Influenza and zoonotic influenza (WHO, 2014). Seasonal

Influenza is a highly contagious respiratory
illness that affects millions of people all over the world (WHO, 2018). Symptoms
of influenza may include fever or chills, cough, sore throat, bodily pain,
headache, and general unwell feeling or malaise, with nausea, vomiting and diarrhea accompanying certain
strains (WHO, 2018; Hirve, 2012). Though commonly known as “the flu,” there are
several strains of the pathogen, the influenza virus, which cause illness (WHO,
2014). Influenza viruses can be categorized as one of four subtypes: Influenza
A, B, C & D (WHO, 2014). However, influenza D occurs mostly in cattle,
while Influenza C viruses are less common, and cause only minor infection and
isolated outbreaks (WHO 2018; WHO 2018). As they represent the lowest burden of
disease, for the purposes of this essay, only subtypes A and B will be discussed.
Of the other two subsets, Influenza A is by far the most common, and possess
the greatest potential to cause a pandemic (WHO, 2018).  

Additionally, Influenza can also be
classified along the axes of seasonal
influenza, pandemic influenza,
and zoonotic influenza (WHO, 2014).
Seasonal influenza viruses (among A, B, and C subtypes) typically occur every winter
in temperate regions (i.e. North America, Europe, and Central Asia) (WHO,
2014). Pandemic influenza infections occur when a strain of the virus that had
not previously been in circulation spreads, en mass, to people who have no
immunity for it (WHO, 2014).

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Influenza is a concern for many reasons.
Due to the severity of its symptoms, the high rates of transmission, the
potential for secondary infections that may arise as a result of the
immunological vulnerability that the virus creates, and the ever-changing
nature of the virus, influenza remains a global threat (even with the creation
of vaccines) (WHO, 2014). Historically, Influenza has caused four global
pandemics since 1900, the most recent, a global outbreak of H1N1 occurring as
recently as 2009 (WHO, 2018).  The
effects of influenza include productivity lost due to illness, economic strain,
and death from respiratory causes (WHO, 2018). Before the 20th
century, influenza pandemics, such as the outbreak of Spanish flu in 1918-1919,
which was responsible for 20-50 million deaths
worldwide (WHO, 2014). The development of vaccines to protect
and prevent against various seasonal and zoonotic strains of influenzas has greatly
reduced of the number of people who get the disease, the disease severity, and
the number of people who die from it (WHO, 2018).

Influenza is classified as an airborne
disease. The case definition of
influenza is the operational definition that is needed in order to adequately
surveil new cases of the disease and its progression within a population. The current
recommended definition by the WHO for influenza is “cough or sore throat, plus measured fever,
shortness of breath and need for hospitalization” (Hirve et al, 2012).
And while certain symptoms typify a case of influenza, such as “measured or
reported fever,” plus “cough” and/or “sore throat,” Hirve et al, (2012) note
that it is difficult to create an international case definition for influenza
because cases are often dependent on the context or location of the outbreak.
For example, a more tailored definition might also to include bodily pain,
headache and malaise (Hirve, 2012). As such national/ and regional surveillance of influenza often
different case definitions than the ones recommended by the WHO.
            People who are most vulnerable
to influenza or complications due to influenza (those who possess certain risk
factors for the disease) include pregnant women,
the elderly (people over 65), children under 59 months, people with compromised
immune systems, people with pre-existing or chronic health conditions (e.g.
diabetes, cancer, chronic respiratory illnesses, coronary heart disease,
obesity, and smoking) healthcare workers, and Indigenous peoples in Canada (WHO
2018; GOC 2017). Additional risk factors
for zoonotic influenza include direct or indirect contact with an infected
animal, living or dead, or being in contaminated environments (WHO, 2018).

In Canada, influenza is one of the top ten
leading causes of death, resulting in an estimated 12 200 hospitalizations, and
3500 deaths per year (GOC, 2017). And according to the WHO (2008), the burden
of disease for influenza in a developed country like the United States was
25–50 million cases per year, 150 000 hospitalizations, and up to 40 000
deaths. This accounts for between 10-20% of the population of developed
countries being affected by the flu virus (WHO, 2008). When extrapolated, this
equates to approximately 1 billion annual cases of influenza; and according to
the WHO (2018), between 290 000 and 650 000
deaths a year globally, caused by seasonal forms of influenza (WHO, 2018). Incidence and prevalence
for influenza can be difficult to measure, however data estimated using
population-based surveillance data, such as hospitalization (Reed, et al,
2015). According the CDC (2018), the preliminary cumulative incidence in 13 US
states, for the 2017-2018 flu season to date is 180 per 100 000 (see: Appendix
A). In
years when a certain strain is worse, or in a pandemic, the incidence will be
high. Factors that contribute to increased incidence are better
screening/reporting and surveillance.

 

Epidemiology
Importance and Study Design

A study by McIntyre et al (2014) was
conducted on elderly people in London, Ontario to examine their perception and
influences of getting seasonal influenza vaccine. As mentioned elderly adults
are at risk for contracting the illness, therefore getting the influenza
vaccine, which provides immunity against a specific yearly strain, is
beneficial for this population. The researchers wanted to see what influenced
their decision to get or not get the vaccine (McIntyre et al, 2014). A series
of focus groups were conducted with a total of 37 vaccine using and non-using
seniors, age 67-91 (McIntyre et al, 2014). The study found that beliefs about
resilience and fear of vaccine side-effects were main influencers of vaccine
non-use (McIntyre et al, 2014). This study is an example of a descriptive study
design. As a cross-sectional analysis, it allows researchers to learn about
people’s attitudes and practices within a population at a given time. It is a
snapshot of what is going on at a particular time in a population, that might
help formulate a hypothesis to be tested later on. Although cross-sectional
analysis can be either descriptive or analytical, in this context, the data
obtained was not used to compare the health outcomes of vaccine using versus
non-vaccine using seniors.

