Mycoplasmal pneumonia (MP) is a pneumonia caused by Mycoplasma pneumonia (MP), once called primary atypical pneumonia. The onset is slow, with fever, paroxysmal irritant cough, and a small amount of mucopurulent or mucopurulent sputum (occasionally bloody sputum). Pulmonary signs are often unremarkable, but the disease is prone to extra-pulmonary multisystem involvement and can be life-threatening or fatal. It occurs in children or adolescents, accounting for about 15%-30% of all pneumonia, and can be as high as 40%-60% in epidemic years; it generally has a good prognosis and is a self-limiting disease.
I. Pathogenesis
Mycoplasma pneumoniae, once named pleuropneumonia-like microorganism (PPLO), is a pathogen without a cell wall between viruses and bacteria, which can pass through bacterial filters. For growth in agar medium, cholesterol-containing yeast leachate and 20% horse serum are required. Its colonies are small, rarely exceeding 0.5mm, and not easily observed by the naked eye. Under the microscope the colonies are round and uniformly granular with a transparent band around the periphery. mp is in various forms such as spherical, rod-shaped and filamentous. There is only a cytoplasmic membrane composed of three layers, Gram stain is negative. The cytoplasm contains ribosomes and double-stranded DNA, and grows more slowly than other mycoplasmas under aerobic or anaerobic conditions, and can be seen only 5-10 days after inoculation. It can ferment glucose to produce lactic acid, and can produce peroxidase-like hemolysins.
The ribosome of Mycoplasma pneumoniae is about 70s in size and contains 3 kinds of rRNAs, 5, 16 and 23s, and about 50 kinds of proteins, of which 16s rRNA has a more conserved repetitive sequence and is species-specific, and is often used for probe hybridization typing and PCR typing. Mycoplasma pneumoniae has a special affinity for respiratory epithelial cells, is resistant to penicillin, and is extremely sensitive to macrolide antibiotics such as erythromycin.
II. Pathogenesis
It is believed that the pathogenesis is mainly due to the adhesion of mycoplasma to mucosal epithelial cells through the mucus cilia layer on the surface of the host respiratory mucosa, and this adhesion is related to the terminal structure of the P1 protein on the surface of Mycoplasma pneumoniae. When this adhesion factor attaches to the mucosal epithelial cells of the respiratory tract, the toxic metabolites released can lead to weakened ciliary motility and cellular damage.
Infection with Mycoplasma pneumoniae induces both humoral and cellular immune responses. The humoral immune response begins with the appearance of specific IgM antibodies, followed by IgG antibodies, which persist for a longer period of time. Secretory IgA antibodies produced locally in the nasopharynx effectively inhibit the binding of Mycoplasma pneumoniae to the respiratory epithelium. Respiratory IgA antibodies are more directly related to the immune status of the host than are antibodies in the serum. Local antibodies are important in the defense against infection. In addition to local immunity to IgA, local cellular immunity also plays a role. Because the initial infection sensitizes the young child, prompting a more severe clinical presentation upon reinfection, this suggests a relationship between the disease and the hypersensitivity of the organism after infection with Mycoplasma pneumoniae.
The pathological changes are mainly bronchitis, capillary bronchitis and interstitial pneumonia. The walls of the tubes are edematous, thickened, and have infiltrative spots. Mucus and even purulent secretions are present in the bronchi and fine bronchi. The microscopy shows acute fine bronchitis with interstitial pneumonia. A small amount of edematous fluid and macrophages are seen in the alveoli. The walls of the fine bronchioles are edematous, congested, and infiltrated with monocytes and lymphocytes, and neutrophils, exfoliated epithelial cells, and cellular debris can be seen in the lumen. Lymphocytes and monocytes infiltrate in the nearby alveolar septum. Diffuse alveolar necrosis and hyaline membrane lesions are seen in severe cases.
