Influenza – Causes, Symptoms and Treatment

Influenza, commonly known as “the flu”, is a common viral respiratory infection caused by the influenza virus causing fever, cough, runny nose, headaches and malaise with seasonal outbreaks.

The influenza virus is an RNA virus, a member of the orthomyxoviridae. The term ‘flu’ is commonly and incorrectly used to refer to similar illnesses caused by other viral respiratory pathogens. 

One such confusion is with parainfluenza, which is a common virus that can cause both upper and lower respiratory infections, including colds, bronchitis, croup, and pneumonia. Despite the name, it is not related to influenza (the flu).

It is caused by an entirely different virus known as the human parainfluenza virus (HPIV). It is also important to know that although many types of viruses can cause a common cold, rhinoviruses are the most common cause.

Types of Influenza Viruses

There are 4 types of seasonal influenza viruses classified as types A, B, C and D. Types A and B viruses are responsible for seasonal epidemics of the disease.

1. Influenza A viruses

They are further classified into subtypes according to the combinations of the proteins on the surface of the virus namely the hemagglutinin (H) and the neuraminidase (NA). 

Hemagglutinin (H) is a glycoprotein on the influenza viral surface that allows the virus to bind to cellular sialic acid and fuse with the host cell membrane while Neuraminidase (NA) is another surface glycoprotein that enzymatically removes sialic acid thus promoting viral release from the infected host cell. 

There are 18 H types and 11 NA types, giving 198 possible combinations, but only a few are human pathogens, thus currently circulating in humans are subtype A (H1N1) and A (H3N2) influenza viruses. Only influenza-type A viruses are known to have caused pandemics.

2. Influenza B viruses

These are not classified into subtypes but can be broken down into lineages. Currently circulating influenza type B viruses belong to either B/Yamagata or B/Victoria lineage.

3. Influenza C virus

This is detected less frequently and usually causes mild infections and does not pose public health challenge.

4. Influenza D viruses

Primarily affect cattle and are not known to infect or cause illness in humans.

What Is Flu (Influenza)?

Influenza is an acute respiratory infection caused by influenza viruses. They were first isolated in 1940 and cause widespread sporadic illness yearly and are usually caused by influenza A and B viruses.

The incubation period for influenza ranges from 1 to 4 days with an average of about 48 hours. Typical, influenza in adults is characterized by sudden onset of chills and after 2 to 3 days, the acute symptoms rapidly subside although, fever may last up to 5 days. Cough, weakness, sweating, and fatigue may persist for several days or occasionally for weeks.

Since 1968, most seasonal influenza epidemics have been caused by H3N2 (an influenza A virus). Between 2009–2010, the H1N1 flu pandemic occurred with the virus spreading to more than 70 countries and all 50 American states with a majority of the deaths occurring in Mexico.

The virus was initially referred to as a swine flu virus but was noted to be a  combination of swine, avian, and human influenza viruses. Thus, the virus name was standardized to influenza A(H1N1)pdm09 to denote the pandemic and distinguish it from the H1N1 seasonal strains.

What Is The Difference Between Flu And Common Cold?

The flu and the common cold can have similar symptoms, like runny nose and cough. But common cold symptoms are usually mild while flu symptoms can be severe and lead to serious complications and their causative agents are different.

How Is It Spread?

Influenza viruses can be spread by:

• Airborne droplets.

Airborne spread appears to be the most important mechanism.

• Person-to-person contact
• Contact with contaminated items.

Who Is At Risk Of Contracting Influenza?

Certain patients are at high risk of complications from influenza viz:

1. Children.

Less than 5 years (with the highest risk in under 2

2. Adults > 65 years
3. Persons with chronic medical disorders.

• • Cardiopulmonary disease.
  • Diabetes mellitus.
  • Renal or hepatic insufficiency.
  • Haemoglobinopathy.
  • Immunodeficiency.

4. Pregnant women 

In the 2nd or 3rd trimester of pregnancy.

5. Patients with disorders that impair the handling of respiratory secretions.

• • Cognitive dysfunction.
  • Neuromuscular disorders.
  • Stroke.
  • Seizure disorders.

6. Patients ≤ 18 years taking Aspirin (because Reye syndrome is a risk).

Morbidity and mortality in these patients may be due to exacerbation of underlying illness, acute respiratory distress syndrome, primary influenza pneumonia, or secondary bacterial pneumonia.

What Are The Symptoms Of Influenza?

1. Common cold-like symptoms 

• • Sore throat.
  • Catarrh.
  • Mild conjunctivitis may also occur.

2. Fever. 
3. Prostration. 
4. Cough.
5. Generalized aches and pains (especially in the back and legs).
6. Headache often with photophobia and retro bulbar aching. 
7. Respiratory symptoms may be mild at first, with a scratchy sore throat, substernal burning, nonproductive cough, and sometimes coryza. 
8. Later, lower respiratory tract illness becomes dominant; cough can be persistent, raspy(hoarse or harsh sounding), and productive.
9. Gastrointestinal symptoms may occur and appear to be more common with the 2009 pandemic H1N1 strain. Children may have prominent nausea, vomiting, or abdominal pain, and infants may present with a sepsis-like syndrome.

