Health

Epidemiology assignment: food borne diseases

Introduction

Infectious Diseases have been the cause of millions of deaths in the past; however, because of the invention of vaccines and antibiotics these diseases have by far reduced the number of deaths. The examples include sexually transmitted diseases, pneumonia and flu. Non-infectious diseases are not contagious, although some can be inherited from parent to offspring. An outbreak is a sudden eruption of disease that afflicts a group of individuals at a specific time and place. Foodborne disease outbreak is an event where a group of people are affected with a similar illness or disease due to the consumption of contaminated food (3). It is a serious issue where there are high numbers of people who are affected for such contaminated food consumption and it is necessary to analyse the issue so that the issue can be mitigated further.

Food borne diseases affect millions of people each year. From a mild reaction such as diarrhoea, vomiting, abdominal cramps to severe problems such as haematuria, long lasting health problems and possible death (9). The most common bacteria that are responsible for foodborne illnesses are: Staphylococcus aureus, Salmonella, Norovirus, Shigella Campylobacter, and Clostridium perfringens. These organisms can be found in different habitats such as in water, soil, on the skin and inside the nostrils of humans as well as on raw meat or poultry (10). It is a serious disease for which people are suffering a lot from severe pain or other chronic symptoms in the human body. It is hereby, necessary to conduct in depth research so that it is possible to identify the contaminated food for which the people suffer a lot.

Foodborne pathogens propagate due to various factors and these factors consist of lack of hygiene, undercooked/raw food, and unpasteurised fruits juices. Foodborne pathogens cause illness by contaminating food without changing its smell, appearance or taste. Thus, it is difficult to determine whether the food is safe to consume or not (4). People are likely to be susceptible to foodborne diseases including pregnant women, young children, older adults, and people with weak immune systems due to various medical conditions or old age (5).

In the following experiment, a foodborne outbreak had occurred in which university students and their grandparents got ill. It was a warm summer morning, and they were heading to Taronga zoo for a day’s outing. For lunch they took cut sandwiches and cartons of rice pudding prepared at the university canteen. Before arriving at the zoo, many had started eating their sandwiches. Members of the group started getting sick before arriving at the zoo. As they arrived at the zoo several people were sick by then and they were admitted to the local hospitals. A case study was conducted thereafter which included taking remnants of food and stool specimens from those taken to the hospital. There were two reasons why this outbreak occurred. The first that the cook handled the food without wearing gloves with infected hands and the second that the refrigerator motor failed so that conditions were warm inside the cabinet conditions were ideal for multiplication- warm summer weather and lack of refrigeration.

In order to find out the causative organism, biochemical tests such as oxidase, catalase, gram stain and latex coagulase tests were taken. The samples of chicken, ham, rice pudding with custard and roast lamb were plated on agar plates. Food specimens were plated on Baird Parker agar plates, faecal specimens were plated on Mannitol Salt agar and MacConkey agar plates. Swabs were also taken from the pus of cook’s thumb, the nose of the cook and his assistant and plated on MSA and HORSE BLOOD AGAR PLATES.

Aim

The aim of the study is to determine the cause of the organism that is responsible for contaminate food which has resulted illness as well as the study helps to discover how these organisms got in contact with the food, spread and multiplied.

Methods As illustrated in Epidemiology and public health microbiology lab manual

Results

The questionnaires were distributed among the people who were involved in the outbreak for such contaminate food and there were 221 respondents who returned the questionnaires with proper feedback and information. The recorded data was obtained from groups that were separated according to the type of sample given to them.

The odds ratio, of all the food types: Chicken, rice pudding with custard, ham and beef are shown respectively. As can be seen in the table above, ham appears to have the highest odds ratio compared to the rest of the food types. As per the above table, it has been seen that, there is high presence of Ham which raises organism among the people for consuming high quantity of ham. apart from that, the ratio of Beef is also high as compared to Chicken and Rice pudding and custard, where people consume high quality of Beef and they were suffering from serious illness due to contaminate food. Chicken and rice pudding and custard have low odds ratio and it indicates that these foods have low effects of such contaminate disease and illness among the people.

