Researchers have assessed the factors behind an increase in Shiga toxin-producing E. coli (STEC) infections in Switzerland.

Since 2015, a large rise in cases has been observed. This coincided with introducing multiplex polymerase chain reaction (PCR) panels for stool analyzes in standard laboratory practice in Switzerland suggesting that the increase in notified patients was because of a change in test practices and numbers.

While it is clear that changes in diagnostics influence surveillance data and trend monitoring, scientists believe this change only explains part of the increase in STEC notifications in Switzerland.

The rise in notified STEC cases is a combination of changing test practices and a real increase in incidence of infections among the Swiss population, according to a study published in the journal Eurosurveillance.

Milder infections
Prior to gradual introduction of multiplex PCR to routine diagnostics between 2011 and 2015, STEC was only specifically tested for in Switzerland upon physician request, and this rarely happened, said researchers. Since their introduction, these tests have become the largest proportion of all diagnostic checks for STEC and have led to an increase in test numbers.

The team analyzed routine data from 11 labs, which reported 62 percent of all STEC cases from 2007 to 2016 to calculate the positivity, i.e. the rate of the number of positive STEC tests divided by the number of tests performed.

The impact of changes in diagnostic approaches on public health surveillance has been highlighted before, especially the switch from culture-dependent to culture-independent diagnostics for foodborne diseases. A trend of culture-independent diagnostic tests means more detection of less virulent strains. However, the 11 Swiss diagnostic labs in the study switched to culture-independent methods for STEC detection before 2007.

Researchers hypothesized that if the increase in new STEC cases was because of the introduction of multiplex PCR only (leading to less targeted screening) there would be a decrease in positivity because of a lower pre-test probability for a positive test outcome. But this was not reflected in the data. Instead, the increase in STEC cases is disproportionally higher compared with the rise in test volume, resulting in the observed increase in positivity.

More tests and positives
E. coli infections continued to climb in 2019, according to the country’s surveillance report on zoonoses and foodborne outbreaks.

The number of cases of a type of kidney failure associated with E. coli infections, hemolytic uremic syndrome (HUS), remained relatively constant from 1999 to 2015. So, the increase in STEC notifications is likely to represent mainly mild cases and/or asymptomatic co-infections that might have been present but undetected in the past.

A changing distribution of STEC serogroups among patients could be an explanation for the change in disease severity, said researchers.

The final dataset comprised 86,043 records, of which, 1,149 were positives. Median age of the tested population increased from 30 to 43 years between 2007 and 2016. Among the STEC-positive population, the median age also went up significantly.

Between 2007 and 2016 the number of multiplex PCR panels performed rose by 42 percent. The positivity of this test increased from 80 of 7,617 in 2014 to 418 of 24,190 in 2016.

The number of tests for STEC increased sevenfold from 2007 to 2016 from 3,711 to 26,639 while the amount of positive test results increased 13-fold from 33 to 440.

Positivity increased for all age groups. Calculated over the study period it was highest for children 1 to 4 years old and increased from 11 of 809 in 2007 to 51 of 1,734 in 2016. The largest relative increase was in those older than 80, from no case among 146 in 2007 to 45 of 2,449 in 2016.

Researchers said the current Swiss surveillance system for STEC could be improved by incorporating strain typing information that would guide intervention and control measures.

“We believe it is critical that all cases of STEC infections, regardless of clinical relevance, are reported in order to identify clusters and sources and thus support outbreak control.”