Water-borne transmission of chloramphenicol-resistant salmonella typhi in Mexico

1. Summary
2. Introduction
3. Methods
5. References

Unit of Epidemiology and Enteric Diseases Laboratory, institute of Health and Tropical Diseases, Secretaria de Salubridad y Asistencia, and Zacatecas State Public Health Service, Mexico

SUMMARY

In mid 1972 an outbreak of typhoid due to a chloramfenicol resistant strain of Salmonella typhi occurred in a small village in central Mexico. 83 cases were recorded, with 6 deaths. The highest attack-rates were for the age-groups 1-14 and 45 and above. Most patients lived in an area of the village with the highest population density and the lowest income leve1s, close to an irrigation canal which traverses the village. Just before the epidemic the municipal water system had not been operational because of a defective pump. Food-specific attack-rates implicated the drinking of water from the canal as the source of the disease, and restoration of the water system just before the start of the investigation promptly halted the outbreak.

INTRODUCTION

IN early 1972 an epidemic of typhoid fever due to a chloramphenicol-resistant strain of Salmonella typhi began in central Mexico. At first an area including Mexico City and the States of Hidalgo and Mexico was involved, but the epidemic quickly spread to the adjacent States of Puebla and Tlaxcala, and in sub sequent months thousands of cases were reported. Epidemiological studies thus far have failed to reveal the source of the epidemic.l-3

Beginning in May, 1972, outbreaks of typhoid due to the chloramphenicol-resistant strain were recorded in communities outside of the Valley of Mexico. We describe here one such out break which occurred in the community of Ciudad Cuauhtémoc in the State of Zacatecas. In this instance, the source of the outbreak was related to the ingestion of contaminated water from an irrigation canal.

Ciudad Cuauhtémoc is a rural community of 3973 inhabitants about 90 km. south of Zacatecas City. The village is on a high plain with a temperate and dry climate. Most of the inhabitants work on nearby communal farms where the principal crops are com and beans. The land is not very fertile, and the income of the villagers is low.

The village has a municipal water system to which almost all of the houses are connected. The source of water for this network is a deep well 1 km. east of the village. The water is neither filtered nor chlorinated. The village also has a sewage system to which les s than 15 % of the houses are connected. Most of the population use outhouses or latrines or, quite commonly, defecate on the open soil.

The town is divided by a river which runs from west to east. This river is dry for most of the year; however, at several points the municipal sewage system discharges untreated waste directly into the river bed, producing stagnant pools. The community is al so bisected by a cement-lined irrigation canal which parallels the course of the river, branching once. This canal usually contains water, the source of which is a nearby dam and reservoir.

There is a single health centre which has a 4-bed infirmary staffed by a doctor and a licensed nurse. There are neither other doctors nor hospital facilities. Patients requiring hospital admission must travel north to Zacatecas or south to Aguasealientes.

As elsewhere in Mexico, enteric disease is hyper endemic in Ciudad Cuauhtémoc. Although typhoid fever has been recorded, it is not common. During 1971 no cases were reported from the community and only 41 cases from the entire 5tate of Zacatecas (population 951,000). During the first 5 months of 1972, only 2 cases were reported in Ciudad Cuauhtémoc.

METHODS

All households in the community were contacted in a house-to-house survey. People with histories of recent illness were examined and questioned to determine if they had had typhoid fever. Criteria for the diagnosis of typhoid included sustained fever, gastrointestinal symptoms, prostration, hepatoesplenomegaly, and where possible isolation of S. typhi or significant rises in anti-O and anti-H titers.

A series of questionnaires were administered to people with typhoid and to healthy individuals selected at random from surrounding households. Samples of food and water were collected for bacteriological analysis and were 83 cases of typhoid fever were identified in the Ciudad Cuauhtémoc outbreak. From them a total of 19 strains of S. typhi were isolated, 13 from heces and 6 from blood. 17 of the 19 strains were resistant to chloramphenicol and 2 were sensitive, as shown by the disc sensitivity test. 3 of the resistant strains and the 2 sensitive ones were phage typed. One of the sensitive strains was identified as phage type E1 and the other was a W form. The three resistant strains were c1assified as Vi degraded approaching phage type A, the characteristic strain found in most of the outbreaks in Mexico during 1972.5-7

In addition, 56%, of the cases had Widal anti-O title of 1/160 or greater by the plate method. In the remaining cases in which there was neither bacteriological nor serological confirmation, the diagnosis was based on c1inical grounds only.

Most patients (68 %) were between the ages of 1 and 14 years, but al! age-groups were affected (table r). Similarly, attack-rates were greater for the 1-4 and 5-15 age-groups, although rates were also raised for the groups 45 years of age and above. The attack rate was 1'74 per 100 and the ratio of affected males to females was 1/1'4 (table r).

The attack-rate was appreciably greater for females than for males (2.3 per 100 inhabitants v. 1·5 per 100 inhabitants). The difference was largely produced by disparities in the attack-rates for the two sexes in the 5-14 and 45 + age-groups, especially that of the 5-14 group in which the rates were 4.2 per 100