ARCHIVED - DIFFICULTIES ASSOCIATED WITH PERTUSSIS SURVEILLANCE

Introduction

The reporting of pertussis is mandatory in all the Canadian provinces and throughout the United States. Even though the incidence rate for pertussis is now barely 10% of the pre-vaccination rate, outbreaks of this disease continue to occur^(1-3). Between July 1998 and February 1999, the city of Montreal recorded an increase in the number of cases of pertussis. The annual incidence rate for the month of November reached 34/100,000, a six-fold increase from the previous year for the same month (non-published surveillance data, Montreal Department of Public Health [DPH]).

A university-affiliated pediatric hospital in Montreal has a computer system that registers emergency department use, ICD-9 coding of any diagnosis, and the automatic reporting to the DPH of cases whose code corresponds to a reportable disease. The emergency department of this hospital treats 75 000 patients per year.

According to a specific records agreement, the hospital uses the 033.1 code for both pertussis and pertussis-like syndrome. We therefore sought to determine to what extent the cases noted by the system corresponded to the definition of a clinical case, whether the case profiles corresponding to the definition differed from the profiles of the cases that did not correspond, and whether antibiotics had been prescribed for the index case and for any close contacts in accordance with the usual recommendations⁽⁴⁾.

Methods

All visitors to the emergency department between 1 September and 9 October, 1998, for whom the 033.1 code was recorded, with or without another code, were identified. If someone had visited the emergency department more than once during the period under study, only the initial visit was examined.

The files were reviewed by a member of the team by means of a standard questionnaire dealing with age, sex, symptoms, vaccine status, any diagnosis recorded, hospitalization, use of a culture medium or polymerase chain reaction (PCR) testing for confirmation of diagnosis, test results, and prescription of an antibiotic. The usual clinical case definition of pertussis was used: >= 2 weeks with paroxysmal cough, wheezing, vomiting or apnea in the absence of any other apparent cause⁽⁵⁾.

Results

The 033.1 code was found to have been used in 146 files. Of the 142 files that were available for examination, 139 (97.9%) contained sufficient information to warrant analysis.

The frequency of primary symptoms is presented in Table 1. Of the 139 cases, 72 (51.8%) met case definition requirements. Children aged >= 5 years and those for whom a confirmation test had not been carried out were more likely to have symptoms that met the case definition (Table 2).

Table 1. Frequency of symptoms

Table 2. Number and percentage of cases corresponding to the case definition according to certain variables

Of the 86 children undergoing culture, PCR testing or both, 22 (25.6%) had a positive result (Table 3). Among the 81 children undergoing culture, 14 (17.3%) had a positive result while 12 (26.1%) of the 46 undergoing PCR testing had a positive result. Since the results of culture medium and PCR testing combined were considered to be a diagnostic standard, the sensitivity, specificity and positive predictive value (PPV) of the case definition were 50.0%, 54.7% and 27.5% respectively. Among those cases that did not meet case definition requirements, 23.9% tested positive.

Table 3. Performance of clinical case definition according to culture and PCR test results: sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV)

Appropriate antibiotic therapy was prescribed in 58 cases (64.4%) out of the 90 for whom such therapy was indicated. The most frequently prescribed antibiotic was clarithromycin (60.5%) followed by erythromycin (20.9%).

Discussion

The computerized system installed in this hospital for reporting diseases should supplement reporting by emergency department doctors and therefore decrease any underreporting. However, such a system can present some difficulties.

Only slightly more than 50% of the cases initially fit the case definition of pertussis. Among those children whose symptoms did not fit the definition, more than 90% had been coughing for < 2 weeks. During a period when an upsurge of pertussis cases is being closely followed by the media, a number of parents will consult a physician at an early stage, thus contributing to an artificial increase in the number of reports. Even when it corresponded to the correct case definition, in barely half of the cases was the diagnosis of pertussis the only one noted in the file. In the remainder, the pertussis diagnosis appeared alongside other diagnoses, such as upper respiratory tract infection, bronchitis, asthma or, as annotated by the clinician, a pertussis-like syndrome.

Although the sensitivity of a culture medium can leave much to be desired^(6-8) and that of a PCR test has barely exceeded 60% in recent studies^(9-11), the rate of positive results of the diagnostic tests in our study (25.6%) appears to have been much lower than the expected rate. In our more northern latitudes, it is not infrequent to find upper respiratory tract infections mimicking pertussis. Also, other micro-organisms may have been circulating during the reporting period.

In our study, the case definition is not very effective. In the literature, the sensitivity of a case definition can vary from 43.6% to 96.0%, its specificity from 35.0% to 80.0%, and its PPV from 44.0% to 82.0%^(6,12,13) . The effectiveness of a case definition can vary according to the standard of comparison used (serology⁽¹²⁾ or culture^(6,12,13)), according to other types of micro-organisms⁽¹²⁾ that may be circulating in the community, and to the age and vaccine status of the patients^(14-16). The use of the new acellular vaccines against pertussis and the possibility of their use in adolescents and adults^(17,18) could decrease the number of cases of pertussis even further in the future, thus lowering the PPV of a cough of >= 2 weeks' duration.

Difficulties associated with the surveillance and control of pertussis have been previously noted: a lack of specificity in the case definition^(12,13), a failure on the part of doctors to diagnose pertussis even in the presence of classic symptoms^(19,20), a significant amount of underreporting^(19,21), an accompanying circulation of other organisms capable of causing a severe cough^(12,22,23), and poor culture sensitivity^(6-8) .

