Epidemic salmonellosis from unmonitored water trucks

From July 1993 through 1994, surveillance of salmonellosis in a Saudi Arabian city revealed a steady increase in cases reported. The epidemiologic pattern of these cases was unusual in three respects: 80% were single cases in families without known contact with another case, there was no apparent time and space relationship between cases, and 85% were among children under 6 years.

During 16 Jamada-Awal to 10 Rajab 1415H (Oct. 21 to Dec. 12, 1994), we conducted a case-control study of 52 cases and 104 age-matched controls visiting the same medical facilities for other reasons during the same week as the case. A case was defined as positive culture for Salmonella in a person with gastroenteritis.

Salmonellosis was associated with drinking water trucked to the house in 37 case-patients (71%) and 17 controls (16%) (odds ratio [OR]=30, 95% confidence interval [CI] 6.6-225), whereas obtaining water in person from government supplies was protective (OR= 0.2, 95% CI 0.03-0.5). Among houses using trucked water, obtaining drinking water from independent trucks coming from unknown sources had a high risk of salmonellosis (OR=13, 95% CI 5.1-33) compared with getting water from trucks coming directly from government supplies (OR=0.2, 95% CI 0.04-0.7) or from any of four government-monitored private companies. The median price of water purchased by case-households was 7 riyals per cubic meter, compared with 9 riyals per cubic meter for controls (p<0.001).

Consumption of raw or undercooked eggs was related to salmonellosis among children under 2 years (OR=5.0, 95% CI 1.4-20). In the same age group, breast-feeding (OR=0.81, 95% CI 0.24-2.6) and bottle-feeding (OR=1.6 95% CI 0.6-4.2) were not related to salmonellosis.

All household contacts were negative for Salmonella. Chlorination bacteriology and chemistry of water in government supplies and four private companies had been checked daily; however, no testing of residual chlorine had been done on trucked water.

Waterborne salmonellosis is not commonly reported. A large infective dose (105-106 of Salmonella is required to cause symptomatic disease in healthy adults. Salmonella in water tend to be more dilute and unlike in food do not multiply extensively. Waterborne salmonellosis is more difficult to identify than foodborne since cases tend to be scattered in time and place. Moreover, water exposure is common to all persons, making it difficult to single out a specific water exposure during interviewing.

In this investigation, the wide distribution of salmonellosis in the affected city and the epidemiologic association with a special water source (independent trucks) make it plausible to implicate a vehicle such as water. The majority of bacteriologically confirmed cases of salmonellosis were among children with no known contact with similar cases. In infants and young children, hypochlorohydria, rapid gastric emptying (ingested fluids or water) and abnormalities in normal flora increase the susceptibility to lower inocula of Salmonella. Children under 2 years are more likely to be fed with infant formula or powdered milk. Salmonella may multiply in these after they are reconstituted with water; they are buffering agents that may lower the infective dose of Salmonella.

Chlorination of water remains the most effective and least expensive measure to control waterborne disease. Epidemiologic data indicated that chlorination of water from government sources and private desalination stations protected against salmonellosis. In contrast, water purchased from independent water trucks that obtained water from other, unidentified sources, was found to be associated with salmonellosis. The price of this water was below the cost of water adequately treated and chlorinated.

In Saudi Arabia, health authorities in municipalities must identify all sources of water and maintain vigilance over the traffic of water trucks and the chlorination of water in these trucks.

Despite the relatively less important role of eggs in this investigation, the public should be aware that eggs can be infected transovarianally. In the USA about 1 in 10,000 fresh eggs is infected in this way. Eggs may also be contaminated through cracks in their shells. The public should be aware that soft-cooked or raw eggs carry a risk of infection with Salmonella. In addition, they should avoid buying cracked eggs. Eggs should not be stored for a long period at ambient temperatures at the market place or at home.