Diphtheria Outbreak -- Saraburi Province, Thailand, 1994

Following the introduction of diphtheria toxoid in Thailand in 1978 and the acceleration of vaccination efforts by the Expanded Program on Immunization (EPI), the incidence of diphtheria in Thailand decreased substantially. A total of 1021 diphtheria cases were reported nationwide in 1984, compared with 25 cases in 1993. However, on July 19, 1994, a cluster of cases of diphtheria was reported in Saraburi province (1994 population: 565,067) to the Division of Epidemiology (DOE), Thailand Ministry of Public Health; no cases had been reported during the previous year in Saraburi. This report summarizes the outbreak investigation by the DOE, which identified 18 cases during April-August 1994 and identified carriage rates of Corynebacterium diphtheriae of 4% and 8% among household contacts and school contacts, respectively.

To determine the extent of the outbreak in Saraburi province, trainees in the Thai Field Epidemiology Training Program conducted chart reviews of patients admitted to the Saraburi provincial hospital during January-August 1994. Persons with laboratory-confirmed or clinically diagnosed diphtheria were included in the case counts. In addition, active surveillance for diphtheria cases was conducted among household contacts and among students attending the same school as one person with confirmed diphtheria with onset during August 9-10.

Based on chart reviews, a total of 18 cases of diphtheria were identified from Prabhuddabath District (the locus of the outbreak) during April-August. Of these, three (17%) cases were fatal; nine (50%) cases occurred among males, and the median age of persons with diphtheria was 6.5 years (range: 2 years-37 years). Three (17%) cases occurred among children aged less than 5 years, and 12 (67%) among children aged 5-14 years. Manifestations in all cases included fever, sore throat, tonsillitis, and pseudomembranes. A history of completing the recommended series of three or more doses of diphtheria and tetanus toxoids and pertussis vaccine (DTP) was reported for six cases. The probable index patient had onset of symptoms on April 12, was a member of the Hmong hilltribe, and had migrated from Northern Thailand to Saraburi Province within the previous month; however, most (16 {89%}) cases occurred among ethnic Thais.

To further characterize transmission, public health officials examined and collected throat swabs from 23 household and 74 school contacts. Although no additional symptomatic cases of diphtheria were detected, carriage rates of Corynebacterium diphtheriae were 4% (one of 23) among household contacts and 8% (six of 74) among school contacts.

In response to the outbreak, DOE and the Saraburi provincial health office initiated outbreak control among household contacts and school contacts by providing treatment with antibiotics and vaccination. Active case-finding was followed by active surveillance for 6 months. In addition, laboratory surveillance has been established in the Saraburi provincial hospital.

Reported by: P Prempree, MD, S Chitpitaklert, MD, N Silarug, MD, Field Epidemiology Training Program, Div of Epidemiology, Ministry of Public Health, Bangkok, Thailand. National Immunization Program, CDC.

Editorial Note

Editorial Note: Based on the recent resurgence of diphtheria in the New Independent States of the former Soviet Union (1,2) and outbreaks in other countries (Ecuador {3}, Algeria, and elsewhere {4}), the epidemiology of diphtheria in the vaccine era has changed -- adolescents and adults now are most likely to be affected during outbreaks (5). Following the introduction and widespread use of diphtheria toxoid-containing vaccines in many developing countries during the late 1970s and early 1980s, the incidence of diphtheria and the circulation of toxigenic diphtheria decreased rapidly (6-9). However, concomitantly, the number of susceptible persons began to increase because of incomplete vaccination coverage, suboptimal vaccine efficacy, and waning immunity among vaccinated persons in the absence of periodic booster doses and natural infection. The accumulation of susceptible persons over time may be sufficient to sustain epidemic diphtheria transmission.

In Thailand, the introduction and widespread use of DTP resulted in a greater than 97% decrease in the incidence of diphtheria during 1984-1993. However, despite high vaccination coverage among infants (90%), the outbreak in this report probably resulted from the introduction of C. diphtheriae by a Hmong migrant. The age distribution of outbreak-associated cases suggests that many preschool-aged and school-aged children in Saraburi Province, and possibly in other areas, had remained susceptible to diphtheria. This partially susceptible cohort may not have been vaccinated because of suboptimal coverage during the early years of the EPI program and may not have been exposed to natural C. diphtheriae infection because of rapid decreases in the circulation of toxigenic strains following the implementation of infant vaccination programs (10).

To reduce the risk for diphtheria epidemics, policy makers and public health officials should evaluate the feasibility of bringing unvaccinated preschool- and school-aged children up-to-date with diphtheria and tetanus toxoids after age 1 year. In addition, a policy of regular diphtheria-tetanus toxoid booster vaccination should be considered to ensure protection of adolescents and adults against diphtheria and tetanus and to enhance population immunity against diphtheria.

References

1. CDC. Diphtheria epidemic -- New Independent States of the former Soviet Union, 1990-1994. MMWR 1995;44:177-81.

2. CDC. Diphtheria outbreak -- Russian Federation, 1990-1993. MMWR 1993;42:840-1,847.

3. Pan American Health Organization. Diphtheria epidemic in Ecuador. EPI Newsletter 1994;16:5-6,8.

4. Galazka AM, Robertson SE, Oblapenko GP. Resurgence of diphtheria. Eur J Epidemiol 1995; 11:95-105.

5. Galazka AM, Robertson SE. Diphtheria: changing patterns in the developing world and the industrialized world. Eur J Epidemiol 1995;11:107-17.

6. Expanded Programme on Immunization. Information system: summary for the WHO Western Pacific Region. Geneva, Switzerland: World Health Organization, 1994; publication no. WHO/CEIS/94.2 WP.

7. Expanded Programme on Immunization. Information system: summary for the WHO South East Asian Region. Geneva, Switzerland: World Health Organization, 1994; publication no. WHO/CEIS/94.2 SEA.

8. Expanded Programme on Immunization. Information system: summary for the WHO African Region. Geneva, Switzerland: World Health Organization, 1994; publication no. WHO/CEIS/94.2 AF.

9. Expanded Programme on Immunization. Information system: summary for the WHO American Region. Geneva, Switzerland: World Health Organization, 1994; publication no. WHO/CEIS/94.2 AM.

10. Pappenheimer AM. Diphtheria: studies on the biology of an infectious disease. In: The Harvey lectures, series 76. New York, New York: Academic Press, 1982:45.

    
    

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