Changing carriage rate of Neisseria meningitidis among university students during the first week of term: cross sectional study

Introduction

During the 1990s there have been major increases in the incidence of invasive meningococcal disease in many developed countries, 1^–3 with serogroup C disease most noticeable, especially among teenagers and young adults. It has also been shown that university undergraduates have higher rates of invasive meningococcal disease than young adults of the same age who are not attending university.4 The provision of places in catered halls seems to be an important factor in differences in rates of invasive meningococcal disease between universities.4 In the United Kingdom, several clusters of invasive meningococcal disease have been reported; a large outbreak occurred in November 1996 at the University of Wales in Cardiff5 and another in October 1997 at the University of Southampton.6

No studies have been published on the epidemiology of meningococcal carriage or acquisition among university students in situations where there are no outbreaks.7 We therefore performed a longitudinal study in first year university students to determine rates of carriage and acquisition of Neisseria meningitidis, together with risk factors for both.

Participants and methods

Follow up

All participants in catered halls were selected for a further pharyngeal swab in either the first week of November 1997 or the first week of December 1997 on the basis of odd or even study numbers. Pharyngeal swabs were taken from students in the only self catered hall in the study in December. At the time of reswabbing the questionnaire was repeated. We therefore had paired data available for these students for October and either November or December.

One case of serogroup C disease occurred in a catered hall in late October, and one third of the students in this hall were therefore given ciprofloxacin to eradicate meningococcal carriage. Although students in this hall were reswabbed according to the prearranged schedule, we excluded them from the main analysis. Two other cases of invasive meningococcal disease (different serogroup B infections) occurred in the study population but these were in the spring term.

Table 1.

Carriage rate of Neisseria meningitidis during first week of term, 1997

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Schedule for pharyngeal swabbing, October to December 1997

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Results

Overall, 2507 first year students attended the university's health centre in the first week of the first term, of whom 2453 (97.8%) agreed to participate. A rapid increase in carriage of N meningitidis occurred during the first week (table 1). Date of swabbing, type of hall, active and passive smoking, and intimate kissing were all independent risk factors for meningococcal carriage during the first week (table 2).

In November, 714 of the 939 eligible students (76.0%) were reinvestigated for meningococcal carriage and social behaviour. We could not process 172 swabs owing to a problem with an incubator, leaving 542 students (57.7%). In December, 653 of 933 students (70.0%) in catered halls who had participated in the first round were reinvestigated along with 149 of 358 (42%) students in the self catered hall. The figure shows the schedule of swabbing from October to December.

The carriage rate of N meningitidis increased during the first term (table 3). Six weeks after widespread treatment with ciprofloxacin the carriage rate in the excluded hall was 40 of 142 (28.2%, 95% confidence interval 20.8 to 35.6). The carriage rate of group C meningococci among students resident in catered halls was 0.5% (0.2 to 1.0) in October, 1.9% (0.9 to 3.9) in November, and 3.1% (2.0 to 4.4) in December.

At some point during the first term, 349 students with initially negative pharyngeal swabs in October acquired meningococci. table 4 shows the independently significant factors associated with meningococcal acquisition during this period. Overall, 300 of the 325 index strains isolated in October (92%) and 333 of the 349 strains acquired by students during the autumn term (95%) were fully typed. table 5 shows the full typing of the commonest isolates in the index round and the acquired strains. Although non-C strains predominated on October 1997 (mainly B and non-groupable meningococci), C:2a:P1.5 (P1.2) was the commonest strain acquired (21 of 333) during the first term (χ² 10.8, P=0.001.)

Discussion

Our results show that meningococcal carriage increases rapidly among university students in the first month of the academic year and that much of this increase probably occurs during the first week. Rapid acquisition rates have previously been found among military recruits; however, these studies were generally smaller and fundamental differences in sleeping arrangements existed compared with students.9^–11

Several explanations for the rapid increase we observed can probably be discounted. The first was an improvement in swabbing techniques over the first week of the study. Although we were unable to identify the person who took each swab, most were taken by one person (KRN) with considerable experience.12 KRN also supervised the technique of the other operators. Furthermore, on each day during the first week, different operators assisted with swabbing in the morning and afternoon sessions yet there were no differences between morning and afternoon carriage rates on any day. We therefore believe that reliability was high between operators taking the swabs. The alternative explanation is that students who were more likely to be carrying meningococci on arrival at university were recruited later in the week. This seems unlikely as over 99% of students attended at their allotted time, and it seems unlikely that any systematic bias would have been introduced by choice of degree course. Furthermore, students are not allocated to halls of residence by course or faculty groups. The association of carriage with markers of social mixing also supports a causal link with acquisition after arrival at university.

Our main finding was a rapid increase in meningococcal carriage from 8% to 23% during the first week. Although the initial carriage rate was surprisingly low (8%), this finding has now been replicated by a subsequent study performed in October 1999 with a different population of students, who had pharyngeal swabs taken both on arrival and one week later (data available on request). Therefore, we do not believe the initially low carriage rate to be artefactual and speculate that clearance of meningococci occurs during the summer holiday between leaving school and starting university. This may arise from dispersal of the sixth form group, resulting in lower rates of recolonisation.

We also noted that during the first week carriage was higher in catered halls. This agrees with a previous study, which identified an increased risk of invasive meningococcal disease at universities offering comparatively more accommodation in catered halls. We speculate that this may be due to fundamental differences in the physical structure and pattern of social interaction between catered and self catered halls at Nottingham University.