Chapter 31 Avian Influenza

INFLUENZA Including avian

Influenza• Febrile respiratory disease with systemic symptoms

caused by a variety of other organisms often called ‘flu• Birds, swine, other mammals also affected• Highly infectious and can spread rapidly from person to

person• Some strains cause more severe illness than others

History of influenza

• 412 BC - first mentioned by Hippocrates

• 1580 - first pandemic described

• 1580-1900 - 28 pandemics

Types of influenza viruses

• Influenza viruses divided into 3main types: influenza A, B, and C

• A viruses• infect birds and other animals, as well as humans• source of seasonal influenza epidemics and all pandemics• causes an average 30,000 deaths per year

• Especially dangerous for the elderly

• B and C viruses• infect humans only and do not cause pandemics• virus C infections - much milder

Influenza

• Mutates frequently• Antigenic drift: small mutations that require a new vaccine every

year• Antigenic shift: emergence of new strains (such as H1N1 become

common when H3N2 used to be about the only human influenza circulating)

• Co-infection with 2 viruses is possible• Recombination of 2 segments can create a different virus

Migratory water birds

Domestic birds

Where does influenza A virus come from?

Humansand other animals

Human influenza A viruses start as avian (bird) influenza viruses

Seasonality: minor changes - antigenic drift

• influenza in humans is closely tied to seasonal outbreaks, typically associated with winter months

• Occurs among influenza A viruses resulting in emergence of new variants of prevailing strains every year

• New variants result in seasonal influenza each winter• Some years are worse than others – partly related to

degree of ‘drift• avian influenza: seems tied to bird migration patterns and

social behavior

Transmission• Spreads easily from person to

person through coughing and sneezing

• Transmitted by:• inhaling respiratory aerosols containing

the virus, produced when infected person talks, coughs, or sneezes• 100,000 TO 1,000,000 VIRIONS PER

DROPLET

• touching an infected person or an item contaminated with the virus and then touching your eyes, nose, or mouth

• expelled respiratory droplets can contaminate surfaces, and be transmitted to mucous membranes through direct contact

• Avian: handling of infected bird feces

symptoms• Sudden onset• both systemic and respiratory symptoms can occur to varying

degrees• systemic - fever, headache, myalgia, malaise• respiratory - cough, sore throatdifficulty breathing

• Complications:• recently some increase in morbidity and mortality - possible factors?

• more elderly people• more high risk neonates• more immunosuppressed patients

• bacterial superinfection can be severe• Streptococcus pneumoniae the most common pathogen involved• Staphylococcus aureus increasing, especially severe pneumonia

due to MRSA

prevention• Vaccine:

• current CDC guidelines recommend immunizing the elderly, pregnant women, and high risk patients (health care and lab workers, immunosuppressed)

• Antivirals:• E.g Tamiflu : insufficient quantities, effectiveness unclear

• Disease containment measures:

What is an influenza pandemic?

• Influenza pandemics are worldwide epidemics of a newly emerged strain of influenza

• Few, if any, people have any immunity to the new virus

• This allows the new virus to spread widely, easily, and to cause more serious illness

What causes a pandemic?• Pandemics occur when a new avian influenza strain acquires the ability infect people and to spread easily person to person

• This can occur in 2 ways:• Reassortment (an exchange of

seasonal and avian influenza genes in a person or pig infected with both strains)

• Mutation (an avian strain becomes more transmissible through adaptive mutation of the virus during human avian influenza infection)

Pandemic influenza: major changes - antigenic shift

• Major changes occur in the surface antigens of influenza A viruses by mutation or reassortment

• Changes are more significant than those associated with antigenic drift

• Changes lead to the emergence of potentially pandemic strains by creating a virus that is markedly different from recently circulating strains so that almost all people have no pre-existing immunity

Global response

Seasonal vs. pandemic influenza• Pandemic influenza is not just a “bad flu,” it is a wholly

new threat to humans• A severe pandemic would cause social disruption unlike

anything most persons now alive have ever experienced• Compared to seasonal influenzas, pandemic influenzas

infect more people, cause more severe illness, and cause more deaths

• Seasonal influenza viruses most often cause severe disease in the very young, the very old, and those with chronic illnesses, but pandemic influenza strains can infect and kill young, healthy people

• The highest mortality rate in the 1918-19 pandemic was in people aged 20-40 years

