How Canada estimates food-borne illness

Why do we estimate food-borne illness in Canada?

Public health surveillance systems only record a small portion of the total number of food-borne illnesses. This is because:

  • many people are never diagnosed with a food-borne illness because:
    • they do not seek care and get better on their own
    • they do not have a sample tested (stool, urine or blood)
    • a laboratory test may not identify the bacteria, parasite or virus that is causing the food-borne illness
  • some illnesses are not reported to the various public health surveillance systems

It is important to understand the true burden of food-borne illness in Canada. To help us estimate with greater accuracy, we use a set of formulas.

How do we estimate food-borne illness in Canada?

Canada estimates food-borne illness caused by bacteria, parasites and viruses as well as from unknown causes. For recent estimates, we used 2000 to 2010 data from several national, provincial and territorial public health surveillance systems.

Estimating illnesses for known bacteria, parasites and viruses

We use 2 methods to estimate the number of food-borne illnesses for:

  • known bacteria, parasites and viruses for which there are surveillance numbers
  • viruses and bacteria that are not part of standard surveillance

To correct for under-reporting and under-diagnosis, we developed a formula (shown below).

Formula 1 - First approach: For pathogens where laboratory-confirmed cases were scaled

Laboratory confirmed illness
× Proportion domestically acquired
× Under-reporting multiplier: Laboratory to local public health and local public health to provincial and territorial public health
× Under-diagnosis multiplier: Proportion severe, Care seeking, Specimen submission, Laboratory testing, Laboratory sensitivity
× Proportion food-borne
= Estimated annual number of domestically acquired food-borne illnesses by pathogen

The formula also involves multiplying by the fraction:

  • of illnesses acquired in Canada (not acquired during international travel)
  • obtained by eating food

Bacteria, parasites and viruses reported through national surveillance systems using this method include:

  • Brucella
  • Campylobacter
  • Clostridium botulinum
  • Cryptosporidium
  • Cyclospora cayetanensis
  • VTEC O157
  • Giardia

They also include:

  • hepatitis A virus
  • Salmonella, non-typhoidal
  • Salmonella Typhi
  • Shigella
  • Vibrio cholera
  • Vibrio, other
  • Vibrio vulnificus

 Bacteria identified through provincial surveillance systems include:

  • Trichinella
  • Listeria monocytogenes
  • Vibrio parahaemolyticus
  • Yersinia enterocolitica

For viruses and bacteria that are not reported by standard surveillance systems, we estimate the:

  • number of Canadians who may have symptoms (like diarrhea)
  • percentage whose symptoms are caused by a particular virus or bacterium

We multiply this estimate by the:

  • amount of illnesses acquired in Canada
  • fraction obtained by eating food

This process is shown below.

Formula 2 - Second approach: For pathogens where Canadian population was scaled down

Canadian population
× Rate of symptoms per person per year
× Proportion of symptoms attributed to pathogen
= Total estimated episodes of pathogen
× Proportion domestically acquired
× Proportion food-borne
= Estimated annual number of domestically acquired food-borne illnesses by pathogen

The bacteria, parasites and viruses included in this category are:

  • adenovirus
  • astrovirus
  • norovirus
  • rotavirus
  • sapovirus
  • Toxoplasma gondii
  • Clostridium perfringens

Bacteria estimates that are calculated using other methods include:

  • Escherichia coli, other diarrheagenic
  • ETEC
  • VTEC non-O157
  • Bacillus cereus
  • Staphylococcus aureus

Estimates for each bacterium, parasite and virus are added together. This gives us an overall total estimate for food-borne illnesses caused by known pathogens.

Estimating illnesses from unknown causes

We estimate the number of people with food-borne illnesses from unknown causes. To do this, we collect data on symptoms from different surveys to estimate the total number of episodes of acute gastrointestinal illnesses.

We subtract the number of illnesses caused by known causes from the total number of acute gastrointestinal illnesses. We then multiply this number by the fraction:

  • of illnesses caught in Canada
  • obtained by consuming food

The formula is shown below.

Formula 3 - Approach for unspecified agents

Total acute gastrointestinal illnesses (AGI)
− Total illnesses related to 25 known AGI pathogens
= Total illnesses related to unspecified agents
× Proportion domestically acquired for 25 known AGI pathogens
× Proportion food-borne for 25 known AGI pathogens
= Estimated annual number of domestically acquired food-borne illness related to unspecified agents

Note: Estimated proportions were based on 25 known pathogens that cause acute gastrointestinal illness. Not included in these estimates are 5 pathogens whose main symptoms were not acute gastrointestinal illness.

