Exclusion of the first sampling campaign.
Altogether, five sampling campaigns were performed at 3-month intervals, starting in October 2008 (Fig. ). Results from the first campaign (October and November 2008) yielded an N. meningitidis carriage prevalence of only 0.87% in the 1- to 29-year-old Burkinabes (0.75%, 0.51%, and 1.35% in the districts of Bogodogo, Dandé, and Kaya, respectively). Evaluation of the initial study concluded that the unexpectedly low carriage prevalence was due to poor selectivity of the commercial modified Thayer-Martin agar plates purchased for that campaign, leading to contamination and difficulties in selecting colonies from the plates. Improvements of the study standard operating procedures and quality control procedures were undertaken, and local production of selective agar plates was successfully established in Ouagadougou. The first campaign was therefore considered a pilot study.
Samples and overall carriage prevalence.
The remaining four sampling campaigns were conducted in 2009 and were designated S1 through S4. Data collection for each round occurred in January-February, April-May, July-August, and October-November for the respective four campaigns. They yielded a total of 20,326 oropharyngeal samples (range, 5,024 to 5,121 samples per campaign). After isolation and primary identification in Burkina Faso, 1,049 isolates were sent to NIPH for confirmation and molecular characterization (range, 191 to 357 isolates per campaign). Of these, 811 (77.3%) isolates were confirmed to be N. meningitidis; 809 were registered in all the data sets and could be included in the data analysis, giving an overall meningococcal carriage rate of 3.98% (Table ).
Carriage rate and serogroup prevalence of N. meningitidis in Burkina Faso at four sampling times in 2009
Regional and seasonal variations in carriage prevalence.
The N. meningitidis carriage prevalence rates were 4.10%, 5.27%, 3.37%, and 3.17% at the time points of S1, S2, S3, and S4, respectively (Fig. ). Overall carriage prevalence was higher during the dry season (S1 and S2) than during the rest of the year (S3 and S4) (P < 0.001), although the difference was not significant in the Bogodogo District. Seasonal variation was also statistically significant between S1 and S2 (P < 0.01), S1 and S4 (P < 0.05), S2 and S3 (P < 0.001), and S2 and S4 (P < 0.001) but not between S1 and S3 or between S3 and S4.
Carriage prevalence of N. meningitidis in three districts in Burkina Faso at four sampling times in 2009.
We found a significant difference in carriage rates when the rate for the urban district (average, 1.82%) was compared to the rates for the two rural districts (average, 5.07%) (P < 0.001). The results also showed a geographic variation among the two rural districts, with the carriage prevalence in Kaya (average, 6.23%; range, 4.71 to 8.77%) being significantly higher than that in Dandé (average, 3.91%; range, 3.06 to 4.94%) (P < 0.001) (Fig. ). This district ranking was the same for all the campaigns.
Determination of serogroup.
Of the 809 isolates, 574 were assigned to a serogroup on the basis of the result from slide agglutination, while 235 isolates were nonserogroupable and further analyzed by capsule gene PCR. After PCR, 102 isolates remained nonserogroupable, while 16 isolates were NmA, 2 isolates NmC, 18 isolates NmX, 60 isolates NmY, and 37 isolates NmW135. Thus, over half of the isolates that were nonserogroupable by serum agglutination harbored a capsule gene that was not expressed phenotypically. In addition, some isolates for which agglutination did not match the other molecular characteristics were checked by capsule gene PCR. The serogroup of 14 isolates was then changed according to the PCR result.
NmA carriage prevalence.
NmA carriage prevalence rates at the time points of S1, S2, S3, and S4 were 0.42%, 0.62%, 0.24%, and 0.29%, respectively (Fig. ), corresponding to an average NmA carriage prevalence of 0.39%. Seasonal variation was statistically significant (P < 0.01) when the rates during the dry season (S1 and S2) and the rest of the year (S3 and S4) were compared. The overall NmA carriage prevalence rates in each of the districts were 0.06%, 0.21%, and 0.94%, for Bogodogo, Dandé, and Kaya, respectively. NmA carriage prevalence follows the same pattern of variation described for the overall carriage prevalence, with a higher level in the rural districts than in the urban district (P < 0.001) and with significant differences between the rural districts (P < 0.05). Interestingly, the NmA carriage prevalence in S4 was the same as that in the pilot study, executed almost exactly 1 year earlier (0.29% in both).
Carriage prevalence of serogroup A N. meningitidis in three districts in Burkina Faso at four sampling times in 2009.
Prevalence of other serogroups.
The carriage prevalence of the different serogroups, presented by district and campaign, showed a predominance of NmY in all three districts at most time points (Fig. ). The increase in NmX prevalence seen between S1 and S2 (Table ) was significant (P < 0.01) and was essentially due to an increase in the district of Kaya. In all the districts, carriage prevalence of NmA, NmY, and NmX increased from S1 to S2 (significantly for NmY [P < 0.05] and NmX [P < 0.01]) and decreased from S2 to S3 (significantly for NmA [P < 0.01] and NmY [P < 0.01]), while Nm W135 and nonserogroupable strains had changes in the opposite direction (significant decrease of NmW135 between S1 and S2 [P < 0.01]).
Serogroup distribution of meningococcal isolates in three districts in Burkina Faso at four sampling times in 2009.
