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Influenza
Overview
Every
year in the United States, millions of people get sick with influenza (the
flu). Influenza epidemics in the U.S. usually occur during the winter months.
According to the Centers for Disease Control and Prevention (CDC), an estimated
23,607 (range 3,349-48,614) influenza-associated deaths and over 200,000
influenza-associated hospitalizations occur every year in the United States.
The highest rates of influenza infection occur among children, but the risks
for serious health problems, hospitalizations, and deaths from influenza are
higher among people 65 years of age or older, young children, pregnant women,
and people of any age who have medical conditions that place them at increased
risk for complications from influenza. Anyone though, including healthy people,
can get influenza, and serious health problems from influenza can occur at any
age. The severity of an influenza season varies from year to year and depends
on many things, including the strains of circulating influenza viruses,
how much flu vaccine is available, when the vaccine is available, how well the
flu vaccine is matched to flu viruses that are causing illness, and the levels
of protective antibody in the population.
A
primary feature of the influenza virus is that it regularly undergoes genetic
and/or recombination changes, which if dramatic enough, can result in the
creation of an influenza virus never seen before in humans. Since the
population would not have antibody protection against this new form of influenza
virus, and if it were highly contagious and infectious, the potential for a
worldwide epidemic (pandemic) would be increased. During most pandemics in the
past, the rates of illnesses and deaths from influenza-related health problems
have increased dramatically worldwide. During the 1918-19 "Spanish
Flu" pandemic, it is estimated that ≈50 million deaths occurred worldwide,
including over a half-million Americans. Influenza can have a very serious and
severe impact on public health.
About
Influenza
Influenza
is a contagious respiratory illness caused by the influenza virus. There are
three types of influenza viruses: A, B, and C. Influenza type A viruses can
infect people, birds, pigs, horses, seals, cats, whales, and other animals, but
wild birds are the natural hosts for these viruses. Influenza A viruses are
divided into subtypes based on two proteins on the surface of the virus. Only
some influenza A subtypes (i.e., H1N1 and H3N2) are currently in general
circulation among people. Other subtypes are found most commonly in other
animal species. Influenza B viruses are normally found only in humans. Unlike
influenza A viruses, these viruses are not classified according to subtype.
Although influenza B viruses can cause human epidemics, they have not caused
pandemics. Influenza type C viruses cause mild illness in humans and are not
thought to cause epidemics.
Influenza
is not the same illness as a cold. Different viruses cause colds.
Influenza tends to be worse than the common cold, and symptoms such as fever
and body aches are more common and intense. Colds are usually milder than the
flu. People with colds are more likely to have a runny or stuffy nose. Colds
generally do not result in serious health problems, such as pneumonia.
Influenza attacks the respiratory tract of the nose, throat, and
lungs. Cold viruses attack the mucous linings of the nose and
throat. Sometimes, cold viruses attack the eye.
Organism,
Causative Agent, or Etiologic Agent
Influenza
virus
Symptoms
and ILI Definition
Influenza
usually comes on suddenly, one to four days after the virus enters the body,
and may include these symptoms:
- Fever
or feeling feverish/chills
- Cough
- Sore
throat
- Runny
or stuffy nose
- Headache
- Muscle
or body aches
- Tiredness
(can be extreme)
Among
children, otitis media (ear infection), nausea, vomiting, and diarrhea are
common. Some infected persons are asymptomatic.
Influenza-like
illness, or ILI, is defined as fever ≥100°F AND cough and/or sore throat (in
the absence of a known cause other than influenza).
Transmission
Human
to Human
Influenza
viruses are spread from person to person by respiratory droplets generated when
an infected person coughs, sneezes, or talks in close proximity to an
uninfected person. Sometimes, influenza viruses are spread when a person
touches a surface with influenza viruses on it (e.g., a doorknob), and then
touches his or her own nose or mouth.
Most
healthy adults who are ill with influenza may be able to infect other people
beginning 1 day before symptoms develop and up to 5 to 7
days after becoming sick. Children and persons with weakened
immune systems might be able to infect other people for even a longer period of
time. The virus can also be spread by people who are infected but have no
symptoms.
