—from Epidemiological Bulletin, Vol. 22 No. 3, September 2001—

Diphtheria Outbreak in Cali, Colombia, August-October 2000

Background
The incidence of diphtheria has declined in recent years in countries such as Cuba, Canada, or the United States that have a functional vaccination program where effective coverage with diphtheria toxoid and Diphtheria-Tetanus-Pertussis (DTP) vaccines have been achieved, especially in the infant population. In addition to the reduction in incidence, a change in the epidemiological profile of the disease was also registered. Indeed, its marked predilection for infants and young children has shifted in the last decade, to involve more young and older adults. On another hand, although mortality has increased during pandemics, the relation of this phenomenon to the causes of the pandemics is not known. Hypothesis on these changes include possible transformations in the microorganism or in the host population that diminish the protective antibodies as age increases.

Santiago de Cali, the capital of the “Valle del Cauca” province, is located at 995 meters above sea level. The average temperature is 25 degrees Celsius. It is the city that attracts the most people in Southwestern Colombia. It has a population of two million people distributed in 20 urban and 3 rural districts (“comunas” or communes), with marked differences in their socioeconomic levels and living conditions.

For health services delivery, Cali relies on a public network of institutions that are part of the local health systems (Silos), of which 6 are urban and one rural, and on “Empresas Promotoras de Salud”, or health promoting companies, with their own network of services (a).

The Municipal Public Health Service is the regulatory entity for surveillance and control in the city. It follows the guidelines and policies laid out by PAHO/WHO and the Ministry of Health with regard to the activities of the Expanded Program on Immunization and Surveillance of vaccine-preventable diseases that are object of eradication, elimination and control. There exists a network of Reporting Units formed by all institutional health service providers.

The last diphtheria case recorded in Cali itself was in 1988. However, an outbreak occurred at the level of the province, in the municipality of Buenaventura, in 1992. On that occasion, a total of 88 people with symptoms compatible with diphtheria were examined. The ages ranged between 1 and 47 years. Laboratory tests were conducted in only 51 of these patients. The bacteriological results were as follows: virulent Corynebacterium diphtheriae was identified in 28 (54.9%) of the patients, and non-virulent C. diphtheriae was identified in 4 (7.8%) of them. Of the total of cases, 4 were fatal.

Figure 1 shows the coverage of a selection of vaccines in children under 1 year in Cali between 1990 and 2000. It can be observed that an increase in the coverage had been achieved between 1990 and 1996, but starting in 1997, the coverage began to decline until 1999, when they reached around 60% in all the biologicals shown here. The reasons for this decline cannot be determined with certainty without a more in-depth study of the factors that had an influence on the vaccination coverage. However, a possible factor in this decline may have been the change in the health services delivery in general - and of immunization in particular - introduced after the application of a law called “Ley 100” in Colombia. The purpose of this law, which was enacted in 1993, is to transform the old National Health System into a mixed general social security system with two contribution regimens (contributory and subsidized). Before 1997, when the new National Health System was consolidated, the public network was responsible for children immunization, which guaranteed an effective coverage with massive strategies, channeling and extramural activities. Starting in 1997, other actors of the social security system, among them nonprofit or for-profit private or semipublic companies, entered the network of service providers. Regardless of whether or not the vaccination services were appropriate in the years preceding the law, it is certain that the resulting change in services - shown by the vaccination coverage - does not meet the vaccination needs. This can explain in part the observed decline in coverage.

On the other hand, the economic crisis that affected the national and local health sector worsened in recent years and affected the Expanded Program on Immunization. This impact is reflected in the lack of supplies, and also resulted in the decline of vaccination coverage at the end of the 1990s.

Materials and results of the investigation
In 2000, an outbreak of diphtheria occurred in Cali between epidemiological weeks 30 and 42, with 8 confirmed cases. The first reported case was a 3 year-old girl. It was the only fatal case. A bacteriological study of a total of 458 suspected cases was carried out. Operational definitions used during the outbreak investigation can be found in Box 1. These cultures were completed for symptomatic persons who consulted health services spontaneously and to direct and indirect contacts of suspected cases. In 1.8% (8) of these samples, C. diphteriae was isolated with positive toxigenicity test. In 32 cases (7%), non-toxigenic C. pseudodiphthericum was isolated. No bacteria of the Corynebacterium sp. was found in the remainder of the patients. All clinically-compatible cases were confirmed by laboratory with isolation of the bacteria. The population under 20 was the most affected by the outbreak (65%).

Of the 8 confirmed cases, 75% were younger than 10 years of age and 25% were 11 to 18 years old. In addition, 5 cases were found in men and 3 in women. In 2 cases (25%) non-virulent C. diphtheriae was found and as a result they were classified as healthy carriers.

