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