Foodborne illness outbreak investigation amongst party guests in Highfield suburb, Harare City, Zimbabwe, 2020: A Retrospective Cohort Study

Linda Nyasha Kanzara¹, Portia Manangazira², Emmanuel Govha³, Tsitsi Patience Juru³, Addmore Chadambuka^3,&, Notion Tafara Gombe^1,4, Maurice Omondi⁵, Mufuta Tshimanga¹

 

¹University of Zimbabwe, Department of Primary Health Care Sciences: Family Medicine, Global and Public Health Unit, Harare, Zimbabwe, ²Ministry of Health and Child Care, Epidemiology and Disease Control Unit, Harare, Zimbabwe, ³Zimbabwe Field Epidemiology Training Program, Harare, Zimbabwe, ⁴African Field Epidemiology Network, Harare, Zimbabwe, ⁵African Field Epidemiology Network, Nairobi, Kenya

 

 

^&Corresponding author
Addmore Chadambuka, Zimbabwe Field Epidemiology Training Program, Harare, Zimbabwe, 3-68 Kaguvi Building, Corner 4th/Central Avenue, Harare, Zimbabwe

 

 

 

 

A foodborne disease outbreak is defined as an incident in which two or more persons experience a similar illness resulting from the ingestion of common food [1]. Contact with contaminated recreational water, infected animals or environments, or an infected person are other ways in which the disease can be transmitted [2]. These points can be anything from growth, harvesting, transport to preparation in unhygienic environments and without proper temperature/environment [2,3]. Foodborne illnesses often present as a condition called gastroenteritis or gastrointestinal illness (GI). Common symptoms are sudden onset diarrhoea, nausea, vomiting, stomach cramps, and fever [3].

 

Food and water can be contaminated at any point between farm to fork [4]. The World Health Organization (WHO) reports that the true magnitude of the burden remains largely unknown [5]. Globally about 600 million, or almost 1 in 10 people in the world, fall ill after consuming contaminated food. Of these, 420 000 people die, including 125 000 children under the age of 5 years [6]. Africa makes up a third of these deaths. Additionally, children are also disproportionately affected by gastrointestinal illnesses. WHO report from 2015 indicates that almost 30% of all deaths from foodborne diseases are in children under the age of 5 years, despite being only 9% of the global population [6]. In Zimbabwe diarrhoea is a leading cause of mortality in children under five years, with more than 900 children succumbing to rotavirus induced diarrhea [7]. According to WHO diarrheal diseases deaths in Zimbabwe reached 5,323 or 4.48% of total deaths in 2018. The Age Adjusted Death Rate was reported to be 38.44 per 100,000 population [8].

 

On the November 2, 2020, five patients presented at Highfield Polyclinic with fever, diarrhoea, abdominal pain, vomiting and some with dehydration. All five patients were members of two households residing in the same compound. Of these cases, two were immediately referred to Harare Central Hospital due to the severity of their conditions. One of the two patients, a 9-year-old male died later that day. On Tuesday November 3, 2020, an additional 16 cases presented at Highfield Polyclinic in the morning with similar symptoms and all reported having attended the same party with the cases from the previous day. A rapid response team (RRT) was deployed to conduct a preliminary outbreak investigation. The RRT deployed to investigate the outbreak was composed of an epidemiologist, environmental health officer and technician, laboratory technician, and a clinician. This team was organized and deployed by the Harare City Health Department in collaboration with the Ministry of Health and Child Care. By the end of the day, November 3, 2020, 32 cases had been reported to the health authorities, all of which were guests at a party held on October 31, 2020. We conducted an analytic outbreak investigation to establish the risk factors for contracting suspected foodborne illness and identify a possible etiological agent for the outbreak.

 

 

Study setting

 

The study was conducted in Highfield, a densely populated, low- to middle-income suburb in the Southwestern district of Harare City, Zimbabwe, with an estimated population of 200,000 residents. Highfield is divided into primary zones known as Old Highfield and New Highfield, with three main municipal public health facilities, including Highfield Polyclinic, located approximately 2 km from the party venue and serving as the nearest health facility. The party, held at a household with seven residents, was attended by guests who primarily reside in the same locality. On the day of the event, both municipal water and electricity were available, although the area experiences frequent power outages. Private healthcare facilities in Highfield are limited, so most residents rely on public clinics for primary healthcare.

