14.11.1 Risk factors (including foods commonly involved)
Individual susceptibility to Shigella infection varies according to age and the presence of predisposing conditions. Infection is rare, however, in neonates and very young children owing to lack of receptor sites on colon epithelial cells. Immunocompromised persons are of particular susceptibility and there is a greater likelihood of bacteraemia, with consequent high mortality.
Shigella is not an environmental organism and the overwhelming source of contamination of foods is humans. The organism is readily destroyed by processing and foods involved are invariably consumed without cooking, or processing, after contamination. Any food may, in principle, be contaminated and a wide range has been implicated as vehicles of shigellosis (Table 14.3).
In practice, foods that receive significant handling during preparation are of greatest risk where contamination is direct from handlers. However, the bacterium can survive for extended periods on foods and contamination of salads, etc. can occur before harvest from soil, water, etc., containing infected faecal material. In either case, risk is greatest where hygiene standards are poor and the incidence of Shigella infection is high among the general population. In developed countries, shigellosis can be associated with localised failure of hygiene, leading to sporadic outbreaks in schools, military establishments, summer camps, religious communities, etc. A very large outbreak in the USA involved more than 1300 culture-confirmed cases of Sh. sonnei infection during 1986-87, most of the affected being tradition-observant Jews. In developed countries, shigellosis is also
Table 14.3 Foods implicated as vehicles for Shigella infection
Tuna salads Lettuce
Shrimp cocktail Potato salad
Soft cheese Cooked rice a disease of poverty and in the USA and Australia, the urban poor, migrant workers and native peoples are at a continuing high risk (Varnam & Evans, 1996).
It is not general practice to examine foods for Shigella in non-outbreak situations. Examination for the organism on a regular basis is unlikely to be of any benefit in safety assurance and is a diversion of resources from management of critical control points.
Isolation is difficult owing to relatively poor growth on commonly used media, especially when other members of the Enterobacteriaceae are present. A number of media have, however, been re-formulated to improve recovery of shigellas, although use is primarily with clinical samples. Direct plating of food is unlikely to be successful, but little attention has been given to enrichment media. Selen-ite broth and Gram-negative (GN) broth have been used, in combination with selective plating. Salmonella-shigella agar, deoxycholate-citrate agar, xylose lysine-deoxycholate agar, various MacConkey agars and eosin-methylene blue agar have all been used. Increased interest in foodborne shigellosis in the 1980s led to attempts to develop new media. The most promising approach has been supplementation of various media with novobiocin. Despite this, the scope for further improvement of traditional, cultural methods appears limited.
Both serological and genetic approaches have been used in attempts to develop alternative methods for detection of Shigella in foods. The major concern has been detection of shigellas in stool samples and, while possibly useful, none is fully satisfactory with foods. Some effort has been made with the fluorescent antibody technique, but non-specific reactions limit use. A more promising method, the Bactigen® slide agglutination test, separately detects Salmonella and Shigella, but has not been fully validated for foods. A further approach is use of enzyme-linked immunosorbent assay (ELISA) to detect the virulence marker antigen of virulent shigellas and enteroinvasive E. coli, or Shiga toxin itself, but use in foods has been limited. DNA hybridisation methods using invasion-essential gene segments as probes are successful with virulent strains, but problems occur due to loss of the plasmid and selective deletion of invasion associated genes during cultivation and storage of isolates. A preferred approach is detection of the ipaH gene sequence which is present on both plasmid and chromosome (Venkatesan et al., 1989). Invasive strains of Shigella may also be detected in foods using PCR-based assays (Lampel et al., 1990; Vantarakis et al., 2000). None is available commercially.
The epidemiology of foodborne Shigella infections is such that control procedures must be directed against contamination from human sources. Shigellosis in communities is difficult to control, owing to a high incidence of person-to-person infection, multiple exposure to the bacterium and significant secondary spread.
Recognition that a problem exists is often considered the first step in control (ICMSF, 1996), accompanied by hygiene education and measures such as supervised hand washing by children.
With respect to shigellosis associated with contamination of particular foods, the Critical Control Points are prevention of contamination of salads and other crops eaten without cooking and control of contamination during handling, especially after heating or other lethal processing has been applied. In contrast to previous opinion that shigellas are delicate and have only limited survival capability in foods, the organism can survive refrigerated and frozen storage on a variety of foods. Illness may therefore occur at a time remote from the point of contamination. At the level of primary agriculture, risk has been associated with use of nightsoil as manure, or irrigation with polluted water. Leafy salad vegetables, such as lettuce, are most commonly involved and washing is unreliable even where extensive use is made of disinfectants.
Contamination during handling almost invariably involves direct human contact, although there has been one, apparently authentic, case of Sh. flexneri infection caused by a monkey touching a child's ice cream (Rothwell, 1981). Control involves minimising hands-on procedures, exclusion of potential excre-tors of Shigella from handling food and ensuring good standards of personal hygiene. Several outbreaks of shigellosis attributable to contamination by food handlers have been described by Smith (1987). These included an outbreak of Sh. boydii infection, which affected 176 persons following a pasta meal. The situation can be complicated by atypical symptoms making shigellosis difficult to recognise. The greatest risk is from convalescent excretors and return to work must be carefully monitored, three consecutive negative stool samples usually being required. Healthy excretors are considered to be of limited importance.
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