One article review: etiology, clinical manifestations and treatment of influenza

Influenza is an acute respiratory infectious disease caused by influenza virus. Influenza viruses are distributed globally, and outbreaks of varying intensity occur every year. In view of the heavy burden of global influenza, this article reviews the information on the causes, transmission routes, clinical manifestations and treatment of influenza.

Pathogeny

Influenza viruses belong to the orthomyxoviridae of RNA viruses. Influenza virus infects the host by adhering to the host cell and penetrating the cell membrane. Hemagglutinin (NA) and neuraminidase (NA) are two major glycoproteins on the surface of influenza virus. Hemagglutinin (HA) is an enzyme that helps viruses replicate and release from host cells. Therefore, viral glycoproteins play an important role in the virulence and pathogenesis of influenza viruses.

Influenza virus strains can be divided into A, B, C and D. Most influenza is caused by one or two influenza viruses. Influenza A virus is the most common infection in the high incidence season of influenza, causing mild to severe diseases, and can infect humans and animals. Most known influenza A viruses can infect birds. Influenza B virus is not as common as the former and can cause local influenza outbreaks. Influenza C virus causes mild upper respiratory symptoms, sporadic cases and local outbreaks, mainly infecting humans, and can also cause disease in pigs and other animals. Influenza D virus mainly infects pigs and cattle. It is not clear whether it infects humans.

Route of transmission

Influenza viruses are usually transmitted from person to person through sneezing, coughing, and air borne droplets when talking. Direct contact with infected persons or contact with objects contaminated by viruses, as well as inhalation of aerosols containing viruses, may lead to infection. After infection, the virus will fall off about 24-48 hours before symptoms appear. Covering your nose and mouth when coughing and sneezing, and washing your hands with alcohol, soap or hand sanitizer are essential to prevent the spread of the virus.

Clinical manifestation

The incubation period of influenza is 2 days on average, usually 1-5 days. Typical symptoms of influenza include high fever, chills, headache, general weakness, muscle and joint pain, and respiratory symptoms such as sore throat and dry cough. Systemic symptoms usually last for 7 days, but weakness and cough can last for several weeks. Adult fever (38~41 ° C) usually lasts for about 3 days. The common severe headache of influenza is usually frontal lobe headache, which is aggravated when the head suddenly moves. Influenza cases usually recover from simple infection after a few days, but long-term or persistent symptoms may indicate complications. For example, persistent febrile headache may be caused by sinusitis. Patients may have eye symptoms, such as eye movement pain and photophobia; The eyes can become red, watery and congested. The sore throat caused by influenza lasts about 5 days. Influenza symptoms in children are similar to those in adults, but other symptoms may occur, such as nausea, stomach pain, otitis media, and vomiting.

Pneumonia is a serious complication, especially in the elderly. Complex influenza infection often presents as primary viral pneumonia, secondary bacterial pneumonia or mixed pneumonia. Symptoms of primary viral pneumonia include high fever, dry cough, headache, sore throat, fatigue, dyspnea and cyanosis.

Secondary bacterial pneumonia may be caused by Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae and other gram-negative bacteria. Severe secondary bacterial pneumonia may lead to hypoxemia, expectorant cough, high white blood cell count, and multiple cavities and infiltration can be seen on chest radiograph.

Children and adults at high risk may also have some extrapulmonary complications, such as heart disease, myositis, myoglobinuria, sinusitis, and ear infections. Neurological complications of influenza include Reye's syndrome, aseptic meningitis, encephalomyelitis and Guillain Barre syndrome (GBS). Studies have shown that influenza virus infection is related to myocarditis and pericarditis. Influenza A, influenza B and enteroviruses are the most frequently reported viruses associated with rare myositis.

Treatment

The symptomatic treatment of influenza usually includes antipyretic and analgesic drugs. In particular, acetaminophen or non steroidal anti-inflammatory drugs are used to relieve fever, pain or other systemic symptoms. Aspirin should be avoided in children because it is associated with Reye's syndrome, a liver and nervous system complication of influenza. Antitussive drugs are usually used to relieve coughing. There is no evidence that antibiotics have the possibility of shortening the course of disease or reducing complications, and their application should be limited to bacterial complications.

For severe influenza like cases or those with high risk factors of severe influenza, empirical anti influenza virus treatment should be given as soon as possible. Antiviral treatment within 48 hours of onset can reduce complications, mortality and hospital stay; Severe patients with onset time of more than 48 hours can still benefit from antiviral treatment. Patients who are not severe and have no risk factors for severe influenza should fully evaluate the risks and benefits, and consider whether to give antiviral treatment.

At present, there are three kinds of anti influenza virus drugs: neuraminidase inhibitor, hemagglutinin inhibitor and M2 ion channel blocker. The neuraminidase inhibitor is effective for both type A and type B influenza. See Table 1 for specific drug recommendations.

Haemagglutinin inhibitor Abydol blocks virus replication by inhibiting the fusion of influenza virus lipid membrane and host cells. It can be used for upper respiratory tract infections caused by influenza A and B viruses. Adults take 200mg orally once, three times a day, and five days. However, its safety for the elderly, children, and pregnant women is still unclear. However, the M2 ion channel blockers amantadine and rimantadine are resistant to the current influenza virus strains and are not recommended.

Influenza vaccination is still the most effective way to prevent influenza, which can reduce the risk of influenza and serious complications. The elderly aged 60 years and above, children aged 6 months to 5 years, pregnant women, family members and caregivers of children under 6 months of age with chronic diseases, medical staff and other key groups are recommended, and influenza vaccine is given priority every year.

Epilogue

The national influenza detection results show that every October is the winter and spring influenza epidemic season in China, which is just the time of influenza epidemic season. Please pay attention to protection, keep good personal hygiene habits, and try to reduce activities in crowded places to avoid contact with patients with respiratory tract infection.

Reference:

1. Javanian M, Barary M, Ghebrehewet S, et al. A brief review of influenza virus infection. J Med Virol. 2021 Aug; 93(8):4638-4646.

2. Chen Rongchang, Zhong Nanshan, Liu Youning, et al. Respiratory Diseases, 3rd Edition. Beijing: People's Health Publishing House, 2022

3. Diagnosis and Treatment Plan for influenza (Version 2020) [J]. Official website of the National Health Commission of the People's Republic of China