Influenza
What is it?
Influenza Virus
Understanding Terminology
Epidemic: serious outbreak in a single community, population or region
• Pandemic: epidemic spreading around the world affecting hundreds of thousands of people, across many countries
More on pandemics Usually a novel strain, the majority of the
population has not been exposed to it before (no immunity)
highly contagious
old flu vaccines are not effective
Influenza Family orthomyxoviridae Enveloped viruses – 80-120 nm, negative
stranded RNA with 8 different segments. Allows for genetic re-assortment when >1 virus infects a single cell
Types A, B, and C: Significant differences in structure, genetics, organization, host range, epidemiologic and clinical characteristics
Covered with surface projections or spikes – Hemagglutinin and neuraminidase—used to subtype influenza A virus types.
Influenza Types
Influenza Types Hosts
Type A Humans, birds,
pigs and horses
Type B Humans only
Type C Humans only
Naming of influenza viruses
Named by type / place isolated / culture # / year of isolation
For example: – A/Fujian/411/2002 (H3N2) – B/Shanghai/361/2002-like
Spread direct transmission (infected person sneezes
mucus into the eyes, nose or mouth of another person)
through people inhaling the aerozolized droplets produced by infected people coughing, sneezing and spitting
hand-to-mouth transmission from either contaminated surfaces or direct personal contact, such as a hand-shake (doorknobs, light switches, bank notes…)
Symptoms of the cold and flu
Influenza – Epidemiology (study of health and illness at the population level)
Incubation period 1-4 days Virus first detected just before onset of
illness. Virus usually not detected after 5 - 10 days. More prolonged shedding in children, immunosuppressed hosts Transmission: via respiratory droplets
– person to person,
– direct contact,
Epidemiology continued
Attack rates 10-20% general population, selected populations 40-50%
• Typical Season: 200,000 hospitalizations and 36,000 deaths
How Does Influenza Change?
Particular characteristic that enables
influenza A viruses to cause annual
epidemics, even pandemics Type A viruses undergo frequent
changes in their surface antigens or proteins
• Minor changes - antigenic drift
• Major changes - antigenic shift
Antigenic Drift
Occurs among influenza A viruses resulting in emergence of new variants of prevailing strains every year
New variants result in seasonal flu each winter
Some years are worse than others – partly related to degree of ‘drift’
Antigenic Shift
Occurs in two ways: Sudden ‘adaptive’ change during
replication of a normal virus OR
From an exchange of genes between human strain of an influenza A virus and an animal strain (e.g. avian flu)
Influenza Diagnosis Most often a clinical diagnosis Laboratory tests
– molecular detection of virus in clinical
specimens
– culture of virus
– serology Rapid ‘near patient’ tests – detect the
presence of flu within 30 minutes – cannot determine the specific virus
Treatment options Let the immune system deal with it
If symptoms severe – antiviral drugs
Antiviral drugs Prevent the flu virus from reproducing
Treatment can shorten the illness by a day and reduce hospitalisations by an estimated 50% (based on seasonal flu)
To be effective, must be taken within 48 hours of the onset of flu symptoms
Some common anti-virals for the flu are zanamivir and oseltamivir (Tamiflu)
Prevention
1) Basic measures to reduce the spread of infection Hand washing: washing hands frequently with soap
and water reduces the spread of the virus from the hands to the face, or to others
Respiratory hygiene: covering the mouth and nose when coughing or sneezing; using a tissue when possible; disposing of dirty tissue promptly and carefully – bag and bin
Avoiding non essential travel: non attendance at large gatherings such as concerts, theatres, cinemas, sports arenas etc
Prevention2) Vaccines Vaccines can prevent influenza infection,
particularly important for the elderly and infants
Vaccine coverage is generally poor for infants – less than 10 percent; for the elderly, it can be above 60 percent
Influenza Vaccine Vaccine takes 6-8 months to produce
following the emergence of a new virus •Supplies will be limited, if available at all
– Priority groups are usually established for use of limited vaccine (like who?)
– 2nd dose after 30 days will likely be required
– Need to monitor vaccine safety and efficacy
Priority groups for vaccinations
Children 6-23 months of age
Adults >65 years
Persons 2-64 years of age with underlying chronic medical conditions
Women who will be pregnant during influenza season
Examples of influenza outbreaks
Bird and human influenza
Birds to humans or birds to animals to humans
• Hong Kong1997, H5N1• HK, China1999, H9N2• Netherlands2003, H7N7• Hong Kong2003, H5N1•Viet Nam andThailand, 2004H5N1
Bottom shows bird to animal to human influenza
H1N1/09 virus
thought to have originated in Asia
mix of swine, avian, and human flu, with pigs as the “mixing vessel”
H1N1 Viral Structure
an enveloped RNA virus
has Hemagluttinin and Neuraminidase glycoproteins on its surface
RNPs: ribonucleoproteins, transcribe viral RNA into cellular RNA
Life Cycle influenza attaches itself to
cell’s surface using hemagluttinin (H)
it is brought into the cells entire and disassembles in it
once the virus has made copies of itself it uses nueraminidase (N) to leave the cell