Pharyngitis, more commonly known as sore throat, is an inflammatory disease of the pharynx, or the back of the throat, and it’s most often caused by an invading pathogen like a virus or bacteria, and rarely by fungi.
Now, let’s quickly review a bit of anatomy and physiology of the respiratory tract. When we breathe in, air enters the respiratory tract through the nose or mouth, respectively into the nasal and oral cavities, and then into the pharynx. At each side of the back of the throat, there is a pair of structures called adenoids and tonsils, which are small clumps of lymphoid tissue that act as the body's first line of defense that swallow harmful foreign particles and pathogens that enter through the nose or mouth. The lower part of the pharynx is continuous with the larynx or the voice box. Now, the pharynx also connects the oral cavity to the esophagus. So, at the top of the larynx sits a spoon-shaped flap of cartilage called the epiglottis, which acts like a lid that seals the airway off while eating or drinking, so that anything we consume can only go one way, down the esophagus and towards the stomach. In contrast, during breathing, the epiglottis stays open, so that air can make its way into the larynx, and then continue its journey through the trachea, or windpipe, towards the lungs.
Zooming in, the epithelium lining respiratory tract consists of goblet cells that release mucus, which is sticky and contains enzymes to help trap and destroy harmful foreign particles and pathogens; as well as columnar epithelial cells that have hair-like projections called cilia, which work to sweep the harmful particles up and out of the airways. In addition, the epithelial layer that lines the pharynx is actually thicker than elsewhere in the respiratory tract, as it has to protect the tissues from any harmful particles from the air we breathe in, as well as from anything we consume.
Okay, so pharyngitis is caused by any harmful particle that manages to breach these defense mechanisms and irritates the epithelial layer that covers the pharynx. In most cases, the cause is infectious. Typically, the pathogen is transmitted from person to person via respiratory droplets or aerosolized particles expelled from an infected individual when talking, coughing, or sneezing. Most often, these droplets land in the eyes, nose, or mouths of people nearby, infecting them; less frequently, the pathogen is transmitted indirectly when an individual touches a contaminated surface and then, prior to washing their hands, they touch their eyes, nose, or mouth.
In most cases, pharyngitis is caused by a virus, such as rhinovirus, influenza, adenovirus, coronaviruses, and parainfluenza. Less frequent viruses include coxsackievirus, herpes simplex virus, Epstein-Barr virus, and HIV. Pharyngitis can also be caused by bacteria, such as Streptococcus pyogenes, which is often referred to as group A beta hemolytic streptococcus or GAS for short; this infection is commonly known as strep throat. Other less frequent bacteria include other streptococci, as well as Chlamydia pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae, Neisseria spp, and Corynebacterium diphtheriae. Pharyngitis can also be rarely caused by fungi, such as Candida albicans.
Risk factors for infectious pharyngitis include being between 5 to 15 years of age, having frequent contact with children, close contact with infected individuals, or being in small spaces with large groups of people, as well as prolonged or inappropriate use of antibiotics, inhaled corticosteroids, or clients who are immunosuppressed, such as those who have HIV.
Now, regardless of the cause, pharyngitis occurs when there’s irritation or damage to the pharynx. In response to this, the immune system launches an inflammatory response, which results in visible pharyngeal redness and edema, as well as increased mucus discharge, as these cells start to produce more mucus that can help trap and eventually expel any pathogens. The inflammatory response can be local, or it can spread to nearby areas of the respiratory tract, such as the nasal cavity, as well as the adenoids and tonsils.
Viral infections tend to also involve the mucosa lining the nasal cavity, causing rhinopharyngitis or the common cold. On the other hand, strep throat can spread to the tonsils, causing tonsillitis and even a peritonsillar abscess, which is a potentially life threatening complication. Additionally, clients with untreated strep throat may develop rheumatic fever, a delayed systemic complication that typically occurs two to four weeks after the original infection. Rheumatic fever arises because Streptococcus pyogenes has a bacterial protein that mimics the structure of some human proteins. So, when the immune system produces antibodies against the bacterial protein, these antibodies also end up attacking the client’s own tissues. This can ultimately result in fever and tissue destruction, especially involving the heart, joints, skin, and brain.