Asthma Triggers: The Respiratory Virus (Cold and flu)
Respiratory viral infections are the most common asthma trigger. Worse, they're responsible for the most severe asthma attacks and are the most common cause of hospital admissions for asthma. So, what are viruses, and why are they so hard on asthmatics?
What are some statistics?
One study showed respiratory viral infections were responsible for a whopping 85% of asthma attacks.1
Another showed they were responsible for 85% of asthma attacks in children and 50% of asthma attacks in adults.2 Yet another showed they were responsible for a whopping 70% of all hospital admissions for asthma. Of those, 18.8% had both a viral and a bacterial infection, making them the most likely to be readmitted once discharged.3
What are viruses?
There are over 5,000 identified species of viruses, although over 400,000 are thought to exist. They are so small they cannot be seen by the naked eye. They are so small that, even though scientists identified them in 1892, they were unable to actually see them until the electron microscope was invented in 1931.4,5
Where are they?
They are seemingly everywhere, and easily enter our bodies when we inhale, eat, or touch surfaces and then our eyes, noses, or mouths.6
Are viruses complex?
No. Actually, they are very simple structures. All they consist of is a genome and a protein coat called a capsid. That’s it! The genome consists of nucleic acid and is either a strand of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The capsid protects the virus and gives it its shape.4,5
Are they living structures?
Viruses are considered pathogens because they are capable of replicating and causing disease. However, they cannot replicate on their own. To do this, they must invade and use the components of an animal or human host cell. For this reason, they are not considered living organisms. Essentially, they are the smallest substances capable of causing disease.4,5
Why do they exist?
The sole purpose of a virus's existence is to invade a cell and replicate. Due to their simple structures, they don't have the means of hunting for cells to invade. For this reason, when they come into contact with a cell they like, it’s merely a coincidence. So, it's not like they are out and about hunting for respiratory cells to invade.
What are asthma respiratory viruses?
While viruses are responsible for causing many diseases, the viruses we are concerned about are the ones that cause the common cold and flu. They include the rhinovirus, influenza/parainfluenza virus, adenovirus, and coronavirus.7 So, of the 400,000 species of viruses thought to exist, we are only concerned with four (or 0.001% of all viruses).
How do respiratory viruses cause infection?
Each species of virus is partial to a specific host, and many are partial to specific cells. Proteins on the capsid of respiratory viruses are “organized into enzymes” making them attracted to cells lining the respiratory tract. More specifically, they bind with receptors on the membranes of respiratory epithelial cells. As noted above, this only happens if there is a chance meeting with the virus and the cell. The virus is now said to have "infected" the cell.4,5
What does a virus do to the cell?
It essentially turns it into a virus making a factory. The Rhinovirus is the most common virus to cause asthma symptoms. So, to keep it simple, we’ll just focus on the Rhinovirus. Once it binds to a respiratory epithelial cell, it forces the cell to take it in. Once inside, the capsid separates itself from the genome (in this case, a strand of RNA). This RNA strand enters the cell’s nucleus and integrates itself to the cell’s DNA. The cell then replicates the viral RNA over and over and over and over and over and over and over and over and over again.4,6
What happens next?
At the same time this is going on, new capsids are being made outside the nucleus. This replicated viral RNA then leave the nucleus and are covered with a new capsid. In this way, the cell is turned into a virus-making factory, making virus after virus after virus after virus after virus after virus after virus.4,6
How do viral infections spread?
As they continue to increase in number, tension is created within the cell, which ultimately bursts (lysis), releasing up to 10,000 rhinoviruses to invade and infect other cells.8 Each newly infected cell, in turn, becomes a virus-making factory. This is how an infection spreads.4,6,9
How does the immune system respond?
Thankfully, our immune systems are equipped with built-in defense mechanisms to stop the spread of viruses. Infected respiratory epithelial cells release chemicals that tell other cells of the invasion. Some of these chemicals cause inflammation of your upper airways, mainly your nose and throat. Other chemicals travel through the bloodstream to recruit immune cells that enhance inflammation.2 This inflammation is necessary because it traps viruses. The downside is that it causes cold symptoms.
How does inflammation cause cold symptoms?
Inflammation irritates goblet cells to cause them to increase mucous production. This is needed to ball up viruses and move them to the back of your throat where they can be swallowed and incinerated by acidic stomach juices. The downside is that this causes a runny nose, nasal congestion, and sinus drainage. Inflammation also irritates nerve endings, causing that annoying itchy, scratchy feeling in your nose and throat. It may also cause coughing and sneezing.
How does inflammation trigger asthma symptoms?
If you have asthma, these chemicals may travel to your lower airways worsening inflammation that already exists. 2 This can even happen if your asthma is controlled. Irritated goblet cells increase mucus production, and irritated nerve endings cause chest tightness. Inflammation also irritates smooth muscles that wrap around smaller airways, causing them to constrict and squeeze airways. This makes you feel short of breath. This may also cause coughing and wheezing.
Can colds be prevented?
It's not easy preventing yourself from getting a respiratory virus. However, a good strategy begins with good hand washing and making sure you get your annual flu vaccination. It's also helpful to have your family members do the same, along with covering their mouths when they cough or sneeze.
What is the treatment?
If you have asthma and develop a cold, it's important that you follow your asthma action plan, if you have one. Otherwise, call your doctor immediately if you experience worsening asthma symptoms, especially those that do not respond to your typical asthma treatment regimen. There is no proven treatment for respiratory viruses, although your doctor may want to tweak your asthma treatment regimen to help you breathe easier while you recover.
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