Foodborne Disease Outbreak Investigation Team Training: Epilogue to the Investigations We have talked about the environmental health, epidemiologic, and laboratory investigations associated with a foodborne disease outbreak. It is easy to get caught up in these investigations. But remember …

Implement control measures whenever sufficient information is available! The primary goal of an outbreak investigation is to implement appropriate control measures to halt transmission as quickly as possible, limit the morbidity and mortality resulting from the outbreak, and prevent similar outbreaks in the future. Don’t let the investigation get in the way of this. Control measures should be implemented whenever sufficient information is available to make sure the control measures are appropriate and well-directed. But sometimes trying to implement control measures as soon as possible backfires. Mistakes do occur.

Mistakes Do Occur Nationwide outbreak of cyclosporiasis One site announces outbreak due to California strawberries Strawberry market devastated True source Guatemalan raspberries One such example was an outbreak of cyclosporiasis that was linked in error by a state health department to the wrong food item. The state health department had undertaken a cohort study that suggested that California strawberries (which were served as a garnish on a dessert) were the source of the outbreak that had occurred at a conference dinner. However, investigators did not have a full menu from the dinner and were not aware that the implicated dessert garnish included both raspberries and strawberries. (And it was the raspberries not the strawberries that were the source of the outbreak.) Media reports based on the erroneous conclusions led to millions of dollars in lost strawberry sales, even though the error was rapidly corrected. Supermarket chains took California strawberries off their shelves in response to pressure from consumers. Consumers stopped buying strawberries from all sources. Truckloads of strawberries headed for market rotted as they were turned away by produce and grocery store managers. Strawberry sales around the United States and Canada crashed, costing the industry $40 million and the loss of 5,000 jobs. (This situation probably could have been avoided if investigators had considered results from simultaneous investigations as this was a multijurisdictional outbreak with cases occurring across the nation. Other sites were consistently implicating raspberries as the source of the outbreak.) Concern about mistakes should not lead us to inaction. Inaction might mean more disease and even deaths. But investigators do need to consider the consequences of both implementing and not implementing control measures on the health of the public and businesses and industry and act accordingly.

How to Avoid Mistakes Use body of evidence to make decisions Follow the tenets of causation Exposure precedes illness Strength of association Dose-response relationship Consistency of data with other investigations Biologic plausibility Balance public health consequences of not taking action with quality of data and potential damage to business/industry So how do you make decisions? Consider the entire body of evidence (results from various studies) and build a case for the implicated food being the source of the outbreak. (Decisions are never based on one study or one statistically significant finding.) Use the tenets of causation to drive your decisions. Look at : Whether exposure preceded illness - The time from consumption of the food to disease development must be consistent with the known incubation period of the illness. Strength of association - The higher the odds ratio or relative risk, the more likely it is that the relationship between an exposure and a disease is causal. Biologic gradient of response (dose response) - Individuals who ate more of a suspected food item should be more likely to become ill than those who ate only a little bit. Consistency of findings across independent studies – Findings from all studies (epidemiologic, environmental health, laboratory) should support each other as well as studies from other sites. Biological plausibility - A finding is biologically plausible if the association between the exposure and the disease is supported by our currently accepted understanding of the causative agent including the modes of transmission of the agent as well as its ability to contaminate a certain food and survive and proliferate in the food (or produce toxin). If these tenets are met, you will have a strong case for a particular food as the source of an outbreak. Finally, balance the potential public health impact of a problem (consequences of not taking action) with the known quality of available data and the potential damage to business or industry. If the disease is life threatening and exposure to the causative agent is ongoing, it is likely you will take action even when the quality of the data is limited or the risks to business are high. If the disease is less severe and exposure no longer seems to be occurring, you will be less likely to proceed if the data are questionable and the risks to business are high.