Transmission of Streptococcus pneumoniae in rural Gambian villages: a longitudinal study.

BACKGROUND. To prepare for national introduction of a pneumococcal vaccine of restricted valency, we studied the pattern of nasopharyngeal carriage of Streptococcus pneumoniae and its transmission in Gambian villages over time. METHODS. We collected nasopharyngeal swab specimens every 2 weeks from 158 villagers in 19 households in 2 villages over 1 year. We studied the prevalence and duration of S. pneumoniae carriage, the effect of household size and composition on carriage, and sequence type-specific carriage within and between households. RESULTS. Ninety-seven percent of children and 85% of adults carried S. pneumoniae at some time. Fifty-three serotypes were represented among 1522 isolates. Carriage was more common among children than adults for all serotypes studied except 9V. There was an overall trend toward shorter carriage with increasing age (P = .043) and significant differences in carriage duration between serotypes. For most serotypes, the odds of being a carrier were greater if there were other carriers in the household. The prevalence of carriage varied by serotype. Most notably, serotype 5 carriage occurred in only 1 village and was transient. Multilocus sequence typing of serotype 6B isolates from 1 village revealed 8 different sequence types and strong evidence of nonrandom distribution among households (P < .001). Study by sequence type suggested household spread starting most commonly in children, followed by spread to adults. CONCLUSIONS. This longitudinal carriage study in Gambian villages provides unique information on the pattern of spread of S. pneumoniae in rural Africa and a baseline for evaluating the impact of the introduction of pneumococcal conjugate vaccine into the region.

Nasopharyngeal carriage of Streptococcus pneumoniae in Gambian infants: a longitudinal study.

BACKGROUND: To prepare for national introduction of a pneumococcal conjugate vaccine of restricted valency, we studied nasopharyngeal carriage of Streptococcus pneumoniae in Gambian infants. METHODS: We studied 236 infants in 21 villages. We collected nasopharyngeal swab samples at birth, twice per month for 6 months, and every second month until 1 year of age. We studied time to acquisition and duration of pneumococcal carriage according to serotype. RESULTS: All infants carried S. pneumoniae at some point. Sixty-five serotypes were found, and the 5 most common serotypes (6B, 19F, 6A, 14, and 23F) accounted for 51% of isolates. The mean age at first acquisition of carriage was 33 days (95% confidence interval, 29-36 days). There were no significant differences in acquisition rates between the 6 most common serotypes (P = .067) or between vaccine serotypes, vaccine-related serotypes, or nonvaccine serotypes (P = .317). However, the duration of carriage differed significantly between the 6 most common serotypes (P = .004). The rate of reacquisition of carriage and the duration of carriage did not differ significantly between the 6 most common serotypes (P = .229 and P = .669 respectively). However, nonvaccine types were acquired faster (P = .004) and were carried for a shorter duration (P < .001) than were vaccine serotypes. A previous episode of serotype 14 carriage was associated with delayed reacquisition of this serotype (P = .005) and longer duration of carriage (P = .017). CONCLUSIONS: The data provided in this study regarding time to acquisition and duration of pneumococcal carriage in Gambian infants provide an important baseline for evaluating the impact of the introduction of a pneumococcal conjugate vaccine in The Gambia and elsewhere in Africa.

Seasonality and outbreak of a predominant Streptococcus pneumoniae serotype 1 clone from The Gambia: expansion of ST217 hypervirulent clonal complex in West Africa.

BACKGROUND: Streptococcus pneumoniae serotype 1 causes > 20% of invasive disease, among all age groups combined, in The Gambia. In contrast, it is rarely detected in carriage studies. This study compares the molecular epidemiology of S. pneumoniae serotype 1 causing invasive disease in The Gambia between 1996 and 2005 to those carried in the nasopharynx between 2004 and 2006. RESULTS: A total of 127 invasive and 36 nasopharyngeal carriage serotype 1 isolates were recovered from individuals of all age groups and were analyzed by serotyping, antibiotic susceptibility testing and MLST. MLST analysis revealed 23 different sequence types (STs), 18 of which were novel. The most prevalent clone among the 163 isolates was ST618 (70.5%), followed by ST3575 (7.4%), ST2084 (2.5%) and ST612 (2.5%). A single ST (ST618), previously shown to belong to the ST217 hypervirulent clonal complex, was frequent among carriage (61.1%) and invasive (72.7%) serotype 1 isolates. ST618 causing both paediatric and adult disease peaked annually in the hot dry season and caused outbreak in 1997 and 2002. CONCLUSION: For over a decade, isolates of ST618 have been the dominant lineage among serotype 1 carriage and disease isolates circulating in the Gambia. This lineage shows similar epidemiological features to those of the meningococcus in the African meningitis belt being able to cause outbreaks of disease.

Nasopharyngeal carriage of Streptococcus pneumoniae in Gambian villagers.

