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Shigella

What are Shigella and shigellosis?

Shigella bacteria cause illness in people and in primates such as monkeys and chimpanzees. Overseas the bacteria are frequently found in water contaminated by human sewage. While person to person spread is the most common route of infection, people who eat food or drink water contaminated by Shigella can become ill.

There are four species (serogroups) of Shigella - Shigella sonnei, S. bodyii, S. flexneri and S. dysenteriae. They all cause the illness known as shigellosis (also known as bacillary dysentery). Shigellosis is a notifiable disease to the Medical Officer of Health. The local Public Health Unit will usually investigate and take appropriate action to prevent the disease from spreading.

The annual incidence in New Zealand is approximately 4 cases per 100,000 people. There were 183 cases notified in 2005, with 50% of victims having recently returned from overseas where they may have contracted the illness.

Approximately 14% of people with the illness need hospital treatment, and occasionally the disease is fatal. Antibiotics treatment may be required for more severe cases.

What are the symptoms of shigellosis infection?

The symptoms of shigellosis are diarrhoea, fever and abdominal cramps. These appear within 12 hours to four days after consuming contaminated food or water. Diarrhoea may contain blood or mucus. Within one-three days the symptoms progress to include intense cramps as well as frequent and painful bowel movements. Illness can last for three to 14 days.

How do the bacteria spread?

The bacteria are transferred between people via the faecal-oral route and on to food in contaminated water or from soiled hands. As few as 10 cells may cause infection.

Shigella multiplies best on foods at room temperature (including sliced fruit) but can also last for several days at refrigeration and sub-zero temperatures. It is among the most acid-resistant of foodborne pathogens and has been shown to persist in fruit juices for up to a week, and for months on certain butter, cheese, herbs and salads.

Who is most at risk?

Outbreaks are common in conditions of overcrowding or poor sanitation. People at most risk of serious complications are young children (especially day care centres), frail elderly in nursing homes, pregnant women or people with low immunity. Complications can result in blood poisoning and kidney failure (haemolytic uraemic syndrome).

What foods have been implicated in outbreaks of shigellosis?

Most meals implicated in causing shigellosis comprise cooked food that is served cold and has been contaminated by the person handling the food. Bacteria may also be carried by flies that have been in contact with sewage.

In New Zealand, foodborne outbreaks (many people affected) have been reported in various locations, including a childcare centre, a church gathering and a marina that served contaminated oysters. The five outbreaks identified in 2005 were all considered to be person-to-person spread. Overseas, outbreaks have been associated with salads (pasta, potato, shrimp, tuna, chicken, turkey, macaroni, fruit, lettuce), hard to wash herbs such as parsley, dips, chopped turkey, deli meats, pudding, fresh produce such as strawberries or spinach, raw oysters and unpasteurised milk.

Will cooking destroy the bacteria?

Yes. Like many other harmful bacteria that could be in our food, Shigella is destroyed when food is cooked to a safe internal temperature (Shigella is readily killed if the core temperature of food reaches 65^oC). Use a food thermometer to measure the internal temperature of your food.

For foods that are served cold, or raw, essential safety measures include proper personal hygiene, safe food handling, good handwashing and preparation practices (see below).

Can I reduce my exposure to Shigella?

YES! Handwashing is one of the best ways to prevent the spread of foodborne illness, including Shigella. Before and after handling food, wash your hands for at least 20 seconds with soap and warm water and dry them for a further 20 seconds. (20 seconds is about as long as it takes to sing “Happy Birthday” twice through). Wash your hands again when you switch from one food to another.

Follow the 4C’s rule:

Clean:

• don’t prepare food if you’re showing symptoms of vomiting or diarrhoea and for at least three days afterwards. Wipe all surfaces including handles with bleach-based household cleaner (follow manufacturer’s instructions)

• make sure your own hygiene is meticulous if you are caring for someone who is sick

• irrigate gardens of fresh produce eg, vegetables, fruits and herbs, with clean water

• wash all fruits and vegetables thoroughly in drinkable water

• keep your kitchen hygienic and surfaces clean

• use separate utensils and chopping boards for meat and ready-to-eat foods

Cook:

• make sure your food is cooked thoroughly, especially if someone in your household is sick

• use a food thermometer to check the core temperature is high enough to kill Shigella, (65^oC)

• reheat cooked food until it is steaming hot throughout

Cover:

• keep ready-to-eat foods wrapped and in your fridge away from raw foods, meat drips and flies

Chill:

• make sure your fridge is operating at 2-4^oC. Bacteria can grow in the danger zone between 4^oC and 60^oC

• freezing food will only halt the growth of bacteria but not destroy them, so proper hygiene when preparing and handling food for freezing and when thawing food is important to prevent contamination.