Avian
Influenza Update
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.
Article courtesy
of Center for Disease Control