Diagnosis

Several tests are available to detect flu viruses in respiratory specimens viz:

1. Rapid influenza diagnostic tests (RIDTs).

This is the most common and works by detecting the parts of the virus (antigens) that stimulate an immune response. These tests can provide results within approximately 10-15 minutes but may not be as accurate as the other flu tests. Therefore, one could still have flu, even though the rapid test result is negative (False Negative).

Despite a negative rapid test result, flu can still be diagnosed based on the symptoms and the clinical judgment of the physician.

2. Rapid molecular assays.

This flu test detects the genetic material of the flu virus and produces results in 15-20 minutes and is more accurate than RIDTs.

3. Reverse transcription polymerase chain reaction (RT-PCR)

This technique is used as a rapid and sensitive method for the detection of influenza viruses in both clinical samples and isolates.

4. Viral culture.

This does not yield timely results to inform clinical management.

5. Immunofluorescence assays. 

They are antigen detection assays that generally require the use of a fluorescent microscope to produce results in approximately 2-4 hrs.

All of these tests require that a healthcare provider swipe the inside of the nose or the back of the throat with a swab and then send the swab for testing. 

During an outbreak, a positive rapid flu test is likely to indicate a flu virus infection. Rapid tests appear to be better at detecting flu in children than in adults.

Treatment of Influenza

Treatment of influenza virus can be by the following measures viz:

1. General measures

• • Rest and sleep.
  • Keep warm.
  • Drink plenty of water to avoid getting dehydrated.

2. Medications

• • Analgesics

Take paracetamol or ibuprofen to lower the temperature. This also helps with aches and pains.

• • Antiviral

Antiviral drugs can make illness milder and shorten the time one is sick. They might also prevent serious flu complications like pneumonia when treatment is started early.

• • • Oseltamivir phosphate (Tamiflu®)

Is taken by mouth as a pill or a liquid for several days.

• • • Zanamivir (Relenza®)

This drug is taken as an inhaler for several days. It is not recommended for people with breathing issues, like asthma or COPD.

• • • Peramivir (Rapivap®)

This is an intravenous medication usually given as a single dose.

• • • Baloxavirmarboxil (Xofluza®)

This is taken orally as a single dose. This is not recommended during pregnancy breastfeeding, being hospitalized or having certain medical conditions.

3. Cough syrups and nasal decongestants

• • Using a spray or oral decongestants like phenylephrine or pseudoephedrine can help with a runny or stuffy nose.
  • Taking cough suppressants (antitussives) like dextromethorphan can help calm a nagging cough.
  • Using expectorants like guaifenesin makes it easier to clear mucus out of your lungs.

How Can One Prevent Influenza?

1. Vaccination

The best way to prevent the flu is to get the flu vaccine every year. They are safe and effective and help train the immune system to recognize infections and fight them off early before one gets sick.

 The influenza virus changes (mutates) a little bit every year, which is why one, needs to get vaccinated every year. The best time to get a shot is during autumn (September, October and November).

 In Children, the flu nasal spray vaccine is used but Babies and children aged 6 months to 2 years with certain health conditions will be offered a flu vaccine injection instead of the nasal spray.

Adults that can be vaccinated include viz:

• Are 50 and over.
• Are pregnant.
• Are in long-stay residents in care homes.
• Have certain health conditions.
• Live with someone more likely to get a severe infection. due to a weakened immune system (such as someone living with HIV, someone who had a transplant, or is having certain treatments for cancer, lupus or rheumatoid arthritis.

The children who can be vaccinated include viz.

• Children aged 2 or 3 years of age.
• All primary school children.
• Some secondary school children.
• Children aged 2 to 17 with certain health conditions.

Even if you get sick with a different strain of the flu than the one in the vaccine, vaccination reduces the risk of getting seriously ill. 

2. Hygiene

Other ways to reduce your risk of getting the flu include:

• Hand washing often with soap and water or the use of an alcohol-based hand sanitizer If one is unable to use soap and water.
• Covering the nose and mouth when sneezing or coughing. Cough or sneeze into the elbow or a tissue rather than the bare hands.
• Avoiding being around people when you have flu or sick persons with the flu or other infectious diseases being around you.
• Consider wearing a mask if you’re sick especially if you can’t avoid being around others.
• Avoid touching your face, eyes, nose and mouth.
• Avoid sharing eating utensils (forks, spoons, cups) with others.

Conclusion

Influenza (also known as “flu”) is a contagious respiratory illness caused by influenza viruses that can cause mild to severe illness, and at times can lead to death.

Patients with mild flu symptoms do not need medical care or antiviral drugs. But if symptoms worsen, stay home and avoid contact with other people except to get medical care.