Foodborne Outbreak Investigation

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1.Using the date of onset for each case from Table 1, construct a simple ‘cases against time’ chart of the outbreak. From THIS data suggest when common exposure to infection may have occurred and explain your reasoning.

Table 1. Cases against time.

Date| Number of cases by area.| Total number of cases|
| Town A| Town B| Village Y| Farm K| |
5th July| | | 1| | 1|
11th July| 2| | 2| | 4|
12th July| 3| | 1| 1| 5|
13th July| 5| | 3| | 8|
14th July| 1| 1| 1| | 3|
15th July| 1| | | | 1|
16th July| 1| | | | 1|
18th July| | | 2| | 1|
19th July| | | 1| | 1|

From the data summarised above, it is clear that statistically most reported cases were notified to the Proper Office between the 11th and 14th July, peaking with a modal value on the 13th July.

At the point in time when the above data was released there was no information available on the potential bacterium that could be leading to the outbreak, however there was information on symptoms, with nearly all patients reporting a sudden onset of diarrhoea and only half or less from vomiting and headaches. Therefore before using this data to suggest when a common exposure date may have occurred it is important to consider 2 points; firstly the incubation period (the time between consuming a contaminated product and the first signs of illness) and secondly the symptoms commonly seem as a result from the ingestion of food poisoning bacterium.

Using the information available and symptom charts from Sprenger, R. (2008: 24-25) it would be reasonable to rule out the infections being caused by bacterium such as Staphylococci, Bascillus or Campylobacter spp. due to the fact that diarrhoea is not a typically common symptom for these strains of bacteria, yet was experienced by 98% of the known suffers. On the same grounds the infection is also unlike to have been caused by bacterium of Shigella or Escherichia spp. as although diarrhoea is a common symptom in result of consumption of both these pathogens, it is normally found to contain or be blood stained, which has not in this case been reported by the involved general practitioners.

Causes of the outbreak in question are more likely to be due to clostridium or salmonella spp. of bacteria in which the reported symptoms are comparable to those that would be expected from foodborne infections of these types. Incubation periods for these bacteria vary with clostridium spp. having a typical onset period of 8 to 12 hours and salmonella spp. 1 to 3 days.

Based on the evidence on hand and the knowledge above it is reasonable to suggest that common exposure was likely to have been around the 8th – 13th July dependant upon the actual bacteria responsible, and the dosage levels that the patients were initially exposed too.

2.Study Tables 2a & 2b and Table 3 and determine which item of food was the probable vehicle of infection.

Table 2. AR % Derived from tables 2a & 2b.

Foods| Eaten| Not Eaten|
| Total No.| No. positive| AR%| Total No.| No. positive| AR%| Chicken legs| 22| 11| 50.0%| 19| 11| 57.9%|
Milk| 29| 21| 79.5%| 6| 2| 33.3%|
Sandwiches| 32| 19| 59.3%| 9| 6| 66.6%|
Crisps| 25| 15| 60.0%| 16| 10| 62.5%|

At the point of the above data being released it had been confirmed that Salmonella was present in faecal samples from all 25 persons who had had clinical illness. Sprenger, R. (2009: 48) describes Salmonella as gram negative, non-spore forming bacterium that are capable of surviving for weeks outside a living host and cannot be destroyed by freezing, they are however heat sensitive perishable after being heated to 60°C for 12 minutes.

18 of the 25 persons diagnosed attended a berry-picking outing, consuming the food indicated above. The outing held on the 9th – 10th September falls consistent with the common exposure dates proclaimed in the answer to the prior...
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