AVIAN INFLUENZA
The term “influenza” originally referred to epidemics ofacute, rapidly
spreading catarrhal fevers of humans caused by viruses in the family
Orthomyxoviridae (114). Today,orthomyxoviruses are recognized as the
cause of significant numbers of natural infections and disease, usually of
the upper respiratory tract, in humans, horses, domesticpigs, and various
bird species and sporadic cases of naturallyoccurring disease in mink and
a variety of marinemammals (71, 136, 244). Infection of domestic poultry
byavian influenza (AI) viruses typically produces syndromesranging from
asymptomatic infection to respiratory diseaseand drops in egg production
to severe, systemic diseasewith near 100% mortality (65). The latter form
of disease is the result of infection by highly pathogenic (HP)AI viruses.
Disease is usually absent with AI virus infectionin most wild bird
species.For more detail, see recently published reviews on HPAI (229),
outbreaks of AI in the 1990s (167), immunologyof AI (210), AI in
different bird species (9), epidemiology and control (7), and evolution
and ecology of AI viruses(212, 244).
Definitions and Synonyms
Avian influenza initially was recognized as a highly lethal,systemic
disease (i.e., highly pathogenic or highly avian influenza). From the late
1870s to 1981, HP AI wasknown by various names including fowl plague
(most common), fowl pest, peste aviaire, Geflugelpest, typhusexudatious
gallinarium, Brunswick bird plague, Brunswick disease, fowl disease, and
fowl or bird grippe (205, 206). In1981 at the First International
Symposium on Avian Influenza, the terminology “highly pathogenic
avianinfluenza,” was adopted as the official designation for the highly
virulent form of avian influenza. The Office International des Epizooties
(OIE), a World Trade Organization allied group that codifies sanitary and
health standards, specifies HP AI as a List A disease (8). OIE List A
contains transmissible diseases that have the potential for very serious
and rapid spread, irrespective of national borders,which are of serious
socio-economic or public health consequenceand which are of major
importance in the internationaltrade of animals and animal products
(158). Milder forms of AI were first recognized in variousdom estic
poultry species between 1949 and the mid- 1960s and have been termed
low pathogenic, pathogenic,nonhighly pathogenic and mildly pathogenic
(MP) AI (5, 68, 181). Their impact on poultry production and tradehas
been much less severe than with HP AI. They have notbeen listed by OIE
as either a List A or B disease (158). Forthis publication, MP AI will be
used to refer to this groupof diseases that are non-HP.
Economic Significance
Economic losses from avian influenza have varied depending on the
strain of virus, species of bird infected, number of farms involved, control
methods used, and the speed of implementation of control or
radicationstrategies.In most developed countries, HP and MP AI have
notbeen endemic diseases in the commercial poultry industries.Most
outbreaks and economic losses have occurredfrom epizootics of HP or
MP AI in commercially raised poultry, predominately chickens and
turkeys. Some developingcountries have had MP AI endemics in
commerciallyraised poultry and, in some developed countries,MP AI has
been endemic in backyard and live poultry market (LPM) systems that
serve ethnic populations oflarge metropolitan areas. The terms live bird
market andLPM have been used interchangeably.Generally, the greatest
losses have occurred during epizooticsof HP AI in domestic poultry
raised on commercialfarms of intensive production areas or in poultry in
LPM systems. Direct losses in HP AI outbreaks haveincluded
depopulation and disposal costs, high morbidity and mortality losses,
quarantine and surveillance costs,and indemnities paid for elimination of
marketing birds. During the 1924—1925 U.S. HP AI outbreak, direct
losseswere in excess of $1 million ($10 million adjusted to 2001) (206).
This cost included cleaning and disinfectionof 2,718 slaughter plants,
8,140 railroad cars, 352,525 transportation coops, and 124,997 pieces of
miscellaneousequipment (206). The 1983—1984 HP AI H5N2 outbreakin
the northeastern United States involved morethan 17 million poultry and
449 commercial farms (75). The U.S. federal government spent more
than $63 million($106 million adjusted to 2001) in direct eradication
costsincluding $40 million in indemnities (75, 129). In addition,producers
absorbed $15 million in nonindemnified losses, and consumers
experienced $349 million ($588million adjusted to 2001) in increased
food costs (129). However, if the federal government had not
implementedan eradication program, the projected cost was $500
millionU.S. for losses to poultry farmers and $5.5 billion inincreased
customer costs (129). In the 1999—2000 outbreakof HP AI H7N1 in
Italy, the government paid farmers$100 million U.S. in compensation for
18 million poultry from 413 flocks, and total indirect losses were$500
million U.S. (I. Capua, personal communication, 2001). In 1997, the
outbreak of HP AI H5N1 in the HongKong LPM system cost $13 million
U.S. for depopulationand indemnities for 1.4 million poultry (L. Sims,
personalcommunication, 2001). Outbreaks of HP AI on a morelimited
scale have occurred in Australia and England (seeTable 5.1). For
example, the 1985 outbreak on onechicken farm cost $2 million AU to
eradicate (55)Mildly pathogenic AI outbreaks have caused significant
economic losses for producers of turkeys and ducks, especiallywhen
accompanied by secondary bacterial or viral pathogens. In general, losses
have been less than with HPAI outbreaks because infected flocks have
been depopulatedthrough a controlled marketing program, the
mortalityrates have been lower, and national and international
tradeusually have not been disrupted. For example, the 1978 epizootic of
MP AI in Minnesota turkeys cost producers $5 millionU.S. ($13.5 million
U.S. adjusted to 2001), and losses from seasonal outbreaks in Minnesota
turkeys between1978 and 1996 total $22 million U.S. ($34 million U.S.
adjusted to 2001) (87). Losses for the 1995 MP AI H7N2 outbreakin Utah
turkeys cost $2 million (87). These costs include mortality losses,
condemnations at slaughter, medicationagainst secondary bacteria, and
cleaning and disinfection.One of the most costly outbreaks of MP AI was
the1999 H7N1 in Italy that affected 199 flocks of poultry (230;I. Capua,
personal communication, 2000). Similar costlyand ongoing MP AI
outbreaks include H9N2 in Asia and theMiddle East and H5N2 in Mexico
and Central America.