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Avian influenza in poultry
Avian influenza is a viral
disease affecting the respiratory, digestive and/or nervous system of many species
of birds. Avian influenza virus infection can occur in most, if not all,
species of birds, both domestic and wild. Influenza viruses vary widely in
their ability to cause disease (pathogenicity) and their ability to spread
among birds. Wild species of birds usually do not develop clinical disease, but
some influenza viruses cause severe illness or death in chickens, turkeys and
A highly pathogenic form of
avian influenza was known as "fowl plague". It first appeared in
Italy more than 100 years ago (around 1878). Pathogenic avian influenza was
first recognized in the United States in 1924-25. It occurred again in 1929. It
was eradicated both times.
A major epidemic of highly
pathogenic avian influenza occurred in the northeastern United States in
1983-84. It took more than 2 years to eradicate, at a cost of more than 70
million dollars. Approximately 17 million birds had to be destroyed during the
The United States has not
had a major outbreak of highly pathogenic avian influenza since 1986, although
less pathogenic strains of avian influenza virus are present and have caused
significant losses in the poultry industry. Another major outbreak of highly
pathogenic avian influenza would be disastrous to the American poultry
In 1996-97 a number of
table-egg farms in Lancaster and Lebanon Counties, PA tested positive for H7N2
avian influenza. Thus far, the avian influenza virus which has been detected by
serologic means and/or virus isolation has been characterized as nonpathogenic
to chickens, but the outbreak has had devastating effects on the local poultry
industry. Between the first week of December 1996 and June 6 1997 nine flocks
were depopulated. The Pennsylvania Agricultural Department imposed a quarantine
on a 75-square-mile area restricting movement of poultry or poultry products
into or off of operations in the area of the quarantine.
The severity of the disease
ranges from inapparent (mild) to rapidly fatal. Lethal strains of the virus can
strike so quickly, particularly in young chickens, that there may be no
clinical signs other than sudden death.
Avian influenza viruses of
low to moderate pathogenicity are identified regularly in the United States in
the domestic poultry populations. Avian influenza virus is reintroduced into
domestic poultry by migratory waterfowl, which are carriers of the influenza
Clinical signs vary greatly
and depend on many factors including the age and species of poultry affected,
husbandry practices, and the inherent pathogenicity of the influenza virus
strain. Clinical signs may include:
- ruffled feathers
- soft-shelled eggs
- depression and droopiness
sudden drop in egg
- loss of appetite
coloring) of wattles and comb
edema and swelling of head,
eyelids, comb, wattles, and hocks
blood-tinged discharge from
loss of ability to walk and stand
pin-point hemorrhages (most
easily seen on the feet and shanks)
- respiratory distress
increased death losses in a
The clinical signs of avian
influenza are similar to those of other avian diseases. Avian influenza may be
confused with infectious bronchitis, infectious laryngotracheitis, fowl
cholera, and the various forms of Newcastle disease.
Typical history, signs, and
lesions may be suggestive of mild forms of avian influenza. Confirmation of a
diagnosis is by serologic testing and virus isolation and identification.
Because virulent strains of avian influenza are considered to be exotic to the
United States, they are reportable to the USDA. Virulence level is evaluated by
virus isolation and controlled laboratory challenge of experimental chickens.
Lesions vary greatly
depending on pathogenicity of the virus, age of the bird, type of poultry, etc.
Lesions may include swelling of the face and area below the beak. Removing skin
from the carcass will show a clear straw-colored fluid in the subcutaneous
Blood vessels are usually
engorged. Hemorrhage may be seen in the trachea, proventriculus, beneath the
lining of the gizzard, and throughout the intestines. The lining of the gizzard
may be easily removed.
Other areas likely to show
swelling and hemorrhages include the muscle along the breast bone as well as in
the heart, gizzard fat, and abdominal fat.
Young broilers may show
signs of severe dehydration with other lesions less pronounced or absent
There are many different
strains (serotypes) of the avian influenza virus. Some of the highly virulent
strains evolved from milder strains following repeated chicken to chicken
passages. The avian influenza virus has been shown to mutate at an extremely
high rate as it serially infects poultry. Chickens are not the normal host for
avian influenza, so the virus they pick up from other birds has a tendency to
mutate and become pathogenic. In 1994, an avian influenza outbreak in Mexico
started out mildly, but mutated into a "killer" virus that decimated
many poultry flocks. This same scenario had occurred in the northeastern United
States in the mid-1980s. Today, extreme biosecurity precautions prevent spread
of the virus to the United States and neighboring countries in Central America.