Pandemic influenza in the 20th Century

1920 1940 1960 1980 2000

H1N1 H2N2 H3N2

1918 “Spanish Flu” 1957 “Asian Flu” 1968 “Hong Kong Flu”

20-40 million deaths 1 million deaths 1 million deaths

1918 Pandemic

Highest mortality in people 20-40 years of age- 675,000 Americans died of influenza - 43,000 U.S. soldiers died of influenza

Lessons from past pandemics

• Occur unpredictably, not always in winter • Great variations in mortality, severity of illness, and pattern of illness or age most severely affected

• Rapid surge in number of cases over brief period of time, often measured in weeks

• Tend to occur in waves of 6 - 8 weeks, subsequent waves may be more or less severe

Key lesson – unpredictability

What about an avian influenza pandemic?

• A highly pathogenic avian influenza strain (A/H5N1) emerged in Hong Kong in 1997, reemerged in birds and humans in 2003, and is now circulating widely in birds in many countries

• Since 2003, this strain has spread from birds to humans and as of August 23, 2006 has infected 241 people (141 deaths) in 10 countries

• This strain has also been documented (rarely, so far) to spread from person to person

• Reassortment or mutation could allow this strain to become easily transmissible between humans – there is no way to know if or when this will happen

Would the next pandemic be severe?

• Past pandemics provide clues as to how humans may be affected by a new influenza virus and how societies would react to a pandemic

• Info n from past pandemics used in economic and disease models to predict impact of future pandemics

• In US , up to 1.9 million people could die, up to 9.9 million could be hospitalized, and up to 90 million could become ill

• Intense pressure on healthcare• Disruption to many aspects of

daily life

Pandemic waves

Past experience teaches us that following

emergence of a new pandemic virus: • More than one wave of influenza is likely• Waves typically last 6-8 weeks • Gaps between the waves may be weeks or months

• A subsequent wave can be worse than the first

What can be done to slow spread of a pandemic?

• Vaccine:• “pre-pandemic” H5N1 vaccines are in development, but would

have reduced efficacy in a pandemic due to antigenic drift

• Antivirals• Disease containment measures:

• may be the only measures available in the early stages of a pandemic

• may be helpful in slowing the spread of a pandemic, allowing more time for vaccine production

Vaccine

• Because virus will be new, there will be no vaccine ready to protect against pandemic influenza at the start of a pandemic

• Specific vaccine cannot be made until virus strain has been identified; takes at least 4-6 months to produce

Antiviral drugs

• Likely to be only major medical countermeasure available early in a pandemic

• Uncertainty about effectiveness for treatment or prevention

• U.S. goal is to stockpile enough antiviral drugs to treat 25% of the U.S. population

Reproduced with permission from Roche Products Ltd. Tamiflu ®

Disease containment measures• Isolation: restriction of

movement/separation of ill infected persons with a contagious disease

• Quarantine: restriction of movement/separation of well persons presumed exposed to a contagious disease

• Self-shielding: self-imposed exclusion from infected persons or those who may be infected

• Social distancing: reducing interactions between people to reduce the risk of disease transmission

• Snow days: days on which offices, schools, transportation systems are closed or cancelled, as if there were a major snowstorm

Other methods to reduce transmission

• Hand hygiene (cleaning hands with soap and water or an alcohol-based hand rub)

• Respiratory hygiene, e.g., “Cover your cough”• Cleaning and disinfection of contaminated objects, surfaces

• Physical barriers (e.g., glass or plastic “windows” to protect front desk workers)

• Use of personal protective equipment (PPE) in some settings (e.g., healthcare) such as gowns, gloves, eye, and respiratory protection

Global surveillance and planning

• Global surveillance is essential; international cooperation critical• International Health Regulations• Pandemic Alert System• http://www.who.int/influenza/preparedness/pandemic/h5n1phase/e

n/

• Planning for a possible pandemic is occurring nationally and internationally

• National, state, local, and individual preparedness are all important

Current WHO phase of pandemic alert for avian influenza A(H5N1) is: ALERT

•

Alert phase: phase when influenza caused by a new subtype has been identified in humans. Increased vigilance and risk assessment, at local, national and global levels, are characteristic of this phase.• If risk assessments indicate that new virus is not developing into a

pandemic strain, a de-escalation of activities towards those in interpandemic phase may occur.