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Diagnosing Foodborne Illness Outbreaks


Foodborne illness outbreaks result when two or more persons develop similar symptoms of illness (gastroenteritis) after eating a common food, or become ill after consuming food from a common source. Such events occur relatively frequently and create various degrees of crises for the firms and victims involved.

While food can be contaminated with a chemical or physical agent, the vast majority of foodborne illness outbreaks are due to contamination by microbes. There are at least 200 of such foodborne pathogens, including bacteria, virus and parasites. Some are less common, such as Plesimonas shigelloides, and there are a handful of common ones, including Norovirus, Listeria monocytogenes, Salmonella spp. and E. coli O157:H7.

Foodborne illness outbreaks often require the intervention of the agencies tasked with public health protection. State and local health departments are usually the first to field reports and investigate. Federal agencies such as the Centers for Disease Control and Prevention, the Food and Drug Administration, the U.S. Department of Agriculture and other federal agencies will also respond as necessary upon notification. These agencies work together during large multi-state investigations to uncover the sources of product contamination with the intention of removing the contaminated products from commerce as soon as possible.

While federal authorities may become involved, most of the work is done by local sanitarians, epidemiologists, public health nurses and clinicians in county and state health departments, with only occasional assistance from the federal agencies.

Epidemiologic investigations seek to uncover the “time-place-person” factors associated with an outbreak, such as the vehicle (food or beverage), the source of contamination, the exposed population, the number of ill persons and their characteristics and associated timelines. Various entry points for contamination need to be identified and include farms, packinghouses, slaughterhouses, processors of all types, manufacturers, foodservices, institutions, and community events, just to name a few.

Sanitarians (inspectors) working in public health agencies (environmental health units) evaluate associated environmental health factors, wherever contamination is suspected in the supply chain with the purpose of identifying the source of the offending agent and cutting off its means of spread. Inspections may uncover a variety of factors such as a contaminated food source, contaminated water supply (such as in produce washing operations), time and temperature abuse (typically in foodservice), poor hygiene, ill workers, contaminated equipment, environmental contamination, cross-contamination or ineffective cooking or heating.

Public health professionals analyze the findings of the investigation and make decisions to notify the public, recall products, stop the sale of products and/or close affected businesses. Even though agencies rarely pursue administrative action against an operation causing an outbreak, the political fallout from such interventions is often intense, causing agencies to act judiciously. Unfortunately, such realities may result in outbreaks continuing for some time before an agency is legitimately able to take action. Some have questioned the timing of agencies when “going public” and argue that a standard method is needed in that decision-making process.

The process of identifying the causes of a foodborne illness outbreak currently rests heavily on the results of molecular or traditional microbial analysis coupled with statistical techniques. In the past, before health departments had the benefit of molecular genotyping (DNA analysis), outbreak investigations were not always as conclusive. With sophisticated tools such as pulsed-field gel electrophoresis now in hand, investigators are increasingly able to identify common links in cases and thus determine the sources of contamination, the implicated products and the exposed populations with increasing accuracy.

When analysis reveals patterns of genetically and clinically associated subtypes of known pathogens, agencies should recognize that an outbreak is occurring and take action, including notification of the public when called for.

A process data collection and analysis ensues whenever there are verifiable reports of outbreaks. This includes an environmental assessment of operations and facilities to pinpoint the most likely mechanisms for the propagation and spread of the pathogen.

Finding the suspect microorganism in either the incriminated food, the victims, or, ideally, in both, helps to confirm an outbreak, and this information also provides a framework for the environmental assessment of a facility. For example, when a strain of Salmonella is found in a specific food product, the sanitarian’s work is then simply to determine the source of food and the production methods for that item that allowed Salmonella to contaminate, grow and survive. By identifying the sources of foods and ingredients, storage conditions, food handling, preparation and service, it is possible for a sanitarian to determine the most likely causes of an outbreak. The sanitarian uses the guidance of the FDA Food Code to identify lapses in sanitation likely associated with the outbreak. The code specifies the hygienic standards, sanitation procedures and process controls necessary for the safe production of food, while deviations from the code indicate that operators may not be in control.

Compliance with the code itself provides a basis for determining the factors likely associated with an outbreak; however, the value of the findings may be limited by certain factors. Those limitations include that the investigation may occur some time after the outbreak is identified, the operation may not be producing the incriminated food at the time of inspection, or operators may sell, discard or destroy the food item of interest before the inspection can take place.

The inspection of an operation can provide convincing evidence for causation when both the pathogen and vehicle are known; however, associated factors may be less obvious when pathogens have not been confirmed, or when epidemiological evidence is lacking, e.g., specific foods lack a strong association with cases.

Public health agencies may identify a foodborne illness outbreak when a sharp increase in cases of gastroenteritis occurs in a population, or when investigators find patterns in the incidence of cases of reportable diseases such as E. coli O157:H7 linked in time. Outbreaks may be short-lived episodes where victims recover quickly with no sequelae, or such events may be protracted over several weeks and affect a large number of victims with severe symptoms. Large, persistent outbreaks tend to be well-documented. Numerous cases result in more data and investigators can make a more accurate analysis when there are better food histories and clinical reporting. In addition, extended timeframes often allow for in-depth facility inspections to take place. Such well-documented larger events lend themselves to theories of causation. When there are limited data on victims or products consumed, causation is less clear, but a successful environmental assessment can still reveal important clues and uncover probable causes of contamination that help to strengthen a hypothesis.

