Infectious diseasesepidemiology and surveillance

Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures

Victorian statutory requirement

Shigellosis (Group B disease) must be notified in writing within five days of diagnosis.

School exclusion: exclude until after diarrhoea has ceased.

Infectious agent

The genus Shigella consists of four species:

• Group A: Sh. dysenteriae
• Group B: Sh. flexneri
• Group C: Sh. boydii
• Group D: Sh. sonnei

Groups A, B and C are further divided into approximately 40 serotypes, designated by numbers.

Identification

Clinical features
Shigellosis is characterised by an acute onset of diarrhoea, fever, nausea, vomiting and abdominal cramps. Typically the stools contain blood, mucus and pus, although some persons will present with watery diarrhoea. Complications include toxic megacolon and reactive arthritis. Rarely haemolytic uraemic syndrome can occur. The infectious dose required to produce disease is low and may be as few as ten organisms.

Illness is usually self-limited and lasts from several days to weeks with an average of four to seven days. The severity of infection depends on host factors such as age and nutritional status and the serotype. Infections with Sh. sonnei usually result in a short clinical course and low case fatality rate. In contrast, Sh. dysenteriae is often associated with serious disease and a high case fatality rate.

Asymptomatic infections occur and carriage may persist for months.

Method of diagnosis
Diagnosis is made by isolation of Shigella spp. from a clinical specimen.

Incubation period

The incubation period depends on the serotype. It varies from twelve hours to seven days but is usually one to three days.

Public health significance & occurrence

Shigella infection occurs worldwide however the incidence of specific serotypes varies by country. Sh. sonnei is the most common type reported in Victoria and Australia. Sh. dysenteriae and Sh. flexneri are usually acquired overseas and are often resistant to multiple antibiotics. In Victoria outbreaks have occurred in child care centres and amongst men who have sex with men.

Two-thirds of the cases and most of the deaths worldwide are in children less than ten years. The disease is rare in infants under six months of age, particularly those who are breastfed.

Secondary attack rates in households may be as high as 40%.

Reservoir

Humans.

Mode of transmission

Faecal-oral transmission is the most important mode of transmission of Shigella however infection may be spread via contaminated food, water, milk or by flies.

Period of communicability

Shigella is communicable during the acute phase and while the infectious agent is present in faeces which is usually no longer than four weeks. Asymptomatic carriage and excretion may persist for months.

Susceptibility & resistance

Everyone is susceptible to infection, with infection following ingestion of a small number of organisms. In endemic areas the disease is usually more severe in young children. The risk of infection is increased in men who have sex with men, those with immune deficiency disorders, by attendance at child care or contact with a child in child care, and in international travellers who do not take adequate food and water safety precautions.

Control measures

Preventive measures
Good personal hygiene is the single most important preventive measure. Frequent and thorough hand washing is important before eating and food handling and after toilet use, especially in young children who may not be completely toilet trained.

Educate travellers on the need for safe food and water consumption.

Control of case
Treatment is usually supportive for mild illnesses. Antibiotics may shorten the duration and severity of illness however their use should be based on the serotype, severity of illness and host characteristics, for example if they are a child in child care, food handler or suffer chronic illness. Multi-drug resistance is common, particularly for overseas-acquired strains. The choice of antibiotic should be based on the antibiogram of the serotype. Anti-motility drugs are thought to increase the risk of prolonged carriage.

Cases should be educated on the importance of personal hygiene, particularly after using the toilet and before and after food handling.

Food handlers should be excluded from work until two negative stools have been obtained, or until at least 48 hours after the diarrhoea has ceased and rigid personal hygiene measures can be assured.

Cases in institutions should be separated from non-infected residents if possible.

Control of contacts
The diagnosis should be considered in symptomatic contacts however stool cultures may be confined to food handlers and those in situations where the spread of infection is particularly likely (child care centres, hospitals, institutions).

Symptomatic contacts of shigellosis patients should be excluded from food handling and the care of children or patients until investigated.

Control of environment
Remove contaminated food and/or water sources. Strict attention should be paid to environmental hygiene in child care centres, institutions and food premises.

Outbreak measures

Two or more related cases should be considered indicative of an outbreak and require investigation. These cases should be reported immediately to the Department of Health. Attempt to determine a common source of infection and identify those at risk of infection.

Refer to the guidelines for the investigation of gastrointestinal illness for further advice and management of outbreaks.