 

Study population

 

The study population was all people who had attended the party on October 31, 2020 in Highfield. A total of 80 guests attended the party.

 

Outbreak and investigation timeline

 

The first patients presented at Highfield Polyclinic on 2 November 2020 with symptoms after attending a party on 31 October. Two of the initial five patients were referred to Harare Central Hospital, where one, a 9-year-old, later died. On 3 November, 16 additional cases with similar symptoms presented at the health facility, prompting the collection of stool and water samples and deployment of a Rapid Response Team (RRT) to investigate the outbreak and initiate active case finding.

 

Preliminary investigations

 

We conducted active case finding in the community through door-to-door visits, interviewing party guests about symptoms consistent with the outbreak, guided by the party guest list. This enabled the rapid identification of additional cases linked to the outbreak and provided insight into potential transmission pathways, possible etiology and development of a hypothesis. In addition, a review of medical records was conducted for cases who had presented at Highfield Polyclinic to gather information on symptom onset, clinical presentation, and treatments administered. No food samples were collected, as no leftovers were available at the time of the investigation.

 

Hypothesis

 

We hypothesized that the outbreak was associated with the consumption of specific food items served at the party, with transmission likely resulting from improper food handling or inadequate storage practices.

 

Epidemiological investigations

 

A retrospective cohort study was conducted. The incubation period was estimated based on the time between the party on 31 October 2020 and the onset of symptoms in the first reported cases on 2 November 2020. A case was any person who attended the party in Highfield on October 31, 2020, and consumed any of the food or beverages, and developed either diarrhoea, abdominal pain, vomiting or all of these symptoms within 72 hours of having attended the party. Any person who attended the party with a prior diagnosis of diarrhoea (consisting of at least three loose motions in a 24-hour period), or at least two of the following: abdominal pains, fever, vomiting or nausea, or any person who did not provide their consent or whose parents or legal guardian did not provide consent was excluded from the study. A pretested questionnaire was used to collect data on demographics, food consumption history, behavioral practices, potential exposures and risk factors and occurrence of symptoms. Key informant interviews were guided by an interview schedule. The key informants were the primary food handlers responsible for preparing the food, including individuals who assisted with food handling and serving at the party, and the hosts of the party. These individuals provided detailed information on food preparation, handling practices, and environmental conditions, which were essential for understanding potential sources and transmission pathways within the outbreak.

 

Laboratory investigations

 

We collected 15 stool specimens from suspected cases before the administration of any antibiotic treatment, confirming through patient or guardian interviews and, when available, reviewing health facility records to ensure no prior antibiotic use. The stool specimens were sent for bacteriological analysis at Beatrice Road Infectious Diseases Hospital (BRIDH) Laboratory. Water samples were collected from all four boreholes in the neighborhood and analyzed for bacterial coliforms at Government Analyst Laboratories (GAL). No swabs were taken from food handlers, kitchen and utensils as they had already been cleaned by the time investigations commenced.

 

Environmental investigations

 

We conducted visual environmental inspections using a checklist at the food preparation and party venues to assess food handling practices, cleanliness, waste management, water supply, storage and availability of sanitary facilities.

 

Outbreak Preparedness and Response

 

We assessed the City Health Department´s outbreak preparedness and response using the 2012 Zimbabwe IDSR Guidelines, which recommend detecting outbreaks within 48 hours to ensure timely response.

 

Data analysis

 

We performed a descriptive analysis of the outbreak data by person, place, and time. Categorical variables were expressed as frequencies and relative frequencies, while continuous variables were described using measures of central tendency and dispersion. To test the hypothesis, we conducted a retrospective cohort study, comparing dichotomized exposure variables (specific food items consumed) among ill and non-ill individuals who attended the party. Attack rates were calculated for each exposure, and relative risks were determined by comparing the attack rate among those who consumed specific food items and those who did not with corresponding 95% confidence intervals (CIs). An epidemic curve was generated to illustrate the outbreak's magnitude and progression over time. Bivariate analysis was conducted to identify potential risk factors, calculating RRs and their 95% CIs associated with the consumption of each food item. Covariates yielding a p-value less than 0.25 in the bivariate analysis were selected for inclusion in the multivariate logistic regression model. We then used backward stepwise multivariate logistic regression to identify independent risk factors associated with foodborne gastrointestinal illness, with statistical evaluations conducted at a significance level of 0.05. Data analysis was performed using Epi Info version 7.2.4.0.