III. Symptoms and signs
1. Incubation period 6 to 35 days, average 3 weeks.
2. Symptoms and signs The disease varies in severity and can range from asymptomatic to severe interstitial pneumonia. Typical case presentation: The onset of the disease is slow, with only headache (69.8%) and malaise at the beginning of the disease, followed by fever and chills (56%) after 2-3 days, followed by sore throat (52.7%), myalgia (41.8%) and cough (10% of patients have no cough). The cough is initially dry, but later becomes a stubborn spasmodic cough, light day and heavy night, even affecting sleep. The cough can lead to facial edema, chest tightness, chest pain, dizziness, headache, dry cough without sputum or cough followed by white mucus sputum and pus sputum, sometimes with blood or hemoptysis. A small number of patients present with retrosternal pain. Fever is seen in more than 80% of patients, with variable fever pattern, body temperature often around 39℃, and fever duration about 1 to 2 weeks.
Pulmonary signs are mostly unremarkable, and older children often do not have any positive pulmonary signs throughout the course of the disease. In a few patients, pulmonary signs appear only at the end of the week, mainly croup and dry and moist woven grass (12) can be heard in the lungs.
IV. Examination methods
Laboratory tests.
1.Blood picture The total white blood cell count is often within the normal range, but occasionally it can be increased. 25% of patients have white blood cells over 10.0×109/L, and a few can reach (25.0-56.0)×109/L. The classification is slightly increased by neutrophils or eosinophils. Thrombocytopenia. Direct Coombs test may be positive. Blood sedimentation may increase in the early stages of the disease.
2, culture method Because of the high nutritional requirements of Mycoplasma pneumoniae and slow growth, it needs to be observed for 10 to 30 days or more, which is not very helpful for clinical diagnosis. At present, foreign countries still use the United States Center for Disease Control (CDC) recommended Hayflied medium, the domestic use of the Capital Institute of Pediatrics to Martin’s medium or pig lung digestive fluid-based medium.
3, serological methods Complement binding test: is a widely used serological diagnostic method for the diagnosis of Mycoplasma pneumoniae infection, take the acute and recovery period double serum potency is 4-fold increase, or a single serum potency ≥ 1:32 is judged as positive. Its sensitivity is up to 90%, specificity is 94%, only the first infection appears positive, re-infection often does not appear positive reaction.
Indirect hemagglutination test: mainly detects IgM antibodies. It appears positive 7 days afterwards, peaks in 10-30 days, and gradually decreases in 12-26 weeks. Blood should be collected early in the acute phase, otherwise it is not easy to detect a 4-fold increase in antibodies. The specificity is not yet satisfactory and is similar to that of the complementary junction test.
ELISA: It is used to detect IgM and IgG antibodies. The method is sensitive, specific, rapid and economical, and is a practical and reliable means of diagnosing Mycoplasma pneumoniae infection. ELISA kits are now available for sale.
Cold agglutination test: It is a non-specific test for the diagnosis of Mycoplasma pneumoniae infection, which is positive in 33% to 76% of infected patients (potency ≥ 1:32). The higher the potency the greater the likelihood of the disease, often appearing positive at the end of the first week or early in the second week of illness and lasting about 2-4 months. This test can also show false positive reactions in pneumonia and respiratory infections caused by adenovirus, parainfluenza virus, etc. in infants and children.
4, nucleic acid hybridization test Mycoplasma pneumoniae detection using radioisotope (32P, 125I, etc.) labeled nucleic acid probe technology. Although this method is sensitive and specific, but the requirements are also high, the need to use isotopes, so it is difficult to promote in the clinic.
5, polymerase chain reaction (PCR) Since 1992 the method is used to examine clinical specimens of Mycoplasma pneumoniae infection. From the comprehensive results, the positive rate of the PCR method is significantly higher than the culture method (sensitivity 10-100 times higher than the ordinary culture method), but also significantly higher than the serological and probe hybridization method. It is also more specific, has no cross-reactivity with other mycoplasmas, and is not interfered by contamination with other bacteria in the oral cavity. The time required is shorter, thus the use of PCR method can strive for early diagnosis to guide the rational clinical use of drugs. A rapid and reliable diagnosis is even more necessary when symptoms appear in the central nervous system. Since this method is quite sensitive, special care should be taken during the experimental operation to avoid contamination.
Other auxiliary examinations.
X-ray examination of the lungs reveals vague cloudy or uniform shadows, denser near the hilum, gradually becoming lighter outward, with indistinct margins, usually without invading the whole lobe. The vast majority of the lobe is involved, with the lower lobe being the most common, the lower left being the most common, the lower right being the next most common, with a small amount of pleural effusion in about 20% of the lateral position, with pulmonary atelectasis in about 10%, and occasional pleurisy, with lung lesions usually absorbed in 2-3 weeks, with complete absorption taking 4-6 weeks. About 30% of children have enlarged hilar lymph nodes.
