World Journal Of Pediatrics-articles

Ying-Chun Xu, Lan-Juan Zhu, Dan Xu, Xiao-Fen Tao, Shu-Xian Li, Lan-Fang Tang, Zhi-Min Chen

Author Affiliations: Department of Pulmonology, Children's Hospital, Zhejiang University School of Medicine, Hangzhou 310003, China (Xu YC, Xu D, Tao XF, Li SX, Tang LF, Chen ZM); Hangzhou Meteorological Bureau, Hangzhou 310051, Zhejiang Province, China (Zhu LJ)

Corresponding Author: Zhi-Min Chen, Department of Pulmonology, Children's Hospital, Zhejiang University School of Medicine, 57 Zhugan Xiang, Hangzhou 310003, China (Tel: +86 571 87061007 ext 61711; Fax: +86 571 87033296; Email: drchenzm@163.com)

Background: Mycoplasma pneumoniae (M. pneumoniae) is an important pathogen of pediatric respiratory infections and the relation of M. pneumoniae pneumonia (MPP) with meteorological factors remains obscure. This study aims to investigate the epidemiological characteristics of childhood MPP and observe if there is a relationship between epidemiological characteristics and meteorological factors in Hangzhou.

Methods: M. pneumoniae DNA in nasopharyngeal aspirates of hospitalized pneumonia children were detected by polymerase chain reaction from January 1, 2007 to December 31, 2009. The positive rates of MPP (MPP rates) in different years, seasons and ages were compared. The relationship between MPP rates and meteorological data, including mean air temperature (ºC), mean relative humidity (%), monthly precipitation (mm) and raining days were analyzed.

Results: In 14 799 pneumonia cases found from 2007 to 2009, the MPP rate was 18.5%. Altogether 1610 boys (16.9%) and 1134 girls (21.4%) suffered from MPP with a significant difference between both genders (χ²=45.68, P<0.001). In children younger than 1 year, 1-2 years, 3-6 years, and older than 7 years, the MPP rates were 9.8%, 21.1%, 44.4% and 61.6%, respectively. The MPP rates were significantly higher in older children than in younger ones (trends test χ²=46.72, P<0.001). In a descending order, the MPP rates in summer, autumn, spring, and winter were 27.8%, 23.9%, 18.0% and 11.6%, respectively (χ²=372.75, P<0.001). The MPP rates in 2007 to 2009 were 12.9%, 19.3% and 23.6%, respectively (trends test χ²=13.72, P<0.001). Of the four meteorological factors, only monthly mean air temperature was included in the multiple linear regression model (P<0.001).

Conclusions: This study showed that the MPP rate was higher in older children than in younger ones. Girls had a higher positive rate of MPP than boys. In Hangzhou, MPP was more prevalent in summer and autumn. Air temperature was the only meteorological factor affecting the prevalence of MPP.

Key words: meteorological factor; Mycoplasma pneumoniae; pneumonia; temperature

Mycoplasma pneumoniae (M. pneumoniae) is a kind of prokaryotic microbe with a size between virus and bacteria. It is a common pathogen of primary atypical pneumonia and other respiratory infectious diseases. Except respiratory tract infections, M. pneumoniae also causes multiple system complications and immune dysfunction or immune damage.^[1,2] In recent years the incidence of M. pneumoniae pneumonia (MPP) has risen, and reports on macrolide-resistant M. pneumoniae have been increasing.^[3,4] Refractory or critical MPP cases are also seen increasingly, with a great harm to children, especially those with weak immune function. Different weather conditions affect microorganisms, including M. pneumoniae. There are studies on the relationship of meteorological factors and M. pneumoniae infections, but none has been conducted in Hangzhou. Different studies have different conclusions on the seasonal or climate effects.^[5-8] The present study aims to investigate the epidemiological characteristics of MPP in Hangzhou and to observe the relationship between meteorological factors and MPP.

This retrospective study was conducted in the Children's Hospital, Zhejiang University School of Medicine, Hangzhou, China, from January 2007 to December 2009. Hangzhou, which has a population of 7 million, is situated in southeast China at 120.2E, 30.3N. Located in subtropical monsoon climate district, it has four distinctive seasons: spring (March to May), summer (June to August), autumn (September to November), and winter (December to the next February). The average annual temperature varies from 15.9ºC to 17.0ºC, with a maximum of 28.4ºC in July and a minimum of 4.3ºC in January. The relative humidity is 71.0%±6.0%.