 

Ethical considerations

 

Permission to carry out the study was obtained from the Health Studies Office, Harare City Health Department, Ministry of Health and Child Care, the District Medical Officer of West-Southwest District and residents of the affected area in Highfield. An official approval letter was provided, though no formal approval number was issued. The Harare City Ethics Committee reviewed and approved the study, approval number (3/7). Written informed consent was obtained from participants. Assent was sought from participants under the age of 18 who are legally considered children. Confidentiality was assured and maintained. Records of the data, pictures taken during the investigation and the consent forms were kept under safe and lock.

 

 

Descriptive epidemiology

 

A total of 63 out of the 80 guests (78.7%) were interviewed. Thirty-two of these had reported to the health facility and 31 were found through active case finding. The median age of the party guests was 16 years (Q₁=10; Q₃ =32). Forty (63.5%) of the investigated guests fell ill after consuming food served at the party. Of these, 45% (18 /40) were males and 55%(22/40) were females. The overall attack rate (AR) was 63.5% (40/63) .

 

The primary case began experiencing symptoms on October 31, 2020 between 0600 and 0000 hours while the index case was reported to the health facility on November 2, 2020 around 0900 hours. Based on this timeline, we estimated a median incubation period of 12 hours (Q₁=6; Q₃=30). The health facility notified the next level on November 3, 2020 and the rapid response team was activated immediately. The shape of the curve is suggestive of a point source outbreak. The last case was on November 3, 2020, between 0600 and 0900 hours. The median incubation period was 12 hours (Q₁ = 6; Q₃ = 30).

 

Clinical presentation of cases

 

All cases, 40 (100%) reported having experienced abdominal cramps. The majority of the cases 36 (90 %) presented with watery diarrhoea and fever. Thirty (75.0%) reported vomiting, 25 (62.5%) experiencing a headache, 11 (27.5) fatigue, 3 (7.5%) and 1 (2.5%) chills and bloody diarrhoea.

 

Case management

 

A greater proportion 23 (57.5%) sought treatment at the local health facility while 12 (30%) cases sought treatment at both a public and private health facility. Five (12.5%) cases sought treatment at either a pharmacy or private healthcare provider. Most of the cases that visited the health facilities were managed with ciprofloxacin (92.5%, 37/40), oral rehydration solution (90%, 36/40) and paracetamol (80%, 32/40).

 

Environmental health assessment

 

Food was supplied and prepared at one of the food handler´s houses on October 31, 2020. It was transported in batches in a pick-up truck to the party venue with the first batch consisting of potatoes, coleslaw, beef and macaroni. The coleslaw was not stored in a refrigerator neither were the potatoes, beef or macaroni. Food preparation was done by one hired food handler and served by an additional three handlers. There were functional refrigerators, hand washing facilities with liquid soap and constant supply of municipal water at the residence where food was prepared and at the party venue. Hot water was not provided in the kitchen. Hand operated bins were available at both the residence where food was prepared and at the party venue. There was one toilet each at the food preparation and party venues. The toilets were provided with municipal water. None of the food handlers had medical certificates or had received any training in food handling.

 

Laboratory investigations results

 

No pathogens were isolated in the four water samples sent for analysis at GAL and the fifteen stool specimens that were collected from the cases for analysis at BRIDH laboratory.

 

Outbreak preparedness and response

 

The outbreak was detected after the stipulated 48 hours. Harare City was adequately stocked with oral rehydration sachets (5000) and ringers´ lactate (6 × 10 litre boxes). Normal saline (2 × 10 litre boxes), 300 fluid administration sets and intravenous cannulas, 20G (200), 18G (100), and 16G (16) were available. Oral drugs such as Cotrimoxazole (10 boxes ×100), Metronidazole (1 box ×100) were available. Essentials such as latex gloves (19 000 pairs) and aprons (420) were available. Standard treatment protocols for diarrhoea at were also available at the health facility.

