Microbes and food 5. Food poisoners
5.4 Protozoa
Cryptosporidium parvum and Giardia lamblia are parasitic protozoa which live in the guts of animals. Contaminated faeces can get into water that is used by people for drinking or cooking. The protozoa produce cysts which are resistant to the chlorine used to kill other microbes in drinking water. They have to be removed by filtration. Only a few cysts are needed to cause infection.

The symptoms are acute pain and diarrhoea and the illness can be severe in the immuno-compromised. Occasionally these organisms get into domestic water supplies and cause big outbreaks of illness. Salad crops and raw vegetables can also become contaminated from infected irrigation water.

Photomicrograph of viruses
Picture 5.5a Individual virus particles. Each one is about 30 millionths of a millimetre across.
5.5 Viruses
Viruses are different from other microbes. They are very small and can only multiply inside the cells of animals or plants. This process harms the host, resulting in a viral disease.

Many enteric viruses can be transmitted in food as well as by direct contact. Well known examples are:

  • polio, which causes nerve damage and paralysis
  • hepatitis A and B which harm the liver.

In the developed world these conditions are prevented by vaccination and good hygiene, but they still pose severe threats to health in poorer nations.

Gastroenteritis caused by enteroviruses (such as Small Round Structured Viruses, SRSVs) is probably much commoner than records show. Nevertheless, outbreaks involving thousands of people have been reported. The symptoms are diarrhoea and vomiting, which normally last for 12–48 hours.

Shellfish are prime sources of enteroviruses, as they are often bred in water contaminated with sewage and might be eaten raw. Other foods are contaminated in the kitchen or factory by people suffering from viral gastroenteritis themselves. It is difficult to monitor viral food poisoning as there are no simple tests for viruses.