Influenza – causes, symptoms, diagnosis, treatment, pathology

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much more. Try it free today! Influenza, the virus that causes the flu,
is one of the most common infectious diseases. Now, there are three types of influenza that
infect humans, called type A, type B, and type C, each one with slightly different genome
and proteins. Influenza belongs to the virus family Orthomyxoviridae
– and type A and B have genomes that are made up of eight RNA segments, whereas type C,
has a seven-segment RNA genome, with each segment containing a few genes. Now, type A, the most common type of influenza
virus, can be further subdivided based on two of the glycoproteins on its protective
envelope surface; H protein, or Hemagglutinin, and N protein, or neuraminidase. Hemagglutinin and Neuraminidase can vary a
bit in their structure, so different versions are identified by a number. For example, type A subtype H3N2, sometimes
just called H3N2, has hemagglutinin number 3 and neuraminidase number 2 on its surface. H3N2 and H1N1 are the most common type A subtypes
to infect humans, but they both also infect various animals. To give the full name of a virus, we use the
type, the original host that it came from, the location where the virus was first identified,
which is usually a city, the strain number, the year of origin, and—for type A influenza—the
subtype named by the H and N glycoproteins. For example, an H1N1 type A flu virus of duck
origin from the province of Alberta, Canada, that is the 35th strain discovered in 1976
would be called A/duck/Alberta/35/76 (H1N1). Type B influenza is less common, it only infects
humans and doesn’t mutate as often as type A. Type B influenza only has a few types of
H and N glycoproteins on its surface. Therefore the naming pattern is similar to
type A influenza without the H and N subtype included at the end or the host type, since
it only infects humans. For example, a type B virus found in Yamagata,
Japan, which is the 16th strain discovered in 1988 would be called B/Yamagata/16/88. Finally, there’s type C influenza which is
only one species, and is the least common and least likely to mutate of the three. Influenza C usually causes mild disease in
children, and unlike type B, it can affect both humans and pigs. Rather than hemagglutinin and neuraminidase,
type C influenza uses a hemagglutinin-esterase-fusion protein to enter and exit cells. So, type C influenza is named without the
HN subtype, similar to how type B’s written. For example, a type C virus found in Sao Paulo,
Brazil which is the 37th strain discovered in 1982 would be called C/Sao Paulo/37/82. Of the three types, type A is the most common
and causes the most severe illness. One reason is that the virus has a tendency
to mutate its H and N glycoproteins during replication, and this allows daughter viruses
to form that are slightly different from one another and from the parent virus. Over time, if enough of these small changes
happen, even if somebody’s immune to the original virus, the mutated virus may have H and N
glycoproteins that are different enough to allow it to evade antibodies, and therefore
infect people who were immune to previous strains. This process is called genetic drift, and
is why individuals can get sick from influenza year after year or from two different strains
of influenza in the same year. In addition, a process called antigenic shift,
is where sometimes a virus will be circulating among animal populations like pigs or chickens
and then will suddenly change in a way that allows the virus to infect humans as well. This happens when the same cell, let’s say
a pig cell, gets infected with two similar flu viruses, for example a flu strain that
usually infects humans and one that usually infects pigs. Since the viral genome is in segments of RNA,
the pieces might reassort, or mix, allowing new viruses to have a mix of RNA segments
in them. Reassortment results in viruses that have
entirely new hemagglutinin, neuraminidase, or both. When a virus is produced that can infect human
cells and has entirely new proteins, people have little or no protection against it, and
it can rapidly spread through the population. This antigenic shift process is responsible
for three major influenza pandemics in the 20th century, including the Spanish Flu in
1918, which killed 3 percent of the world’s entire population at the time. The flu is transmitted when an infected person
sneezes or coughs, which spreads thousands of droplets containing the virus into the
local area, up to about two meters or six feet away. These droplets can then land in the mouths
or noses of people nearby, or get inhaled into the lungs. The virus can also survive on surfaces for
a few hours, so it’s possible to get the virus by touching a surface, like a contaminated
doorknob, and then touching your own eyes, nose, or mouth. When the flu virus enters the body, it uses
hemagglutinin to bind to sialic acid sugars on the surface of epithelial cells in the
upper respiratory tract. Once bound, the cell swallows up the virus
in a process called endocytosis. That viral RNA is negative-sense, meaning
that each piece first has to be transcribed by RNA polymerase into positive-sense mRNA
strands, before it can be translated into proteins and assembled into viruses. These viruses leave the cell by simply budding
out from it by using the neuraminidase, which cleaves the sialic acid sugars in the membrane,
releasing the newly created viruses from the cell. Influenza symptoms start 1-4 days after infection
and include fever, headache, runny nose, sore throat, and a cough. Most of these symptoms get better in a week,
but the cough often persists for up to two weeks. There can be sometimes be complications, though,
like acute otitis media, bronchiolitis, croup, sinusitis, and pneumonia, including antibiotic-resistant
strains caused by Staphylococcus aureus and Streptococcus pneumoniae. The highest risk of complications is among