 

Factors associated with contracting foodborne illness

 

On bivariate analysis, eating coleslaw (crude RR 2.9, 95 % CI 1.1‒7.8, p =0.01) and cold macaroni (crude RR 2.3, 95% CI 1.3‒3.9, p =0.00) at the party on October 31, 2020 were significant risk factors for contracting foodborne illness. Food-specific attack rates were 73 % (37/51) for coleslaw and 82% (31/38) for macaroni, both of which were higher than the overall attack rate of 63%. Eating cold beef (crude RR 1.2, 95 % CI 0.8 ‒1.8, p =0.31), cold boiled eggs (crude RR 1.2, 95 % CI 0.8-1.8, p =0.31), hot potatoes and (crude RR1.6, 95 % CI 1.3‒2.0, p =0.27) were not statistically significant risk factors for contracting foodborne illness (Table 1).

 

Independent factors associated with contracting foodborne illness

 

The independent risk factors for contracting foodborne illness were eating coleslaw adjusted RR= 7.4 (95% CI: 1.1- 50.0; p-value = 0.04) and eating cold macaroni adjusted RR=4.1 (95% CI: 1.1- 14.7; p-value = 0.03) (Table 2).

 

Measures of impact for foodborne illness

 

If eating coleslaw caused foodborne illness, the excess risk of illness among the total study population was 38 per 100. About 27 % of the illness among total study population was attributable to eating coleslaw at the party and could be eliminated if coleslaw was not served at the party. If eating cold macaroni caused foodborne illness, the excess risk of illness among the total study population was 60 per 100. About 43 % of the illness among total study population was attributable to eating cold macaroni at the party and could be eliminated if cold macaroni was not served at the party (Table 3).

 

 

The overall attack rate indicates serious public health implications of foodborne outbreaks. The epidemic curve was suggestive of a common point source outbreak as most of the cases fell within one incubation period. Our findings suggest that the possible vehicles of transmission of the pathogens were the coleslaw and cold macaroni served at the party. From the epidemic curve, the median incubation period in hours was 12 (Q₁= 6; Q₃ = 30). The short incubation period, and presence of upper GI symptoms including nausea and vomiting are indicative of a toxin-producing pathogen.

 

The evidence on the etiological agents that cause foodborne illnesses shows that they exist in three broad categories which are biological, physical and chemical [9]. The most common bacterial toxins involved in food poisoning are bacteria such as Bacillus cereus, Staphylococcus aureus, Clostridium botulinum, Vibrio cholerae, Escherichia coli, Salmonella and Shigella species [10]. In this study, no etiological agents were identified from the collected stool samples and one or a mixture of these microorganisms could be responsible for this outbreak. In conducting analytic epidemiology eating coleslaw had a three times risk of contracting foodborne illness. Additionally, eating cold macaroni had a two times risk of contracting foodborne illness. Raw foods are listed by the CDC as high risk foods for all three categories of contamination [11].

 

In this study, the probable etiological agent could have been either one of or a mixture of the following microorganisms, Staphylococcus aureus and Salmonella species which are among the most common foodborne pathogens. The short incubation period, common symptoms of stomach cramps and vomiting reported amongst cases are both common with Staphylococcus aureus and Salmonella infections [1]. The fresh produce in the coleslaw implicated in this study have been reported to be responsible vehicles of transmission for foodborne outbreaks and these reports are consistent with the findings of this outbreak [12]. Salmonella is reported to be associated and transmitted through foods that include raw vegetables, eggs and sometimes starches [13,14]. While food items likely to be infected by staphylococcal food poisoning include egg products, salads such as egg, tuna, chicken, potato, and macaroni [15].

 

The environmental assessment highlighted critical gaps in food safety practices that may have contributed to the outbreak. A possible explanation for the food contamination is that inadequate cooling and/or leaving foods at room temperatures and storing them in large containers while being refrigerated was associated with the outbreak. Food prepared for the event, including coleslaw and macaroni, was not refrigerated during transport or storage, remaining at room temperature for extended periods. This is particularly concerning, as coleslaw and pasta are high-risk foods that can support bacterial growth when not stored at appropriate temperatures. Although the food preparation site was equipped with functional refrigerators, these were not utilized to store the prepared food before transport. This is in line with the reports by the European Centres for Disease Control which indicate the three most frequently identified factors that contribute to salmonellosis are improper cooling, contaminated raw products, and inadequate heating [16].

