Respiratory syncytial virus

Definitions & Key takeaways

Respiratory syncytial virus (RSV) is most known to infect the human respiratory tract and cause bronchiolitis, which is an inflammation of the bronchioles, the smallest air passages of the lungs. It usually occurs in children less than two years of age with the majority being aged between three and six months. It presents with coughing, wheezing, and shortness of breath which can cause some children difficulty in feeding. Bronchiolitis is typically diagnosed clinically, and the treatment is supportive with supplemental oxygen and giving fluids to prevent dehydration.

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Bronchiol- refers to the small airways of the lungs and -itis means inflammation, so bronchiolitis is inflammation of the small airways in the lungs.
It’s most often caused by infection from the respiratory syncytial virus or RSV. It mostly affects young children - causing illness in nearly every child at some point in their life.
The name respiratory syncytial virus comes from the virus causing cells lining the respiratory tract to merge, forming a large multinucleated “cell” called a syncytia.
Respiratory syncytial virus is part of the Pneumoviridae family, and it’s transmitted when an infected person sneezes or coughs, spreading 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 get inhaled into the lungs. The virus can also survive on surfaces for a few hours, so it’s possible to get the virus by touching a surface, like a contaminated doorknob, and then touching your own eyes, nose, or mouth.
Upon entering the body, the virus encounters the epithelial cells lining the nasopharynx which is the part of your throat nearest your nose.
It creates some local damage, and then works its way down the respiratory tree kind of like a secret agent rappelling down a rope of mucus.
It goes down past the trachea and main bronchi to eventually reach the bronchioles - it’s primary target. Respiratory syncytial virus is an enveloped virus with a linear negative-sense strand of RNA, which means that once the virus enters its RNA into a respiratory epithelial cell, that strand has to get converted into a complementary sense strand in order to get translated.
The cell is forced to use it’s energy and organelles to make viral proteins - basically turning into a virus factory. The new viruses invade neighboring cells, creating multinucleated syncytia out of some while destroying others.
The cellular destruction attracts nearby immune cells, like natural killer cells whose job it is to kill the virus-infected cells.
Immune cells release various chemokines which creates an inflammatory reaction that makes epithelial cells secrete more mucus and makes the blood vessels in the walls of the airways more leaky.
More immune cells and more fluid enters the damaged areas, creating inflammation and swelling. The extra fluid in the walls of the airway, makes the walls thicker and narrows the airway.
Children typically have narrower airways than adults to begin with, so this additional narrowing of the airways affects them the most, and largely explains why they disproportionately suffer from bronchiolitis.
In addition, dead cells and mucus slide into the airway forming mucus plugs which can trap air behind the plug. Over time, trapped air slowly diffuses into the bloodstream, and tiny airways collapse - a process called atelectasis.
Sometimes, the mucus plugs end up acting like one way valves, allowing air to enter, but not escape the bronchioles. In other words, air keeps going in with each inhalation until the lungs are really inflated, but the air can’t escape during exhalation - this is called air-trapping.
Both atelectasis and air-trapping can sometimes be seen in the different regions of the lungs at the same time like in this chest X-ray.
Atelectasis and air-trapping both reduce the lung’s ability to bring in oxygen and get rid of carbon dioxide. Over time, a serious RSV infection can lead to hypoxemia which is a decreased oxygen content in the blood.
Although the main cause of bronchiolitis is respiratory syncytial virus, it can be caused by other viruses like adenovirus, human bocavirus, and human metapneumovirus.
It can also be caused by other pathogens or viruses, like the bacteria Mycoplasma pneumoniae. Sometimes a severe bout of bronchiolitis may be due to more than one of these invaders attacking at the same time.
Initially, bronchiolitis symptoms are similar to the common cold - congestion, pharyngitis, sore throat, and cough. If the infection becomes more severe, it can lead to symptoms like difficulty breathing, wheezing, and fever.
If there’s hypoxia it can be especially dangerous for a child’s developing brain, and the body’s natural response is to increase heart rate and breathing in an attempt to deliver oxygen to the brain more quickly.
Over time, this can lead to exhaustion, and require hospitalization. Young infants with bronchiolitis can also experience central apnea which is where they have short periods of time where they stop breathing altogether.
The diagnosis of bronchiolitis is largely a clinical diagnosis based on whether RSV is known to be circulating at a given time of the year, the child’s age, and the presence of classic signs and symptoms.
Children at risk for bronchiolitis include those who weren’t breastfed, those born prematurely, and those with neuromuscular disorders that cannot easily clear their airways.
Although it’s not always needed, diagnostic testing for RSV can be done by swabbing epithelial cells in the nasopharynx and looking for the presence of viral antigens.
There’s no proven antiviral therapy for bronchiolitis, so treatment usually consists of supplemental oxygen and giving fluids to prevent dehydration.
For some kids at high risk of serious complications like those born very premature, with significant pulmonary disease like bronchopulmonary dysplasia or with congenital heart disease, monthly injections of a premade antibody against RSV called palivizumab has been shown to be beneficial.
Alright, as a quick recap, bronchiolitis is inflammation of the small airways that typically affects infants and young children.
Inflamed airways along with atelectasis and air trapping due to mucus plugs can lead to difficulty breathing and hypoxia.
Bronchiolitis is typically diagnosed clinically, and the treatment is supportive with supplemental oxygen and hydration.