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 subtypesCan be highly pathogenic or low pathogenicH5 infections have been documented among humans, sometimes causing severe illness and deathInfluenza A H7Potentially nine different subtypesCan be highly pathogenic or low pathogenicH7 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 H9Potentially nine different subtypesDocumented only in low pathogenic formAt least three H9 infections in humans have been confirmedInfluenza Type BInfluenza 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 CInfluenza 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.

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Defra project – modelling avian influenza in UK poultry industry

• Network model based on UK poultry farms (41,000)

• 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

Grandparent

stock

Parent stock

Rearing farm

Parent stock

Prod farm

H

H

Broiler farm

SH

Parent stock

Rearing farm

Parent stock

Prod farm

H

Broiler farm

SH

Broiler farm

Independent Catchers

SH

SH

Catchers

SH

Broiler farm

Other species

Esp turkeys

SH

Other species

SH SH

Feed mill

Feed mill

Vaccination

teams

Imported?

Feed mill

SHCleaning teams

Cleaning teams

BROILERS

Arrows indicate movement of :

movement of birds

movement of eggs

movement of personnel

movement of feed

Obviously each type of movement will also have associated movements, i.e. vehicles

END