Describe the food-borne diseases and the principles of food hygiene.

Food-borne Diseases and Food Hygiene

• The incubation period IS the most useful single discriminator:
• Under about 6 hours suggests a preformed toxin — staphylococcus aureus (1 to 6 hours, with prominent vomiting) and bacillus cereus emetic type (1 to 5 hours, classically from reheated rice)
• 6 to 24 hours suggests clostridium perfringens, classically from meat cooked in bulk and allowed to cool slowly
• Over 24 hours suggests an infection — salmonella, Shigella, Campylobacter, E. Coli, Vibrio and viruses
• Neurological symptoms suggest botulism — Clostridium botulinum toxin from improperly canned or preserved food, causing descending flaccid paralysis with diplopia, dysphagia and dysarthria, and NO fever; it is a medical emergency with respiratory failure
• The major agents — bacterial: Salmonella, Staphylococcus, Clostridium, Bacillus cereus, Shigella, Campylobacter, E. Coli including O157, Vibrio cholerae and parahaemolyticus, Listeria
• Viral: norovirus, hepatitis A and E, rotavirus
• Parasitic: Taenia, Trichinella, Ascaris, Entamoeba, Giardia, Echinococcus
• Non-infective: natural toxins (mushroom, fish), chemicals, adulterants and aflatoxin
• The factors that permit an outbreak, and these are what an investigation looks for — preparing food too far in advance; storing at room temperature; inadequate cooling and refrigeration; inadequate reheating; undercooking; cross-contamination between raw and cooked food; an infected food handler, particularly one with a septic skin lesion or who is a carrier; and contaminated raw ingredients or water
• The danger zone is 5 to 60°C, within which bacteria multiply rapidly; the rule is to keep hot food hot (above 60°C) and cold food cold (below 5°C)
• Investigation of a food-borne outbreak follows the standard epidemic sequence, with three points specific to food:
• Construct a food-specific attack rate table, comparing the attack rate among those who ate each item with those who did not; the implicated food shows a high rate in eaters, a low rate in non-eaters, and the largest difference between them
• Collect specimens from patients (stool, vomitus), from the food, and from food handlers, and examine the kitchen and its practices
• Use the incubation period distribution to narrow the likely agent before the laboratory reports
• The principles of food hygiene, expressed by WHO as the five keys to safer food, which should be listed:
• 1. Keep clean — hands, surfaces and equipment
• 2. Separate raw and cooked — including separate boards and knives
• 3. Cook thoroughly — especially meat, poultry and eggs
• 4. Keep food at safe temperatures
• 5. Use safe water and raw materials
• Food handlers require particular attention — medical examination and exclusion of carriers; exclusion from work while suffering from diarrhoea, vomiting, jaundice or a septic skin lesion; hand hygiene and covering of cuts; no smoking, spitting or handling money while serving food; and training
• Milk deserves separate mention — pasteurisation, by the holder method (63°C for 30 minutes) or HTST (72°C for 15 seconds), kills pathogens including Mycobacterium bovis, Brucella, Salmonella and Coxiella while preserving nutritive value and flavour
• Its adequacy is tested by the phosphatase test, since alkaline phosphatase is destroyed at just above the pasteurisation temperature
• The HACCP system (Hazard Analysis and Critical Control Points) is the modern approach for the food industry — identifying hazards, determining critical control points, setting limits, monitoring, taking corrective action, verifying and record-keeping
• Its principle is preventive rather than inspectional — controlling the process rather than testing the finished product, since end-product testing can never sample enough to give confidence
• Keep raw meat below cooked food in a refrigerator, so that drip cannot contaminate what will not be cooked again
• Thaw frozen poultry completely before cooking, since a partly frozen bird will not reach a safe temperature at the centre