Influenza
A Viruses Between Animals and Humans
Influenza
A viruses normally seen in one species sometimes can cross over and cause
illness in another species. Influenza viruses from different species can mix
and create a new influenza A virus if viruses from two different species infect
the same person or animal. For example, if a pig were infected with a human
influenza virus and an avian influenza virus at the same time, the viruses
could reassort (exchange genetic material) and produce a new virus. The
resulting new virus might then be able to infect humans and spread from person
to person, but it would have surface proteins not previously seen in influenza
viruses that infect humans. Most people would have little or no immunity
against this type of major change in the influenza A virus. If this new virus
caused illness in people and was transmitted easily from person to person, an
influenza pandemic could occur. It also is possible that the process of
reassortment could occur in a human. For example, a person could be infected
with an avian influenza strain and a human strain of influenza at the same
time. These viruses could reassort to create a new virus that had a protein
from the avian virus and other genes from the human virus. While it is unusual
for people to get influenza infections directly from animals, sporadic human
infections and outbreaks caused by certain avian influenza A viruses and pig
influenza A viruses have been reported.
Severity
of Illness
Most
people generally recover from illness in a few days to less than two weeks, but
some people develop complications (such as pneumonia) and may die from
influenza. The highest rates of influenza infection occur among children;
however, the risks for serious health problems, hospitalizations, and deaths
from influenza are typically greatest among people 65 years of age or
older, children aged <5 years especially those aged <2 years,
pregnant women, and people of any age who have medical conditions that place
them at increased risk for complications from influenza.
In
people with chronic medical conditions such as heart or lung disease, influenza
can lead to pneumonia and other life-threatening illnesses. Persons 65 years of
age and older account for approximately 90% of deaths attributed to pneumonia
and influenza. Young children with influenza can develop high fevers, and a
small percentage of children hospitalized with influenza can have febrile
seizures. Deaths from influenza are uncommon among children, but do occur.
Influenza has also been associated with neurological problems, Reye’s syndrome,
muscle inflammation, and heart inflammation.
Treatment
& Prevention
Most
people who develop influenza illness will recover on their own by
getting rest and will not need medication. Antiviral medications can
shorten the duration and severity of illness if given within the first 48 hours
of the illness. These medications are usually prescribed to persons who have a
severe illness or to those who are at higher risk for developing serious
illness or complications due to influenza.
The
best way to prevent influenza is to get an influenza vaccine each year as soon
as the vaccine is available to the public. Vaccination is associated with
reductions in influenza-related respiratory illness and physician visits among
all age groups, hospitalizations, and deaths among persons at high risk, otitis
media (ear infection) among children, and work absenteeism among adults.
Other
forms of prevention include:
- Hand
washing and using alcohol-based hand sanitizers,
- Covering
your coughs and sneezes with a disposable tissue or your arm or sleeve,
- Avoiding
touching your eyes, nose, or mouth,
- Avoiding
close contact with persons who are ill,
- Staying
home when you are ill, and
- Taking
antiviral medications if prescribed by your doctor.
- In certain situations (e.g., influenza outbreaks in settings like nursing
homes), antiviral medications may be prescribed to high-risk individuals to
prevent them from developing influenza illness after exposure to infected
individuals.
Avian
Influenza
Influenza
viruses that infect birds are called “avian influenza viruses”. Only influenza
A viruses infect birds. All known subtypes of influenza A viruses can infect
birds, except subtypes H17N10 and H18N11, which have only been found in bats;
however, there are substantial genetic differences between the subtypes that
typically infect both people and birds. Although avian influenza A viruses do
not usually infect humans, several instances of human infections and outbreaks
of avian influenza have been reported since 1997. Most cases of avian influenza
infection in humans are thought to have resulted from contact with infected
poultry or contaminated surfaces. There is still a lot to learn about how
different subtypes and strains of avian influenza viruses might affect humans.
Because of concerns about the potential for more widespread infection in the
human population, public health authorities closely monitor outbreaks of human
illness associated with avian influenza. To date, human infections with avian
influenza viruses detected since 1997 have not resulted in sustained
human-to-human transmission. However, because influenza viruses have the
potential to change and gain the ability to spread easily between people,
monitoring for human infection and person-to-person transmission is important.
From
December 2014 to June 2015, the United States Department of Agriculture’s
(USDA) Animal and Plant Health Inspection Service (APHIS) and the U.S.
