How Do Viruses Cause/Trigger Asthma?
It has now been established that viruses trigger asthma attacks, and they also cause new-onset asthma. They even have their own asthma subgroup. What I would like to discuss today (and I promise to keep it simple) is HOW viruses cause and trigger asthma. So, here’s all you need to know.
What are respiratory viruses?
They include rhinovirus (RV), respiratory syncytial virus (RSV), parainfluenza, influenza, coronavirus, and metapneumovirus1. They most likely infect the upper airway (nose, sinuses, throat) causing common colds. Sometimes these colds become severe, in which case the viruses may infect the lower airways (bronchioles, alveoli) causing bronchitis, bronchiolitis, pneumonia and asthma2.
Can respiratory viruses cause/trigger asthma in anyone?
Although it has not been thoroughly disproved, chances are pretty good that respiratory viruses will probably not induce asthma in those who do not have asthma genes. This means that about 90% of the human population has no chance of developing virus induced asthma. So, only about 10 of individuals will have a predisposition to developing asthma.
How do viruses cause respiratory infections?
Once in awhile, a respiratory virus (say, a Rhinovirus) makes it past your immune defenses and invades a respiratory cell. The cells involved are epithelial cells, or cells lining airways. When this happens, it turns this cell into a virus-making factory. Once a cell is full of viruses it bursts, thereby releasing thousands of viruses to invade other respiratory cells. Infected cells have their own built in defense mechanisms. Once invaded, they release mediators of inflammation into the bloodstream.
What are the mediators of inflammation?
These are chemicals, or proteins, called cytokines, chemokines, leukotrienes, and histamine. They warn other airway cells of the invasion and tell them to prepare for battle. They also "mediate" inflammation. Some directly cause airway inflammation by causing airway epithelial cells to release some of their fluid into intracellular spaces. This inflammation is needed to trap viruses, although it’s also responsible for cold, allergy, and asthma symptoms. Some indirectly cause inflammation by recruiting granulocytes (eosinophils and neutrophils) to airways. Granulocytes secrete more mediators to enhance the inflammatory response to cause worsening cold symptoms and persistent asthma symptoms.
How do viruses cause cold symptoms?
Respiratory viruses typically cause upper airway inflammation, resulting in your typical cold symptoms. This inflammation irritates nerve endings to cause a scratchy throat and irritating feeling in your nose and sinuses. They irritate goblet cells to cause increased mucus production to cause a stuffy and runny nose. This also results in nasal drainage. This mucus, while uncomfortable, is needed to move trapped viruses to the back of your throat.
How do viruses cause asthma symptoms?
There are three basic theories here.
- Indirect Infiltration. One theory speculates that the same mediators released during upper airway infections (Rhinovirus) may directly penetrate lower airways, causing lower airway inflammation that way. This probably only happens when colds are severe. Interestingly (as I detail below), individuals with allergies are increasingly likely to have severe colds2,4. Airway inflammation irritates smooth muscles causing bronchospasm and asthma symptoms.
- Parasympathetic nerve stimulation. Viruses may enhance the effect of the neurotransmitter acetylcholine, thereby causing what is called “reflex bronchospasm.” This effect may be directly stimulated by viruses or indirectly by the mediators of inflammation. Anticholinergic medicine (such as atropine, Atrovent, and Spiriva) block this effect, thereby opening airways.
- Direct Infiltration. Some respiratory viruses (RSV, influenza, parainfluenza) may directly infect lower airways and cause damage to airway epithelial cells. This results in direct parasympathetic nerve stimulation and the release of inflammatory markers2,4. This theory may explain why the flu may really hit asthmatics hard.
How do viruses cause new-onset asthma?
It appears that viruses, especially those that infect children early in life (and especially the RSV virus) when the immune system is still maturing, may cause changes to the immune system, making these children more likely to develop allergies and asthma. This is especially true if the infection is severe2. In effect, viruses may act as “environmental factors,” or keys, that turn asthma genes on, thereby causing new-onset asthma.
Does asthma make colds more severe?
There is also a theory that speculates that the allergic response may enhance viral replication, along with the inflammatory response mentioned above to make cold symptoms more severe2,4.
Do colds make asthma more severe?
Another theory speculates that viruses may alter the immune system in such a way that makes an individual more likely to develop allergies, thereby making that person more likely to develop an allergic subgroup of asthma (like Allergic Asthma). Furthermore, viral replication may enhance allergic inflammation, including lower airway inflammation, to trigger asthma symptoms even in individuals considered to have controlled asthma2,3,4.
How do viruses impact your asthma? Do viruses trigger your asthma? Is a virus responsible for you developing asthma in the first place? The first question can be answered by observing asthma symptoms when you have colds. The second question is most likely an educated guess. What is the impact of viruses on your asthma? Let us know in the comments below.
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