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Food Safety Focus (32nd Issue, March 2009) – Incident in Focus

Food Safety and People with Lower Immunity

Reported by Dr. Ken CHONG, Scientific Officer
Risk Assessment Section, Centre for Food Safety

On 11 February 2009, the Queen Mary Hospital reported several cases of intestinal mucormycosis caused by Rhizopus species under the order Mucorales in blood cancer patients. Mucormycosis is the collective name given to several different diseases caused by Mucorales. Investigation is still ongoing to find out whether there is a common source of infection.

Immunity is one's ability to defend against infection. The patients stated above are severely suppressed in their immunity and needed to be hospitalised. There are other population groups with lower immunity though not to the same degree as the above patients. In this article, we shall focus on the general food safety advice for those people with lower immunity. As for patients whose immune systems are severely suppressed, e.g. blood cancer patients undergoing immunosuppressive treatments, special advice should be sought from the attending doctor.

Infections in People with Lower Immunity

Some patients may have weakened immune system due to immunological disorders, such as HIV infection and some chronic diseases (e.g. cancer), or immunosuppressive therapy they receive that are used to decrease the body's immune responses, such as drugs given to prevent transplant rejection. These patients are at high risk of infections caused by microorganisms which will not lead to any or only minor adverse health effects in healthy persons. These microorganisms can be bacteria, fungi, viruses or parasites from either the environment or food, for example, Enterobacter bacteria and Aspergillus fungi. These pathogens may result in serious complication in patients with weakened immune system.

Elderly, pregnant women, young children and infants are also relatively susceptible populations in comparison to healthy adults. These people, if infected by some common foodborne pathogens, may develop more severe symptoms than just mild ones such as diarrhoea and vomiting.

The natural defences against or ability to withstand an infection decline gradually as people age. The immune system of some elderly may further be weakened if they suffer from certain chronic diseases, such as diabetes, as a result of the disease or its therapy.

Hormonal and immunological changes during pregnancy result in weakening of mother's immune system and hence she is more prone to foodborne disease. On the other hand, the developing foetus is susceptible to foodborne pathogens that may not cause symptoms in the mother. Listerosis is a well-known risk in this group of people.

The natural defences of infants and young children against foodborne pathogens are weaker, because the immune system is not well developed and the protection afforded by resident gut flora may not be as effective as that in adults. The stomach of newborns is less acidic than that of adults; bacteria and other pathogens survive relatively easier in newborns. In addition, infants and young children consume more food in proportion to their weight than adults. Hence, they can proportionately consume more toxins or contaminants if present in food.

Food Safety for People at Risk

Strict observance on food safety and hygiene is important to prevent exposure to pathogens that can cause disease in people at risk. The "5 Keys to Food Safety: Choose, Clean, Separate, Cook and Safe Temperature" are fundamental to safe preparation and handling of food. In addition, people with lower immunity should choose food including repackaged food carefully. Below are examples of food that they should avoid or consider:

Food type Food to Avoid Lower Risk Choice
Dairy Products
  • Cheeses made from unpasteurised milk
  • Pasteurised milk
  • Cheeses made from pasteurised milk
Meat and Poultry
  • Undercooked or raw meat and poultry
  • Cold meats
  • Thoroughly cooked meat and poultry
  • Cold meats reheated thoroughly
  • Undercooked or raw seafood e.g. sashimi
  • Refrigerated smoked seafood
  • Precooked seafood
  • Thoroughly cooked seafood
  • Smoked seafood and precooked seafood reheated thoroughly
  • Canned fish and seafood
Fruits and Vegetables
  • Unwashed fresh fruits and vegetables, including lettuce/salads
  • Thoroughly washed fresh vegetables and fruits
  • Cooked vegetables
  • Food that contain raw/undercooked eggs
  • Use pasteurised egg for recipes that call for raw or undercooked eggs
  • Fully cooked eggs

Source: USDA Fact Sheets for At-Risk Populations

People with lower immunity (and their caregivers) should follow specific food safety advice given by doctors or dietitians. In particular, patients who are severely suppressed in their immune system should follow the advice strictly.

Key Points to Note:

  1. People with HIV infection, some chronic diseases, immunosuppressive therapy have lower immunity.
  2. Immunity of elderly, pregnant women, young children and infants is also lower.
  3. People with lower immunity should strictly observe proper food safety and hygiene practices to avoid foodborne diseases.

Advice to People with Lower Immunity

Advice to Trade