A case control study of food poisoning outbreak in Andaman and Nicobar Island

 

 

Abstract              Background: An outbreak of food poisoning occurred among a government establishment of Andaman and Nicobar Island. Investigation of the outbreak was carried out to ascertain the probable cause for the occurrence and to suggest remedial measures to control the outbreak. Methods: A case-control study was undertaken. Odds ratios were calculated to determine the most likely food item responsible for the outbreak for each food item of the suspected meal. An environmental assessment was also carried out to identify the factors that may have contributed to the occurrence of the outbreak. Results: Out of 287 individuals who consumed food in the mess, 24 personnel were affected in this outbreak in 12 hours. The overall attack rate was 6.2%. Only 15 individuals reported seeking medical help and were detained for observation and treatment. Nine cases did not report to Medical authorities but were detected during a rapid search of cases. All had complete resolution of symptoms with no hospital admission or any complications. Results from the case-control study implicated eating egg curry were highly associated with the outbreak (OR 33.33, 95% CI 7.40-124.27, P-value <0.05). Discussion: The consumption of egg curry prepared in the dinner of the previous day was found to be as the most likely vehicle of food poisoning. The eggs reaching kitchen needs to be checked by the Food handlers scrupulously. Various practical and easy methods needs to be employed to differentiate spoilt eggs from good. Conclusion: Prompt measures controlled the outbreak effectively and prevented further spread. Food handlers need to be trained in quality assessment and refrigerating the food items in recommended temperature.

 

INTRODUCTION

The World Health Organisation (WHO) defines foodborne disease outbreaks as when the observed number of cases exceeds the expected number. In the context of foodborne disease, "outbreak" refers to two or more cases resulting from the ingestion of a common food.¹ Food poisoning has been defined as acute gastroenteritis caused by ingestion of food or drinks contaminated with either living bacteria or their toxins or inorganic chemical substances and poisons derived from plants and animals.² According to WHO almost 1 in 10 people in the world fall ill after eating contaminated food and 420,000 die every year, resulting in the loss of 33 million healthy life years (DALYs). ³ As per the CDC estimates, 5.5 million (59%) foodborne illnesses were caused by viruses, 3.6 million (39%) by bacteria, and 0.2 million (2%) by parasites. The pathogens most commonly involved were Norovirus (5.5 million, 58%), non-typhoidal Salmonella spp. (1.0 million, 11%), C. perfringens (1.0 million, 10%), and Campylobacter spp. (0.8 million, 9%).⁴ Incidents are commonly reported in training establishments where a large number of individuals dine-in from a common kitchen.⁵ Incidences of the outbreak had been reported from establishments where the personnel living without their family have their meals from a common mess.⁶On a fine summer morning of April, the Duty Medical Officer (DMO) of the Hospital in Andaman and Nicobar Islands received information stating that three cases of a particular Government establishment had been reported with clinical features suggestive of food poisoning, and few more cases were also detained at establishment’s Primary Medical Care setup. Due to reporting a large number of cases in a short period, an outbreak of food poisoning was suspected. Immediately on receipt of a call, an outbreak investigation team was rushed to the hospital and concerned establishment. Owing to its geographical location, Andaman and Nicobar Islands has limitations in terms of natural as well as logistic resources. In addition, the outbreak happened at a time when the entire nation was gripped by the first wave of Covid-19 pandemic. This paper deals with the outbreak investigation carried out in the resource constrained setting in the remote location of Andaman and Nicobar Island and discusses the various steps taken to manage it. An epidemiological investigation was carried out to confirm the outbreak, ascertain the probable cause, establish the epidemiological association between the illness and consumption of a particular food and suggest remedial measures to control the outbreak and prevent further cases.

 

METHODOLOGY

Following notification of the cases, the epidemiological investigation team (Preventive Medicine specialist, Pathologist, Laboratory technician, and Sanitary inspector) visited the location and the hospital to collect all relevant data. The list of food items in the menu for the last 48 hours was recorded from the cookhouse along with the list of personnel reported to the hospital and detained in the Medical Inspection room at establishment. The diagnosis of Food Poisoning was confirmed based on the history and clinical findings. An outbreak of food poisoning was confirmed when a large number reported similar gastrointestinal symptoms and signs. Data related to the prevalence of gastroenteritis was collected from the establishment’s Primary Medical Care setup for confirmation of the existence of an outbreak. A chronological listing of all known cases was carried out which helped in delineating the population at risk and in the preliminary definition of the transmission dynamics of the outbreak according to place and time. Apart from the unmarried personnel eating in the establishment’s mess, many married personnel also had meals (breakfast and lunch) in the same mess. None of the married personnel who had breakfast and lunch were affected the current outbreak and hence for the study, a case was defined as any member of the mess who was previously well and had presented with symptoms of loose motions or nausea or vomiting or abdominal pain with or without fever, headache, and weakness after having dinner in the mess on the previous night.

