The Bug
| Influenza viruses form part of the family of ORTHOMYXOVIRIDAE
(from the Greek ortho-standard, myxo-mucus) which refers
to their capacity to infect the respiratory tract. The
human influenza virus was described for the first time
in 1933 by a group of researchers in London, England
through the help of electron microscopy.
The influenza viruses are classified into types A,
B and C by identifying complement-fixing antibodies
to the nucleoproteins and matrix protein. Type A is
the most common strain of influenza virus. It has an
extremely wide range of hosts. In addition to humans,
it infects many animals, including seals, pigs and birds.
Influenza Types B and C infect only humans. |
Picture taken from
Viruses: From Structure to Biology - The Influenza Virus
Hemagglutinin
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The flu viruses are enveloped minus strand RNA viruses. Unlike
most viruses, the shape of influenza viruses is highly variable;
however, their surface is consistently covered with protein
spikes .There are eight RNA segments which encode 10 proteins.
Two of the proteins, heamagglutinin and neuraminidase, make
up the surface spikes. These proteins are antigenic, and antibody
to them is what protects humans from influenza. Haemagglutinin
(H) is the protein by which the virus attaches to its host
cell. At present, there are 15 immunologically and genetically
distinct haemagglutinin subtypes. Neuraminidase (N) is an
enzyme that plays a role in releasing virions from their host
cell, promoting the spread of infection. Nine neuraminidase
subtypes have been identified. Only three haemagglutinin and
two neuraminidase subtypes are commonly associated with human
infections.
The H and N proteins of influenza virus undergo continuous
mutations in ther RNA, leading to antigenic variation in these
proteins, and thus the evolution of new strains. Scientists
call this change in virus strains, "antigenic drift". Drift
is an ongoing process and is one way the virus evades the
body's natural immune system. Immunity to one strain of influenza
virus confers only partial immunity to a new strain which
has undergone antigenic drift. Thus, influenza viruses can
continue to infect humans, and cause annual epidemics of disease.
This is the reason why yearly influenza vaccination against
new strains of influenza virus is necessary.
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In contrast to antigenic drift, "antigenic shift"
is characterized by major changes in surface antigens.
Antigenic shift occurs only in influenza A viruses,
and is associated with severe illness and worldwide
pandemics. Antigenic shift originate from the genetic
recombination of strains of virus from two different
species, and is facilitated by the segmentation of the
RNA genome. Recombination, or antigenic shift, results
in the creation of a virus strain with complete new
hemagglutinin and neuraminidase proteins. Because the
new virus is antigenically completely different from
other human influenza viruses, there is a complete lack
of contemporary human immunity. This sets the stage
for worldwide spread of disease.
Influenza viruses are named according to their type
(i.e., influenza A, B or C), the place where the virus
was first identified, the laboratory number, and the
year it was collected. The influenza A virus is also
named by the type of H and N proteins it carries and
the animal from which it was isolated (if not of human
origin). For example, Influenza A/Sydney/5/97/(H3N2)
is a human influenza A virus that was isolated in Sydney,
Australia, in 1997, and has type 3 hemagglutinin and
type 2 neuraminidase.
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Influenza B and C viruses are mainly found in humans. Although
these types have also been isolated from pigs, it is not clear
whether pigs are a natural host or if the isolates represent
single occurrences of virus transmission from humans. In contrast,
the influenza A viruses have been isolated from a wide range
of wild and captive warm-blooded animals (birds and mammals).
Chronic infection or virus latency (a carrier state) have
not been found in any human or animal. Wild aquatic birds
provide one reservoir for influenza A. Influenza can be isolated
from duck populations throughout the whole year. Wild ducks
and other aquatic birds show no clinical symptoms. Influenza
infects primarily the intestinal tissue of these water birds
and is associated with fecal shedding of virus for 2-4 weeks.
Juvenile ducks, which have not established protective immunity
to influenza, are preferentially infected. Since new generations
of immunologically naïve birds are brought up every year,
the virus continues to circulate in the population.
