Influenza – Cause, Symptoms, Diagnosis, Treatment

Influenza – Cause, Symptoms, Diagnosis, Treatment

Influenza also called flu or grippe, an acute viral infection of the upper or lower respiratory tract that is marked by fever, chills, and a generalized feeling of weakness and pain in the muscles, together with varying degrees of soreness in the head and abdomen.

Influenza, commonly known as “the flu”, is an infectious disease caused by an influenza virus.[rx] Symptoms can be mild to severe.[rx] The most common symptoms include high fever, runny nose, sore throat, muscle and joint pain, headache, coughing, and feeling tired.[rx] These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks. In children, there may be diarrhea and vomiting, but these are not common in adults.[rx]

Influenza, or flu, is a contagious respiratory infection caused by several flu viruses that infect the nose, throat and lungs. People infected with the seasonal flu virus feel miserable with fever, chills, muscle aches, coughing, congestion, headache and fatigue for a week or so. Most people who get the flu get better within two weeks, but some people may develop serious complications, such as pneumonia. Pandemic influenza is when a new flu virus strain occurs that can spread easily from person-to-person and the virus is one for which most people have no immunity.

Types of Influenza

There are four types of influenza viruses: types A, B, C and D:

  • Influenza viruses – infect humans and many different animals. The emergence of a new and very different influenza A virus with the ability to infect people and have sustained human to human transmission can cause an influenza pandemic.
  • Influenza B viruses – circulate among humans and cause seasonal epidemics. Recent data showed seals also can be infected.
  • Influenza C viruses – can infect both humans and pigs but infections are generally mild and are rarely reported.
  • Influenza D viruses –  primarily affect cattle and are not known to infect or cause illness in people.

There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known as the flu season) almost every winter in the United States. Influenza viruses are the only influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease. A pandemic can occur when a new and very different influenza A virus emerges that both infect people and has the ability to spread efficiently between people. Influenza type C infections generally cause mild illness and are not thought to cause human flu epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

Influenzavirus A

This genus has one species, influenza A virus. Wild aquatic birds are the natural hosts for a large variety of influenza A.[53] Occasionally, viruses are transmitted to other species and may then cause devastating outbreaks in domestic poultry or give rise to human influenza pandemics.[53] The influenza A virus can be subdivided into different serotypes based on the antibody response to these viruses.[54] The serotypes that have been confirmed in humans are:

  • H1N1, which caused Spanish flu in 1918, and Swine Flu in 2009
  • H2N2, which caused Asian Flu in 1957
  • H3N2, which caused Hong Kong Flu in 1968
  • H5N1, which caused Bird Flu in 2004[55][56]
  • H7N7, which has the unusual zoonotic potential[57]
  • H1N2, endemic in humans, pigs, and birds
  • H9N2
  • H7N2
  • H7N3
  • H10N7
  • H7N9, rated in 2018 as having the greatest pandemic potential among the Type A subtypes[58]
  • H6N1, which only infected one person, who recovered[rx]

Influenzavirus B

This genus has one species, influenza B virus. Influenza B almost exclusively infects humans[rx] and is less common than influenza A. The only other animals known to be susceptible to influenza B infection are seals[rx] and ferrets.[rx] This type of influenza mutates at a rate 2–3 times slower than type A[rx] and consequently is less genetically diverse, with only one influenza B serotype.[rx] As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible.[rx] This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.[rx]

Influenzavirus C

This genus has one species, influenza C virus, which infects humans, dogs, and pigs, sometimes causing both severe illness and local epidemics.[rx][rx] However, influenza C is less common than the other types and usually only causes mild disease in children.

Influenzavirus D

This genus has only one species, influenza D virus, which infects pigs and cattle. The virus has the potential to infect humans, although no such cases have been observed.[8]

Structure of the influenza virion. The hemagglutinin (HA) and neuraminidase (NA) proteins are shown on the surface of the particle. The viral RNAs that make up the genome are shown as red coils inside the particle and bound to ribonucleoproteins (RNP).

Naming Influenza Viruses

CDC follows an internationally accepted naming convention for influenza viruses. This convention was accepted by WHO in 1979 and published in February 1980 in the Bulletin of the World Health Organization, 58(4):585-591 (1980) (see A revision of the system of nomenclature for influenza viruses: a WHO Memorandum pdf icon[854 KB, 7 pages]external icon). The approach uses the following components:

