Comparing foodborne illness rates between countries can be misleading because of differences in methodology and data sources, according to a study.

Foodborne illness estimation models are country-specific, making international comparisons problematic. Some disparities in estimated rates can be attributed to variations in methodology rather than real differences in risk, said scientists.

Researchers looked at foodborne disease estimates for the United Kingdom, Australia, Canada, and the United States. Findings were published in the journal BMJ Open Gastroenterology.

Published illness rates in the UK were lower than elsewher. However, when adjusted to a more like-for-like approach to the other countries, differences were smaller and often overlapped.

Misleading or distorting the facts
Following the UK’s decision to leave the European unio, one of many questions raised was whether changes in trading partners might increase food safety risks. Several commentators have quoted publicly available foodborne illness rates, especially in the UK and the United States.

The U.S. Centers for Disease Control and Prevention (CDC) estimates that 48 million people, or one in six, get ill from food each year. In the UK, the Food Standards Agency (FSA) says 2.4 million cases, or 1 in 28 people, get sick every year. 

This supports the assertion that the risk of foodborne illness in the United States is over four and a half times greater than in the UK. However, such figures hide differences in the ways data are collected, the pathogens included, the manner in which overall disease burden is attributed to foodborne transmission, and other calculations. Comparisons can be misleading or used to distort the facts deliberately, said scientists.  

For Salmonella, the United States and Canada had significantly higher rates than the UK. This difference may be because of mass vaccination of poultry in the UK beginning in the late 1990s, said researchers.

In the UK models for E. coli O157, it was assumed there was no under-reporting of the seriousness of the illness, which explains the lower estimates compared with other countries, wher adjustments for underreporting were included in the models.

Studies in Australia, Canada, and the United States used telephone surveys with a 28-day recall to estimate infectious intestinal disease. The UK produced three estimates, a cohort study plus two phone studies with 7-day and 28-day recall, respectively. In the UK, estimates are based on 2018 data, this was 2010 for Australia and 2006 for the United States and Canada.

Looking at individual pathogens
Comparisons for individual pathogens between the four studies are possible but need careful interpretation. One consideration is the year the estimates are for and seeing if any intervention measures were undertaken.

UK estimates of infectious intestinal disease, also known as acute gastroenteritis illness, are based on a cohort study, while the other three main studies reviewed used one of two different models for each pathogen depending on data availability.

Australian and Canadian studies used expert elicitation to estimate foodborne proportions while the UK and the United States used outbreak data to judge what proportion of cases from outbreaks was foodborne.

The biggest impact on estimates for foodborne illness is the approach, such as a cohort study or telephone survey. For the latter, the length of the recall period is a major contributor to differences. Scientists recommended that only estimates using the same approach are compared, and ideally undertaken at a similar period of time. 

“Genuine differences in foodborne illness rates between countries are also likely to be influenced by factors such as consumer preferences in terms of which foods are consumed and how they are prepared, diet, hygiene practices both in the food business and at home and wider environmental factors such as climate,” said researchers.

Responding to the study, Elaine Walter, Martyn Kirk, and Shannon Majowicz said comparing estimates between countries should be done with caution but is feasible and can provide important insights.

The trio, who are members of the World Health Organization’s (WHO) Foodborne Disease Burden Epidemiology Reference Group (FERG), cited previous studies wher it had worked.

They said there is more that should be done to improve interpretability, comparability, and reproducibility. This would benefit countries wanting to compare estimates over time and contribute to larger, international efforts to estimate the burden of foodborne disease. FERG is in the process of updating foodborne illness estimates published in 2015.