Introduction

Parainfluenza is a respiratory infection caused by the human parainfluenza virus. It is most dangerous in immunocompromised individuals, those with a chronic lung condition, children under five years old, and the elderly. As a respiratory infection, it affects the respiratory tract, which includes the nose, trachea, bronchi and lung components like bronchioles and alveoli.^{1, 2}

Mechanism of parainfluenza infection

Transmission

The human parainfluenza virus is transmitted via aerosols in the air, which tend to be large droplets. If a person comes into contact with these contaminated droplets, they are likely to contract the infection. This is possible when someone infected coughs or sneezes, introducing the contaminated particles into the air as well as onto any surfaces the droplets may settle on. Once on a surface, the virus may survive up to ten hours and be transmitted to anyone who touches their eyes, nose, or mouth after touching a contaminated surface.¹

Airway entry

Parainfluenza virus has a protein on its surface that binds to a protein found on a host cell, which acts as a receptor for viral attachment. The virus also has another protein on its surface called the fusion protein, which will mediate fusion of the virus with the host cell, allowing the virus to enter the airway cells. Once inside the cell, the virus will replicate by exploiting various host cell machineries. It will use the host cell to replicate its genetic information, which is in the form of single-stranded ribonucleic acid (RNA). RNA is a nucleic acid, like DNA, and just differs slightly in structure as it has a different type of sugar and contains uracil rather than thymine. The parainfluenza virus will also use the host cell to produce its necessary proteins. Once all is completed, the new viruses will assemble and exit the cell.¹ 

Symptoms

Symptoms of parainfluenza tend to be on the mild side and bear a lot of similarity with other respiratory infections. They include:

• Fever
• Coughing
• Sore throat
• Blocked nose
• Sneezing
• Hoarseness
• Wheezing

In more serious cases, the infection caused by the parainfluenza virus may lead to pneumonia or croup, which is characterised by a barky-sounding cough. Both of these may cause people to require hospitalisation.  

Pathophysiology of parainfluenza in the airway

Inflammation

When the virus enters and replicates in a host cell, the host cell is able to recognise viral components. Upon doing so, a pathway is activated, which leads to the production of proteins called interferons. These proteins will signal the presence of infection to surrounding host cells, leading to the activation of antiviral defences. They will also recruit immune cells to the site of infection to further tackle the virus. These immune cells will produce cytokines, proteins that will signal to other immune cells, recruiting them to the area. An increase in immune cells in the area as well as widening of blood vessels (vasodilation) to increase blood flow to the area, will cause inflammation.³ 

Mucus production

The airways will also experience an increase in mucus production as well as inflammation. This is due to increased epithelial cell turnover in the airways. This means that there will be more cell death, and so there will be shedding to try to replace the dead cells with new, fresh cells. As a response to the virus, the body will also increase its number of mucin-producing cells.^3,4 Mucin is a protein released by epithelial cells, which largely makes up the mucus. 

Airway blockage

Due to the increase in inflammation, the airways will swell up, leading to narrowing, making it more difficult to breathe. This will be furthered by the increase in mucus, increasing the blockage of the airways.⁴  

Airway function impact

The effects of parainfluenza infection will affect the key airway function, namely, gas exchange. The lungs are an important site for gaseous exchange, where carbon dioxide is removed from the blood and excreted via exhalation. Oxygen is inhaled and will diffuse into the blood, making it oxygenated. This will allow oxygen to be carried all around the body, providing oxygen to all body tissues to allow for respiration (a chemical process within the body that allows for energy release). This is impaired during parainfluenza infection due to increased inflammation and mucus production. This furthers the blockage caused by mucus, preventing air from reaching the necessary capillaries for gaseous exchange.⁵ Furthermore, the excess mucus will cause some irritation to the airways, leading to the body’s natural reflex of coughing.⁶ 

Diagnosis and prevention

Diagnosis 

Diagnosis will begin with a physician’s examination, who will be able to diagnose the illness by listening to your heart, lungs and the list of symptoms experienced by the patient. They may also take a look inside the throat or measure blood oxygen using a clip that goes on your finger. In some cases, although unlikely, the doctor may do a swab of the throat or nose and send it off to a laboratory for testing. At the laboratory, tests will be carried out to try to identify the virus. These may include doing a viral culture, carrying out a type of assay specific to the parainfluenza virus, or doing a PCR.¹

Treatment

Usually, there is no treatment for parainfluenza as it tends to be a mild illness, which is self-limiting. People may choose to treat their symptoms with over-the-counter medications, as well as by resting and increasing fluid intake. However, those experiencing serious complications from the infection will receive treatment to tackle the complications. For example, someone who has developed croup due to their parainfluenza infection may receive anti-inflammatory corticosteroids.¹

Prevention

The most effective way to prevent catching parainfluenza is by utilising good hygiene practices. These include:

• Handwashing with soap and warm water before touching your face or before eating. If washing hands is not possible, an alcohol-based hand sanitiser may be used 
• Surfaces should also be regularly cleaned, especially if used for eating 
• It is also important not to share any utensils like knives or forks, or drink from the same bottle as someone ill 
• If infected, people should try to reduce the spread by covering their nose and mouth with a tissue when coughing or sneezing and disposing of it 
• Those infected may also be encouraged to wear a face mask if forced to be in public, otherwise, people should stay home if unwell 

Summary

Parainfluenza is a viral infection that affects the airways and associated organs. It is usually mild and does not require treatment. However, in some cases, it may lead to more serious conditions like croup or pneumonia, which is seen most often in individuals who are elderly, immunocompromised, have a chronic lung condition, or are under the age of five. The virus causes disease by attaching to, entering and replicating in host cells in the airways and lungs, triggering inflammation and mucus overproduction, leading to symptoms such as cough, fever, sore throat and sneezing.

FAQs

What is parainfluenza vs influenza?

Parainfluenza and influenza are both respiratory viruses which cause similar symptoms. The difference between them is that they are different viruses, which belong to different viral families. 

How long does parainfluenza usually last?

Parainfluenza typically lasts seven to ten days. 

Does parainfluenza show up on a flu test?

Parainfluenza will not show up on a flu test. This is because the flu is caused by influenza, and the test is made to detect the influenza virus. Since parainfluenza is a different virus, it will not be detected by the test.

How long does parainfluenza last on surfaces?

Parainfluenza may last up to 10 hours on surfaces.

Can steroids help parainfluenza?

Steroids may help in conditions that develop as complications of parainfluenza, for example, croup. The steroids work by decreasing inflammation, therefore helping to widen the airway.

Can you get parainfluenza twice?

Yes, you can get parainfluenza twice.

Do antibiotics help parainfluenza?

No, antibiotics would not be an effective treatment against parainfluenza. This is because antibiotics are made to kill bacteria, which are completely different organisms from viruses.