V. Disease diagnosis
This disease must be distinguished from the following diseases.
1, viral pneumonia Pneumonia caused by respiratory syncytial virus, parainfluenza virus and adenovirus is more common in children under 5 years of age. Influenza viral pneumonia can be seen in patients with influenza.
2, bacterial pneumonia pneumococcal pneumonia, the onset of acute, often cold, rain, upper respiratory tract infections and other triggers, chills, high fever, chest pain, rust sputum, pulmonary solid signs are obvious. The blood picture shows a significant increase in white blood cells, and the isolation of pathogenic bacteria in sputum and blood may be positive.
3, parrot fever History of contact with birds (parrots, pigeons) or poultry. The onset of the disease is rapid, with fever, relatively slow pulse, headache, and chills. Definitive diagnosis depends on serological examination.
4, rickettsial disease mainly with Q fever, because Q fever sometimes with pneumonia as the main manifestation. q fever patients have contact with cattle, sheep, goats and their dairy products or dietary history, serum complement binding test and rickettsial agglutination test can confirm the diagnosis.
5, fungal infection Candida, Cryptococcus, Trichoderma, Histoplasma, Bacillus, etc. Sputum and urine can be taken for culture and smear; serum complement binding test, agar diffusion method, etc. If positive results are detected, it can be identified.
6.Tuberculosis Tuberculosis has a slow onset and long duration, and Mycobacterium tuberculosis can be detected in sputum.
7, other actinomycosis, nocardia, pulmonary infarction, pulmonary atelectasis, bronchopulmonary cancer, pneumoconiosis and central nervous system diseases must also pay attention to the differential diagnosis.
6. Complications
Some cases may be associated with pleurisy, otitis media, meningoencephalitis, acute polyneuritis, acute cerebellar ataxia, acute psychosis, pancreatitis, pericarditis, myocarditis, arthritis, hemolytic anemia, hepatic and renal impairment, etc. In conclusion, mycoplasma pneumonia is characterized by diversity and easy misdiagnosis.
Seven, medication treatment
1, general treatment respiratory isolation, rest, supply sufficient water and nutrition. Symptomatic treatment: avoid salicylic acid drugs to prevent hemolysis. Generally use antipyretic and analgesic drugs with slow and long-lasting effects, such as acetophen, calcium carbapenem, lysine, Chaihu, etc., supplemented by physical cooling in case of high fever. Reduce phlegm and cough. Remove intranasal secretions and keep the airway open. If necessary, nebulized inhalation can be used.
2, antibacterial treatment Clinical preference erythromycin 30-50mg/(kg? d), divided into 4 oral doses, adults 1.5g/d, divided into 3 oral doses, the course of treatment 2-3 weeks. New macrolides, such as roxithromycin, have less gastrointestinal side effects, high body fluid concentration, strong cell penetration, long half-life, small dosage, 5mg~10mg/(kg?d), divided into 2 oral doses. The new drug azithromycin capsule is 10mg/(kg?d) for the first dose and 5mg/(kg?d) for the next dose, once orally, for 5 days as a course. Because of the long half-life, the effect can last for 1 week after stopping the drug. Norfloxacin or ciprofloxacin is also used, 0.4g per dose, 2 times/d, 5-7 days of treatment.
3, Chinese medicine treatment Chinese medicine identification, pneumonia belongs to the category of warm-heat disease, such as “lung heat and cough”, “wind and temperature offending the lung”. Commonly used herbs include: ephedra, almond, gypsum, licorice, cold water stone, silver flower, forsythia, suzy, etc. If there is fever and sweating, add Scutellaria baicalensis, or reuse raw gypsum; if there is fever and no sweating, add fresh rhizome; if there is cough and asthma with phlegm, add Tianzhu Huang and Lycopodium.
VIII. Prognosis
It is a self-limiting disease with generally good prognosis and the death rate is usually less than 0.1%. However, the prognosis is worse in cases of complicated central nervous system infection.
IX. Preventive care
Erythromycin prophylaxis appears to be effective in densely susceptible populations. The vaccine is not yet commonly used.