We have become accustomed to the fact that, every winter, due to the annual flu waves, hospitals appear on the news packed with people in the hallways suffering from the ravages of the worst symptoms of the flu, including pneumonia.
For most patients, fortunately, flu symptoms are milder than those of Covid-19 and do not require hospitalization. The most common symptoms are fever that causes chills and cold sweat, headache, persistent or dry cough accompanied by sore throat, stuffy nose, weakness and… terrible muscle and joint pain.
Why does the flu virus enter our body through the respiratory tract, affecting the mucous membranes of the nose, throat and lungs?
The immune reaction is a double-edged sword
In most respiratory viral infections, the symptoms are not directly due to the action of the virus but to the immune system’s response to eliminate it. Sore throat, cough, increased mucus and fever are simply symptoms associated with the body’s response to the action of the immune system. That is why they are common to a host of viral and bacterial diseases.
In general, almost any disease that causes inflammation will eventually lead to these common symptoms, which also include muscle weakness, pain, and joint pain. It is the inflammatory response that causes all of these symptoms, regardless of the pathogen that causes them. Muscle inflammation accompanied by weakness is known as myositis, while myalgia is the name given to pain in muscles and joints.
When an immune response to a pathogen occurs, the different cells involved release a series of small proteins known as cytokines. Cytokines come from different families that regulate multiple activities of the cells of the immune system and other organs. Some serve to signal the location of the infection, others regulate the type of response that should be produced depending on the pathogen that has infected, others control the intensity of the immune response and others even regulate the repair of damage to organs and tissues.
Cytokines are part of a complex signaling system that immune system cells need to control the immune response, but which also influence other tissues and organs. Many of these cytokines produce inflammation not only at the site of infection but also in other organs, including muscles and joints. This is why flu and other respiratory infections, including Covid-19, cause muscle pain, quite similar to that suffered by mild arthritis.
Among these cytokines, interleukin 1-beta (IL-1b), which causes fever, and interleukins 6 (IL-6) and 17 (IL-17), together with tumor necrosis factor-alpha (TNF-a), are the most relevant in explaining muscle pain. In cases of influenza and other viral infections, high levels of IL-6 in the muscle have been associated with pain.
This pain is probably associated with the loss of muscle that occurs with influenza but also with other viral infections that can cause muscle fiber tears or rhabdomyolysis. These muscle tears have been associated with high levels of IL-6 or TNF-a after these infections.
In response to these cytokines, the muscle produces elevated levels of prostaglandin E₂ (PGE₂), a lipid molecule associated with inflammation and pain. For this reason, non-steroidal anti-inflammatory drugs are prescribed to treat muscle pain and weakness, whose mechanism of action consists of inhibiting the production of PGE₂, such as ibuprofen or paracetamol.
Stopping pain, yes or no?
We have just explained that both muscle and joint pain are side effects of the immune system’s inflammatory response in its attempt to stop the infection. And that regardless of the origin of the inflammation, it is the presence of inflammatory cytokines that causes the most serious symptoms in the muscles and joints. Symptoms that include not only pain but also the loss of muscle fibers.
But there is a paradox. Inflammation is the first strategy used by the immune system when it tries to stop a viral infection. What is more, the inflammatory response is essential to keep the infection under control, and also to repair the damage caused by viruses and bacteria. If these responses are part of the battery of weapons that our body uses in its continuous fight against pathogens, does it make sense to disable them?
Muscle pain and weakness in a viral infection can be bearable collateral damage. It is quite another thing when they become a chronic symptom associated with an imbalance in the immune system that is causing chronic muscle inflammation without a clear origin. And this is possibly what happens in other types of myalgia and fibromyalgia, although there is still much discussion about it.
It is necessary to find the keys that affect chronic muscle pain accompanied by weakness, and an uncontrolled release of proinflammatory cytokines could be the cause. Hence the importance of controlling the levels of these cytokines in the treatment of this type of disease.
Guillermo López Lluch, Professor in the area of Cellular Biology. Associate researcher at the Andalusian Centre for Developmental Biology. Researcher in metabolism, ageing and immune and antioxidant systems. Pablo de Olavide University
This article was originally published on The Conversation. Read the original.
#muscles #hurt #flu
