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