The table above shows the food-specific attack rates of all the food types, from the persons eating food and not eating food and the ill percentages are given for each food type. From the people that ate food, the highest ill percent rate was for ham (59.2%) and from the persons not eating ham had the least ill percentage (25%). It further indicates that, high consumption of Ham raises the chances of getting ill and it also implies that the chance of getting ill is low for the people who do not consume ham. On the other hand, the person, who eat chicken, have 51.6% chance of getting ill for high consumption of chicken and there is 70% of chance of attack rate who are not consuming chicken. Additionally, the person who consume rice pudding with custard and roast beef, they have attack rate approximately 52.4% and 56.1% respectively. Hereby, it can be said that, all the above mentioned food has impacts on the health of the people and there is high attack rate of consuming the contaminate food.

The food samples that are shaded grey in table 3; which indicates that, Chicken, beef, and rice pudding with custard had no growth of colonies on the BPA plate, therefore no further tests were conducted. However, ham grew in BPA plate and its colony morphology was 3 millimetres with black colonies, a zone of cloudiness and halo of clearing respectively. The colony-forming unit (CFU) for ham was 2.5 x 107 (CFU/g) for the first group that conducted the experiment and 4.6 x 107 (CFU/g) for the second group. The cell morphology of ham was gram positive cocci (GPC) in clusters. Additionally, biochemical tests were done to further assist in the identification of the pathogen such as the, catalase test which had a positive result. There were some suspicious colonies, it was latex coagulase and Dnase positive. The analytical profile index (API) id is 6736153.

Chicken, beef and rice pudding with custard had no growth. On the other hand, Ham had growth on both Mannitol agar plate (MSA) and MacConkey (MAC) agar plate. In MSA, the colony morphology for ham was less than 1 millimetre, circular and yellow, which means it is a mannitol fermenter. The cellular morphology was GPC in clusters. The catalase test was positive. There were suspicious colonies. However, in MAC, ham had 2 different organisms grown on it, therefore it had 2 different results. The first column included 1 mm circular pink colonies, with GPC in chains and negative for catalase test and there were no suspicious colonies. The second had 2-3-millimetre circular pink colonies, with gram negative rod (GNR) singular, and a catalase test negative and with no suspicious colonies.

Table 5 represents the results for cook’s nose on HBA and MSA. The results for all were the same on HBA. Although on MSA they were different. For the cook’s nose on MSA the colony morphology was less than 1 millimetre, circular, with yellow colonies (+ve mannitol). The cellular morphology included GPC in clusters, with a positive catalase test, latex coagulase test as well as DNase test. API STAPH ID is 6736153. S.aureus was sensitive to all antibiotics.

Cook’s assistant nose: On the MSA THE COLONY MORPHOLOGY WAS LESS THAN 1 millimetre, circular pink colonies and (-ve mannitol). Cellular morphology was GPC in clusters. And a positive catalase test, with no suspicious colonies and With a negative latex coagulase test and DNase test. API staph is 6706113

Pus from cook’s thumb: On the MSA, the colony morphology was less than 1 millimetre, circular yellow colonies (+ mannitol). The cellular morphology was GPC in clusters. With a catalase positive. With suspicious colonies, a positive coagulase test and DNase test and the Bacteria was sensitive to all antibiotics. API id is 6736153 (same as cook’s nose)

In table 6, all the antibiotics, that were used in this experiment, represented. S.aureus is sensitive to all the antibiotics. However, as shown in the results Vancomycin appears to have a diameter below 15 millimetres which shows S.aureus is not sensitive to it, however it is but this was an incorrect result due to vancomycin being a large disc that diffused into the agar plate very slowly.

The questionnaires

The biochemical tests are beneficial for the presence of certain enzymes that also determines which nutrition and which isolate can or can’t use and determines oxygen requirements. The number of pensioners which were ill was 200 and 3 of them died. Their percentage morbidity was 90.6% and mortality was 1.3%, with a percentage case fatality rate of 1.5%. The incubation period was 3.04 hours as shown in figure 1.