Conclusion

It is imperative that public health organizations examine their methods of ensuring the surveillance and control of pertussis. During a time of limited resources, how can public health organizations justify their involvement in every single reported case when a large number of them are not likely to be pertussis, while those cases that may be pertussis likely represent only the tip of the iceberg.

Acknowledgements

The authors would like to thank Dr. R. Allard, Ms. C. Duchesne and Ms. L. Marcotte of the Direction de Santé publique de Montréal-Centre, Montreal, Quebec for their contribution to the study.

References

1. Health Canada. Notifiable diseases summary. CCDR 1996;22(24):205-6.

2. De Serres G, Boulianne N, Douville Fradet M et al. Pertussis in Quebec: ongoing epidemic since the late 1980s. CCDR 1995;21:45-7.

3. Centers for Disease Control and Prevention. Pertussis - United States, January 1992-June 1995. MMWR 1995;44:525-9.

4. American Academy of Pediatrics. Pertussis. In: Peter G, ed. 1997 red book: report of the Committee on Infectious Diseases, 24th ed. Elk Grove Village, IL: American Academy of Pediatrics, 1997: 394-407.

5. Centers for Disease Control and Prevention. Case definitions for infectious conditions under public health surveillance. MMWR 1997;46(RR-10):25.

6. Strebel PM, Cochi SL, Farizo KM et al. Pertussis in Missouri: evaluation of nasopharyngeal culture, direct fluorescent antibody testing, and clinical case definition in the diagnosis of pertussis. Clin Infect Dis 1993;16(2):76-85.

7. Loeffelholz MJ, Thompson CJ, Long KS et al. Comparison of PCR, culture, and direct fluorescent-antibody testing for detection of Bordetella pertussis. J Clin Microbiol 1999;37(9):2872-76.

8. Halperin SA, Bortolussi R, Wort AJ. Evaluation of culture, immunofluorescence, and serology for the diagnosis of pertussis. J Clin Microbiol 1989;27:752-57.

9. Lind-Brandberger L, Welinder-Olsson C, Lagergard T et al. Evaluation of PCR diagnosis of Bordetella pertussis and Bordetella parapertussis infections. J Clin Microbiol 1998;36(3):679-83.

10. Heininger U, Schmidt-Schläpfer G, Cherry JD et al. Clinical validation of a polymerase chain reaction assay for the diagnosis of pertussis by comparison with serology, culture, and symptoms during a large pertussis vaccine efficacy trial. Pediatrics 2000;105(3):e31.

11. Mastrontonio P, Stefanelli P, Giulano M. Polymerase chain reaction for the detection of Bordetella pertussis in clinical nasopharyngeal aspirates. J Med Microbiol 1996;44(4):261-6.

12. Davis SF, Sutter RW, Strebel PM et al. Concurrent outbreaks of pertussis and Mycoplasma pneumoniae infection: clinical and epidemiological characteristics of illnesses manifested by cough. Clin Infect Dis 1995;20:621-28.

13. Patriarca PA, Biellik RJ, Sanden G et al. Sensitivity and specificity of clinical case definition for pertussis. Am J Public Health 1988;78:833-36.

14. Yaari E, Yafe-Zimerman Y, Schwartz SB et al. Clinical manifestations of Bordetella pertussis infection in immunized children and young adults. Chest 1999;115(5):1254-58.

15. Bortolussi R, Miller B, Ledwith M et al. Clinical course of pertussis in immunized children. Pediatr Infect Dis J 1995;14(10): 870-74.

16. He Q, Viljanen MK, Nikkari S et al. Outcomes of Bordetella pertussis in different age groups of an immunized population. J Infect Dis 1994;170(4):873-77.

17. National Advisory Committee on Immunization. Statement on pertussis vaccine. CCDR 1997;23(ACS-3)1: 1-16.

18. National Advisory Committee on Immunization. Statement on adult/adolescent formulation of combined acellular pertussis, tetanus, and diphtheria vaccine. CCDR 2000;26(ACS-1):1-8.

19. Deeks S, De Serres G, Boulianne N et al. Failure of physicians to consider the diagnosis of pertussis in children. Clin Infect Dis 1999;28:840-46.

20. Sotomayor J, Weiner LB, McMillan JA. Inaccurate diagnosis in infants with pertussis. An eight-year experience. Am J Dis Child 1985;139(7):724-27.

21. Sutter RW, Cochi SL. Pertussis hospitalizations and mortality in the United States, 1985-1988. Evaluation of the completeness of national reporting. JAMA 1992;267(3):386-91.

22. He Q, Viljanen MK, Arvilommi H et al. Whooping cough caused by Bordetella pertussis and Bordetella parapertussis in an immunized population. JAMA 1998;280(7):635-37.

23. Hagiwara K, Ouchi K, Tashiro N et al. An epidemic of a pertussis-like illness caused by Chlamydia pneumoniae. Pediatr Infect Dis J 1999;18(3):271-75.

Source: P Rivest, MD, MSc, Direction de Santé publique de Montréal-Centre, Montreal, Quebec; F Richer, MD, MSc, Direction de Santé publique de la Montérégie, Longueil, Quebec; L Bédard, MSc inf, MPH, Direction de Santé publique de Montréal-Centre, Montreal, Quebec.