BACKGROUND: To prepare for the introduction of a pneumococcal conjugate vaccine of restricted valency, we studied the nasopharyngeal carriage of Streptococcus pneumoniae in Gambian villagers. METHODS: A cross-sectional survey was conducted in 21 villages after a census. We recorded demographic characteristics, information on medical history, and data on possible risk factors for carriage from subjects. We collected a nasopharyngeal swab specimen from each subject for isolation and serotyping of S. pneumoniae and for antibiotic susceptibility testing. RESULTS: The prevalence of S. pneumoniae carriage among 2872 villagers was 72%. It was highest among infants (i.e., children aged <1 year; 97%); the rate was 93% among babies aged <1 month and decreased with increasing age (P<.001). Prevalence of carriage was linked to proximity to another village. Sixty-three percent of isolates recovered from children aged <5 years were covered by the 7-valent vaccine or were of a vaccine-related serotype, compared with 43% of isolates overall. Forty-three isolates (14.3%) tested were initially penicillin resistant; none had high-level resistance, and 4 had intermediate resistance. The rates of resistance to other antibiotics were as follows: trimethoprim-sulfamethoxazole, 39%; tetracycline, 32.3%; chloramphenicol, 6.3%; cefotaxime, 0.3%; and erythromycin, 0%. The rates were highest for isolates of vaccine serotypes. CONCLUSIONS: Pneumococcal carriage rates among Gambian villagers are very high. A pneumococcal conjugate vaccine of restricted valency should reduce the pool of antibiotic-resistant pneumococci. The large reservoir of pneumococci of nonvaccine serotypes will require close monitoring when the vaccine is introduced.

Elimination of Haemophilus influenzae type b (Hib) disease from The Gambia after the introduction of routine immunisation with a Hib conjugate vaccine: a prospective study.

BACKGROUND: Routine immunisation of infants in The Gambia with a Haemophilus influenzae type b (Hib) polysaccharide-tetanus toxoid conjugate vaccine began in May, 1997. We investigated the effectiveness of the vaccine when delivered through the expanded programme on immunisation and the effect of national immunisation on incidence of Hib disease. METHODS: Surveillance for Hib disease was maintained in the western half of The Gambia using standard methods with an emphasis on meningitis. We estimated vaccine efficacy using the case control method, and vaccine coverage and population denominators for incidence rates using a cluster sample survey. Prevalence of Hib carriage in a sample of 1-2-year old children attending health centres for vaccination was ascertained with oropharyngeal swabs plated onto antiserum agar. FINDINGS: Between May, 1997, and April, 2002, a total of 5984 children were examined for possible Hib infections. 49 children had Hib disease, 36 of whom had meningitis. The annual incidence rates of Hib meningitis before any use of the vaccine (1990-93) dropped from over 200 per 100,000 children aged younger than 1 year to none per 100,000 in 2002, and from 60 to no cases per 100,000 in children younger than 5 years. The prevalence of Hib carriage decreased from 12% to 0.25% (p<0.0001). Two doses of vaccine were needed for direct protection from Hib disease (vaccine efficacy 94%, 95% CI 62-99). Since most children received a protective dose after the age of greatest disease risk, indirect effects were important in reducing disease incidence. INTERPRETATION: The Gambian Hib immunisation programme reduced the occurrence of Hib disease despite irregular vaccine supply. The effect of the programme in The Gambia has important implications for the introduction of the vaccine into routine immunisation programmes of other developing countries.

Serotype and antimicrobial susceptibility patterns of isolates of Streptococcus pneumoniae causing invasive disease in The Gambia 1996-2003.

OBJECTIVES: To describe the characteristics of pneumococcal isolates obtained from patients with invasive pneumococcal disease in The Gambia. METHODS: Pneumococcal isolates were obtained from children aged < or =6 years with invasive pneumococcal disease during a Haemophilus influenzae vaccine effectiveness study (1997-2002) and from patients with invasive pneumococcal disease admitted to the MRC hospital, Fajara, for routine care (1996-2003). Isolates were identified, serotyped and tested for antibiotic susceptibility. RESULTS: Five hundred and thirty one pneumococcal isolates were obtained from 518 patients; 55 (10.6%) patients died; 415 isolates (79%) were from blood culture, 84 (16%) from CSF, and 42 (8%) from lung aspirates. Forty serogroups and serotypes were identified; six accounted for 64% and 16 for 86% of all episodes; 33.7% were of serotypes 1 and 5. 23.5% were of a 7-valent vaccine serotype, 57.1% were of a 9-valent vaccine serotype; 56% were of a 7-valent serogroup and 78% were of a 9-valent serogroup. There was a significant increase in the proportion of isolates of non-vaccine serogroup with increasing age (P < 0.0001). Antibiotic resistance had not significantly increased over time; but intermediate non-susceptibility to penicillin had risen and resistance to chloramphenicol had fallen in isolates of vaccine serotype compared with those of non-vaccine serotype. CONCLUSIONS: The majority of invasive pneumococcal disease in The Gambia is caused by pneumococci of relatively few serogroups. A conjugate vaccine would be expected to reduce the pneumococcal disease burden substantially and to have a beneficial effect on pneumococcal antibiotic resistance to penicillins.