 The population at risk were all the personnel having food in the same mess. A structured questionnaire was developed to collect information about the socio-demographic characteristics, food consumption, and clinical data. All additional cases with mild signs and symptoms that have not been reported for treatment were traced with the help of the team and administrative authorities of the establishment by an extensive block to block survey of the population at risk. The final line listing of cases was prepared. Information regarding possible exposure controls was also collected. A control was defined as a member who had dinner in the mess, the previous night but had not presented with any symptoms. Cases and controls were selected in a ratio of 1:2. All cases and selected controls were investigated and interviewed by the team and a record of the same was maintained. An environmental assessment was undertaken regarding food preparation processes. The kitchen, storage area, distribution area and surroundings of the cookhouse were inspected. Galley premises were scrutinized to identify the conditions that may have contributed to the occurrence of the outbreak. The water supply system was checked for cross-contamination. The weather conditions were also noted. The flow chart of various activities undertaken is depicted in Fig 1. Stool samples were collected for bacteriological analysis from two patients detained in the Primary Medical Care setup. Water and food samples were also collected as required and sent for bacteriological analysis.

Microsoft Excel was used to compile the collected data, to carry out descriptive and inferential statistics. An epidemic curve was drawn with a two-hour time interval. The distribution of onset of symptoms and demographic characteristics were also analyzed. The odds ratio was calculated for each exposure variable of interest (food items) and a P-value of <0.05 was considered statistically significant.

 

RESULTS

Out of 387 dining-in members in the mess, 24 (6.20%) had developed signs and symptoms suggestive of food poisoning. Out of which 15 members had reported to the Primary Medical Care setup /Hospital and were detained for observation and treatment. Nine cases did not report to medical authorities but were found out during active surveillance for cases. Out of 24 cases, 93.83% cases had loose motions, 50% had abdominal pain, 29.17 % had nausea/ vomiting, 20.83% had a fever and none had blood in stools (Table 1). All of them were conservatively managed and had complete resolution of symptoms without any complications. The age group of affected personnel ranged from 18-55 years with a mean age of 30.25 ±10.34 years (Table 2). The control population consisted of 48 personnel with an age range of 18-49 years and with a mean age of 31.4±8.34) years. The Index case of the present outbreak was reported to a hospital at 0330 h. This was followed by a gradual increase in the number of cases. The first two cases had onset of symptoms between 0200 h to 0400 h, twelve cases between 0401 h and 0600 h and similarly, seven cases had onset between 0601 h and 0800 h and three cases had onset between 0801h to 1000 h; the last case had symptom onset at 0930 h. The peak of the epidemic was reached at 0530 h. The epidemic curve (as shown in figure 2) plotted showed clustering of cases that occurred within 7 to 14 hours of dinner intake with a median incubation period of 10 hours and no secondary peaks of occurrence of cases which is a classical "common vehicle point exposure" curve typical of food poisoning. The water from the water service agency responsible for the provisioning of safe and wholesome water was used for the preparation of food. The overhead water storage tank above the galley had particulate sediments in its bottom and water from it was being used for cleaning the utensils. All three water samples, collected from the galley, cookhouse, and mess did not show any growth of a coliform organism. Investigation of stool samples of the two patients was inconclusive. Gram-negative bacilli were seen after incubation of eggshell at 37ºC for 48 hours with the possibility of contamination while egg white showed no growth. In the floating test, eggs were placed in a vessel containing 10% sodium chloride solution. Remaining eggs from similar batches floated suggesting old eggs were no longer fresh to eat while eggs from other batches slowly sank suggesting their freshness.

 All the food handlers denied any recent history of hospitalization, diarrhea before and during the period of the outbreak investigation. They were asymptomatic and did not have any open wounds or visible skin infections. Food handlers had been medically examined by the establishment’s Medical Officer on monthly basis and were found to be Free from Infection during their evaluation for that month. The cookhouse, dining hall, and surrounding places were clean. The storage of the raw materials was found to be satisfactory. There was no evidence of any animals/ rodent menace or cockroach menace in the kitchen, however, a slight fly nuisance was observed. Some of the kitchen staff were not appropriately dressed especially those at the galley. They were not wearing aprons and few had not covered their heads with headcovers due to hot and humid environmental conditions. Twenty-four personnel met the case definition. Their exposures to specified food items (Rice, Dal, Egg Curry, Veg Sabji, and Grapes) served during dinner were compared with the 48 controls. (Table. 3 shows only food items served during dinner). The highest odds ratio was found for consumption of Egg curry. 21 of 24 cases reported eating this food item compared to 09 of 48 controls (p<0.05). Eating egg curry was highly associated with the outbreak (OR 33.33, 95% CI 7.40-124.27). Hence, the consumption of egg curry was considered the most likely vehicle of food poisoning. The occurrence of food poisoning because of the consumption of egg curry is rare among government establishments. However, this odds ratio can be taken as a relative risk estimate i.e. the risk of developing illness was much higher among persons who ate egg curry than among those who did not.

