Carriage of Mycoplasma pneumoniae in the upper respiratory tract of symptomatic and asymptomatic children: an observational study

Abstract

Background: Mycoplasma pneumoniae is thought to be a common cause of respiratory tract infections (RTIs) in children. The diagnosis of M. pneumoniae RTIs currently relies on serological methods and/or the detection of bacterial DNA in the upper respiratory tract (URT). It is conceivable, however, that these diagnostic methods also yield positive results if M. pneumoniae is carried asymptomatically in the URT. Positive results from these tests may therefore not always be indicative of a symptomatic infection. The existence of asymptomatic carriage of M. pneumoniae has not been established. We hypothesized that asymptomatic carriage in children exists and investigated whether colonization and symptomatic infection could be differentiated by current diagnostic methods.

Methods and findings: This study was conducted at the Erasmus MC-Sophia Children's Hospital and the after-hours General Practitioners Cooperative in Rotterdam, The Netherlands. Asymptomatic children (n = 405) and children with RTI symptoms (n = 321) aged 3 mo to 16 y were enrolled in a cross-sectional study from July 1, 2008, to November 30, 2011. Clinical data, pharyngeal and nasopharyngeal specimens, and serum samples were collected. The primary objective was to differentiate between colonization and symptomatic infection with M. pneumoniae by current diagnostic methods, especially real-time PCR. M. pneumoniae DNA was detected in 21.2% (95% CI 17.2%-25.2%) of the asymptomatic children and in 16.2% (95% CI 12.2%-20.2%) of the symptomatic children (p = 0.11). Neither serology nor quantitative PCR nor culture differentiated asymptomatic carriage from infection. A total of 202 children were tested for the presence of other bacterial and viral pathogens. Two or more pathogens were found in 56% (63/112) of the asymptomatic children and in 55.5% (50/90) of the symptomatic children. Finally, longitudinal sampling showed persistence of M. pneumoniae in the URT for up to 4 mo. Fifteen of the 21 asymptomatic children with M. pneumoniae and 19 of the 22 symptomatic children with M. pneumoniae in this longitudinal follow-up tested negative after 1 mo.

Conclusions: Although our study has limitations, such as a single study site and limited sample size, our data indicate that the presence of M. pneumoniae in the URT is common in asymptomatic children. The current diagnostic tests for M. pneumoniae are unable to differentiate between asymptomatic carriage and symptomatic infection.

Figure 1. Enrollment flow diagram.

ED, emergency department; GPC, General Practitioners Cooperative.

Figure 2. Monthly enrollments during the course of the study.

The enrollments for the symptomatic group are represented above by a red dotted line. The enrollments for the asymptomatic group are represented below by a blue dotted line. The solid lines represent the absolute number of M. pneumoniae–positive participants. Enrollment for the asymptomatic group started in January 2009.

Figure 3. M. pneumoniae DNA loads.

(A) Pharyngeal bacterial loads (genomic copy number per milliliter on the y-axis) of M. pneumoniae PCR-positive participants in the asymptomatic group (open squares) and the symptomatic group (filled triangles). (B) Nasopharyngeal bacterial loads (genomic copy number per milliliter on the y-axis) of M. pneumoniae PCR-positive participants in the asymptomatic group and the symptomatic group. The bacterial load distribution was compared using the Mann-Whitney U test. (C) Comparison of the bacterial loads in pharyngeal samples and nasopharyngeal samples for the participants who tested positive for M. pneumoniae in both. Correlation was calculated using the Spearman rank test. (D and E) Distribution of bacterial loads for upper RTIs (URTI) and lower RTIs (LRTI) in the pharyngeal and nasopharyngeal samples. The line in each graph represents the median.

Figure 4. M. pneumoniae DNA loads in the longitudinal study.

This figure shows the bacterial DNA loads in the study participants of the asymptomatic group (A) (open squares) and the symptomatic group (B) (filled triangles) during the follow-up study. Each point represents one visit of one participant and is connected by a line to the point representing the next visit. On the y-axis the bacterial DNA load (genomic copy number per milliliter) is shown. On the x-axis the consecutive visits are represented.

Figure 5. Anti–M. pneumoniae serum antibody levels.

(A and B) Serum IgM (A) and IgG (B) antibody levels (in units/milliliter) are compared between the asymptomatic group and the symptomatic group (using the Mann-Whitney U test). (C and D) IgM (C) and IgG (D) antibody levels are plotted against the bacterial DNA load (genomic copy number per milliliter) in all samples. Open squares indicate asymptomatic participants. Filled triangles indicate symptomatic participants. The horizontal lines in A and B represent the median.

Figure 6. Number of detected viral and bacterial pathogens.

The dot plot shows the percentages of participants with 0, 1, 2, 3, or >3 pathogens present in the URT. On the x-axis the percentages are shown, on the y-axis the number of pathogens is shown. The filled and open triangles show respectively M. pneumoniae (Mpn) PCR-positive symptomatic children (n = 44) and M. pneumoniae PCR-negative symptomatic children (n = 46). The filled and open squares show respectively M. pneumoniae PCR-positive asymptomatic children (n = 57) and M. pneumoniae PCR-negative asymptomatic children (n = 52).