influenza transmission - unisi.it · amino acids in the receptor binding site of ha ... 1900 1910...
TRANSCRIPT
Influenza transmission
Wendy [email protected]
• Modes of transmission of influenza viruses in humans • Consequences for infection control policy in outbreaks/pandemics• Survival of viruses in the environment • Environmental factors that affect transmission (e.g. humidity, temp)• Animal models of transmission• Molecular determinants of transmission• As a risk assessment tool for pandemic potential of novel viruses• Human challenge model of transmission• Pharmaceutical and non‐pharmaceutical interventions of
transmission
Influenza transmission
Escape Survive Enter and Invade
Modes of transmission of influenza
Direct contact
Contaminated fomites
Aerosols and respiratory droplets
Survival of viruses in the environment
• Infectivity is more likely to survive in cold temperatures.
• Environmental pH• Surfaces vary for virus adsorption and survival• Surface decontamination with washing up liquid
Survival of influenza on household surfaces
Greatorex et al. PLoSOne2011
Virus in the air
• Large droplets >5m travel short distances
• Small droplets <5m and aerosols 1mtravel more than 6 feet
• Implications for guidelines on bed spacing in wards
Seasonality of influenza
0
50
100
150
200
250
300
350
400
450
500
89/9090/9191/9292/9393/9494/9595/9696/9797/9898/9999/0000/0101/0202/0303/0404/05
RCG
P co
nsul
tatio
n ra
te p
er 1
00 0
00
pop
ula
tion
0
5000
10000
15000
20000
25000
Num
ber o
f dea
ths
RCGP rate No. of deaths
Figure 6. Variation of Transmission Efficiency with Relative Humidity: A Model
Lowen AC, Mubareka S, Steel J, Palese P (2007) Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature. PLoS Pathog3(10): e151. doi:10.1371/journal.ppat.0030151http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0030151
The contribution of environmental factors to influenza seasonailty
• Absolute humidity and temperature (Lowen and Palese PLoS Path. 2007 in the guinea pig model)
• Vapor pressure: the amount of moisture in the air (Shaman and Kohn PNAS. 2009) affects virus survival
• Humidification of indoor spaces might decrease transmission?
Other ideas about influenza seasonality
• Host susceptibility due to fluctuation in immunity, melatonin and vitamin D
• Host behaviour, crowding indoors during inclement weather
• If low vapor pressure, short days and low temperature are all that is required, influenza should be very rare in subtropical regions!
• Other factors must contribute may in different ways in different regions.
Animal models to study influenza transmission
• The day of infection, dose and exposure route and period are all defined.
• The previous exposure is known (naive animals).
Animal models for influenza virus transmission
Many reagents Infected by inoculationΧMouse adapted virusΧ Different cellular receptors to humansΧ Don’t sneezeΧ Virus does not readily transmitΧ Lower respiratory tract infection
Unadapted human viruses Infected by inoculation Reagents availableΧModel not well characterisedΧ Cellular receptors not characterisedΧ Lower respiratory tract infectionΧ Not been used in transmission study
Can be naturally infectedUnadapted human viruses Virus does transmitΧ New model, not well characterisedΧ Receptors on cells not characterisedΧ No symptomsΧ Lower respiratory tract infection
Mouse Cotton rat Guinea pig
Mouse experiments –Period of optimal transmission
Experimental design: • 2 donor mice co‐housed with groups of 2 sentinel mice every 24 hrs• 80 sentinel mice exposed/time period
Schulman & Kilbourne, 1963 J Exp Med
Transmission did not correlate with viral titre
Titres given as Log10EID50 mean of 5 mice
Schulman & Kilbourne, 1963 J Exp Med
Guinea pig transmission model
• Allows greater numbers than ferrets, and more easily controlled environment
• Assessment of transmissibility of NAI resistant virus (Bouvier and Palese J Virol. 2008), effects of environment (Lowen and Palese. 2007) and genetic determinants.