 

DISCUSSION

An increased number of cases from a training establishment reported to the Hospital in Andaman and Nicobar Islands, with complaints of Gastrointestinal symptoms. The clinical features of the affected personnel, associated with the history of consumption of dinner from a common mess, clustering of cases within 7-14 hours of last meal, absence of a secondary peak in the Epidemic curve, and favorable laboratory reports all confirmed the diagnosis of food poisoning outbreak. In our study, 24 cases presented with symptoms, predominantly with diarrhea, abdominal pain, nausea/ vomiting, and fever. Diarrhea with or without abdominal cramps is a presenting feature of food poisoning due to Salmonella spp, C. jejuni, C. perfringens, and diarrhoeal form of Bacillus cereus.⁷ But fever, seen in 20.8% of cases in the present outbreak, is characteristically seen in Salmonella food poisoning. Hence, the Salmonella species was suspected as the causative agent of the outbreak. All cases occurred within 7-14 h of consumption of dinner, with a median incubation period of 10 h. Such incubation period is seen in food poisonings due to Salmonella spp, Clostridia spp, and some cases of B. cereus. ¹ Microbial analysis of the food sample revealed the presence of Gram-negative bacilli in the eggshell, which further suggested the presence of Salmonella spp (which is a Gram-negative organism) as the etiological agent. Clostridia and B. cereus being Gram-positive were the unlikely cause of the outbreak. Similar reports were found in a study conducted in New Zealand between 2000 - 2009, wherein 204 outbreaks of non-typhoidal salmonellosis were analyzed. The results showed that non-typhoidal salmonellosis was primarily a foodborne disease, but there was insufficient evidence regarding the food vehicle.⁷ An outbreak of food poisoning was reported from a Military establishment in May 2011, when 53 cases reported sick after likely consumption of Potato-bitter gourd vegetables during dinner, which was found to be contaminated with non-typhoidal Salmonella spp. ⁸

Comparison of these 24 cases with 48 controls and subsequent calculation of odds ratio pointed towards the consumption of egg curry as the primary cause for food poisoning. The presence of Gram-negative organisms in the eggshell and positive floatation test further substantiated this finding. Epidemiological investigations revealed that the eggs were not stored at the recommended temperature. Also, egg curry was the first preparation of the dinner menu, which was cooked at 1630 h and laid out at room temperature for a long period before consumption by the personnel from 1930 h onwards.

Another factor implicated for outbreak might be hot weather with temperatures ranging from 35°C and 42°C and consumption of prepared food after a prolonged period of three and half hours without proper storage leading to rapid multiplication of the causative microorganism. Given the above, the etiological agent suspected for the outbreak of food poisoning was Salmonella spp and the likely food item attributed was egg curry. The cases admitted to Hospital were conservatively managed for their symptoms. All of them responded to the therapy and recovered completely without any complications. Any further increment in cases was prevented by prompt action and administration of preventive measures by the Epidemiological Investigation Team. Even though viruses are important causes of food poisoning; clinical samples and food items could not be tested for the presence of viruses due to the non-availability of facilities. In addition, samples are not sent to mainland due to non-availability of flights due to Covid restrictions imposed by Andaman and Nicobar administration. Hence, the lack of advanced laboratory testing for the identification of the specific types of organisms is one of the important limitations during the investigation. Also, the inspection of the environment was done after the preparation of food and therefore information obtained was incomplete regarding food storage, preparation, and transport on the particular day of the outbreak. Preventive measures were also undertaken concurrently to control the outbreak such as patients were isolated and provided with adequate supportive treatment. Affected individuals were removed from the food handler's duty. Measures were instituted to maintain a high standard of sanitization and cleanliness in the storage, cooking, and issue of food articles in the galley and cookhouse. All cooks were instructed to ensure thorough cooking of all items of food derived from animal sources, particularly egg products and meat dishes, and their proper storage to prevent subsequent contamination. Super chlorination of water was also ensured. In addition, certain measures need to be emphasized for the prevention of future outbreaks due to the consumption of eggs and eggs products. The quality and wholesomeness of the food supplied (especially eggs and meat) are to be ensured by Supply Officer in liaison with the doctor of the establishment. Only clean eggs with intact shells are to be collected from the supplier and to be stored between 4-8⁰C. These eggs should be used within 3 weeks for the best quality. Eggs may be subjected to a floatation test before use. Eggs should be cooked thoroughly until both the yolk and white are firm in consistency. Food to be prepared such that it is ready for consumption just before mealtime. Eggs and egg-containing preparations should be served immediately after cooking. Cooked eggs should not be kept out of the refrigerator for more than 2 hours. They may be refrigerated for serving later but should be thoroughly reheated to 165º F before serving. Education of food handlers is essential with special emphasis on the protection of subsequent contamination of prepared food and storage of prepared food.

 

CONCLUSION

Our study showed that the episode of food poisoning among personnel was associated with dinner on the previous night and the most attributable food item was egg curry prepared from non-fresh eggs and the most likely cause for food poisoning was salmonella species. The epidemiological investigation team overcame the challenges posed by the limitation of resources as well as restrictions owing to Covid-19 pandemic, and succeeded in controlling the outbreak. The flow chart of activities undertaken during the investigation of this outbreak could serve as a template for the investigation of similar outbreaks.

 

REFERENCES

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