Department of the Interior (DOI), National Wildlife Health Center detected and
reported highly pathogenic avian influenza (HPAI) H5 viruses in U.S.
domestic poultry (backyard and commercial flocks), captive wild birds, and wild
birds. The HPAI H5 influenza A viruses detected in US domestic poultry,
captive wild birds and wild birds were H5N8, H5N2, and a new H5N1 virus that
had a combination of genes from HPAI H5N1 viruses that spread in Asia and
low-pathogenic avian influenza viruses that circulate in wild birds in North
America. No human infections with these viruses were detected; however, similar
viruses have infected people.
As of June 15, 2017, 859 laboratory–confirmed avian influenza A
(H5N1), not the newly detected H5N1, infections in humans resulting in 453 deaths
have been reported to the World Health Organization (WHO) from 16 countries.
No human infections with avian influenza A (H5N1) have been identified in
the United States. For updates on avian influenza, please see the CDC web site at http://www.cdc.gov/flu/avianflu/ and the WHO website at http://www.who.int/csr/disease/avian_influenza/en/.
There has also been avian influenza (not H5N1) outbreaks reported in the
US. In January 2016, an outbreak of HPAI (H7N8) virus was reported in a
commercial turkey flock in Dubois County, Indiana.
Low pathogenic avian influenza (LPAI) (H7N8) was subsequently
detected in eight nearby turkey flocks. No transmission of HPAI (H7N8) or LPAI
(H7N8) virus to humans was reported. More information is available at Avian Influenza H7N8 Update.
Two outbreaks of high pathogenic avian influenza (HPAI) (H7N9)
were reported in Lincoln County, Tennessee in March 2017. In addition, five
states, Georgia, Wisconsin, Tennessee, Alabama and Kentucky, reported cases of
low pathogenic avian influenza (LPAI) (H7N9) during 2017. One outbreak of LPAI
(H5N2) was reported in Wisconsin in 2017.
In addition, two avian influenza (not H5N1) outbreaks occurred
among poultry populations in southeast and northeast Texas in February and May
2004, respectively. No human cases of influenza occurred from these poultry
outbreaks. For additional information regarding avian influenza, please visit
the Texas Animal Health Commission web site at http://www.tahc.texas.gov/animal_health/poultry/#AI.
In April 2013, China began reporting human cases of avian
influenza A (H7N9). As of June 15, 2017, there have been 1533 laboratory-confirmed
cases of avian influenza A (H7N9), including at least 592 deaths, reported to
the World Health Organization. Most human cases are associated with exposure to
infected live poultry or contaminated environments, including markets where
live poultry are sold. Information to date suggests that these viruses do not
transmit easily from human to human. For more information, please visit the CDC H7N9 website at http://www.cdc.gov/flu/avianflu/h7n9-virus.htm or the WHO H7N9 website at http://www.who.int/influenza/human_animal_interface/influenza_h7n9/en/index.html.
In December 2016, New York City detected cases of
avian influenza A (H7N2) among cats in an animal shelter. There was one
confirmed human case associated with the outbreak. The person had mild symptoms
and recovered. There were no cases of person-to-person transmission reported.
For more information, please visit the CDC website at https://www.cdc.gov/flu/spotlights/avian-influenza-cats.htm.
Influenza
Surveillance
The latest information on influenza surveillance for the current
influenza season and past influenza seasons in Texas may be found at http://dshs.texas.gov/idcu/disease/influenza/surveillance/
School
Exclusion Criteria
Children
with influenza are required to be excluded from school and daycare for at least
24 hours after fever has subsided without the use of fever suppressing
medications. It is recommended that adults with influenza not return to
work for at least 24 hours after fever has subsided without the use of fever
suppressing medications.
Recent Texas Trends
The official influenza reporting season for the United States begins in October and continues through May. In Texas, influenza activity usually peaks in January or February, although the peak of influenza has happened as early as October and as late as March. Individual cases of influenza are not tracked; however, sentinel surveillance partners in the state provide information on when and where influenza viruses are circulating, if circulating influenza viruses match the vaccine strains, if the circulating influenza viruses are changing, where and when influenza-like illnesses are occurring, and the severity of influenza activity. For surveillance reports see the Data link above.