Animal models for human influenza virus transmission: The Ferret
Ferret model is well established. Ferrets are naturally susceptible to
human influenza viruses. Same cellular receptors as humans. Upper respiratory tract infection, just
like humans. The clinical signs of infection closely
resembles those seen in humans. Ferrets sneeze and produce nasal
discharge.
Χ Very few reagents.
Χ Expensive.
Influenza virus transmission between ferrets
Direct Contact
Donor
Respiratory Droplet Exposed
18
Aerosol cage set‐ups and airflow around the cages
Van Hoeven et al, PNAS 2009 Munster et al, Science 2009
Transmission of pandemic H1N1 2009 influenza in the ferret
Contact
Donor
Aerosol1,E+01
1,E+02
1,E+03
1,E+04
1,E+05
1,E+06
1,E+07
0 1 2 3 4 5 6 7 8 9
pfu/ml
Days post inoculation
T15
T16
1,E+01
1,E+02
1,E+03
1,E+04
1,E+05
1,E+06
1,E+07
0 1 2 3 4 5 6 7 8 9
pfu/ml
Days post inoculation
T15
T16
T13
T14
1,E+01
1,E+02
1,E+03
1,E+04
1,E+05
1,E+06
1,E+07
0 1 2 3 4 5 6 7 8 9
pfu/ml
Days post inoculation
T15
T16
T13
T14
F91
F92
Viral determinants of transmission
• Virus must replicate to sufficient titres in the respiratory tract
• Virus must bind to receptors that are abundant in the respiratory tract
• Virus must survive in the environment (including respiratory tract)
• Virus release and morphology
Sorrell et al. Current Opinion in Virology. 2011
The first human influenza pandemic of the 21st century was not H5N1 bird flu
H5N1 does not transmit between people.
The respiratory environment
Avian and human influenza viruses bind to different types of sialic acid receptors.
Cell Cell
Avian influenza virus
α‐2,3 α‐2,6
Human influenza virus
Avian HAHuman HA
Sialic acid distribution varies through the human respiratory tract.
Shinya et al. Nature. 2006.
Upper respiratory tract
Lung
Nasal mucosa paranasal sinus bronchus
2,6 SA
2,3 SA
Amino acids in the receptor binding site of HA absolutely required for transmission
Human H3 L SCTG AGC 6/6
Avian H3 Q GCAG GGC 0/6
226 228 Transmission rate
Tumpey et al. Science 2007Pappas et al. Plos One 2010Roberts et al.J Gen Virol 2011Imai and Kawaoka Current Opinion in Virology 2012
Human H1 D G
Avian H1 E D
190 225
H5N1 ferret transmission controversy.
The Kawaoka paper
pH1N1 2009
H5N1 2004
7:1 reassortant HA library N224K Q226L
Mutations in receptor binding site AND in HA stalk region are required for H5 transmission in ferrets
Imai et al. Nature 2012
HA stability
The Fouchier approach
H5N1Indonesia 2006
H5N1RG Q226L G228SPB2 E627K
Passage in ferrets
A common set of mutations to confer transmissibility
Minimum set of mutationsQ226L G228SPB2 E627K
PlusH103Y T156A in HAH99Y I368V in PB1R99K S345N in NP in F5 virus
4 or 5 changes to natural H5N1 for transmission
C A Russell et al. Science 2012
543
10 nt away
H103YT315I
The importance of HA and PB2
• For H5N1 virus, H9N2 virus and 1918 virus experimental models have shown that residues in HA and in PB2 are key for transmissibility
Herfst et al. Science 2012Imai et al. Nature 2012Wan and Perez, Sorell et al. PNAS 2009Van Hoeven et al. PNAS. 2009
Influenza Pandemics in the 20th Century:Reassortants also required HA mutations
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
H1N1H2N2
H3N2
Avian circulatingviruses
A role for neuraminidase in transmissionHA NA
SA
Cell Cell
HA:NA balance
Reassortment between swine viruses from different continents led to a new human influenza
HA:NA balance is critical for transmissibility
• pH1N1 2009 emerged from swine after acquiring novel NA and M genes by reassortment
• HA:NA balance confers RD transmission of pH1N1 (Yen et al. PNAS 2011)
Human viruses must adapt to the innate barriers in the human nose
Stalk
Enzyme Head
Neuraminidase adaptation to poultry by shortening the stalk length
M gene and transmission
• M and NA were the two new genes acquired by the precursor of the 2009 H1N1 pandemic that seem to have led to human transmission.