high-risk groups like young children under 6 months of age, pregnant women, adults over
65 years old, and those with chronic medical conditions, like chronic heart or lung disease. Young children in particular are more likely
to have neurologic complications like encephalitis and febrile seizures. One unique complication is Reye syndrome,
which is a condition where a after taking aspirin while fighting off a viral infection
like the flu, a child develops encephalopathy and liver disease. Although less common, all of these complications
can occur in previously healthy individuals as well. Although most people don’t show symptoms until
a few days after infection, the disease can be spread to other people 1 day before symptoms
even begin, and up to a week or two afterwards. This means that a person can pass the flu
to someone else even before they have symptoms and feel sick. There are many tests used to detect if an
individual is infected with influenza. Rapid influenza diagnostic tests can detect
the flu virus from nasal secretions within minutes, but often vary in their reliability
and often can detect a certain type of influenza, but not the specific strain of influenza A,
like H1N1, for example. There are two more accurate and reliable ways
to identify influenza. The first is a viral culture, which involves
growing the virus and identifying it, and the second is to use polymerase chain reaction
to detect even a tiny amount of viral RNA. Medical treatment for influenza is usually
used for high-risk patients or if the illness is severe. There are two classes of medicines: the first
is neuraminidase inhibitors, which inhibit neuroaminidase…and stops the viruses from
breaking out of the host cells. The second are M2 proton channel inhibitors,
which are made of a compound called adamantane and prevent viruses from replicating inside
the host cell. It turns out, though, that the virus’s M2
gene mutates frequently, allowing it to become resistant to the M2 proton channel inhibitors. The most effective way to prevent influenza
is through vaccination, and there are two that are commonly offered. One option is a trivalent inactivated influenza
vaccine, TIV, which is a killed virus that is injected into a muscle, and the other is
a live, attenuated influenza vaccine, or LAIV, which is a weakened virus that is sprayed
into the nose so that it can infect the epithelial cells. Both of them are actually trivalent meaning
they contain a mixture of the three strains that are predicted to be the ones that will
dominate the coming season. This prediction is based on what strains typically
dominate year after year – like H1N1, H3N2, and influenza B – as well as real-time data
on which strains are circulating during the winter season on the other side of the planet,
since these ones are likely to spread and cause trouble. This is because countries in the northern
hemisphere, like the US and Canada, experience winter—and therefore the flu season—at
the opposite time than countries, like Australia, in the southern hemisphere! Now, there are a few risks associated with
getting the flu vaccine. Most flu vaccines are produced by growing
the virus inside of a chicken embryo and therefore the vaccine contain small amounts of egg protein,
which can be a problem for individuals with a food allergy. Guillain-Barre syndrome, an autoimmune disorder
of the peripheral nervous system, is another side effect believed to be associated with
the influenza vaccine. Lastly, children under 6 months old are in
the pediatric group at highest risk of serious influenza complications, but they are too
young to get the flu vaccine. That said, the best way to protect these young
infants is to make sure their caregivers are vaccinated – which creates a natural barrier
to infection. On average, the flu vaccine reduces the risk
of illness by half, roughly falling from 10% to 5% in terms of likelihood of getting sick
over the entire winter season, and two factors play an important role in determining if the
flu vaccine will protect a person from illness. First, a person’s characteristics—like being
in a high-risk category—can make that person more susceptible to the virus despite being
vaccinated. Second, the vaccine must match the circulating
flu virus of the season. Since flu viruses are constantly mutating,
the flu vaccine formulation is reviewed each year to keep up with any changes. Finally, it’s important to be vaccinated each
year before the start of flu season to ensure protection against the circulating virus. Now, getting vaccinated is super important
for both yourself as well as the community, since if the majority of a community is vaccinated
against influenza, or any other contagious diseases, it actually protects the other members
of the community, even those few that aren’t vaccinated! Alright, so let’s say one everyone’s not vaccinated,
and one person gets the flu and then comes in contact with two other people. Those two people then get it and each spread
it to two other people. This chain reaction goes on and on, maybe
even causing an influenza outbreak in that community. But now let’s say that the majority of people
are vaccinated, in this case the chain reaction can’t spread through the whole community,
so even if a few people aren’t vaccinated, they’re still protected. This is called herd immunity, because the
herd is protecting the weakest members. This is especially important for people who
can’t get certain vaccines—like infants, pregnant women, or immunocompromised individuals—because
they still get some protection when the spread of contagious disease is contained. Alright, as a quick recap, influenza, also
known as the flu, is a common infectious disease, and can spread from person to person through
tiny droplets in the air or on surfaces. Most people recover from the flu without complications,
though sometimes antiviral treatments can be used. The most effective way to prevent the flu
is through seasonal flu vaccination.


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