A total of 14 799 children were enrolled in this study. Inclusion criteria were as follows: 1) diagnosis of pneumonia; 2) nasopharyngeal aspirate (NPA) specimens collected for M. pneumoniae DNA detection after informed consent was obtained; 3) living in Hangzhou; and 4) age of less than 18 years. Pneumonia was defined by fever, respiratory symptoms (including cough, sputum, and shortness of breath), and parenchymal infiltrates shown on chest radiography. Children with pneumonia who had no NPA specimens collected for M. pneumoniae DNA detection were excluded. The study was approved by the Ethics Committee of Children's Hospital, Zhejiang University School of Medicine.

NPA specimens of each patient with pneumonia were collected with a disposable sputum suction set with controlled pressure on the day of admission. M. pneumoniae DNA was detected by fluorescence quantitative real-time polymerase chain reaction (PCR) using a quantitative diagnostic kit for M. pneumoniae DNA (PCR fluorescence probing) (Da An Gene Co., Ltd. of Sun Yat-sen University, China) based on the TaqMan PCR.^[9] Amplification, detection, and data analysis were carried out with an applied biosystems 7500 real-time PCR system (Applied Biosystems, Inc., CA, USA).

Meteorological data in Hangzhou including air temperature (ºC), relative humidity (%), precipitation (mm), and raining days were obtained from the Hangzhou Meteorological Bureau. Temperature and humidity were read from a thermometer and a humidity meter respectively in instrument shelters 1.5 meters in height each hour. Daily precipitation was measured by a udometer. Readings were calculated into monthly mean value to coincide with the MPP data. A raining day is the day when daily precipitation is more than 0.1 mm.

Data analysis was primarily descriptive and was made using SPSS version 16.0 (SPSS Inc., USA). Multiple linear regression was used to analyze the relationship between meteorological factors and the positive rates of MPP (MPP rates). Proportions were compared using the chi-square test or Fisher's exact test. MPP rates in different age groups and years were compared using trend test. P value <0.05 was considered statistically significant.

In the 14 799 children with pneumonia, 9509 were boys and 5290 were girls, with a male to female ratio of 1.8 (Table 1). M. pneumoniae DNA was positive in 2744 children (18.5%). Altogether, 1610 boys (16.9%) and 1134 (21.4%) girls were MPP positive; there was a statistical significant difference between both genders (χ²=45.68, P<0.001).

The youngest child was 1 month old and the oldest was 17 years old, with a median of 6.9 months (25th-75th percentile: 2.4-21 months). In children younger than 1 year, 1-2 year, 3-6 years and older than 7 years, 944 (9.8%), 569 (21.1%), 727 (44.4%) and 504 (61.6%) were MPP positive, respectively (Table 1). MPP rate was higher in older children than in younger ones with an increasing tendency (trends test χ²=46.72, P<0.001).

The number of M. pneumoniae positive cases in summer, autumn, spring, and winter was 693 (positive rate, 27.8%), 734 (23.9%), 682 (18.0%), and 635 (11.6%), respectively in a descending order (χ²=372.75, P<0.001) (Fig. 1). In 2007, 2008, and 2009, the number of M. pneumoniae positive cases was 654 (12.9%), 935 (19.3%), and 1155 (23.6%), respectively, which showed an increasing tendency (trends test χ²=13.72, P<0.001).

Monthly MPP rates and meteorological data are shown in Table 2. Of the meteorological data, only monthly mean air temperature was included in the model (P<0.001). The regression equation was MPP rate (%) = 5.875 + 0.830 × monthly mean air temperature (ºC), which indicated that MPP rate increased by 0.830% with an increase of 1ºC in monthly mean air temperature (Fig. 2). The other three meteorological factors including monthly mean relative humidity, monthly precipitation, and monthly raining days were excluded from the model.

The effect of gender on M. pneumoniae infection is different. Studies have found that gender is not an important factor for M. pneumoniae infection.^[10-12] In our study, however, the positive rate of M. pneumoniae DNA was higher in girls than in boys.