  • The antigenic type (e.g., A, B, C, D)
  • The host of origin (e.g., swine, equine, chicken, etc.). For human-origin viruses, no-host of origin designation is given. Note the following examples:
    • (Duck example): avian influenza A(H1N1), A/duck/Alberta/35/76
    • (Human example): seasonal influenza A(H3N2), A/Perth/16/2019
  • Geographical origin (e.g., Denver, Taiwan, etc.)
  • Strain number (e.g., 7, 15, etc.)
  • Year of the collection (e.g., 57, 2009, etc.)
  • For influenza, A viruses, the hemagglutinin, and neuraminidase antigen description are provided in parentheses (e.g., influenza A(H1N1) virus, influenza A(H5N1) virus)
  • The 2009 pandemic virus was assigned a distinct name: A(H1N1)pdm09 to distinguish it from the seasonal influenza A(H1N1) viruses that circulated prior to the pandemic.
  • When humans are infected with influenza viruses that normally circulate in swine (pigs), these viruses are call variant viruses and are designated with a letter ‘v’ (e.g., an A(H3N2)v virus).
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Causes of Influenza

Some people who get the flu will develop complications. Some of these complications can be serious or even life-threatening. They include

  • Bronchitis
  • Ear infection
  • Sinus infection
  • Pneumonia
  • Inflammation of the heart (myocarditis), brain (encephalitis), or muscle tissues (myositis, rhabdomyolysis)

The flu also can make chronic health problems worse. For example, people with asthma may have asthma attacks while they have flu.

Certain people are more likely to have complications from the flu, including

  • Adults 65 and older
  • Pregnant women
  • Children younger than 5
  • People with certain chronic health conditions, such as asthma, diabetes, and heart disease

Symptoms of Influenza

Seasonal influenza is characterized by a sudden onset of fever, cough (usually dry), headache, muscle and joint pain, severe malaise (feeling unwell), sore throat, and a runny nose. The cough can be severe and can last 2 or more weeks. Most people recover from fever and other symptoms within a week without requiring medical attention. But influenza can cause severe illness or death especially in people at high risk (see below).

Symptoms of the flu come on suddenly and may include

  • A fever between 38°C and 40°C (about 100-104°F) or higher
  • Muscle and joint pain throughout the entire body (myalgia and arthralgia)
  • Headache
  • Severe fatigue and a general feeling of being very ill
  • Dry cough
  • Stuffy and/or runny nose
  • Loss of appetite
  • Extreme tiredness
  • Fever or feeling feverish/chills
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Fatigue (tiredness)

Common signs and symptoms of the flu include

  • Fever over 100.4 F (38 C)
  • Aching muscles
  • Chills and sweats
  • Headache
  • Dry, persistent cough
  • Fatigue and weakness
  • Nasal congestion
  • Sore throat

Emergency warning signs

  • Shortness of breath
  • Chest pain
  • Dizziness
  • Confusion
  • Extreme vomiting
  • Flu symptoms that improve but then relapse with a high fever and severe cough (can be bacterial pneumonia)
  • Cyanosis
  • High fever and a rash.
  • Inability to drink fluids

Not everyone with flu will have all of these symptoms. For instance, it is possible to have flu without a fever.

The symptoms of influenza typically come on suddenly. Initially, a person with flu may experience:

  • a high temperature
  • a stuffy or runny nose
  • a dry cough
  • cold sweats and shivers
  • aches that may be severe
  • a headache
  • fatigue, and a feeling of being unwell
  • a low appetite

Flu symptoms in adults

Adults with the following symptoms should seek medical help urgently:

  • breathing difficulties
  • pain or pressure in the chest or abdomen
  • dizziness, confusion, or loss of alertness
  • seizures
  • not urinating, which may indicate dehydration
  • severe pain, weakness, and unsteadiness
  • a fever or cough that goes away and then comes back
  • a worsening of other existing health conditions

Flu symptoms in children

Children often have similar symptoms to adults but can also have gastrointestinal symptoms, such as nausea, vomiting, and diarrhea.

If a child has the following symptoms, they need emergency medical care:

  • breathing difficulties
  • rapid breathing
  • bluish face or lips
  • chest pain or ribs pulling inward as they breathe
  • severe aches
  • dehydration, for example, not urinating for 8 hours and crying dry tears
  • lack of alertness or interaction with others
  • a fever above 104°F or any fever in a child under 12 weeks of age
  • a fever or cough that goes away but then comes back
  • a worsening of any other medical conditions

Should children have flu medication? Find out more here about Tamiflu and its effects on children.

Flu symptoms in babies

Flu can be dangerous for babies. If symptoms appear, a parent or caregiver should seek medical help.

A baby with flu may:

  • be very tired
  • have a cough and sore throat
  • have a stuffy or runny nose
  • have a fever of 100°F or more
  • have vomiting or diarrhea

The baby needs emergency medical attention if they:

  • do not want anyone to hold them
  • have a blue or gray skin color
  • are breathing fast or have difficulty breathing
  • have a fever with a rash
  • have symptoms that go away but come back again
  • show signs of dehydration, for example, not urinating
  • do not wake up or interact
  • have severe and persistent vomiting

Some people may also have vomiting and diarrhea. This is more common in children.

