INFLUENZA

VIROLOGY OF INFLUENZA

Subtypes:A - Causes outbreakB - Causes outbreaksC - Does not cause outbreaks

PATHOGENICITY

• High pathogenicity avian influenza (HPAI)– Causes severe disease in poultry– Contains subtypes H5 or H7

• Low pathogenicity avian influenza (LPAI)– Causes mild disease in poultry– Contains other H subtypes

• Includes non-HPAI H5 and H7

• LPAI H5 or H7 subtypes can mutate into HPAI

Center for Food Security and Public Health, Iowa State University, 2013

Immunogenic Components of Immunogenic Components of the Influenza Virusthe Influenza Virus

• Surface glycoproteins, 15 hemagglutinin (H1-H15), nine neurominidases (N1-N9)

• H1-H3 and N1N2 established in humans• Influenza characterized by combination of H and N

glycoproteins 1917 pandemic - H1N1

2004 avian influenza - H5N1

2009 H1N1

• Antigenic mix determines severity of disease• Human response specific to hemagglutinin and

neurominidase glycoproteins

Antigenic Drift and Shift

• Antigenic drift– Small changes in influenza virus due to

point mutations accumulated during virus replication

• Antigenic shift– Abrupt change in virus subtype

• Genetic reassortment between subtypes• Direct transfer of virus • Re-emergence of virus

Center for Food Security and Public Health, Iowa State University, 2013

RESERVOIRS

Domestic and wild birdsWaterfowl and shorebirds

Natural reservoirs for influenza ACarry all known H and N antigens

– Usually in the LPAI form

Predominant subtypes change periodically Pigs Wet markets (live animals)

Center for Food Security and Public Health, Iowa State University, 2013

Figure 1. Natural hosts of influenza viruses

Nicholson et al. Influenza. Lancet 362:1734, 2003

Nicholson et al. Influenza. Lancet 362:1735, 2003

Figure 2. Origin of antigenic shift and pandemic influenza. The segmented nature of the influenza A genome, which has eight genes, facilitates reassortment; up to 256 gene combinations are possible during coinfection with human and non-human viruses. Antigenic shift can arise when genes encoding at least the haemagglutinin surface glycoprotein are introduced into people, by direct transmission of an avian virus from birds, as occurred with H5N1 virus, or after genetic reassortment in pigs, which support the growth of both avian and human viruses.

Morbidity and Mortality: Birds

• Domesticated poultry– HPAI morbidity and mortality rates approach

90-100%• Wild birds

– Typically asymptomatic• Some H5N1 viruses may

cause death

Center for Food Security and Public Health, Iowa State University, 2013

Influenza Epidemics

• Epidemic requirements1. New influenza subtype must emerge in

species with little to no immunity2. Virus must produce disease in that species3. Sustainable transmission must occur in new

species• Note: Asian lineage H5N1 has NOT met

third criteria in humans

Center for Food Security and Public Health, Iowa State University, 2013

Transmission in Poultry

• In an infected flock, virus can spread in multiple ways– Fecal-oral– Aerosol– Fomites– Mechanical vectors

• Virus introduction– Migratory birds– Infected poultry, pet birds

Center for Food Security and Public Health, Iowa State University, 2013

Influenza Virus Survival

• Virus persistence in aquatic environments– Weeks to months – Preferred conditions

• Low temperatures, brackish water– May survive indefinitely when frozen

• Virus persistence in feces– Weeks to months

Center for Food Security and Public Health, Iowa State University, 2013

SURVEILLANCE FOR FLU

http://www.cdc.gov/h1n1flu/updates/us/

http://www.cdc.gov/h1n1flu/updates/us/ - real-time

Update: influenza activity – U.S., Sept 29-Dec 7, 2013. MMWR 62(50):1034, 2013.

The figure shows peak influenza activity for the United States by month for the 1976-77 through 2008-09 influenza seasons. The month with the highest percentage of cases (nearly 50%) was February, followed by January with 20% and March and December, with approximately 15% of all cases.

Prevention and control of seasonal influenza with vaccines. MMWR 58(RR-8):5, 2009

http://www.milbank.org/reports/0601fauci/0601Fauci.pdf

CLINICAL OUTCOMES OF INFLUENZA INFECTION

•• AsymptomaticAsymptomatic

•• SymptomaticSymptomatic

Respiratory syndrome - mild to severe

Gastrointestinal symptoms

Involvement of major organs - brain, heart, etc.

Death

Factors Influencing the Response to Influenza

• Age• Pre-existing immunity (some crossover)• Smoking• Concurrent other health conditions• Immunosuppression• Pregnancy

Treatment in Humans

• Antiviral drugs– Amantadine– Rimantadine– Zanamivir– Oseltamivir

• Currently circulating H5N1 viruses may be resistant to amantadine, rimantadine

Center for Food Security and Public Health, Iowa State University, 2013

The H1N1 Epidemic

Kaplan K. How the new virus came to be. LA Times, 14 Sept, 2009; latimes.com/health

RECOMMENDATIONS TO PREVENT FLU

STRATEGIES TO PREVENT FLU (1)• COVER MOUTH AND NOSE WHEN

SNEEZING

• WASH HANDS FREQUENTLY WITH SOAP AND WATER OR ALCOHOL

• AVOID TOUCHING EYES, NOSE AND MOUTH

• AVOID CONTACT WITH SICK PEOPLE

• AVOID CROWDED CONGESTED ENVIRONMENTS

STRATEGIES TO PREVENT FLU (2)• IF SICK STAY HOME, DON’T EXPOSE

OTHERS

• FOLLOW PUBLIC HEALTH ADVICE; e.g. school closures etc.

• GET FLU SHOT(S)

• TAKE ANTIVIRAL DRUGS IF PHYSICIAN RECOMMENDS