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Definition
Avian influenza is flu infection in birds. The disease is of
concern to humans, who have no immunity against it. The virus
that causes this infection in birds can mutate (change) to
easily infect humans. Such mutation can start a deadly worldwide
epidemic.
Causes, incidence, and risk
factors
Historically, avian influenza
viruses infected pigs and mixed with pig influenza viruses. The
viruses exchanged genetic information, which led to the
formation of a new virus. This new virus could then infect
humans and easily spread from person to person. Previous flu
pandemics (worldwide epidemics) have started this way.
The first avian influenza
virus to infect humans directly occurred in Hong Kong in 1997,
during an avian flu epidemic on the island. This outbreak was
linked to chickens and classified as avian influenza A (H5N1).
Since the Hong Kong outbreak,
the bird flu virus has spread across Asia, and in October 2005
was discovered in poultry in Turkey and Romania. So far, over
161 people have been infected by H5N1. More than half of the
confirmed cases have died.
The wider the area over which
the avian flu virus spreads, the greater the chances of a
worldwide outbreak. There is tremendous concern that H5N1 poses
an enormous pandemic threat.
Farmers and other people
working with poultry, as well as travelers visiting affected
countries, have a higher risk for getting the bird flu. Handling
an infected bird can cause infection. People who eat raw or
undercooked poultry meat are also at an increased risk for avian
influenza. Highly infective avian flu viruses, such as H5N1,
have been shown to survive in the environment for long periods
of time, and infection may be spread simply by touching
contaminated surfaces. Birds who recover from the flu can
continue to shed the virus in their feces and saliva for as long
as 10 days.
Healthcare workers and
household contacts of patients with avian influenza may also be
at an increased risk of the bird flu.
Symptoms
Symptoms of avian flu
infection in humans depend on the particular strain of virus. In
case of the H5N1 virus, infection in humans causes more classic
flu-like symptoms, which might include:
- Cough (dry or productive)
- Sore throat
- Fever > 100.4°F (38°C)
- Difficulty breathing
- Diarrhea
- Runny nose
- Headache
- Malaise
- Muscle aches
Signs and tests
If you think you have been
exposed to avian influenza, call your health care provider
before your visit. This will give the staff a chance to take
proper precautions that will protect them and other patients.
In February 2006, the U.S.
Food & Drug Administration approved a new, faster test for
diagnosing strains of bird flu in people suspected of having the
virus. The test is called the Influenza A/H5 (Asian lineage)
Virus Real-time RT-PCR Primer and Probe Set. The test gives
preliminary results within 4 hours. Older tests required 2 to 3
days.
Your doctor might also perform
the following tests:
- Chest x-ray
- Nasopharyngeal culture
- Blood differential
- Auscultation (to detect abnormal breath sounds)
Other tests may be done to
look at the functions of your heart, kidneys, and liver.
Treatment
Treatment with the antiviral
medication oseltamivir (Tamiflu), and perhaps zanamivir (Relenza),
may decrease the severity of the disease, if started within 48
hours after symptoms begin. Oseltamivir may also be prescribed
for household contacts of people diagnosed with avian flu.
Samples of H5N1 from human infections proved resistant to
the antiviral medications amantadine and rimantadine. Therefore
these medications cannot be used if an H5N1 outbreak occurs.
People with severe infection
may need breathing assistance with mechanical ventilation. It is
currently recommended that people diagnosed with H5N1 infection
be put in isolation.
Because different types of
avian flu virus may cause different symptoms, treatment may
vary.
Currently, there is no
available vaccine against avian influenza. However, a vaccine
against H5N1 is being tested in clinical trials.
Doctors recommend that people
get an influenza (flu) shot to reduce the chance of an avian flu
virus mixing with a human flu virus, which would create a new
virus that may easily spread.
Expectations (prognosis)
Prognosis depends on the severity of
infection and the type of avian influenza virus that caused it.
The current death rate for patients with confirmed H5N1
infection is more than 50%. The H7N7 avian flu outbreak in the
Netherlands resulted in 89 confirmed human cases but only one
death. An avian flu virus designates H9N2 infected 3 children in
Asia; all three recovered.
Complications
Pneumonia and acute
respiratory distress are seen with H5N1 infections. Infection
with this virus may also lead to sepsis and organ failure.
Calling your health care
provider
Call your health care provider
if you develop flu-like symptoms within 10 days of handling
infected birds or traveling to an area with a known avian flu
outbreak
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Avian flu is becoming more dangerous
Influenza A viruses naturally occur in wild birds. Although these
birds aren't affected by the virus, domestic poultry such as chickens
and turkeys are - and so are people.
However, the strain found in the Vietnamese boys seems to be more
dangerous. More than 90 per cent of birds who get H5N1 die, and
mortality among humans is also high. The H5N1 virus was first shown to
have passed from birds to humans in 1997, during an outbreak of avian
influenza among poultry in Hong Kong. The virus caused severe
respiratory illness in 18 people, of whom six died. In the past year or
two there have been a stream of cases reported by the World Health
Organization (WHO) in Asia. Up to November 2005, worldwide130 people had
caught the infection, as a result of close and direct contact with
infected birds. Sixty-seven of these have subsequently died. More
worryingly, recent research has shown that H5N1 has changed so that it's
even more deadly in chickens and mice, and can now infect cats too. H5N1
is also resistant to some of the drugs used to treat flu (such as
amantadine).
H5N1 has become common among birds in Asia, who shed the virus in
their saliva, nasal secretions and faeces. Millions of chickens and
ducks have been slaughtered across South East Asia in recent months in
an effort to prevent spread of the virus from birds to humans. Work is
also underway to make a vaccine against H5N1. But it will take time and
that is in short supply. If H5N1 becomes able to pass from human to
human then the situation will be even more serious as most people have
little immunity to the strain and there will be rapid spread. In
September 2004, the first possible case of human-to-human transmission
was reported in Thailand.
Flu viruses continually change
One of the most worrying features of the viruses which cause flu is
the way that they are continually changing over time, through small
changes in their make-up called antigenic drift and occasional abrupt
major changes called antigenic shift.
This can mean that, although you might have fought and won a
miserable battle against flu a year or two ago, the next time the virus
appears your body won't recognize it. The antibodies your body made
against flu last time won't work. So the immune system must learn to
fight it all over again. This is why people need to be immunized against
flu each year, using the most up-to-date strains of the virus.
There are three types of influenza viruses: A, B, and C. Influenza C
only causes mild problems in humans. Influenza B can cause more serious
illness and seasonal epidemics, but because it only changes through the
slower process of antigenic drift there is little risk of a pandemic,
where millions of people are suddenly exposed to a new, dramatically
different virus. That threat is more likely to come from Influenza type
A which does undergo shift.
New strains of flu tend to emerge in Asia and the East, where people
live in closer quarters with their animals and different flu viruses may
mix to cause new strains, or pass between species. For example, domestic
ducks may carry H5N1 without symptoms and release the virus for long
periods, acting as what is known as a silent reservoir. In rural areas
free-ranging ducks and chickens often mingle and share the same water
supplies. The virus may then be passed from one animal to another and
possibly even to humans.
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