Sometimes people have trouble figuring out whether they have a cold or the flu. There are differences between them. The symptoms of a cold usually come on more slowly and are less severe than symptoms of the flu. Colds rarely cause a fever or headaches.

Sometimes people say that they have a “flu” when they really have something else. For example, “stomach flu” isn’t the flu; it’s gastroenteritis.

Diagnosing of Influenza

  • Before treating your condition, your doctor will need to check for the influenza virus. The preferred test is the rapid molecular assay. In this procedure, your doctor swabs your nose or throat. The test will detect influenza viral RNA within 30 minutes or less.
  • Results aren’t always accurate, and your doctor may have to make a diagnosis based on your symptoms or other flu tests.
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Treatment of Influenza

Patients with uncomplicated seasonal influenza

Patients that are not from a high-risk group should be managed with symptomatic treatment and are advised, if symptomatic, to stay home in order to minimize the risk of infecting others in the community. Treatment focuses on relieving symptoms of influenza such as fever. Patients should monitor themselves to detect if their condition deteriorates and seek medical attention Patients that are known to be in a group at high risk for developing severe or complicated illness, (see above) should be treated with antivirals in addition to symptomatic treatment as soon as possible.

Evidence suggests that some antiviral drugs, notably neuraminidase inhibitor (oseltamivir, zanamivir), can reduce the duration of viral replication and improve prospects of survival, however ongoing clinical studies are needed. The emergence of oseltamivir resistance has been reported.

  • In suspected and confirmed cases, neuraminidase inhibitors should be prescribed as soon as possible (ideally, within 48 hours following symptom onset) to maximize therapeutic benefits. However, given the significant mortality currently associated with A(H5) and A(H7N9) subtype virus infections and evidence of prolonged viral replication in these diseases, administration of the drug should also be considered in patients presenting later in the course of illness.
  • Treatment is recommended for a minimum of 5 days but can be extended until there is satisfactory clinical improvement.
  • Corticosteroids should not be used routinely unless indicated for other reasons (eg: asthma and other specific conditions); as it has been associated with prolonged viral clearance, immunosuppression leading to bacterial or fungal superinfection.
  • Most recent A(H5) and A(H7N9) viruses are resistant to adamantane antiviral drugs (e.g. amantadine and rimantadine) and are therefore not recommended for monotherapy.
  • The presence of co-infection with bacterial pathogens can be encountered in critically ill patients.

Patients with severe or progressive clinical illness associated with suspected or confirmed influenza virus infection

(i.e. clinical syndromes of pneumonia, sepsis, or exacerbation of chronic underling diseases) should be treated with antiviral drugs as soon as possible.

  • Neuraminidase inhibitors (i.e. oseltamivir) should be prescribed as soon as possible (ideally, within 48 hours following symptom onset) to maximize therapeutic benefits. The administration of the drug should also be considered in patients presenting later in the course of illness.
  • Treatment is recommended for a minimum of 5 days but can be extended until there is satisfactory clinical improvement.
  • Corticosteroids should not be used routinely unless indicated for other reasons (eg: asthma and other specific conditions); as it has been associated with prolonged viral clearance, immunosuppression leading to bacterial or fungal superinfection.
  • All currently circulating influenza viruses are resistant to adamantane antiviral drugs (such as amantadine and rimantadine), and these are therefore not recommended for monotherapy.

WHO GISRS monitors resistance to antivirals among circulating influenza viruses to provide timely guidance for antiviral use in clinical management and potential chemoprophylaxis.

  • Neuraminidase inhibitors – interfere with release of influenza virus from infected cells and thus halt spread of infection.
  • The endonuclease inhibitor baloxavir – interferes with viral replication by blocking viral RNA transcription. It is active against influenza A and B and may be an important new treatment option should resistance to neuraminidase inhibitors develop.
  • Zanamivir – is given by an inhaler, 2 puffs (10 mg) 2 times a day; it can be used in adults and children  7 years. Zanamivir sometimes causes bronchospasm and should not be given to patients with reactive airway disease; some people cannot use the inhalation device.
  • Oseltamivir 75 mg – orally 2 times a day is given to patients > 12 years; lower doses may be used in children as young as 1 year. Oseltamivir may cause occasional nausea and vomiting. In children, oseltamivir may decrease the incidence of otitis media; however, no other data clearly show that treatment of influenza prevents complications.
  • Peramivir – is given IV as a single dose and can be used in patients > 2 years who cannot tolerate oral or inhaled drugs. Studies of its use for influenza B are limited.
  • Baloxavir – is given as a single 40 mg dose orally to patients ≥ 12 years and 40 to 80 kg or a single 80 mg dose for patients >80 kg. It can be used in patients ≥ 12 years with uncomplicated influenza who have been symptomatic for ≤ 48 hours and who are otherwise healthy and not at high-risk. It has not been studied in patients who are hospitalized, immunocompromised, or pregnant. or have severe pneumonia (rx).
  • Adamantanes (amantadine and rimantadine) – were previously used; however, more than 99% of current and recent circulating influenza viruses are resistant to adamantanes, so these drugs are currently not recommended for treatment. Adamantanes block the M2 ion channel and thus interfere with viral uncoating inside the cell. They were effective only against influenza A viruses (influenza B viruses lack the M2 protein).