Estimating hospitalizations and deaths

We use 2 methods to estimate hospitalizations and deaths from known causes of food-borne illness.

  1. For each bacterium, parasite and virus with hospitalization data in the Canadian Institute for Health Information Hospital Morbidity Database, we identify the annual number of hospitalizations and deaths. We adjust both the number of hospitalizations and deaths to take into account under-diagnosis and under-reporting. See the figure below.

Formula 4 - First approach: hospitalizations and deaths estimates

Annual number of hospitalizations and deaths, Canadian Institute for Health Information - Hospital Morbidity Database (CIHI-HMDB)
× CIHI-HMDB under- reporting multiplier
× Under-diagnosis multiplier: lab testing and test sensitivity
× Proportion domestically acquired
× Proportion foodborne
= Estimated number of hospitalizations and deaths related to domestically acquired, food-borne illness

  1. For bacteria and viruses not included in this database, we look to the number of cases reported to national surveillance systems. We then apply a fraction to those cases that resulted in hospitalizations or deaths. We adjust this number to take into account under-diagnosis. See the figure below.

Formula 5 - Second approach: hospitalizations and deaths estimates

Laboratory-confirmed cases
× Under-reporting multiplier
× Proportion hospitalized or died
× Under-diagnosis multiplier: lab testing and test sensitivity
× Proportion domestically acquired
× Proportion foodborne
= Estimated number of hospitalizations and deaths related to domestically acquired, food-borne illness

Both methods involve multiplying the adjusted number by the fraction:

  • of illnesses acquired in Canada (not acquired during international travel)
  • obtained by eating food

Hospitalizations and deaths associated with unknown causes were also estimated. A similar approach to the one used for estimating food-borne illness from unknown causes was used.

Accounting for uncertainty of estimates

All formulas use:

  • an uncertainty model to account for variability and data uncertainty
  • this generates a point estimate and a 90% probability interval to arrive at upper and lower limits

What improvements have been made in how Canada estimates food-borne illness?

Canada’s recent estimates of 4 million illnesses occurring yearly from food-borne diseases are more accurate than the 11 million published in 2008. There are a number of reasons for this.

  • Earlier estimates were based on data and methodologies used by the U.S. Centers for Disease Control and Prevention in 1999.
  • Improvements in our methodology and data mean more accurate estimates.
  • The estimate now includes pathogen-specific estimates. We can now identify and rank the pathogens responsible for causing the highest number of food-borne illnesses.
  • The estimates are now limited to food-borne illnesses acquired only in Canada.
  • A more precise definition for acute gastrointestinal illness tries to factor out:
    • chronic illnesses such as Crohn’s disease
    • symptoms related to a respiratory infection
  • A smaller proportion of food-borne illness is applied to unknown causes of acute gastrointestinal illness.
  • We are accounting for uncertainty.

How does Canada compare to the United States?

In Canada, the overall yearly estimate of illnesses caused by foods is slightly less than the U.S. estimate from the Centers for Disease Control and Prevention:

  • 1 in 8 Canadians is affected by food-borne illness
  • 1 in 6 Americans is affected by food-borne illness

The same 4 pathogens cause the highest number of illnesses in both countries. However, the top illnesses in the U.S. are ranked in a different order than for Canada:

  1. norovirus
  2. Clostridium perfringens
  3. Campylobacter spp.
  4. Salmonella spp., non-typhoidal (ranked second in the U.S.)

In the U.S., norovirus, Salmonella spp. and Campylobacter spp. were also found to be leading causes of bacterial food-borne hospitalizations.

Listeria monocytogenes and Salmonella spp.ranked high in pathogens causing death in both countries.

While the methods used by Canada and the U.S. are very similar, there are key differences.

  • In Canada, we developed estimates for adenovirus and excluded Mycobacterium bovis and Streptococcus Group A from our estimates. The U.S. included these agents in its estimates. 
  • For our definition of severe symptoms, we included both bloody diarrhea and duration (symptoms lasting longer than 7 days). The U.S. considered bloody diarrhea only.
  • For rotavirus, astrovirus and sapovirus, we came up with an estimate based on the total population. The U.S. estimates were for individuals under 5 years of age.
  • For hospitalizations and deaths, different calculations were used to account for under-reporting and under-diagnosis.
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