Overall, nonserogroupable isolates were significantly more dominant in the urban district of Bogodogo (33% of the isolates) than in the rural districts of Dandé and Kaya (11% and 9%, respectively) (P < 0.001) (Fig. ). Kaya had the highest proportion of serogroup A isolates, as they represented 15% of all isolates, while the proportions of NmA in Dandé and Bogodogo were lower and comparable (5% and 3%, respectively). Although NmY dominated in all three districts, it represented 76% of the isolates in Dandé but only 51% in Kaya and 34% in Bogodogo. The proportions of NmX isolates were comparable in Bogodogo and Kaya (15% and 17%, respectively), but NmX was almost nonexistent in Dandé (<1%, corresponding to a single carrier).
Overall serogroup distribution of meningococcal isolates in three districts in Burkina Faso in 2009.
The 809 isolates confirmed to be N. meningitidis
were assigned to 29 different STs, of which 15 STs belonged to 5 different clonal complexes. A total of 51 different porA
combinations were identified, and the isolates were assigned to 65 different combinations of serogroup, ST, and porA
. During the pilot and the carriage studies, we found 17 new STs, 5 new PorA variants, and 1 new FetA variant; all have been submitted to the MLST database (http://pubmlst.org/neisseria/
All 80 NmA isolates were ST-2859 (ST-5 complex), and all had the porA-fetA combination P1.20,9/F3-1 (Table ). This single clone of serogroup A has been present for the whole period from October 2008 to November 2009, as all 14 NmA strains retrieved from the pilot study also had this characteristic. In addition, two isolates with ST-2859, P1.20,9/F3-1, were nongroupable both by serogrouping and by PCR.
Characteristics of N. meningitidis retrieved from the carriers in Burkina Faso according to serogroup
The four serogroup C isolates originated from the districts of Bogodogo and Dandé at the time point of S2, belonged to the ST-41/44 clonal complex, and contained the FetA variant F5-2 (Table ). Three of these isolates were ST-206, and one was a new ST (ST-7929), a single-locus variant of ST-206. During the pilot study, a single NmC isolate was found and attributed to another new ST (ST-7376) of the ST-41/44 clonal complex, also a single-locus variant of ST-206.
NmW135 (n = 70) was represented in all three districts by the ST-175 complex, almost exclusively by ST-2881 (n = 69), but also by a single isolate of a new ST, ST-7928, which is a single-locus variant (adk) of ST-2881. The porA-fetA combination P1.5-1,2-36/F5-1 was found for 97% of the NmW135 isolates.
The serogroup X isolates (n = 90) belonged to STs not assigned to any clonal complex: ST-181, ST-751, and ST-5789. ST-5789 is a single-locus variant (adk) of ST-181, while ST-751 is a double-locus variant from ST-181 (gdh and phdC). Carriage of NmX was mainly observed in the districts of Kaya (n = 70) and Bogodogo (n = 19). NmX ST-181 (n = 67) was present in both Kaya and Bogodogo, while ST-5789 was present only in Bogodogo (n = 12). ST-751 was present only in Kaya (n = 10) and Dandé (n = 1). Of the NmX isolates, 96% had the PorA variant P1.5-1,10-1, of which 63% harbored the F1-31 FetA variant and 27% harbored F4-23 (Table ).
The NmY (n = 457) isolates belonged to the ST-23 complex (n = 403) and the ST-167 complex (n = 54). The ST-23 complex was mainly represented by ST-4375 (n = 396), the dominant ST in our study, but 3 new STs were identified, all of them single-locus variants of ST-4375. Among the isolates assigned to ST-4375, eight different porA-fetA combinations were identified, but 97% of the ST-4375 isolates had the porA-fetA combination P1.5-1,2-2/F5-8, making this clone the dominant strain circulating in Burkina Faso during the study period (47% of all isolates). The ST-167 complex was more diverse, represented by four STs and 10 porA-fetA combinations. However, 96% had PorA variant P1.5-1,10-8 and 77% had FetA variant F1-3.
Age and gender distribution.
Among the 20,326 participants, 43.7% were males and 56.3% were females. Overall carriage prevalence in males was higher than that in females for NmA (P < 0.05), NmY (P < 0.01), and nonserogroupable meningococci (P < 0.05). The only STs with significant differences in prevalence between genders were ST-2859 (P < 0.05) and ST-4375; both were more common among males (P < 0.01). Carriage of N. meningitidis was the highest among the 10- to 14-year-olds when both male and female participants were considered. Among male participants, the 15- to 19-year-olds had the highest carriage prevalence (Fig. ). For the female participants, the carriage prevalence was the highest among the 10- to 14-year-olds. The difference in age distribution between genders was mostly due to serogroup Y, since this serogroup dominated, but it was also substantial for serogroup A and nonserogroupable meningococci.
Carriage prevalence of N. meningitidis, NmA, and NmY by age group and gender.
The highest prevalence of NmA was found among 5- to 9-year-olds (overall, 0.58%; male, 0.66%; female, 0.51%). Among the 10- to 14-year-olds, males had an even higher prevalence (0.80%), while the prevalence among females in the same age group dropped (0.15%). The oldest recorded person carrying NmA was 26 years of age, although a 29-year-old NmA carrier was identified during the pilot campaign.
Except for NmC, all the detected serogroups were represented in all age groups, but the variation by age group was high for NmY (range, 1.22 to 2.95%) compared to that for the other serogroups (Fig. ). The increase in prevalence of nonserogroupable isolates with age, peaking in the 15- to 19-year-olds, was observed for both male and female participants.
Carriage prevalence of each meningococcal serogroup by age group.
By investigating the carriage rates in relation to the age, in years, of the participants, we found a local peak of NmA and NmY prevalence among 5-year-olds (0.70% and 3.33%, respectively), as well as a local peak of NmA and NmX prevalence among 9-year-olds (1.22% and 0.82%, respectively).