A recent study aiming to describe the impact of the COVID-19 pandemic on cases of foodborne enteric diseases in Canada found a considerable reduction in cases in 2020 compared to pre-pandemic levels.

For the period of March–December 2020, weekly counts of laboratory confirmed cases of Salmonella, Shigella, Shiga toxin-producing Escherichia coli (STEC), and Listeria monocytogenes were obtained from Canada’s National Enteric Surveillance Program (NESP). The study also collected epidemiological information from PulseNet Canada regarding the suspected source of illness for clusters of cases identified by whole genome sequencing (WGS). Next, using the data, researchers calculated incidence ratios for each pathogen and compared the data against the pre-pandemic period.

Compared with the average number of cases (9,053) reported by provincial public health laboratories in the five years prior to 2020, there was a 33 percent reduction in the total number of cases (6,054) of enteric pathogens reported nationally in 2020. Decreases in the number of reported cases in 2020 compared with the previous 5-year period were noted for Salmonella, Shigella, E. coli O157, and non-O157 STEC. However, the reported number of cases for L. monocytogenes in 2020 remained similar to those of the previous 5-year period.

Overall, total count of reported cases of enteric illness in Canada declined in 2020 compared with historical averages, and was the lowest total count in 23 years of monitoring enteric illness at the national level. Similar declines in reported enteric illness incidence in 2020 were observed in other countries, including the U.S., which observed a 26 percent decrease in reported enteric illness incidence in 2020.

There are several hypotheses as to what caused such declines in incidences of enteric diseases related to the COVID-19 pandemic, such as:

  • Avoidance of healthcare settings
  • Changes in behaviors related to exposure
  • Decreases in international travel.

The study’s findings provide support for these hypotheses. Regarding avoidance of healthcare settings, of the pathogens included in this analysis, L. monocytogenes is the most likely to cause severe disease. As health care seeking behavior is associated with disease severity, the lack of reduction in L. monocytogenes’ 2020 reported incidence is possibly due to the pathogen being likely to cause more severe illness compared with the other bacteria assessed.

Additionally, the fact that Shigella cases saw the sharpest decline compared to the other pathogens assessed suggests that changes in behaviors leading to minimal person-to-person contact directly affected the spread of enteric pathogens. This is because, in Canada, a majority of Shigella cases (52.4 percent) are believed to be acquired through person-to-person transmission.

The largest reduction observed across all analyses conducted was the decline in cases added to multijurisdictional clusters that were associated with international travel. The researchers believe that  it is possible that the bulk of the reductions in observed incidence rates is attributable to reductions to international travel.