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Avian Influenza
(plagiarised)
Roger Bowers
Avian Influenza
Bird flu
Avian influenza is a disease of
birds caused by influenza viruses
closely related to human
influenza viruses.
Transmission to humans in close
contact with poultry or other birds
occurs rarely and only with some
strains of avian influenza. The
potential for transformation of
avian influenza into a form that
both causes severe disease in
humans and spreads easily from
person to person is a great
concern for world health.
(DNA)
Typical infectious cycle.
Influenza viruses classified into types A, B or C based on differences
between their nucleoprotein and matrix protein antigens (substance that
stimulates an immune response) .
Avian Influenza … type A
.
Further classified by proteins haemagglutinin (H) and neuraminidase (N)
… projections on their surfaces. There are 14/15 H subtypes and 9 N
subtypes of influenza A
To date all highly pathogenic AI viruses that cause generalised rather
than respiratory disease belong to either the H5 or H7 subtypes; classic
fowl plague H7N7
The pathogenicity of AI viruses is correlated with cleaving of
haemagglutinin molecule into two subunits … use to determine whether
or not an isolated virus is potentially pathogenic.
Influenza Type A
•Can infect people, birds, pigs, horses, seals, whales, and other animals, but
wild birds are the natural hosts
•Only some influenza A subtypes (i.e., H1N1, H1N2, and H3N2) are currently in
general circulation among people.
•Within subtypes of avian influenza or A viruses there also are different strains.
H5 and H7 viruses can be distinguished as “low pathogenic” and “high
pathogenic” forms on the basis the severity of the illness they cause in poultry;
influenza H9 virus has been identified only in a “low pathogenicity” form.
•Each of these three avian influenza A viruses (H5, H7, and H9) theoretically
can be partnered with any one of nine neuraminidase surface proteins; thus,
there are potentially nine different forms of each subtype (e.g., H5N1, H5N2,
H5N3, H5N9).
Influenza A H5
Potentially nine different subtypes
Can be highly pathogenic or low pathogenic
H5 infections have been documented among humans, sometimes causing
severe illness and death
Influenza A H7
Potentially nine different subtypes
Can be highly pathogenic or low pathogenic
H7 infection in humans is rare, but can occur among persons who have direct
contact with infected birds; symptoms may include conjunctivitis and/or upper
respiratory symptoms
Influenza A H9
Potentially nine different subtypes
Documented only in low pathogenic form
At least three H9 infections in humans have been confirmed
Influenza Type B
Influenza B viruses are normally found only in humans. Unlike influenza A
viruses, these viruses are not classified according to subtype. Although
influenza type B viruses can cause human epidemics, they have not caused
pandemics.
Influenza Type C
Influenza type C viruses cause mild illness in humans and do not cause
epidemics or pandemics. These viruses are not classified according to subtype.
Influenza viruses can change in two different ways:
•“Antigenic drift," which occurs through small changes in the virus that happen
continually over time. Antigenic drift produces new virus strains that may not be
recognized by antibodies to earlier influenza strains. In most years, one or two of the
three virus strains in the influenza vaccine are updated to keep up with the changes in
the circulating flu viruses.
•The other type of change is called "antigenic shift." Antigenic shift is an abrupt, major
change in the influenza A viruses, resulting in a new influenza virus that can infect
humans and has a hemagglutinin protein or hemagglutinin and neuraminidase protein
combination that has not been seen in humans for many years. Antigenic shift results
in a new influenza A subtype. If a new subtype of influenza A virus is introduced into
the human population, if most people have little or no protection against the new virus,
and if the virus can spread easily from person to person, a pandemic (worldwide
spread) may occur.
•Influenza viruses are changing by antigenic drift all the time, but antigenic shift
happens only occasionally. Influenza type A viruses undergo both kinds of changes;
influenza type B viruses change only by the more gradual process of antigenic drift.
Defra project – modelling avian influenza in
UK poultry industry
• Network model based on UK poultry farms (41,000)
dI j / dt    I i Gij S j
j
• Different Gij , τ for
– Local ‘non-operational’ transmission (air-borne, environmental,
walkers); based on distance
– Global ‘operational’ contact (sharing hatcheries, slaughter-houses, feed
mills,….)
• Stochastic simulation & (like) pair-approximation for epidemics;
parameterisation??
• ‘Natural’ epidemics
• Control – switches off some Gij
BROILERS
SH
SH
Grandparent
Arrows indicate movement of :
H
Feed mill
Cleaning
teams
Cleaning
teams
stock
movement of birds
Imported?
movement of eggs
movement of personnel
SH
Parent stock
Vaccination
Parent stock
Rearing farm
teams
Rearing farm
Parent stock
Parent stock
Prod farm
Prod farm
H
H
movement of feed
Obviously each type of movement will also have associated
movements, i.e. vehicles
Feed mill
SH
Broiler farm
Broiler farm
Feed mill
Broiler farm
Broiler farm
Catchers
Independent
Catchers
SH
SH
SH
SH
Other species
SH
Other species
Esp turkeys
END