Acute respiratory diseases (ARD) account for 75% of acute illnesses in developed countries, of which about 80% are caused by viruses. IgM detection of respiratory pathogens provides timely and effective laboratory evidence for screening and diagnosis of ARD pathogens, guiding clinical diagnosis and rational use of antibiotics.

What are the main factors that cause respiratory infections?

Seventy to eighty percent of upper respiratory tract infections are caused by viruses, including rhinovirus, coronavirus, adenovirus, influenza and parainfluenza viruses, respiratory syncytial virus, Ebola virus, and coxsackievirus. The remaining 20-30% are caused by bacteria.

Lower respiratory tract infections are caused by microbial infections such as viruses, bacteria, mycoplasma, chlamydia, and Legionella, and are the most common infectious diseases.

The respiratory eight-pathogen test mainly includes Mycoplasma pneumoniae, Legionella pneumophila, respiratory syncytial virus (RSV), influenza virus A, influenza virus B, parainfluenza virus, adenovirus, and Chlamydia pneumoniae.

1. Respiratory Syncytial Virus (RSV)

Respiratory syncytial virus (RSV) belongs to the subfamily Pneumovirinae of the family Paramyxoviridae, and it is generally believed to have only one serotype. RSV mainly invades the upper respiratory tract epithelial tissue, causing necrotizing inflammation, which can lead to severe bronchitis and pneumonia, especially in breastfed infants and young children. It is one of the main flu viruses, and the positive rate of RSV can reach 11.58%-36% in some regional respiratory pathogen surveillance. The infectiousness of the disease is strong, and the infection rate in the population is very high, with about 90% of adults having been infected. Although neutralizing antibodies and complement-binding antibodies can be produced after RSV infection, reinfection is still common. Anti-respiratory syncytial virus-specific IgM antibodies can appear as early as one week after onset, and can persist for 2-3 months.

2. Adenovirus

Adenovirus is an unenveloped icosahedral DNA virus that can spread from person to person through aerosols or fomites. Six percent of respiratory infections worldwide are caused by adenovirus. Its main pathologies include respiratory infections, epidemic keratoconjunctivitis (commonly known as "pink eye"), viral gastroenteritis, acute hemorrhagic cystitis, among others, with respiratory infections and "pink eye" being the most common. The virus content in the excreta is highest in the first few days after infection. Anti-adenovirus-specific IgM antibodies can appear about one week after onset and can persist for 2-3 months.

3. Pneumonia caused by Chlamydia pneumoniae

Chlamydia pneumoniae is the third discovered Chlamydia after Chlamydia trachomatis and Chlamydia psittaci. It is a human pathogen that spreads through airborne transmission. About 10% of infectious pneumonia cases are caused by Chlamydia pneumoniae. The infection is widespread worldwide, with anti-Chlamydia pneumoniae antibodies detected in 50% of adults. Serum conversion is most common in the 5-15 age group, and immunity reaches its peak at age 20. About half of Chlamydia pneumoniae infections are asymptomatic. The most common clinical symptoms of Chlamydia pneumoniae infection are mild respiratory infections. The typical symptoms are persistent dry cough, as well as sore throat, headache, and fever. About 10% of infected individuals develop pneumonia. Chronic diseases associated with Chlamydia pneumoniae include bronchial asthma and atherosclerosis. In addition, the relationship between Chlamydia pneumoniae infection and the pathogenesis of coronary heart disease is being studied. IgM antibodies usually appear 2-3 weeks after the initial Chlamydia pneumoniae infection and can persist for 2-6 months. Three to five weeks after the appearance of IgM antibodies, the level of IgG antibodies in the patient's body increases significantly, which means that the detection time of anti-Chlamydia pneumoniae antibodies is later than many other infections. After recurrence of infection for 1-2 weeks, the level of IgG antibodies in the patient's body increases, but some patients do not show an increase in IgG levels. In case of recurrent infection, a slight increase in IgM antibody levels may be observed. Both indirect immunofluorescence assay using infected cells as detection matrix and microimmunofluorescence assay (MIF) using elementary bodies as detection matrix cannot rule out cross-reactivity between Chlamydia pneumoniae and Chlamydia trachomatis or Chlamydia psittaci.

Influenza Virus

Influenza mainly spreads during the cold season, with an incubation period of 1-5 days. Influenza virus belongs to the Orthomyxoviridae family and is divided into three types: A, B, and C. This is what people commonly refer to as influenza A, B, or C. Due to the strong ability of the influenza virus to mutate, a single infection does not confer lifelong immunity. For high-risk populations (such as those with weakened immune systems or elderly individuals), vaccination with relevant virus strains can be used at the beginning of the influenza season, but this only provides protection against the specific subtypes used. Specific IgM antibodies against influenza can appear as early as one week after the onset of symptoms and can persist for 2-3 months.

4. Influenza A Virus

Influenza A virus has a wide range of hosts and can infect a variety of animals, including pigs, horses, dogs, birds, and seals. It has caused several global pandemics in humans in the past. Currently, the main subtypes that infect humans are H1N1 and H3N2. Symptoms of influenza A (H1N1) are similar to those of the common cold, such as fever, cough, fatigue, and loss of appetite. During the 2009 outbreak in the United States, cases were characterized by sudden onset of fever, cough, muscle aches, and tiredness, and some patients also experienced diarrhea and vomiting. Other symptoms may include fever, cough, sore throat, body aches, headache, weakness, poor mood, loss of appetite, chills, and fatigue. Some patients may also experience diarrhea or vomiting, muscle pain or tiredness, and red eyes.