In temperate and sub-arctic regions , influenza circulates
through human populations, as well as pigs and horses every
winter. In tropical and subtropical regions, influenza virus
circulates throughout the entire year. In contrast to aquatic
birds where the intestinal tissue is infected, in humans,
as well as pigs and horses, influenza viruses infect the epithelial
cells of the respiratory tract resulting in acute respiratory
or pulmonary illness.
Outbreaks of influenza occur in poultry, sea mammals and
cattle at unpredictable times. The virus is not maintained
in these animals and epidemics/outbreaks tend to be self limiting
due to the high mortality.
Mice and rabbits are not naturally infected with influenza
but provide a useful experimental model for studies of influenza
virus infection. Ferrets are also very useful in studies of
influenza, producing specific influenza antibodies. Embryonated
hen's eggs provide the major production source of influenza
virus for vaccine and research.
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The influenza virus is transmitted from person to person
very efficiently through droplets of saliva that travel through
the air, usually propelled by the action of coughing, sneezing
or even talking. This mode of transmission is especially effective
in confined or enclosed environments like schools or nursing
homes, environments typically experienced by Canadians in
the winter months. From 20 - 50% of a community population
may be affected by any given outbreak, which usually peaks
at about 3 weeks and recedes within another month. School-age
children are a key source of dissemination for community epidemics
and the major portal of entry for the virus into the household.
The virus replicates within 4-6 hours in the epithelial
cell columns of the respiratory tract. A short incubation
period of 1-4 days makes influenza a prime candidate for large
scale outbreaks of infection. The infected individual remains
infectious for 2-5 days following the appearance of symptoms.
Influenza virus rarely is found outside the respiratory tract.
Isolating people with flu symptoms is not a completely effective
means of disease control because flu can be spread by someone
whose symptoms are not yet apparent.
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Influenza viruses cause Influenza or "flu". Influenza is
a highly contagious, febrile, acute infection of the nose,
throat, bronchial tree and lungs. It causes significant morbidity
and mortality, even in periods between pandemics. Much of
the morbidity and mortality associated with influenza infection
is related not to the disease itself, but to cardiopulmonary
and respiratory complications of the infection. Influenza
affects millions of people of all age groups every year, and
occurs mainly in late fall, winter or early spring.
Anyone can become infected with influenza. However the persons
at highest risk of developing severe disease or complications
from influenza are the following:
- Adults and children with chronic cardiac
or pulmonary disorders (including bronchopulmonary dysplasia,
cystic fibrosis, and asthma) severe enough to require regular
medical follow-up or hospital care. Chronic cardiac and
pulmonary disorders are by far the most important risk factors
for influenza-related death.
- People of any age who are residents of
nursing homes and other chronic care facilities. Such residents
often have one or more of the medical conditions outlined
in the first group. In addition, their institutional environment
may promote spread of the disease.
- People >65 years of age. The risk of severe
illness and death related to influenza is moderately increased
among healthy people in this age group, but is not as great
as among those with chronic underlying disease.
- Adults and children with chronic conditions
, such as diabetes mellitus and other metabolic diseases,
cancer, immunodeficiency, immunosuppression, renal disease,
anemia and hemoglobinopathy.
- Children and adolescents (age 6 months
to 18 years) with conditions treated for long periods with
Aspirin. This therapy might increase the risk of Reye's
syndrome after influenza.
- Persons infected with human immunodeficiency
virus (HIV). There is limited information about the frequency
and severity of influenza illness among HIV-infected persons,
but reports suggest that for some the symptoms may be prolonged
and the risk of complications increased.
- People at high risk of influenza complications
who are embarking on foreign travel to destinations where
the virus is likely to be circulating. For example in the
tropics, influenza can occur throughout the year, in the
southern hemisphere, peak activity occurs from April through
September, and in the northern hemisphere from November
through March.
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