The cooks family got sick the latest because they received the food the latest

Discussion

The discussion section is developed to reach decision regarding the actual reason behind occurrence of the food poisoning among the individuals by analysing the results and information developed. The typical method for tracing bacteria used in this experimental study was done to identify if the foods were contaminated by S.aureus. The Strain typing is used to detect single strains of bacteria to show substantially that the isolates are indistinguishable and therefore may be related in the outbreak because S.aureus is found very commonly in the environment. (8) The other identification methods of S.aureus include in this study was Serological detection, typing of enterotoxins, phage-typed, DNA(RFLP AND PFGE) and multi-locus sequencing typing (MLST). The purpose of the DST (drug sensitivity test antibiogram) in this outbreak was an attempt to show that the isolates are indistinguishable.

Baird parker (BP) agar and MSA are used for selection of S.aureus among the other bacteria present in the sample. The Baird Parker agar plate is a selective and differential agar medium used for isolating and enumerating S.aureus in food samples. This medium contains tellurite and lithium which are selective for inhibiting most other organisms. It also contains glycine and pyruvate which complement the growth of S.aureus (Staphylococcus aureus). The egg yolk in the medium is responsible for the yellow colour of the medium. Moreover, it is responsible for the detection of lecithinase production and proteolytic activity which surrounds colonies as clear zones. S.aureus can be differentiated on a BP agar plate from other bacteria because it will form black colonies with clear zones due to tellurite reduction (7)

Mannitol salt agar (MSA) which was used in the specimens collected from the infected individuals is also a selective and differential medium and has 7.5% NaCl which is considered very high salinity for most bacteria and inhibits most organisms including Escherichia coli (E.coli). However, the Genus Staphylococcus is able to resist the salinity and can grow on MSA because it contains Phenol red dye which differentiates between pathogenic and non-pathogenic staphs. Although only S.aureus is able to grow and ferment on mannitol salt agar plate the PH indicator was changed from red to yellow because it is pathogenic in nature. However, Staphylococcus epidermis (S.epidermis) can also grow but cannot ferment on mannitol and the PH indicator red because S. epidermis is usually non-pathogenic in nature and is part of normal human flora.

Table 1

In table 1 the odds ratio, in the context of all foods is the measurement of the likeliness of an outbreak to occur from different types of mediums (food type’s e.g. Chicken, Ham, beef and rice pudding). Based on these values we can determine the significance and relevance to the clinical case study (food outbreaks). The Odd values were calculated for the types of foods in the table above and this clinical case informs that Ham had the highest value which is 4.46 which shows a significant value greater than 1.0. The value greater than 1.0 odd ratio informs that the exposure might have been the key risk factor for the development of the disease (13). Thus, Ham was contaminated by the cook and it was the vehicle of spread for pathogen as the odd ratio of all the food types showed that the ratio is greater than 1.0 in Ham and in no other food types. Thus, it can be decided that the bacteria spread among the individuals through the eating of the Ham.

Table 2

Table 2 shows the food-specific attack rates of all the food types from the persons that ate the food while going to the zoo. In the results, it can be seen that ham had the highest ill percentage (59.2%) because it was contaminated with bacteria which produced toxins. Thus, Ham acted as the key risk factor for development of food poisoning can be decided from Table 2 as the percentage difference of food-specific attacks were also found to be more with people who ate Ham in comparison to the ones who did not eat Ham. However, the other people who have not eaten the food may have got sick due to the unhealthy environment where most of the individuals were vomiting. Moreover, they could have also got sick through the transmission of the bacteria by coughing or sneezing or any other physical contact with the people who were infected by the bacteria while eating the food.

In figure 1 the major illnesses occurred was between 0.5-6 hours. S.aureus is a gram-positive bacterium and is carried by 30% of the participants either in their nose or skin (6). It is informed that toxins of S.aureus at 1-6 hours incubation period are called enterotoxins. Since the results in figure 1 informed that most of the individuals consumed the food within 6 hours of their preparation thus it can be identified that the food poisoning was spread among the people due to the release of the toxin by the bacteria rather than itself. The other factor which may have contributed to the relapse of the disease was breakage of the refrigerator which results the food to be stored in hot and humid condition resulting the bacteria to have effective environmental condition to grow and release toxin to infect the food.