 

The findings reflect positively on the health-seeking behaviors in the community. Nonetheless, although most of the cases sought medical attention, there was a delay in presenting to the health facility. This could be a result of the mild onset of the illness or having sought medical assistance at the pharmacy first. There is need to explore further on the health seeking behaviors knowledge, attitudes and practices in the community.

 

All the food handlers were not trained in food handling and had no medical certification. Gumbo et. al (2014) found that 80% of the food handlers in a foodborne outbreak in Bulawayo City did not have any training or qualifications [17]. This depicts a gap in food handling practices and regulations which need to be reinforced as food handlers are a crucial link in the food chain from farm to fork and have been implicated in most foodborne outbreaks. Without medical examinations, food handlers may unknowingly carry infectious agents that can contaminate food, particularly in high-density settings like Highfield. This absence underscores the risk of unmonitored health conditions among handlers as potential sources of food contamination.

 

In line with the targets of the IDSR guidelines of 2010, the response to the outbreak was delayed, with detection occurring more than 48 hours after the initial cases were presented [18]. However, Harare City health facilities were well-prepared with sufficient medical supplies, including oral rehydration sachets, intravenous fluids, and antibiotics and standard diarrhea treatment protocols were in place ensuring the adequate and appropriate case management. This preparedness highlights the capacity of local health systems to respond effectively once an outbreak is detected, though the delay in initial detection suggests a need for enhanced surveillance and rapid response mechanisms.

 

This study has limitations that may influence the interpretation of its findings. The time lapse between the exposure event and data collection, which was primarily self-reported, may have introduced recall bias and reporting inaccuracies, particularly in estimating food consumption and symptom onset. Furthermore, laboratory testing was limited to bacteriological analysis of stool and water samples, restricting our ability to detect other pathogens that could also explain the outbreak. Despite reviewing medical records and self-reported non-use of antibiotics, some patients may have received treatment before sample collection, potentially contributing to the negative limiting pathogen detection. No leftover food samples were available for testing, which limited our ability to directly identify or confirm the contamination source. The environmental inspection of the food preparation site was conducted after food handling and cleaning had already been completed, preventing a full assessment of food safety practices on the day of the outbreak. Despite these limitations, the study provides important insights into the outbreak's probable foodborne sources and associated risk factors, offering valuable lessons for future outbreak investigations.

 

 

The outbreak investigation concluded that this was a common point source of the foodborne outbreak among guests who attended a party in Highfield on October 31, 2020. Coleslaw and cold macaroni that were served at this party are the possible vehicles of transmission for the microorganisms. Laboratory investigations did not isolate specific pathogens, but the course of illness largely fit that of a toxin-producing pathogen. The outbreak investigation identified lapses in food safety practices and the absence of trained, medically certified food handlers, which likely facilitated the spread of illness among guests. While there was a delay in outbreak detection, Harare City health facilities were adequately stocked with essential supplies and adhered to standard treatment protocols, enabling effective case management. These findings underscore the need for enhanced food safety education and stricter regulations for food handlers, as well as improved outbreak surveillance and rapid response systems.

 

 

The authors declare that they have no conflict of interest.

 

Funding declaration

 

Funding for this article was from personal sources.

 

Permission to publish

 

Permission to publish the article was obtained.

 

Ethical consideration

 

All ethical considerations protocols were followed in coming up with this article.

 

 

LNK, PM, EM, AC, TPJ, MO, NT, MT- Conception, design and field work. LNK, PM, EM, AC, TPJ, MO, NT, MT- Writing the draft manuscript. LNK, PM, EM, AC, TPJ, MO, NT, MT- Review and approved the final draft.

 

 

We would like to acknowledge the University of Zimbabwe: Health Studies Office, Ministry of Health and Child Care: Epidemiology and Disease Control Directorate, Harare City Health Department and the study participants for making our study a success.

 

 

Table 1: Foods associated foodborne illness outbreak investigation amongst party guests in Highfield Suburb, Harare City, Zimbabwe, 2020

Table 2: Independent risk factors associated with foodborne illness outbreak investigation amongst party guests in Highfield Suburb, Harare City, Zimbabwe, 2020

Table 3: Measures of impact for party guests who ate coleslaw and cold macaroni in Highfield Suburb, Harare City, Zimbabwe, 2020

 

 

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