There is significant concern for this
disease not only because of its potential impact on the worldwide poultry
industry and its markets but, more importantly, its significance to public
health.
Influenza, both Type A and Type B, commonly referred to as
the "flu", manifests itself in susceptible humans with cold-like symptoms aggravated
by fever, malaise, muscle and joint aches, nausea and vomiting. Immuno-compromised,
the young, and the elderly are particularly susceptible if unprotected.
Type A influenzas have been subtyped into 16 H and 9 N
surface proteins. Type B influenzas, are thought to be more stable, and are
more amenable to becoming vaccine candidates. Type A influenza reservoirs are
considered to be migratory waterfowl. Among poultry veterinarians, migratory
waterfowl are considered to be in "4-F" status - "fowl feathered flu factories".
Influenza breaks in domestic poultry have largely been traced to migratory
waterfowl or live bird markets to the extent that some state plans for
emergency disease planning in poultry discourage poultrymen and service
personnel from hunting and processing migratory waterfowl due to the
asymptomatic shedding of type A viruses and the ease of fecal-oral transmission
to poultry. In poultry, signs of influenza may vary from negligible, with no
effect on growth or laying, to extreme mortality, leaving a decimated, cyanotic
flock with digestive and respiratory tract hemorrhages.
In 1997, six human deaths in Hong Kong were attributed to
a pathogenic strain of type A influenza H5N1. Because of intermingling of
migratory and domestic waterfowl and live bird markets, concerns for genetic
mutations allowing bird to human transmission were raised. Since 2003, there
have been 225 confirmed cases of H5N1 influenza in humans with 128 fatalities.
As of this writing, human to human transmission has not been demonstrated.
As of June 1, 2006, H5N1 has been reported from wild and
domestic birds in 49 countries in Asia, the middle East, Africa and some parts
of Europe.
Etiology: Influenza viruses are of the Orthomyxoviridae family and have three
types: type A (found in both humans and animals), type B (found only in humans),
and type C (uncommon strain found only in humans). Type A may be subtyped
according to two surface glycoproteins. There are 16 H or hemagglutinins and
9 N or nueraminadases, allowing 144 different subtype combinations. For the
human, type A provides a more ominous threat due to the assortment of virus
components. Recurrent epidemics of type A flu are a constant challenge to the
CDC in predicting future subtype candidate viruses.
Epidemiology: Type A influenza viruses in domestic poultry are
categorized into Low Pathogenic AI (LPAI), which is the most common, and Highly
Pathogenic AI (HPAI), or the lethal form, uncommon in US poultry.
LPAI viruses cause mild infections (no weight loss or
respiratory signs) in domestic birds and are not considered a threat to public
health. Generally, world marketing agreements do not turn away finished
products from an LPAI geographic area.
HPAI virus strains H5 and H7 can cause alarming losses
with 90-100% mortality in commercial poultry. HPAI has been a problem in live
bird markets of some east coast US cities where the USDA has continually
monitored the eradication efforts. H5N1 subtype is an HPAI. It has cost the
poultry industry of Asia millions of dollars in bird losses and marketing
barriers have been established. H5N1 influenza has been isolated from domestic
cats, captive tigers, and leopards in Asia. In 2005, there was an unusually
high death loss of migratory birds due to H5N1 in central China. In the past 30 years, there have been 24 outbreaks of HPAI worldwide; most of these were
confined to a single flock or farm.
International vigilance in monitoring avian influenza
outbreaks is occurring because LPAI H5 and H7 viruses are capable of evolving
into HPAI. Likewise, there is fear that HPAI viruses could transmit to humans
through genetic mutation. At present, extensive monitoring for H5N1 virus
among waterfowl migrating to US flyways is being carried out across the United States.
Diagnosis: Detection of type A influenza is dependent upon diagnostic
tests. Antigen capture screening tests are available. Nasopharyngeal,
tracheal or cloacal swabs are used, and results can be determined in a short
period of time. Positives are subtyped. ADDL has continuous settings of SPF
chicken eggs for virus isolation attempts from tracheas, bronchi, cloacal, or
nasopharyngeal swabs. Embryo harvest is followed by electron microscopy,
antigen capture immunoassay, and subtyping.
Real time Reverse Transscription Polymerase Chain Reaction
(RT-PCR) test is available at ADDL. This test allows greater sensitivity and
is useful for large scale screening during epidemics. Results are available
quickly. Advantages to this method are small sample size requirement,
timeliness, and the ability to subtype. Suitable samples to submit for this
test are tracheal and nasopharyngeal swabs of Dacron on plastic sticks (1 swab
per tube with 1 ml viral transport media), with no blood contamination, shipped
on cold paks.
Serological type A screening, using the Agar Gel
Precipitin (AGP) or Agar gel Immunodeficiency (AGID) method, requires a minimal
amount of serum and can be read within 1-2 days. In
some instances where only eggs were available, yolk antibodies have been
tested.
Surveillance and Control (Biosecurity): Surveillance of type A influenza
viruses in the Indiana poultry industry is based on monitoring sera for the
presence of type A antibodies. The USDA provides antigen and antibody to
perform the AGP test to private and public laboratories upon request. The
National Poultry Improvement Plan (NPIP) provides protocol and uniform
procedures that are acceptable according to world standards. Seropositive
samples are submitted to the National Veterinary Services Laboratory (NVSL) for
subtyping. Commercial poultry farms exercise biosecurity precautions in
housing that limits access by wild birds, unwanted visitors, vermin, and rodent
entry. Visitors are required to put on disposable suits, head, hand and shoe
covers before entry and removal at exit. Delivery traffic and utility personnel
respect biosecurity protocols. Commercial killed vaccines have been
advocated and stockpiled by the USDA for commercial poultry use. In a 2004
outbreak of type A influenza (LPAI-H7N2) among commercial poultry in the
eastern United States, eradication was the response of choice.
On a regional basis, biosecurity concerns are implemented
according to the location of disease. A Level 1 would be the minimum amount of
acceptable biosecurity for poultry, while Level IV would indicate that poultry
are in imminent threat and biosecurity is at its highest level. A
communication network between the Indiana State Poultry Association and poultry
producers is being updated in order for sound and current information to be
made available. Outdoor poultry that can be exposed to migratory waterfowl,
directly or indirectly, as well as farm ponds, remain a threat to disease
prevention programs.
Strict surveillance of poultry populations is enforced in
most countries in an attempt to contain the H5N1 outbreaks. Continuous
research on methods of identifying the constantly changing AI virus is key to
finding a solution to this impending global problem.
-by Maria Solacito, ECFVG Student
-edited by Dr. Tom Bryan, Avian Diagnostician, Heeke ADDL
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