.htm
To determine the type of influenza virus causing a flu outbreak in a swine
herd, secretions are obtained and processed. Current common varieties of swine
flu include H1N1, H3N2 and H1N2 (more on this below).
What is the function of HA and NA viral surface projections anyway? The function
of the HA protein is to bind virus particles to susceptible cells in the host
animal. The function of the NA protein is mainly at the end of the life cycle
of the virus at which time it facilitates the release of the virus particles
from the infected cell surfaces during the budding processes. The HA and NA
proteins are the antigens against which neutralizing antibodies are directed
by the host animal’s immune defense system.
The hallmark of the Influenza A virus is the ability of its HA and NA proteins
to undergo change—either by “drift” or “shift”, allowing the influenza virus
to have tremendous variability and survivability. Virologists have identified
fifteen (15) distinct HA and nine (9) distinct NA subtypes of the influenza
A virus to date. That’s a lot of potential variation. The epidemiology of influenza
is essentially a story of the dog-fight between host defense and the wily influenza
virus that attempts to dodge the host defense through drift and shift. This
fact has also been the most difficult problem in making of a vaccine against
influenza.
Drift (also called antigenic drift) means that point mutations
in the HA and/or NA genes accumulate during viral replication. Antibody produced
in response to an influenza virus infection is very specific for the influenza
virus type that stimulated antibody development in the first place. If sufficient
drift occurs, previously developed host antibody will be ineffective against
the new “drifted” virus type.
Shift (also called antigenic shift) occurs when an influenza
A virus containing an immunologically new HA or NA (or both) is introduced
into an immunologically naïve population of, say, pigs. Shift occurs via three
routes:
1. A virus bearing a new HA/NA can arise through genetic reassortment between
species, as when, for example, a pig farmer sick with human influenza becomes
co-infected with the swine influenza while he tends an infected herd. The farmer
is “double infected.” A reassortment of viral genetic material from the human
and the pig within the human host cell can occur, giving rise to a novel HA/NA
strain.
2. A wholly species-specific influenza virus from one species (e.g., birds
or swine) can infect another species (e.g., humans) directly without undergoing
genetic reassortment as described in #1 above. This is exactly what scientists
think is happening during the current avian flu epidemic in Asia in which humans
are becoming ill with wholly avian flu virus (see SEMP Biot #149: “What Is
Avian Flu?” available at: http://www.semp.us/biots/biot_149.html.)
3. An influenza virus can be passed from one species (e.g., birds) through
an intermediate animal host (e.g., pigs) to a third species (e.g., humans).
Pigs have been proposed to the “mixing vessel” for the generation of reassortment
influenza A viruses between humans and birds because—and this is very important—pigs
can easily support replication of both avian and human influenza A viruses
within their cells.
Transmission of the influenza virus through species.
Source: Carolyn Buxton Bridges, MD, Influenza Branch, Division of Viral and
Rickettsial Diseases, Centers for Disease Control and Prevention, Atlanta,
Georgia: “Human influenza viruses and the potential for inter-species transmission” available
at: http://www.xl3.info/pdf/prod13.htm
Notes on the 1918 Flu Pandemic
Influenza A virus was first isolated by RE Shope* in 1931 from swine and by
W. Smith, et al.**, in 1933 from humans, approximately 15 years after the 1918 “Spanish” flu
world pandemic. Recently, RNA sequences of the 1918 virus have been studied
by researchers at the Armed Forces Institute of Pathology (AFIP) in Maryland,
which had stored specimens of 70 human autopsy cases of the 1918 flu pandemic.*
In addition, influenza RNA was extracted from a preserved bird from the 1915-1918
era stored at the Smithsonian Institution. The work done confirms that the
1918 virus was an H1N1 virus and was closely related to swine and human H1N1
viruses that Swope isolated in the 1930s. However, both 1930 human and swine
viruses were genetically distinct from the 1918-era archived wild bird virus
from the Smithsonian. Hence, the researchers hypothesize that the virus causing
the 1918 pandemic was unlikely transmitted directly from birds to humans or
pigs. Rather, they think that the H1N1 pandemic virus likely circulated among
swine and/or humans for some period, undergoing drift, before leading to widespread
illness in 1918.
Emergency hospital during 1918 influenza epidemic, Camp
Funston, Kansas
Source: http://www.vaccineinformation.org/photos/flu_afp001a.jpg
Editor’s Note : Influenza virus A causes yearly epidemics
that result in illness for humans, pigs, and domestic poultry. We now know
that intra-species transmission is the norm. Indeed, pandemics are global epidemics
among humans caused by the transmission of novel influenza A viruses generated
via inter-species transmission. Pandemics are no longer thought of as tornadoes
that suddenly thrust themselves upon human populations. Rather, we now know
that herald epidemics in non-human and human species occur for variable amounts
of time before a pandemic takes root. Thus, surveillance among human, swine
and bird populations has become essential for early detection of viruses with
pandemic potential and for initiation of prevention efforts, particularly vaccine
development.
Sources and Notes:
*According to AFIP’s Dr. Jeffrey Taubenberger, the tissue specimens
were contained in paraffin wax blocks that had sat at room temperature on shelves
for 80 years. Of the 70 cases, 35 lung specimens were examined. Most of the
specimens revealed bacteria because the victims had succumbed to secondary
bacterial pneumonia. Taubenberger wanted to find victims who died very quickly
after the onset of symptoms in order to try to capture the virus while it was
still present and replicating. Of those six cases found, only one was positive.
**Shope RE. Swine influenza III. Filtration experiments and
etiology. J Exp Med 1931; 54:373-385.
*** Smith W, et al. A virus obtained form influenza patients.
Lancet 1933; ii:66-68.
Other excellent background sources:
- National Academies: “ The Threat of Pandemic Influenza: Are We Ready?
Workshop Summary” (2004), available online at http://books.nap.edu/books/0309095042/html/index.html
- Ann H. Reid and Jeffery K. Taubenberger: “ The origin
of the 1918 pandemic influenza virus: a continuing enigma” in J Gen Virol,
Sept 2003; 84: 2285 – 2292; available online at: http://vir.sgmjournals.org/cgi/content/full/84/9/2285?maxtoshow=&HITS;=10&hits;=10
&RESULTFORMAT;=&author1;=taubenberger&searchid;=1105307861918_405&
stored_search=&FIRSTINDEX;=0&sortspec;=relevance&search;_url=http%3A%2F%2
Fvir.sgmjournals.org %2Fcgi%2Fsearch&journalcode;=vir
- Carolyn Buxton Bridges, MD, Influenza Branch, Division of Viral and Rickettsial
Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia: “Human
influenza viruses and the potential for inter-species transmission” available
at: http://www.xl3.info/pdf/prod13.htm.