A Summary Tool for Factors Influencing Case Confirmation in Foodborne Outbreak Investigations: Considerations Across Common Settings
In the context of an enteric illness outbreak, public health investigators often encounter situations where only a subset of exposed individuals become confirmed cases. This variation can be attributed to numerous biological, behavioral, and systemic factors that influence the likelihood of illness onset, medical testing, and case confirmation. Below is a summary of why only some individuals become confirmed cases across different common settings.
Download the alternative format
| Long term care facilities | Events | Restaurants | Households | Hospitals | School/childcare centres | |
| Variable exposure | • Not all residents consume the same foods, especially if there are dietary restrictions for some individuals • Contamination may be sporadic across dishes or portions, leading to uneven exposure |
• Not all attendees consume the same foods, especially at buffet-style events • Contamination may be sporadic across dishes or portions, leading to uneven exposure |
• Some individuals may have ordered a different meal or a well-cooked version of the implicated item • Contamination may be sporadic across dishes or portions, leading to uneven exposure |
• While some households may have purchased product, not everyone in the household may have consumed it • Contamination may be sporadic, leading to uneven exposure in the same shared dish or meal |
• Not all patients consume the same foods, especially if there are dietary restrictions for some patients • Contamination may be sporadic across dishes or portions, leading to uneven exposure |
• Not all children / students consume the same foods, especially if there are various items to choose from in daycare or school lunch program, or if some children are “picky eaters” and remove ingredients from a dish • Contamination may be sporadic across dishes or portions, leading to uneven exposure |
| Dose-response | • Individuals who consumed larger portions of a contaminated item are more likely to ingest a higher infectious dose, increasing their risk of illness • Individuals who consumed a smaller portion of a contaminated item are less likely to ingest an infectious dose, decreasing risk of illness |
• Individuals who consumed larger portions of a contaminated item are more likely to ingest a higher infectious dose, increasing their risk of illness • Individuals who consumed a smaller portion of a contaminated item are less likely to ingest an infectious dose, decreasing risk of illness |
• Individuals who consumed larger portions of a contaminated item are more likely to ingest a higher infectious dose, increasing their risk of illness • Individuals who consumed a smaller portion of a contaminated item are less likely to ingest an infectious dose, decreasing risk of illness |
• Individuals who consumed larger portions of a contaminated item (or additional “leftovers” of a contaminated item) are more likely to ingest a higher infectious dose, increasing their risk of illness • Individuals who consumed a smaller portion of a contaminated item are less likely to ingest an infectious dose, decreasing risk of illness |
• Individuals who consumed larger portions of a contaminated item are more likely to ingest a higher infectious dose, increasing their risk of illness • Individuals who consumed a smaller portion of a contaminated item are less likely to ingest an infectious dose, decreasing risk of illness |
• Children / students who consumed larger portions of a contaminated item are more likely to ingest a higher infectious dose, increasing their risk of illness |
| Food handling | • Variations in food handling (e.g. timing, temperature) can result in uneven contamination across dishes or within a single dish • Some portions may have been served fresh, while others sat out too long prior to serving, increasing risk • Some patients may have eaten early while food was fresh (lower risk), while others saved some of their meal for hours later (higher risk) |
• Variations in food handling (e.g. timing, temperature) can result in uneven contamination across dishes or within a single dish • Some portions may have been served fresh, while others sat out too long prior to serving, increasing risk • Some individuals may have eaten early while food was fresh (lower risk), while others had second helpings hours later (higher risk) |
• Variations in food handling (e.g. timing, temperature) can result in uneven contamination across dishes or within a single dish • Some portions may have been served fresh, while others sat out too long prior to serving, increasing risk • Some individuals may have saved “leftovers” for consumption hours or days later, without proper food handling (e.g. refrigerating, re-heating) |
• Variations in food handling (e.g. timing, temperature) can result in uneven contamination across dishes or within a single dish • Some individuals may freeze, cook, prepare, or reheat the product differently within the same household • Some portions may have been served fresh, while others sat out too long prior to serving, increasing risk • Some individuals in the household may not have been involved with food preparation, which may have been the primary source of contamination (e.g. raw chicken) |
• Variations in food handling (e.g. timing, temperature) can result in uneven contamination across dishes or within a single dish • Some portions may have been served fresh, while others sat out too long prior to serving, increasing risk • Some patients may have eaten early while food was fresh (lower risk), while others saved some of their meal for hours later (higher risk) |
• Variations in food handling (e.g. timing, temperature) can result in uneven contamination across dishes or within a single dish • Some portions may have been served fresh, while others sat out too long prior to serving, increasing risk • Some children / students may have eaten early while food was fresh (lower risk), while others saved some of their meal for hours later (higher risk) |
| Under-reporting | • Staff may not report illness in residents, especially if symptoms are mild • Residents may have communication issues or cognitive impairment, preventing reporting of symptoms to staff • Symptoms may be overlooked or attributed to pre-existing conditions |
• Some individuals may not report illness to others from the event, or may not associate their symptoms with the event, especially if symptoms are mild or delayed • Individuals may not seek care, especially if their symptoms are mild, or if they are out of town for the event |
• Some individuals may not report their illness or seek care, especially if symptoms are mild or delayed | • Some individuals may not report illness or seek care, especially if symptoms are mild or delayed |
• Patients may have complex health issues, preventing accurate reporting of symptoms to staff • Symptoms may be overlooked or attributed to pre-existing conditions or antibiotics |
• Staff may not report illness in children / students for a number of reasons (e.g. under-resourced, lack of training) • Parents may not report illness in children / students for a number of reasons (e.g. not able to keep a sick child at home) • Symptoms may be overlooked or attributed to other common childhood illnesses • Young children may not report symptoms accurately • Parents may not seek care for their children, especially if their symptoms are mild |
| Testing gaps | • Delayed or absent stool testing can prevent case confirmation, even among symptomatic individuals • Testing capacity within facilities may vary over time • Testing criteria may be interpreted differently by staff in the same facility • Testing criteria in certain facilities may be strict and result in clinicians not requesting tests for gastrointestinal symptoms |
• If someone seeks care, a clinician may not order a stool sample • Delayed or absent stool testing can prevent case confirmation, even among symptomatic individuals |
• If someone seeks care, a clinician may not order a stool sample • Delayed or absent stool testing can prevent case confirmation, even among symptomatic individuals |
• If someone seeks care, a clinician may not order a stool sample • Delayed or absent stool testing can prevent case confirmation, even among symptomatic individuals |
• Clinicians may not order stool tests due to other plausible explanations for symptoms • Delayed or absent stool testing can prevent case confirmation, even among symptomatic individuals • Testing capacity within hospitals may vary over time • Testing criteria may be interpreted differently by staff in the same hospital • Testing criteria in certain hospitals may be strict and result in clinicians not requesting tests for gastrointestinal symptoms |
• If a parent seeks care for their child, clinicians may not order a stool sample • Delayed or absent stool testing can prevent case confirmation, even among symptomatic children / students |
| Host factors | • Not everyone exposed will become ill. Individual susceptibility varies based on immune status, age, and other health factors | • Not everyone exposed will become ill. Individual susceptibility varies based on immune status, age, and other health factors | • Not everyone exposed will become ill. Individual susceptibility varies based on immune status, age, and other health factors | • Not everyone exposed will become ill. Individual susceptibility varies based on immune status, age, and other health factors | • Not everyone exposed will become ill. Individual susceptibility varies based on immune status, age, and other health factors | • Not everyone exposed will become ill. Individual susceptibility varies based on immune status, age, and other health factors |