final program.qxd - Parallels Plesk Panel

First, the origin of primary introductions of the virus into farmed poultry populations will be
reviewed, and next the secondary transmission between poultry farms. Subsequently, the
knowledge on the prevention of primary introductions and the control of secondary
transmission will be addressed.
Primary introductions
Primary introductions of Avian Influenza virus in farmed poultry populations may originate
from three sources: free flying aquatic birds, live poultry markets or pet birds. The relative
risk associated with each of these sources varies depending on the likelihood of direct or
indirect contacts with susceptible poultry.
Until recently, the hypothesis was that free flying aquatic birds, especially ducks and
geese, but also shorebirds, gulls, terns and auks, serve as an important reservoir of low
pathogenicity AI virus strains (LPAI). These LPAI virus strains can be transmitted from
waterfowl to poultry by infected feces either through direct contact or indirectly through
contamination of feed, water, or free-range area. If the LPAI strain is of the H5 or H7
subtypes, the next step would be that the virus mutates into a HPAI strain, causing an
outbreak in poultry.
Most of the introductions of H5N1 in Europe, are, however supposed to originate from
migratory birds, either directly from Asian countries, or via an intermediate host.
Since 1959 24 primary isolates of HPAI have been identified, and last year H5N1 was
found in swans, ducks, geese and other water fowl in most of the countries of Western
Europe.
In addition to waterfowl, live bird markets also pose a significant risk to the introduction of
AI viruses into poultry flocks. Because of the continuous marketing process, LPAI and, in
Asian countries probably also HPAI virus, can persistently circulate in these systems.
Indirect contacts through contamination of persons, equipment or vehicles could transmit
the virus to industrial poultry flocks. Additionally, companion or pet birds may also serve
as a source of infection, because AI viruses have been recovered from caged birds,
usually during quarantine. However, transmission from this source to poultry has not been
documented.
Secondary transmission
Once infected, birds excrete AI viruses from both the respiratory and the digestive tract.
Thus, within a poultry house, bird-to-bird transmission is probably by aerosol and
ingestion. Between flocks, infected poultry feces appear to be a most likely source of
transmission, human-associated contacts being the main route.
In several specific accounts strong evidence has implicated the movement of caretakers,
farm owners and equipment, trucks and drivers moving birds or delivering food, and
artificial inseminators in the spread of the virus. Birds or other animals which are not
themselves susceptible to infection may also become contaminated and transmit the
virus. Moreover, shared water or food could become contaminated and serve as an
infection source. Finally, there is some evidence that windborne spread may have played
a role amongst very closely situated farms and that flying insects could become
contaminated with infected feces. Vertical transmission of HPAI viruses is unlikely,
because AI viruses are embryo lethal.
“ Focusing FIRST on PEOPLE “ 66 w w w . i s h e i d . c o m