Investigation of the Outbreak of Food-and Water-borne Diseases in Khansar , Isfahan in 2018 Background

Water-borne infectious diseases remain a leading cause of death worldwide, resulting in more than 2.2 million deaths annually. The incidence of these diseases, such as diarrhea and gastrointestinal infections is on the rise (1). Water and foodborne disease outbreaks occur every year in different parts of the country. Food-borne diseases are one of the most important public health issues which cause illness and death in a significant number of people every year (2). Food safety assumes A-R-T-I-C-L-EI-N-F-O A-B-S-T-R-A-C-T

critical importance in public health; therefore, governments are striving to enhance food safety due to increasing problems in this regard (3).
According to the World Health Organization, when two or more people get the same disease from the same contaminated food or drink, the event is called a foodborne outbreak (4). Common causes of foodborne diseases include bacteria, bacterial toxins, viruses, and parasites (5). Clinical signs and symptoms, including abdominal cramps, nausea, vomiting, and diarrhea, are observed in most food-borne diseases; moreover, fever and headache are detected in some cases depending on the severity of the disease. These symptoms appear about 24-48 h after infection and last for 1-2 days, and the body is able to recover after a little while in most cases.
The incubation period is an important diagnostic key in determining the cause of the disease. The short incubation period with vomiting is recognized as the main symptom of food poisoning since toxins can directly irritate the stomach and lead to nausea and vomiting (5,6). Water and foodborne diseases can range from a mild illness to very serious diseases that sometimes endanger the health of people and lead to death. Such diseases can be much more severe and dangerous in high-risk individuals, such as children and neonates, pregnant women and their fetuses, and people with weakened immune systems (7,8).
Vist is a village in the Central District of Khansar, Isfahan Province, Iran, with a population of 5000 people. Vist is the center of five other villages, and most of its residents work in agriculture, Persian is spoken in this area with Khansari dialect. In spring 2017, a water and foodborne disease outbreak occurred in this region which affected 291 people. Therefore, the present research aimed to investigate this outbreak and study various aspects of the epidemic in terms of prevalence and demographic factors.
This descriptive cross-sectional study retrospectively examined the outbreak of gastroenteritis that occurred in 2017 in the Vist region of Khansar using epidemiological review and reporting mode. This outbreak lasted for 61 days from 22/5/2017 to 23/7/2017, and 261 patients were included in the current study. Acute gastroenteritis is the inflammation of the gastrointestinal tract, which is associated with the sudden onset of diarrhea with or without vomiting, fever or abdominal pain, and nausea (9). Viruses, especially noroviruses, are recognized as the leading cause of this problem in adults (10).
Diagnosis of the disease was made by laboratory physicians and experts of health centers using laboratory results and symptoms. Data were collected using a linear list of patients that included demographic variables, place of residence, onset date, symptoms, and results of human samples. Data analysis was performed using Excel software and SPSS software (version 22). The inclusion criterion was residing in outbreak areas since two weeks before it started, and the incompleteness of recorded data was the exclusion criterion.
The method was developed using the "epidemiological review and reporting" model as follows: 1) Definition of the disease: Gastroenteritis is a disease associated with the sudden onset of diarrhea with or without vomiting, fever, abdominal pain, and nausea. If left untreated, this disease can lead to dehydration, electrolyte, acid-base disorder, and even death.
2) Confirmation of diagnosis: The diagnosis of gastroenteritis was confirmed based on clinical, epidemiological, and laboratory findings. Patients were identified based on the variables of clinical characteristics (common symptoms), time, place, and person. Thereafter, the cases who had more than three episodes of diarrhea, abdominal pain, nausea, and vomiting

