I have the flu and my bones ache all over. What's going on?

It is the end of winter and the temperature is still cold. It is currently the peak season for respiratory infectious diseases. The latest influenza weekly report (as of December 15, 2024) released by the National Influenza Center shows that the positive rate of influenza virus testing in southern and northern provinces across the country has continued to rise recently.

Many netizens who have been infected said that fever is not terrible, but the most torturous thing is the muscle aches all over the body. "The pain goes to the joints of the bones, and the joints all over the body feel like they are going to explode..."

So, what causes bone pain after catching the flu?

Muscle and bone pain all over the body is actually one of the common clinical manifestations of influenza. The degree of pain varies from person to person. Some people only experience ordinary, mild pain, while others experience unbearable pain all over the body. This pain is actually a manifestation of the human immune system fighting the virus.

The main reasons why influenza virus causes body pain are as follows:

1. When the virus invades the human body, it triggers the body's immune defense mechanism, inducing the production of histamine and a series of cytokines. These chemicals can dilate blood vessels, making it easier to quickly attract more immune cells to fight the virus invasion. While the immune system is clearing the virus, it will cause a certain degree of inflammatory damage to the body, leading to pain. Because cytokines are dispersed throughout the body, the pain is often systemic, although some people feel it more severely in the legs and back.

Second, the virus enters the body and is recognized by the immune system, which can activate inflammatory lipid mediators and release cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) that synthesize prostaglandins (PGs). Prostaglandins not only mediate inflammatory pain, but also increase the body's sensitivity to pain and amplify the pain effect.

3. The human body's oxygen consumption increases when it is in a fever state. For every 1°C increase in body temperature, our basal metabolism will increase by about 12%. At the same time, oxygen consumption also increases, and the body is more likely to be in a state of hypoxia, which leads to lactic acid accumulation and causes muscle aches throughout the body.

4. Some people often have diarrhea, vomiting and fever when they have the flu, which can easily lead to fluid loss, dehydration and disrupt the balance of electrolytes in the body. Electrolyte imbalance can cause muscle weakness, cramps or spasms, which can aggravate body aches.

This pain will be more intense in the first three days of fever. As the body temperature drops, the pain will gradually decrease. Under normal circumstances, the pain symptoms will disappear after 5-7 days.

In addition, some children may develop childhood acute benign myositis. Childhood acute benign myositis is a self-limiting disease in which children develop myositis during the course of influenza. The main cause of the disease is viral infection, which causes muscle involvement. It mainly occurs in children aged 4-14 years. The most common symptom is sudden lower limb pain during the course of respiratory tract infection, with bilateral calf myalgia being the most common. The most common and prominent sign is dull pain in the calf gastrocnemius muscle, which often worsens after exercise and can be relieved after rest. Severe children have difficulty walking or refuse to walk. Laboratory tests mainly show elevated serum creatine kinase, which can be as high as 20-30 times the normal value. Most of them return to normal within two weeks as the symptoms disappear.

So, how to relieve the symptoms of body aches?

First, treat the cause. After the diagnosis of influenza is confirmed based on epidemiological history, clinical manifestations and etiological examination, antiviral treatment should be started as soon as possible. In particular, the use of antiviral drugs within 48 hours of onset can alleviate symptoms, reduce complications and shorten the course of the disease. Currently, oral antiviral drugs for influenza viruses are mainly neuraminidase inhibitors, including oseltamivir and mabaloxavir.

Comparison of the similarities and differences between oseltamivir and mabaloxavir

(Excerpt from Shanghai Xinhua Hospital WeChat official account, Chen Jihui, Department of Clinical Pharmacy, Xinhua Hospital)

Secondly, if the body temperature exceeds 38.5 degrees, it is recommended to use antipyretic, analgesic and anti-inflammatory drugs, such as ibuprofen, acetaminophen, etc. However, if the symptoms of body pain are more obvious, even if the body temperature does not reach 38.5 degrees, antipyretic, analgesic and anti-inflammatory drugs can be added.

In addition, you can apply local hot compresses or massage to relieve discomfort when you are in pain. You should also rest in bed and get enough sleep. Pay attention to strengthening balanced nutrition intake, eat small and frequent meals, low-salt, easily digestible diets, mainly semi-liquid or liquid diets rich in high-quality protein, calcium, and vitamins, to ensure sufficient calorie intake. Avoid cold stimulation, excessive fatigue and other inducements. After the symptoms subside, carry out activities and exercises in a planned manner, combining work and rest.

References
[1] https://ivdc.chinacdc.cn/cnic/zyzx/lgzb/202401/t20240118_272013.htm

[2] Ge Hailiang, Li Su, Yu Shaoxiong, et al. Inflammatory response caused by viral infection: a "double-edged sword" for the host to fight against viruses[J]. Acta Microbiologica Sinica, 2022, 62(5): 1571-1586.

[3] Hao Jingxia, Li Bo, Chen Jingshi, et al. Case characteristics and treatment analysis of acute benign myositis in children[J]. Chinese Medicine, 2022, 17(8): 1230-1233.

Author: Shuai Wang, MD Department of Infection and Clinical Microbiology, Beijing Chaoyang Hospital, Capital Medical University