Human parainfluenza viruses

Human parainfluenza viruses, or HPIV, are a group of viruses that commonly cause respiratory tract infections, especially in children.

In fact, they are the second most common cause of acute respiratory infections in children under 5 years of age.

Parainfluenza viruses are part of the paramyxoviridae family of viruses, and there are four types that infect humans - types I, II, III, and IV.

Now, parainfluenza viruses are enveloped viruses which means that they are covered by a lipid membrane.

Inside the lipid membrane is a nucleocapsid which contains a single stranded, negative sense RNA genome and an RNA polymerase enzyme.

And, on the lipid membrane are two glycoproteins - HN, or hemagglutinin-neuraminidase, protein; and F, or fusion, protein.

Parainfluenza viruses are transmitted when an infected person sneezes or coughs, which spreads thousands of droplets containing the virus into the surrounding area up to about two meters, or six feet, away.

These droplets can then land in the mouths or noses of people nearby, or be inhaled into their lungs.

The virus can also survive on surfaces for a few hours, so it’s possible to get the virus by touching an infected surface, like a contaminated doorknob, and then touch your own eyes, nose, or mouth.

When the parainfluenza virus enters the body, it uses hemagglutinin to bind to sialic acid sugars on the surface of epithelial cells in the respiratory tract.

Once bound, the fusion, or F protein helps the virus fuse with the epithelial cell membrane and release the nucleocapsid into the cytoplasm.

In the cytoplasm, an enzyme called RNA polymerase transcribes the negative sense viral RNA into positive-sense mRNA strands, which is then translated by host cell ribosomes into viral proteins and assembled into new viruses.

These viruses leave the cell by using neuraminidase, which cleaves the sialic acid sugars of the cell membrane, allow the newly created viruses to simply bud out of the cell.

The new viruses invade neighboring cells, and destroy them.

Cellular destruction attracts nearby immune cells, which release various chemokines that create an inflammatory reaction; this reaction makes epithelial cells secrete more mucus, and it also increases the permeability of blood vessels in the walls of the airways.

Increased permeability allows more immune cells and fluid enter the damaged areas, causing inflammation and swelling.

The extra fluid thickens the walls of the airway, and narrows it.

Children typically have narrower airways than adults to begin with, so this additional narrowing of the airways affects them the most.

In severe cases, it becomes difficult for air to pass through the airways, resulting in hypoxemia, which means decreased oxygen levels in the blood; and hypercarbia, or increased carbon dioxide levels in the blood.

Parainfluenza viruses can cause a variety of respiratory tract illnesses.

HPIV type I and type II commonly cause croup, also known as acute laryngo-tracheo-bronchitis, which is the inflammation of the larynx, trachea and bronchi.

HPIV III commonly affects the lower airways, resulting in pneumonia or bronchiolitis, which is inflammation of the bronchioles.

HPIV IV can affect both upper and lower respiratory tracts, but the infection is pretty rare, and even when it does occur, it’s very mild.

Now, parainfluenza virus outbreaks occur more frequently during late autumn, and some people have an increased risk of getting an infection.

These are children between 6 months and 3 years of age, people with congenital narrowing of the airway, people with hyperactive airways, like those with asthma; and those with a history of intubation, which is where a tube is introduced into the trachea to help with breathing.