Archives of Hygiene Sciences
Volume 9, Number 4, Autumn 2020 were regarded as patients with gastroenteritis.
3) Confirmation of the epidemic: Considering that this huge number of patients with gastroenteritis was unprecedented in the Vist region during the last few years and there has been a sudden and unexpected increase, we have faced an outbreak. 4) Identification of endangered population: It was found that all 3,567 residents of Vist area were at the risk of this outbreak.
A. Medical examination: Using interviews and examination for, all patients referred to health centers underwent medical examinations, and the necessary laboratory samples were obtained from some of them.
B. Epidemiological examination sheets: These sheets which included demographic information, some epidemiological information, and inclusion criteria were completed for patients and analyzed.
C. Finding other patients with gastroenteritis: Other patients who did not refer to health centers were searched, and they were asked to refer to health centers or report in the event of any other cases of the disease. 5) Evaluation of ecological factors: The necessary investigation was performed about the water situation in the region, the used water wells, the method of water purification, and water supply pipe networks of the region. 6) Assessment of the population at risk: Medical examinations and treatment measures were provided to all patients referring to the comprehensive health center of Vist, along with training on personal hygiene standards. The endangered population referring to health center were informed of the outbreak by health officials. Moreover, in order to improve the environment, the relevant organizations were also informed about this outbreak. 7) Data analysis: At this stage, the attack rate was obtained by dividing the number of people who contracted the disease by the population at risk. On the other hand, the death rate was calculated by dividing the number of deaths from the disease by the population at risk.
Epidemiologic Time Curve and the map of spatial outbreak data were also plotted using Excel software. In the epidemiologic time curve, the days of outbreak and the number of patients were plotted on the x-axis and y-axis, respectively. The map of spatial outbreak data displays the distribution of patients, the involved area, and paint clustering in one area.
Ethical considerations: To observe ethical issues, the names of patients remained confidential, and fecal samples were collected for testing after obtaining oral consent.
This outbreak lasted for 61 days from 22/5/2017 to 23/7/2017, and 261 patients were included in the current study. Regarding gender, the male to female ratio was 1. In terms of age, the age groups of 15-5 years (24.4%) and 25-40 years (23.4%) had the highest frequency of disease, respectively. With respect to occupation, the highest frequency of the disease was related to housewives (30.2%) and students (28.2%), respectively (Table 1).    The population of the study area was 3,567 people, the attack rate was obtained at 81.4 per 1,000, and the death rate was zero. No side effects were reported, and possible ways of transmission were human-to-human and contaminated water. Moreover, environmental factors which caused an unfavorable condition in some water wells in the area and close contact between people were found to contribute to the outbreak.

Results
The affected area is illustrated in Map 1, where each star represents 10 sick people. It showed that the most affected village in the West area was the village itself with 1998 residents and the attack rate of 100 per 1000 population. Disease severity in none of the patients led to hospitalization or death. Three samples were initially taken from the patients and sent to the city laboratory and the provincial health center, and from the provincial health center to Tehran Reference Laboratory (partner laboratory of the Center for Disease Control and Management). The microbial load in the samples was reported to be zero, and they were tested positive for the virus. Thereafter, the type of virus was identified as norovirus in the next three samples.
Noroviruses are recognized as one of the leading causes of acute non-bacterial gastroenteritis which is highly infectious and easily transmitted (9). This virus is responsible for 40% of foodborne diseases in the United States (11). It is noteworthy that noroviruses are the most common known cause of gastroenteritis in adults (10). In related studies, there exist several different types of outbreaks which are classified based on the modes of exposure to the causative agent (1).
In a point source epidemic, cases suddenly rise to a peak and then fall which is due to peoples' exposure to the source of infection in a short period of time. This outbreak will subside quickly at the absence of secondary spread (2). In a prolonged epidemic, the epidemic curve shows an increase in cases at the beginning, followed by a decrease, and then one or more increase in affected cases after a time period equal to the length of the incubation period due to the secondary personto-person transmission (12).
The current epidemiological curve displayed in Figure 1 indicates a propagated epidemic in which secondary cases and person-to-person transmission after epidemic onset cause a rise in the affected cases. On the other hand, a sudden increase in the number of patients at the beginning of the epidemic points to a common source for the present epidemic. Furthermore, the infection of all age groups demonstrates the uniformity of exposure and the sensitivity of all people to this disease. In addition, the type of agent was diagnosed as norovirus, which can be transmitted through food, water, and contaminated surfaces, although infection mostly spread through human-to-human transmission (13).
The test results of six human samples sent to the reference laboratory showed that they were negative for Vibrio cholerae, Salmonella, Shigella, Staphylococcus aureus, Escherichia coli, and Yersinia pathotypes. Regarding other samples that were also examined for microbial load in the city laboratory, all cases were tested negative. Based on these results regarding the nondetection of bacterial pathogen, the hypothesis of bacterial pathogenicity in the outbreak of Vist was rejected, and norovirus was identified as the cause of the outbreak. Considering the population of the region (3,567 people), the infection process continued in a steady-state with the same slope and ended on 2017.5.5.
Nonetheless, there still exists the possibility that outbreak starts from the same region or other areas of the city due to the presence of sensitive people in the region and city. Water was most likely responsible for the transmission of this disease due to the following reasons: the consistency of epidemiological studies, propagated epidemiologic curve, uniformity of exposure, and susceptibility of all people to this disease, as well as the recognition of norovirus as the causative agent in laboratory tests. Therefore, disinfection and chlorination of drinking water sources is recommended to prevent similar outbreaks. Limitations of the study: One of the notable limitations of the present study was the referral of some patients to health centers in other neighboring cities for treatment and their absence in this study.