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Avian Influenza
Type A influenza viruses can infect several animal species,
including birds, pigs, horses, seals and whales. Influenza
viruses that infect birds are called “avian influenza
viruses.” Birds are an especially important species
because all known subtypes of influenza A viruses circulate
among wild birds, which are considered the natural hosts
for influenza A viruses. Avian influenza viruses do not
usually directly infect humans or circulate among humans.
Influenza A viruses can be divided into subtypes on the
basis of their surface proteins — hemagglutinin (HA)
and neuraminidase (NA). There are 15 known H subtypes.
While all subtypes can be found in birds, only 3 subtypes
of HA (H1, H2 and H3) and two subtypes of NA (N1 and N2)
are known to have circulated widely in humans.
Avian influenza usually does not make wild birds sick,
but can make domesticated birds very sick and kill them.
Avian influenza A viruses do not usually infect humans;
however, several instances of human infections and outbreaks
have been reported since 1997. When such infections occur,
public health authorities monitor the situation closely
because of concerns about the potential for more widespread
infection in the human population.
Avian Influenza Infections in Humans
Confirmed instances of avian influenza viruses infecting
humans since 1997 include:
- 1997: In Hong Kong, avian influenza
A (H5N1) infected both chickens and humans. This was
the first time an avian influenza virus had ever been
found to transmit directly from birds to humans. During
this outbreak, 18 people were hospitalized and 6 of them
died. To control the outbreak, authorities killed about
1.5 million chickens to remove the source of the virus.
Scientists determined that the virus spread primarily
from birds to humans, though rare person-to-person infection
was noted.
- 1999: In Hong Kong, cases of avian
influenza A H9N2 were confirmed in 2 children. Both patients
recovered, and no additional cases were confirmed. The
evidence suggested that poultry was the source of infection
and the main mode of transmission was from bird to human.
However, the possibility of person-to-person transmission
remained open. Several additional human H9N2 infections
were reported from mainland China in 1998-99.
- 2003: Two cases of avian influenza
A (H5N1) infection occurred among members of a Hong Kong
family that had traveled to China. One person recovered,
the other died. How or where these 2 family members were
infected was not determined. Another family member died
of a respiratory illness in China, but no testing was
done. No additional cases were reported.
- 2003: Avian influenza A (H7N7) infections
among poultry workers and their families were confirmed
in the Netherlands during an outbreak of avian flu among
poultry. More than 80 cases of H7N7 illness were reported
(the symptoms were mostly confined to eye infections,
with some respiratory symptoms), and 1 patient died (in
a veterinarian who had visited an affected farm). There
was evidence of some human-to-human transmission.
- 2003: H9N2 infection was confirmed
in a child in Hong Kong. The child was hospitalized but
recovered.
Characteristics of Avian Influenza in Birds
Certain water birds act as hosts of influenza viruses
by carrying the virus in their intestines and shedding
it. Infected birds shed virus in saliva, nasal secretions
and feces. Avian influenza viruses spread among susceptible
birds when they have contact with contaminated nasal, respiratory
and fecal material from infected birds; however, fecal-to-oral
transmission is the most common mode of spread.
Most influenza viruses cause no symptoms, or only mild
ones in wild birds; however, the range of symptoms in birds
varies greatly depending on the strain of virus and the
type of bird. Infection with certain avian influenza A
viruses (for example, some H5 and H7 strains) can cause
widespread disease and death among some species of wild
and especially domesticated birds such as chickens and
turkeys.
Symptoms of Avian Influenza in Humans
The reported symptoms of avian influenza in humans have
ranged from typical influenza-like symptoms (e.g., fever,
cough, sore throat and muscle aches) to eye infections,
pneumonia, acute respiratory distress, viral pneumonia,
and other severe and life-threatening complications.
Antiviral Agents for Influenza
Studies to date suggest that the prescription medications
approved for human influenza strains would be effective
in preventing avian influenza infection in humans, however,
sometimes flu strains can become resistant to these drugs
and so they may not always be effective.
Potential for an Influenza Pandemic
All influenza viruses can change.
It is possible that an avian influenza virus could change
so that it could infect humans and could spread easily
from person to person. Because these viruses do not commonly
infect humans, there is little or no immune protection
against them in the human population. If an avian virus
were able to infect people and gain the ability to spread
easily from person to person, an “influenza pandemic” could
begin.
Background on Pandemics
An influenza pandemic is a global outbreak of influenza
and occurs when a new influenza virus emerges, spreads,
and causes disease worldwide. Past influenza pandemics
have led to high levels of illness, death, social disruption
and economic loss.
There were 3 pandemics in the 20th century. All of them
spread worldwide within 1 year of being detected. They
are:
- 1918-19, "Spanish flu," [A
(H1N1)], caused the highest number of known flu deaths:
more than 500,000 people died in the United States, and
20 million to 50 million people may have died worldwide.
Many people died within the first few days after infection
and others died of complications soon after. Nearly half
of those who died were young, healthy adults.
- 1957-58, "Asian flu," [A
(H2N2)], caused about 70,000 deaths in the United States.
First identified in China in late February 1957, the
Asian flu spread to the United States by June 1957.
- 1968-69, "Hong Kong flu," [A
(H3N2)], caused approximately 34,000 deaths in the United
States. This virus was first detected in Hong Kong in
early 1968 and spread to the United States later that
year. Type A (H3N2) viruses still circulate today.
Once a new pandemic influenza virus emerges and spreads,
it typically becomes established among people and circulates
for many years. The U.S. Centers for Disease Control and
Prevention and the World Health Organization conduct extensive
surveillance programs to monitor the occurrence of influenza
activity worldwide, including the emergence of potential
pandemic strains of influenza virus. |