• M and NA contribute to aerosol release, morphology and transmission (Lakdawala et al. PLoSPath 2011)
• pH1N1 2009 M is critical for transmissibility in guinea pigs (Bouvier and Palese. J Virol 2011)
The host range barrier: which animal viruses can contribute to the next pandemic?
Risk assessment for pandemic potential of new viruses
• H3N2 viruses in North American pigs have reassorted with pH1N1 2009 viruses and acquired novel M gene.
• These reassortants are transmissible in ferrets.• The small size of the susceptible population may be all that limits this pandemic.
Risk assessing emergence of antiviral resistance
Kiso et al. PLoSPath 2010.
Seibert, C. W. et al. 2010. J. Virol. 84(21):11219-11226
FIG. 3. Wild-type and oseltamivir-resistant pandemic H1N1 viruses are transmitted efficiently by aerosol contact in the guinea pig model
Competitive fitness of transmission
Duan et al. PLoS Path 2010
The reliability of the ferret model?
• Ferrets are easily infectable and often show mild symptoms (caveat infection dose and route)
• Several viruses that infect and transmit between ferrets do not infect people or cause human disease (classical swine flu; H2, H7;H9)
• Ferret sialic acid containing glycans are similar but not identical to human ones
Human transmission challenge studies
Killingley et al JID 2012:205
• 60 volunteers screened
• 24 eligible
• 9 donors
• 15 recipients
• 30 hrs interaction
• 7/8 donors infected• 5 donors symptomatic
•1 donor had fever
• 10/12 recipients reported symptoms
• 3/12 confirmed influenza
• 25% transmission
Non‐pharmaceutical interventions
Simulating aerosol transmission
Noti et al. CID. 2012
Face mask protecting from inhaled virus
Noti et al. CID. 2012
Fit tested N95 99% protectiveNon fit tested mask = surgical mask reduced 2/3 virus
Pharmaceutical interventions
• Immunization reduced transmission in the guinea pig model (Lowenet al. J Virol 2009)
• Vaccination policy to immunize children
• Prophylactic use of antivirals to interrupt transmission
Infection control policy
R0 depends on• Size of susceptible population• Length of time for which infected is contagious
• Inherent transmissibility of virus
The relationship between symptoms and contagiousness.
0
20
40
60
80
100
No.
of s
neez
es
0
5
10
15
20
No.
of c
ough
s
Days post inoculation
Days post inoculation
Inoculated
2/3
0/3
373839404142
0 1 2 3 4 5 6 7 8 9
Bod
y Te
mpe
ratu
re C
Days post inoculation
Roberts et al. PLoSOne in press
Transmission is not dependent on symptoms
• Infectious virus detected in air exhaled in normal tidal breathing
• Guinea pigs have no respiratory symptoms but transmit efficiently
• pH1N12009 pandemic had a large proportion of asymptomatic individuals, and R0=1.4 but spread quickly
• Viral shedding precedes clinical illness by one day in humans
Patrozou and Mermel. Public Health Reports 2009
Screening at airports to contain a pandemic?
How does transmission occur?
• Estimated infectious dose 1‐10 infectious particles (by aerosol delivery Alford et al 1963)
Transmission proportional to the minimum infectious dose
Inoculated donor
Transmission to sentinel
Wild type virus
Receptor mutant virus
~ 5 viruses
~ 180 viruses
Escape Survive Enter and Invade
Understanding transmission in the future
• Super shedders• Competition experiments• Lung on a chip• Chains of transmission, lengths of contagiousness
• Effect of prior exposure• Non‐pharmaceutical interventions; First Defense.