Influenza viruses are a major cause of respiratory disease and are responsible for periodic epidemics and pandemics worldwide. There are three main types of influenza viruses (A, B, and C) with type A being the most variable and causing the largest epidemics. Antigenic drift allows the virus to evade immunity between epidemics, while antigenic shift of surface proteins can cause pandemics by introducing novel subtypes into humans. Transmission occurs via respiratory droplets or contact. Clinical signs include fever, cough, and muscle aches. Complications can include pneumonia or Reye's syndrome in children. Diagnosis is through antigen detection, virus isolation, or serology. Treatment focuses on antivirals and symptom relief

1. Influenza virus
Orthomyxoviridae

2. Influenza viruses
The Influenza viruses are a major determinant of morbidity &
mortality caused by respiratory diseases. Outbreaks of infection
some times occur in world wide epidemics. Influenza has been
responsible for millions of deaths world wide during the
twentieth century.
Control of influenza is difficult due to it’s high frequency of
genetic reassortment. Influenza A is highly variable antigenically
& is responsible for most cases of epidemic influenza. Influenza
type B may exhibit antigenic changes & some times causes
epidemics. Influenza type C is antigenically stable & causes only
mild illness.

3.  Properties of Orthomyxoviruses: All known
Orthomyxoviruses are influenza viruses, three
immunologic types are known, A,B& C
Antigenic changes continually occur within type A & to
lesser degree in type B , type C is antigenically stable.

4.  Influenza A strains are also known for pigs, horses, birds &
some of them are antigenically similar to strains circulating
in human population.
Virion: Spherical, pleomorphic 80- 120 nm
Single stranded negative sense, RNA virus, genome
consists of 8 segments, Enveloped.

5.  Classification: Antigenic differences exhibited by two of the
internal structural proteins. The NP & M proteins are used
to divide influenza viruses into types A,B & C. Theses
proteins possess no cross reactivity among the three
types.
Antigenic variations in the surface glycoproteins HA & NA
are used to subtype the viruses. 16 subtypes of HA (H1-
H16) & 9 subtypes of NA (N1-N9) in many different
combinations have been recovered from birds, animal or
humans.

6. Influenza virus infections in humans :
Epidemiology : The types of influenza vary markedly in their
epidemiologic patterns.
Influenza C is least significant, it causes mild, sporadic respiratory
disease but not epidemic influenza.
Influenza B some times causes epidemics, but influenza type A
causes massive epidemic & can cross around the world causing
pandemics.
The incidence of influenza peaks during winter.
There is no evidence of latent infection, continuous person – to
person chain transmission must exist for viral survival.
Periodic outbreaks appear because of antigenic changes in one or
both surface glycoprotein of the virus.
Avian influenza ranges from highly lethal infections in chickens &
turkeys to in apparent infections.
It is possible that influenza is a water born infection ( virus is shed in
duck faeces & remains viable in water for days or weeks).

7.  Influenza epidemic occurs in waves (every 2-3 years). Every 10-40 year when
a new subtype of influenza virus appears, a pandemic results
 Past Antigenic Shifts
1918 H1N1 “Spanish Influenza” 20-40 million deaths
1957 H2N2 “Asian Flu” 1-2 million deaths
1968 H3N2 “Hong Kong Flu” 700,000 deaths
1977 H1N1 Re-emergence No pandemic
At least 16 HA subtypes and 9 NA subtypes occur in nature.
Up until 1997, only viruses of H1, H2, and H3 are known to infect and cause
disease in humans.


8. Theories Behind Antigenic Shift
1. Reassortment of the H and N genes between human and
avian influenza viruses through a third host.
2. Recycling of pre-existing strains – this probably occurred
in 1977 when H1N1 re-surfaced.
3. Gradual adaptation of avian influenza viruses to human
transmission.
There is some evidence that this occurred in the 1918 H1N1
pandemic.

9.  Transmission :
 From person to person by air born droplets or by
contact with contaminated hands or surfaces.

10.  Pathogenesis : Few of respiratory cells are firstly infected,
virus spread to adjacent cells, where replication cycles
repeated.
 Viral NA lowers the viscosity of mucous film in the respiratory
tract, within a short time many cells in the respiratory tract
are infected & killed.
 Virus shedding starts one day before onset of symptoms,
peaks within 24 hrs – 2 days then declines rapidly.
 Interferon is detectable in respiratory secretions one day
after viral shedding, this aids in the recovery of the host.
 Specific Ab & cell mediated response can not be detected for
another 1-2 weeks. Influenza virus causes cellular destruction
& desquamation of superficial mucosa of the respiratory
tract, this lowers it’s resistance to secondary bacterial
infections ( Staph, Strept, Haemophilus).

11.  Incubation period : 1 – 4 days.
Clinical signs :
 A :Uncomplicated influenza (type A & B).
Symptoms appear abruptly, (chills, headache, dry
cough followed by high fever, generalized muscular
aches, malaise, anorexia).
The fever lasts for 3 days, respiratory symptoms lasts
another 3-4 days.
The cough & weakness may persist for 1-3 weeks
after subsides of major symptoms.
In children, symptoms are similar to adults & influenza
is an important cause of Croup in under one year of
age.
Type C rarely cause these symptoms.

12.  B : Pneumonia : Serious complications usually occur only
in the elderly & debilitated individuals (specially with
cardiopulmonary or other chronic disease).
Pregnancy appeared to be a risk factor for lethal
complications in some epidemics ( mainly due to
pneumonia, cardiovascular & renal diseases).
Lethal pneumonia is mainly viral, secondary bacterial or
combined infection which is three times more common
than primary influenza pneumonia.
Staph aureus co infection has been reported to cause 42 %
fatality rate.
C : Reye’s syndrome :It is an acute encephalopathy of
children & adolescent ( 2- 16 years of age), fatty liver
degeneration is associated with this syndrome. Mortality
rate is high (10-40 %).

13.  The cause of Reye’s syndrome is unknown but it is
recognized as complication of influenza B, A & herpes virus
(Varicella zoster) infections.
Epidemic cases of Reye’s syndrome have been associated
with influenza B outbreaks.
Immunity : Antibody against HA & NA are important in
immunity to influenza, resistance to initiation of infection
is related to HA Abs Where decrease of the disease
severity & it’s transmission to contacts is related to NA
Abs.
Protection correlates with both serum antibodies &
secretory IgA antibodies in nasal secretions.
No cross protection is seen between the three influenza
types. Serum antibodies persists for months – years &
secretory Abs lasts for several months only).

14. Laboratory diagnosis :
Detection of Antigen by ELISA
Isolation & identification of the virus : and
inoculation in embryonated eggs & monkey
kidney cells.
Identified by CF, FAT
Serology : HI, CF, ELISA .

15. Treatment :

Amantadine hydrochloride, rimantadine antiviral drugs are
useful to prevent influenza A infections (they block
uncoating).
NSAIDs (other than asprin) is used for fever & headache
Control :
Amantidine is effective against influenza A if given early in
the illness. However, resistance to amantidine emerges
rapidly.
Neuraminidase inhibitors are becoming available. They are
highly effective and have fewer side effects than amantidine.
Moreover, resistance to these agents emerge slowly
.

16.  Oseltamivir (Tamiflu)R is neuraminidase
inhibitors serving as competitive inhibitors of the
activity of viral neuraminidase enzyme upon sialic
acid found on glycoprotein on the surface of the
normal host cells. By blocking the activity of the
enzyme, oseltamivir prevent new viral particles
from being released by infected cells.