Table 3

In table 3, chicken, beef, and rice pudding with custard had no growth of colonies on the BPA plate because they were not contaminated and did not contain the organism responsible. This shows that they had no interference in the outbreak but ham had growth in BPA plate which referred it to be contaminated. Biochemical tests were done to further assist in the identification of the pathogen such as the catalase test, which is a test that distinguishes bacteria that are able to produce catalyse enzyme. The only staphylococcus can produce catalyse enzyme and because the test showed catalase positive, it was confirmed that it was S.aureus. There were some suspicious colonies due to the presence of causative pathogen S.aureus. It was latex coagulase positive, which is the test done to identify S.aureus and DNAase positive that is a test that distinguishes microorganism based on their DNase activity. The API gave a percentage of 97.7% correspondence to S.aureus.

Table 4

The Chicken, beef and rice pudding with custard had no growth and Ham had 3 colony morphologies because it included 3 different organisms. On the other hand, Ham had growth on both Mannitol agar plate(MSA) and (MAC) agar plate. In MSA, The colony morphology for ham was less than 1 millimetre, circular and yellow, which means it is a mannitol fermenter. Most pathogens are mannitol fermenters and this indicates that it is most likely Staphylococcus. The cellular morphology was GPC in clusters. The catalase test was positive and there were suspicious colonies. All of this indicates that the organism present was S.aureus. However, in MAC, ham had 2 different results where the first column of results included 1 mm circular pink colonies with GPC in chains and negative for catalase test and there were no suspicious colonies. These results indicate that it was a non-pathogenic bacteria called Enterococci. The second column had 2-3 millimetre circular pink colonies with gram-negative rod (GNR) singular and a catalase test negative and with no suspicious colonies. This also indicates that it was a non-pathogenic bacteria, however, E.coli was the organism this time. The API for ham gave a percentage of 97.7% which corresponds to S.aureus.

Table 5

On the HBA plate, the cook’s nose and the cook’s assistant nose and Pus from the cook’s thumb had the same results for day 1. However, On MSA plate cook’s nose results correspond to the results of the pus from cook’s thumb. This is because they both had the same organism involved which was S.aureus. The reasons are the presence of suspicious colonies and another reason is that they were both mannitol fermenters. The analytical profile index (API) id which is used to identify bacteria gave a best and faster way to identify the bacteria. It gave a percentage of 97.7% correspondence to S.aureus. (11) It was latex coagulase positive, DNase positive which also helped in the identification of S.aureus. However, for the cook's assistant nose, the organism involved was S.epidermis which was different from the rest. This was due to the results being different. The bacteria wasn't a mannitol fermenter and didn't change colour from pink/red to yellow, there weren't any suspicious colonies present. It was also coagulase and DNase negative. And the API gave a percentage of 97.8% correspondence to S.epidemis.

Table 6

In table 6 all the antibiotics that were used in this experiment represented S.aureus which found to be sensitive to all antibiotics. However, as shown in the results Vancomycin appears to have a diameter below 15 millimetres which shows S.aureus is not sensitive to it. Thus, it can be determined that the given antibiotics except Vancomycin can be administered to the patients for treatment purpose to resolve their health issues. This is because in all other antibiotics they showed antibiotic sensitivity meaning the bacteria cannot grow if the following antibiotics are administered.

Thus, from the collected information it can be decided that the key causative agent for food poisoning among the individual was S.aureus. Moreover, it came from the infectious hands and nose of the cook as the person was not covering their mouth or hands while preparation of food. This is also evident as the results informed that the causative agent for the food poisoning was found on the unprotected hands and nose of the cook. Moreover, S.aureus can be regarded as the key causative for food poisoning as it was found on the uncovered nose of the cook which may have been carried into the food through sneezing while preparing food. Therefore, lack of hygiene among the cook became the main concern in this respect that caused relapse of the food poisoning among the individuals.

Conclusion

It can be concluded that, there is high impacts of consuming Ham for which the people are affected, and they are suffering from serious illness. the odds ration and the attack rate, both are high for the consumption of ham and it indicates that consumption of ham affect the people. there are serious organism among the people due to high consumption of ham and the S.aureus affects the health condition of the people. The number of affected people is raising rapidly year to year due to the consumption of contaminated food which is harmful for the people.

References

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