A second study from the literature was
done by Gefenaite et al (2014). The authors conducted a test-negative case-control
study in Lithuania, during the 2012-2013 influenza season, to assess the
effectiveness of seasonal influenza vaccines against severe, lab-confirmed influenza
in risk populations. Cases were hospitalized patients over age 18 with either the
H1N1 and H3N1 strains of influenza A, or influenza
B
strain, and who self-reported that they had received or not received the
seasonal influenza vaccine; controls were influenza-negative individuals
(Gefenaite et al, 2014). Only patients who exhibited certain symptoms met the
eligibility criteria for participation. The results showed that seasonal
influenza vaccination did reduce the occurrence of lab-confirmed influenza (Gefenaite
et al 2014). This is an example of an analytic study design because it attempts
to test a hypothesis, and there is a measure the association between exposure
(getting a seasonal influenza vaccine) and outcome (reduction of lab-confirmed
influenza). However, this is not to say that correlation is causation. There
may be additional factors that influence this observed outcome, such as small sample
size or the imprecise nature of (retrospective) case-control design. 

Influenza may be described in terms of the
three-dimensional interaction between agent, host and environment which influence
the spread of disease. This is known as the “epidemiological triangle” (or
triad). The goal of epidemiologists is to stop the disease from proliferating
by breaking the chain at one of the following axes. At the top of the triad is
the agent, the influenza virus itself. The influenza virus is indiscriminate,
meaning anyone who is not immune can be a host (i.e. can potentially get the
virus), however, some people are more susceptible than others (see also:
“susceptible host”). In addition, zoonotic influenza is passed from animal to
person (WHO, 2018). Well-known strains, or serotypes of the Influenza A viruses
include H1N1 or “Swine flu”, and H5N1, “Avian flu”. Some Influenza B strains
are also zoonotic (Hay, et al, 2001). “Environment” is the third axes
of the triangle. It involves external (situational, geographical, etc.) factors
that facilitate disease transmission. Influenza patterns are determined by
temperature and humidity (Sooryanarain & Elankumaran, 2015). According to
Sooryanarain & Elankumaran (2015), influenza viruses thrive in cool, dry
conditions, which explains why seasonal influenza occurs in the winter months
in temperate regions and is but severe, while the pattern in tropical regions
is year-round but is mild (WHO 2008).

In order for Influenza to be spread,
certain conditions must be met. According to the “chain of infection” the
influenza virus (agent) is transmitted to the reservoir. This can be either a
human host or an animal host, for zoonotic influenza (WHO, 2014). The “portal
of exit” is through the respiratory tract (nose, or mouth via a sneeze, cough,
talking, or runny nose). The Mode of Transmission is direct, indirect; aerosol
and droplet. Influenza is transmitted to the host directly when the mucus of an
infected person goes directly into an uninfected person’s mucus membranes;
through hand-to-mouth transmission, such as a handshake or contact with a
contaminated surface (WHO, 2014); and finally, indirectly, through wither the
droplet-spread or the inhalation of the airborne, aerosolized mist produced by
the spit, cough, or sneeze of an infected person (WHO, 2014). Portal of Entry,
is any mucus membrane (eye, nose, mouth), respiratory tract, GI tract. Because seasonal serotypes of influenza are constantly mutating and
evolving, people must get vaccinated for each new strain, with each season,
therefore people who are not vaccinated with the latest strain are susceptible
hosts (see above: “risk factors”) (WHO, 2014).

The disease progression of influenza
viruses is relatively short. The virus typically only lasts between 2-7 days,
during which time people will be symptomatic, but most people will recover in
7-10 days (GOC, 2017). The latent period of influenza can be short, and the
incubation period can be between 1-4 days (usually 2 days) (GOC, 2017). The
infectious period for adults (i.e. when they are capable of spreading the
illness to others) is anywhere between the first day before flu symptoms begin
until the fifth day after (GOC, 2017).

Communicable Disease Prevention and
Control Principles & Methods

To prevent and
control the spread of influenza, measures can be taken at each stage of the
epidemiological triangle in order to break the chain. At the level of the
agent, antiviral prophylaxis (drugs) can be taken by the host a viable method
of disrupting the virus’s replication cycle, which will disrupt contagiousness
and transferability (Spicknall et al, 2010). According to Bean, et al (1982),
influenza A and B can survive on hard, non-porous surfaces for 24-48 hours, and
on porous surfaces for up to 12 hours; therefore, proper cleaning and
disinfecting of contaminated surfaces will either kill the virus or disrupt its
survivability (Spicknall et al, 2010). Altering the host to stop the spread is
also utilized. Protecting susceptible people through a combination of
vaccination (one of the most effective methods of prevention and control),
social distancing, school closure and restriction of public transport; and
behaviour change such as hand washing, masks, and limited contact with infected
persons will disrupt the chain at this level (Spicknall et al, 2010). Survival
and transmissibility of influenza are greatly enhanced by cool, dry conditions
rather than rainy ones (Sooryanarain & Elankumaran, 2015). Climate change,
border closure, reducing population density, and vaccine promotion during peak
influenza season in temperate zones could break the chain at the environmental
level (Spicknall et al. 2010)