5. Influenza B Virus
Influenza B virus has a low pathogenicity and is usually only found in humans and seals, occasionally causing local outbreaks but generally not leading to worldwide pandemics. It is the pathogen of influenza and can cause serious complications in patients with potential pathology. Because it is easily confused with other respiratory diseases, clinical diagnosis during outbreaks can be difficult, making laboratory diagnosis crucial. At the onset of the disease, patients may experience general flu-like symptoms such as sudden onset, coughing, sore throat, fever, headache, muscle aches, and discomfort, which progress and may lead to high fever, respiratory distress, cyanosis, paroxysmal coughing, and expectoration of a small amount of bloody sputum.

6. Legionella Pneumophila

Legionella pneumophila, also known as Legionnaires' disease bacterium, is a gram-negative bacterium that causes Legionnaires' disease, a severe form of pneumonia. Legionella is an intracellular pathogen that can infect and multiply within macrophages, causing inflammation and damage to the lungs. Legionnaires' disease was first recognized in 1976 when an outbreak occurred among people attending a convention of the American Legion in Philadelphia, Pennsylvania.

Legionella can be found in natural and man-made water systems, such as cooling towers, hot tubs, and decorative fountains. The bacterium is spread through inhalation of contaminated water droplets or mist, but it is not spread from person to person. The disease is more common in people over 50 years of age, smokers, and those with weakened immune systems.

Symptoms of Legionnaires' disease include fever, chills, cough, shortness of breath, muscle aches, and headache. The disease can be severe, and up to 10% of people who contract it die from complications. Diagnosis of Legionnaires' disease is typically confirmed through laboratory testing of respiratory secretions, blood, or urine for the presence of Legionella bacteria or antibodies. Treatment usually involves antibiotics, and severe cases may require hospitalization and supportive care.

7. Chlamydia pneumoniae

Chlamydia is currently the smallest self-replicating cell known, lacking a rigid cell wall (due to wall deficiency), which means antibiotics that target the cell wall have little effect on it. Currently, 12 species of the Chlamydia genus have been discovered in humans. Chlamydia pneumoniae can cause atypical pneumonia and common upper respiratory tract infections. Infection can be transmitted via aerosols, and humans are the only host for this pathogen. The pathological changes in Chlamydia pneumoniae pneumonia are mainly interstitial pneumonia, sometimes accompanied by bronchopneumonia, known as primary atypical pneumonia. It is mainly transmitted by droplets, with an incubation period of 2-3 weeks, and the incidence is highest in adolescents. Clinical symptoms are usually mild, and some patients may even be asymptomatic. If symptoms do occur, they are typically general respiratory symptoms such as headache, sore throat, fever, and cough, but there have also been occasional reports of deaths. It can occur all year round, but it is more common in autumn and winter. In some cases, pharyngitis and mild ear disease may also occur, but most infections have no symptoms. Re-infection is also possible. Histological studies have found that Chlamydia pneumoniae can adhere to the epithelial cells of the trachea, bronchi, and small airways. If it is a Chlamydia pneumoniae infection, adhesion between cells is mediated by surface lipoproteins (adhesins). Adhesins are also an antigen.

One week after infection with Chlamydia pneumoniae, IgM and IgA antibodies may appear, and IgG antibody titers may rise after a few weeks and persist for months. Anti-Chlamydia pneumoniae antibodies can cross-react with other Chlamydia species. When making a differential diagnosis, pneumonia caused by Mycoplasma pneumoniae, viral pneumonia, psittacosis, and Q fever must be ruled out. Chlamydia pneumoniae-specific IgM antibodies can appear as early as one week after onset, and may persist for 3-6 months.

8. Parainfluenza virus

Parainfluenza viruses types 1, 2 and 3 can cause laryngotracheobronchitis (croup) in children aged 2-4 years. type 3 is epidemic and types 1 and 2 are geographic in nature. Parainfluenza virus infections of types 1 and 3 are common in young children. Localized epidemics occur in nurseries, pediatric wards, elementary schools and other children's settings. Type 3 is endemic, highly contagious, and can occur in all seasons, and most children can be infected within one year of age. Epidemics caused by parainfluenza virus type 1 or 2 tend to occur annually and alternate in predominance. The disease caused by type 2 tends to be more disseminated. Types 1, 2, and 3 can be epidemic in the fall. Type 4 causes mild respiratory disease. Early in the disease there is moderate sore throat and dry cough, and in many cases hoarseness and croup are prominent; this croup (acute laryngotracheobronchitis) is the most severe and dangerous form of pediatric parainfluenza virus infection.

In adults, parainfluenza virus infection can cause upper respiratory illness, often accompanied by fever. Parainfluenza virus infections can be endemic, especially between late fall and the following spring. Parainfluenza viruses are transmitted by aerosols. Antibodies against each type of parainfluenza virus are cross-reactive and can be differentiated by comparing antibody titers. Parainfluenza virus-specific IgM antibodies may appear as early as about 1 week after onset and may persist for 2-3 months.

Limitations of the assay

It is limited to qualitative tests and auxiliary diagnosis, and should not be used as the sole basis for clinical diagnosis and treatment. It should be considered in conjunction with information on the patient's clinical symptoms/signs, medical history, other laboratory tests (especially pathogenic tests), response to treatment, and epidemiology.

In the early stages of infection, the absence of pathogen-specific IgM antibodies or very low titers can lead to negative results. If pathogen infection is suspected, patients should be prompted to retest within 7 to 14 days.