While investigators should investigate all reports of outbreaks, the lack of resources available to health departments often limits their efforts. Too often, investigators fail to uncover the cause and the mechanisms at work. This means prevention becomes less effective and repeat occurrences are more likely.

Environmental assessment techniques

When a particular food item is suspected, the sanitarian follows the flow of that food item through production from receipt of the ingredients through to service. If that item is not being produced, a similar recipe or food production process can be examined. When deemed appropriate, the sanitarian may also collect environmental samples, which includes surface swabs and food items.

Hazard Analysis Critical Control Point, or HACCP, is a system of preventive controls that identifies hazards and establishes steps of production where those hazards can be eliminated or reduced to safe levels. Since the HACCP process identifies categories of microbiological hazards in food (contamination, growth and/or survival), sanitarians can use HACCP as an investigative tool. Using HACCP principles, the key steps of production are identified and the investigator can determine if operators properly applied food code standards to the most hazardous production steps. Investigators can also use HACCP to better target microbial sampling.

When the pathogen is not confirmed, the symptomology of victims is important to the environmental assessment. Symptoms of nausea and diarrhea are common to many foodborne illness etiologies, but their frequency in victims, coupled with average incubation period and length of symptoms, can provide clues to specific pathogens. For example, bacterial infections such as salmonellosis, campylobacteriosis, vibriosis and shigellosis are usually accompanied by fever, whereas infections with E. coli O157:H7 and related serotypes cause bloody diarrhea as the predominant symptom. Intoxications caused by Staphylococcus aureus and the spore-forming bacteria Bacillus cereus and Clostridium perfringens produce a rapid onset of symptoms, with nausea and vomiting as the predominant symptoms, while Clostridium botulinum is unique for causing descending bilateral paralysis.

Different levels of the supply chain have different risks. For example, viral infections are often passed between ill workers and patrons in food service settings through the handling of ready-to-eat foods with bare hands, but this rarely occurs at the manufacturing level.

When the contamination with infectious organisms occurs farther up to food chain, for example at the manufacturing or processing level, factors are less likely to involve direct food handling mistakes and more likely to include contaminated equipment, cross-contamination and process failures. When the primary producer level is suspected as the point of exposure, especially when produce is involved, contaminated water supplies become very important.

The distinction between “infection” and “intoxication” is another important consideration, as factors that give rise to bacterial infections through food may be significantly different from those associated with the spore-forming or intoxicating microorganisms. Cross-contamination, a human carrier or a contaminated source of ingredients, are often associated with infectious disease, while time and temperature abuse is always necessary for intoxicating microorganisms to proliferate and/or produce toxins in foods.

Investigators suspect Norovirus as the cause of foodborne illness when clinical specimens do not reveal bacterial agents and victims experience a rapid onset of nausea and vomiting combined with low-grade fever. In light of a Norovirus outbreak, investigations often focus on hygienic standards and the presence of infected workers. Investigators using molecular assay techniques, such as RT/PCR (reverse transcription polymerase chain reaction), can often detect the virus in food handlers and occasionally in surface swabs of equipment and environment. In such viral GI outbreaks, a ready-to-eat food is usually involved.

Sometimes unique symptoms such as septicemia and infected lesions in persons consuming raw shellfish clearly point to the pathogen Vibrio vulnificus. Other unique symptomologies include hemolytic uremic syndrome associated with E. coli O157:H7, the previously mentioned descending bilateral paralysis of botulism and the temperature reversal phenomena in cases of Ciguatera (from ingestion of inshore marine finfish such as barracuda, mackerel and jacks). All of these etiologies, whether known or suspected, will influence how a sanitarian goes about an environmental assessment.

For example, the source of shellfish is always an important issue when vibriosis is identified. Sanitarians in such cases typically collect and analyze the required shellfish tags and may also collect receipts of shellfish deliveries. Infections with Vibrio vulnificus don’t usually result in outbreaks, but investigators should look into such illnesses due to the severity of symptoms, with mortality reaching up to 40 percent of affected cases. Vibrio infection is the direct result of exposure of at-risk consumers to seawater or shellfish harvested from areas where seawater temperatures and other environmental conditions favor the presence of the free-living marine bacteria. While Vibrio vulnificus is an opportunistic pathogen, poor handling of shellfish anywhere in the supply chain increases the likelihood of exposure.

In the case of Ciguatera intoxication, one or several victims may be involved in an outbreak scenario. Ciguatoxin is bio-accumulated in certain finfish and is heat stable, making cooking of no importance to prevention. Ciguatera symptoms involve both the digestive and neurological systems, with some victims experiencing temporary paralysis. As in vibriosis cases, the sanitarian is concerned about the source of the fish, but there is usually no connection between poor handling and cases, except perhaps for cross-contamination that can occur during preparation.

To be effective in the role of an environmental investigator, a sanitarian must be well-versed in several disciplines and have expert knowledge of the characteristics and routes of transmission of a wide host of foodborne illness etiologies.

© Food Safety News
  • Sansher

    great article by a wonderful ex Florida Public Health EH Man.