Children of more than 7 years old in our study had the highest positive rate as reported elsewhere. In Turkey, serum specific IgG was positive only in children of more than 2 years old, with a sudden increase of seropositivity at 7 years and a peak at 10 years.^[13] Similar sudden increase and peak were also reported in Japan.^[14] In Australia, the age group most commonly affected by M. pneumoniae was those of 5-9 years old.^[15] The outbreak of M. pneumoniae is closely related to contact types, e.g., schools, kindergartens, and militaries,^[16] which could partially explain the age trend in the mentioned reports including ours. Some studies reported that most of patients with M. pneumoniae infection were younger than 5 years.^[17] Interestingly, Phares et al^[11] found that the incidence of M. pneumoniae was the highest in infants but the incidence of MPP in pneumonia caused by M. pneumoniae, Legionella longbeachae, Chlamydia pneumoniae was higher in children of more than 5 years old. In the present study, the MPP rate increased as the age of patients grew. This phenomenon may be partly due to the lower incidence of pneumonia caused by other agents in older children.

It is generally accepted that M. pneumoniae infection varies in frequency over the years, and a 3-4 year cycle was observed in North America, Europe, Japan and Australia.^[2,14,15] In our study, however, MPP rates increased significantly from 2007 to 2009. We postulated that the period of our research was part of the cycle. Further observation is needed to confirm our notion.

In Hangzhou, the MPP rate was highest in summer and comparatively lower in autumn, spring and winter. Foy et al^[5] found that M. pneumoniae infection was endemic without significant seasonal fluctuations in a 12-year study in Seattle and Washington. A study on both children and adults in Yugoslavia revealed that MPP was more frequent between August and November.^[18] Another study found that M. pneumoniae infection was more common in winter than other seasons in Istanbul, Turkey.^[6] A peak of M. pneumoniae infection in June or July was observed in Italy and Australia.^[15,19] In a study conducted between 1986 and 2004 in Korea,^[20] the earlier epidemics of M. pneumoniae infection up to 1996 peaked in summer, whereas the later epidemics peaked in autumn or early winter. However, the above data were all reported in different climate areas other than subtropical monsoon climate areas. In order to define more subtle factors influencing MPP, we further explored the effects of meteorological factors on MPP.

The present study revealed a strong positive correlation between M. pneumoniae positive rate and monthly mean air temperature. The survival of M. pneumoniae in aerosols was found to be best at an extremely low relative humidity. The temperature response was mediated by humidity.^[21] The effects of temperature were such that irrespective of relative humidity, an increase in temperature resulted in a decreased airborne survival time in laboratory settings.^[21] In real-life, Onozuka et al^[8] found the number of cases of MPP increased with higher temperature or relative humidity in the weeks preceding disease onset which was opposite to the experimental results. Although humidity was thought to be an important factor for survival of M. pneumoniae,^[8,21] our data proved no correlation between MPP rate and humidity or between temperature and humidity. Nor other meteorological factors including monthly mean precipitation and raining days were found to be correlated to the MPP rate.

There are shortcomings in the present study. First, the study period was not long enough. The epidemiological trend or the cyclical pattern was not fully observed. The relationship between epidemiological characteristics and meteorological factors was not elucidated. Second, the study was conducted only in one city. Large-scale epidemiological studies in other areas are needed to confirm the correlation between M. pneumoniae infection and meteorological factors. Third, control group was not set in the study. Fourth, more detailed clinical features such as laboratory data were not analyzed. However, the present study with its great number of enrolled patients provides useful information to pediatricians.

In conclusion, the MPP rate is higher in older children and it is highest in summer, followed by autumn, spring and winter in Hangzhou. The MPP rate is increased as monthly mean air temperature increases.

Funding: This study was partially supported by grants from the National Natural Science Foundation of China (Nos. 30872798, 81070004 and 81000765), the Zhejiang Medicine & Health Research Fund (No. 2009A123), and the Zhejiang Education Bureau Scientific Research Program (20070248).

Ethical approval: The study was approved by the Ethics Committee of Children's Hospital, Zhejiang University School of Medicine, China.

Contributors: Xu YC proposed the study and wrote the first draft. Zhu LJ, Xu D, Tao XF, Li SX, Tang LF and Chen ZM collected and analyzed the data. All authors contributed to the design and interpretation of the study. Chen ZM is the guarantor.

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