Prevention

The most effective way to prevent the disease is vaccination. Safe and effective vaccines are available and have been used for more than 60 years. Immunity from vaccination wanes over time so annual vaccination is recommended to protect against influenza. Injected inactivated influenza vaccines are most commonly used throughout the world.

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Among healthy adults, influenza vaccine provides protection, even when circulating viruses do not exactly match the vaccine viruses. However, among the elderly, influenza vaccination may be less effective in preventing illness but reduces the severity of disease and incidence of complications and deaths. Vaccination is especially important for people at high risk of influenza complications, and for people who live with or care for the people at high risk.

WHO recommends annual vaccination for:

  • pregnant women at any stage of pregnancy
  • children aged between 6 months to 5 years
  • elderly individuals (aged more than 65 years)
  • individuals with chronic medical conditions
  • health-care workers.

The influenza vaccine is most effective when circulating viruses are well-matched with viruses contained in vaccines. Due to the constantly evolving nature of influenza viruses, the WHO Global Influenza Surveillance and Response System (GISRS) – a system of National Influenza Centres and WHO Collaborating Centres around the world – continuously monitor the influenza viruses circulating in humans and updates the composition of influenza vaccines twice a year.

For many years, WHO has updated its recommendation on the composition of the vaccine (trivalent) that targets the 3 most representative virus types in circulation (two subtypes of influenza A viruses and one influenza B virus). Starting with the 2013–2014 northern hemisphere influenza season, a 4th component is recommended to support quadrivalent vaccine development. Quadrivalent vaccines include a 2nd influenza B virus in addition to the viruses in trivalent vaccines and are expected to provide wider protection against influenza B virus infections. A number of inactivated influenza vaccines and recombinant influenza vaccines are available in injectable form. Live attenuated influenza vaccine is available as a nasal spray.

Pre-exposure or post-exposure prophylaxis with antivirals is possible but depends on several factors e.g. individual factors, type of exposure, and risk associated with the exposure.

Apart from vaccination and antiviral treatment, public health management includes personal protective measures like:

  • Regular hand washing with proper drying of the hands
  • Good respiratory hygiene – covering mouth and nose when coughing or sneezing, using tissues and disposing of them correctly
  • Early self-isolation of those feeling unwell, feverish and having other symptoms of influenza
  • Avoiding close contact with sick people
  • Avoiding touching one’s eyes, nose or mouth

Other useful tips for flu recovery

Other useful flu recovery tips include:

  • Take simple pain-relieving medication such as paracetamol or ibuprofen, as directed on the packet, to ease muscle pain and bring down your fever (unless your doctor says otherwise).
  • Do not give any medications that contain aspirin to children under 18 years of age with flu. The combination of the flu and aspirin in children has been known to cause Reye’s syndrome– a very serious condition affecting the nervous system and liver.
  • Antibiotics are not effective against the flu because the flu is a virus, and antibiotics fight bacteria. However, your doctor may prescribe them if you develop a bacterial infection on top of your flu.
  • Gargle with a glass of warm water to ease a sore throat. Sucking on sugar-free lollies or lozenges also helps.
  • A hot water bottle or heating pad may help relieve muscle pain. A warm bath may also be soothing.
  • Use saline nose drops or spray to help soothe or clear a stuffy nose. These decongestants help shrink swollen blood vessels in the nose. Talk to you doctor or pharmacist about which medication will be the best one for you.
  • Do not smoke – this will irritate your damaged airways.
  • Try warm, moist air inhalation. Boil a kettle and put the water in a bowl on a table. Put your head over the bowl with a towel over your head and inhale the warm air for up to 20 minutes. Don’t put anything in the water.
  • Ask for help if you live alone, are a single parent, or are responsible for the care of someone who is frail or disabled. You may need to call someone to help you until you are feeling better.
  • Remember, if you buy medicine at the pharmacy to treat your symptoms (over-the-counter medications), check with the pharmacist to see if it is the best one for you. Mention if you have a chronic illness or are taking any other medication.

Reference

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