Current research efforts on avian influenza are directed toward understanding
why and how mildly pathogenic viruses become highly pathogenic.
Avian influenza viruses are
subdivided into serotypes based on their hemagglutinin (H) and neuraminidase
(N) surface antigens. The highly pathogenic serotype of avian influenza
responsible for the 1983-84 outbreak in the United States and the 1994 outbreak
in Mexico was H5N2. Historically, serotypes including H5 and H7 are associated
with disease in poultry.
Infected birds shed the
virus in fecal and oculo-nasal discharges. Even though recovered flocks shed
less virus than clinically ill flocks, recovered flocks will intermittently
shed and should be considered infected for life.
Waterfowl (wild and
domesticated) are the primary natural reservoir of influenza viruses. Wild
waterfowl usually do not show clinical signs, but they can excrete the virus
for long periods of time. In addition, waterfowl can be infected with more than
one type of influenza virus. Detection is further complicated by the fact that
they often do not develop a detectable antibody response after exposure to the
Influenza virus has been
recovered from water and organic material from lakes and ponds utilized by
infected ducks. Co-mingling of these birds with range-reared flocks is a factor
in some outbreaks.
The avian influenza virus
can remain viable for long periods of time at moderate temperatures, and can
survive indefinitely in frozen material. As a result, the disease can be spread
through improper disposal of infected carcasses, manure, or poultry
The disease also can be
easily spread by people and equipment contaminated with avian influenza virus.
Avian influenza viruses can be transmitted on contaminated shoes, clothing,
crates, egg flats, egg cases, vehicles, and other equipment. Any object located
on an infected poultry farm must be considered contaminated and should be
completely cleaned and disinfected before it is moved from that premises.
Clothing worn on an infected farm should be laundered.
Insects and rodents may
mechanically carry the virus from infected to susceptible poultry.
Influenza virus has been
isolated from turkey eggs suggesting vertical transmission, although typically
the virus kills the embryo. There is little or no evidence of egg-borne infection
of poults. However, eggshell surfaces can be contaminated with the influenza
virus, and thus are a means of transmission.
Avian influenza viruses have
frequently been isolated from clinically normal, imported exotic birds. These
infected birds are a potential threat to cage birds, wild birds, and poultry.
Live-bird markets are a
reservoir of infection. Such markets serve as a focal point for gathering and
housing many species of bird. These facilities are rarely cleaned or
There is no effective
treatment for avian influenza. However, good husbandry, proper nutrition, and
broad spectrum antibiotics may reduce losses from secondary infections.
It must be remembered that
recovered flocks continue to intermittently shed the virus.
All buildings should be
cleaned and disinfected after an infected flock is removed. The poultry litter
or manure should be composted before application to cultivated lands.
A vaccination program, in
conjunction with strict quarantine, has been used to control mild forms of the
disease in commercial chicken and turkey flocks. With the more lethal forms of
the disease, however, strict quarantine and rapid depopulation of infected
flocks remains the only effective methods of stopping avian influenza. The
success of such a program depends, of course, on the full cooperation and
support of the poultry and allied industries.
With the realization that
there is a reservoir of influenza virus in wild waterfowl, every effort must be
made to prevent direct or indirect contact between domestic poultry and wild
waterfowl. Persons handling wild game (especially waterfowl) must change
clothes completely and bathe prior to entering poultry houses.
There is currently a serious
and ongoing outbreak of avian influenza in Mexico. According to Mexican
authorities, the highly pathogenic influenza virus has been eradicated.
However, as vaccination is widespread, it is difficult serologically to confirm
this report and precautions should be continued.
It is very important to
prevent the spread of this disease into the United States. It is very easy to
spread avian influenza on clothing and through human contact. Do not visit or
go near any poultry flocks in Mexico unless proper biosecurity actions are
Specialty or hobby-type
flocks have an increased risk for direct or indirect exposure to avian
influenza because of their contact with wild birds and other poultry. These
flocks are commonly mixed and marketed through a live auction market
distribution system where proper sanitation is not always practiced. This
system mixes various types of stressed poultry and has been a key link to avian
influenza outbreaks in commercial flocks.
The poultry owner is the
first line of defense in identifying outbreaks of avian influenza. If birds
develop signs of avian influenza, or if exposure is suspected, immediately
notify the state poultry officials.
By J. P. Jacob, G.D.
Butcher, F. B. Mather, and R.D. Miles, University of Florida IFAS Extension