The death of the 3 year-old occurred in a neighborhood of a district located in the eastern part of the city. A total of 26 related contacts were found, who shared the same overcrowded dwelling and lived in precarious hygienic conditions - factors that led to the outbreak.

In spite of the immediate interventions that were carried out as part of the field investigation, 4 more cases with epidemiological link to the fatal case (brothers) were presented in weeks 31 to 34. In week 41, 2 cases (2 brothers) were reported, without epidemiological link to the previous ones. Those were located in a commune of the northeast of the city. This implies that there were multiple sources of infection, indicating the possibility that additional cases were not detected. The last case of the outbreak appeared in week 42, in a 19 year-old adolescent residing in a commune of the southeastern area of the city. It did not have any epidemiological link with any of the previous cases (Figure 2).

The case-fatality rate of the outbreak was of 12.5% (1/8). The proportional distribution by age was as follows: 25% (2/8) in the 0-4 years group; 50% (4/8) in the 5-9 years group, 12.5% (1/8) in the 10-14 years group, and 12.5% (1/8) in the 15-19 age group (Table 1). The proportion of vaccinated among the cases was 12.5% (1/8), at the expense of a child vaccinated with three doses of DPT. Of the cases, 62% had incomplete series of vaccination and 75% (6/8) did not have access to social security.

The 8 cases belong to a socioeconomic stratum characterized by a high percentage of unmet basic needs, shown in overcrowded dwelling conditions, unhealthy conditions, inadequate excreta disposal and difficult access to drinking water and sewerage services. The clusters of cases were found in neighborhoods located in 3 urban communes of the city, all distant from one another.

Interventions
A plan with 7 strategic lines of action was designed: 1) Intensive vaccination activities in risk groups; 2) Monitoring and reporting of suspected cases; 3) Review and adaptation of the case management protocol; 4) Proper management of confirmed cases and contacts; 5) Timely laboratory diagnosis; 6) Mass communication campaign; 7) Interinstitutional and intersectoral coordination for case management and vaccination intensification.

At the beginning the outbreak, immediate hospitalization of all suspect cases that had had direct contact with the fatal case was put in place as a control measure, permitting effective control of the outbreak.

In the control plan, and in accordance with the patterns of the outbreak, it was determined that complete DPT vaccination series were to be carried out in all children under 5 showing incomplete vaccination and to the women of childbearing age from 10 to 49 years, target for the neonatal tetanus elimination plan. It was also decided to give priority to the vaccination with Td of schoolchildren (6 to 18 years), as this group was the most affected. A vaccination coverage of 80% was obtained in this group.

During the investigation, the design and implementation activities were carried out with participation of various institutions and sectors. Further, there was high community participation, and analysis was carried out on a constant basis, which allowed for periodic adjustments to the action plan. In spite of these positive elements, various difficulties arose, linked to the economic and political environments in the country. Firstly, adequate procurement of supplies (among them the diphtheric antitoxin) was complicated for various reasons, including importation processes. Further, a work-related crisis started in this period, due to lack of wage payments and processes of the administrative reform. This implied a decrease in human resources, especially on the operational side, as well as the creation of a situation of uncertainty and the appearance of a related lack of motivation. These circumstances had an impact on the completion of the control plan, which was continued into March 2001. At that time, it was evaluated and guidelines were produced for its continuity and the attainment of the proposed goals. Other difficulties included some delay in the accomplishment of the chronogram of the outbreak investigation activities and lack of clarity in the management of suspected cases with a Corynebacterium report.

Strict surveillance continued after the plan of operation concluded and no new case has been reported since October 2001, which suggests that the outbreak was successfully controlled.

Conclusions
Although one fourth of the cases occurred in people older than 10 years, the presence of cases in the group of 6-10 years suggests an accumulation of susceptible people that coincides with the low coverage found in recent years. A continued decline in the coverage could lead to the appearance of additional diphtheria outbreaks and to a change in the epidemiology of the disease, with a larger proportion of cases in unvaccinated children than in older adults. As a result, it is important to monitor and increase the vaccination coverage in the country.

The conditions of poor basic sanitation and overcrowding in which the persons affected by the outbreak lived led to the decision to hospitalize the symptomatic contacts. This was one of the most effective interventions in the control and management of the outbreak.

Note:
(a) The “Empresas Promotoras de Salud” are denominations created by law in the new Colombian Social Security System. They are companies that manage the ressources of their affiliates and beneficiaries.

Source: Prepared by Ms. Nancy Landazabal García and Ms. María Mercedes Burgos Rodríguez from the Cali Health Secretary and Dr. Desirée Pastor from the PAHO/WHO Representation in Colombia.

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Epidemiological Bulletin, Vol. 22 No. 3, September 2001