A large proportion of E. coli strains that produce deadly Shiga toxin go under-tested and thus under-diagnosed, according to a study by Washington State Department of Health epidemiologists who surveyed health laboratories in the state to determine how extensively they test for E. coli infections.

The study, published in the March 2012 Emerging Infectious Diseases, found that the increased use of testing for Shiga toxin in Washington between 2005 and 2010 directly correlated with increased diagnoses of non-O157 infection. Still, as of 2010, only 40 percent of stool samples tested in state labs were screened for both O157 and non-O157 strains.

E. coli O157 is the most well-known strain of Shiga toxin-producing E. coli (STEC), but many other strains have gained prominence in recent years after causing illness outbreaks of their own. Infections from many of these other strains, often referred to as “non-O157” STEC, are considered clinically indistinguishable from O157 infections, which can lead to severe diarrhea, hospitalization, kidney failure and even death. Recently, the U.S. Department of Agriculture has targeted six additional strains of non-O157 STEC to join O157 as a banned contaminant in meat.
But while indistinguishable from the others in its health impacts, O157 does require its own specific type of culture test to identify it. And up until 2005, it was largely the only type of E. coli strain being tested in clinical labs. A different test for Shiga toxin can detect any of type of STEC, including O157, but it takes longer to perform than the 24-hour O157 test.
In 2009, the Centers for Disease Control and Prevention published official STEC testing recommendations for labs, suggesting that all stool samples be tested for O157 with the culture test and all STEC strains with the Shiga toxin test simultaneously. Even though the Shiga toxin test also detects O157, the CDC recommends performing both tests to more rapidly detect O157, which is still the most common strain of STEC.

Testing for more strains of STEC beyond O157 is the only way to more accurately diagnose E. coli infection and foster more effective epidemiology, the study’s authors argued.

“Detecting a larger proportion of cases means we’re able to run PFGE [genetic fingerprinting] at Washington Public Health laboratories and detect clusters of infection so we can remove the common source,” said Kathleen Stigi, lead author of the study and an epidemiologist at the Washington State Department of Health. “With more detection comes a better understanding of the virulence factors of these strains.”

The authors surveyed each of the 57 clinical labs in Washington that test stool samples, all but one of which routinely performed at least one type of STEC test.

As mentioned already, 40 percent of stool samples in Washington were tested according to the CDC’s recommendation of dual O157 and STEC tests at the end of 2010. Labs that tested only for O157 screened 47 percent of samples, while 13 percent were only screened with the STEC test.

From 2005 through 2010, the number of labs testing for non-O157 strains rose from two to 19. During that same time, the number of detected non-O157 infections shot from eight in 2005 to 76 in 2010. The increases in non-O157 detections happened most sharply from 2008 to 2010, corresponding to the years in which the majority of the labs began testing for STEC beyond O157.

The correlation between increased testing and increased diagnoses is self-apparent, the authors said: If more labs test for non-O157 STEC, more people infected will be properly diagnosed and more outbreaks might be detected.

The authors estimated that in 2010, half of all non-O157 STEC infections that could have been detected were not, simply because the labs testing potentially infected stool samples did not perform the Shiga toxin test.

Stigi said the biggest factors holding back labs from performing both types of tests were costs, procedural changes and staffing constraints. Along with that, more STEC cases result in more epidemiology work: Every reported STEC infection requires an epidemiological investigation for local health officials. Testing for more pathogens means requiring more man-hours on the investigation side during a time when agencies are pulling their purse strings tighter.

Regardless, the authors recommended labs adopt the CDC’s dual testing strategy for the sake of public health.

“The potential virulence of non-O157 STEC infections underscores the need for enhanced laboratory testing and epidemiologic research,” the study read. “To encourage adherence to STEC testing recommendations, healthcare providers should request Shiga toxin testing if it is not routinely performed at their laboratory.”

To the authors’ knowledge, their study is the first of its kind to specifically proportion STEC testing rates in terms of stool samples tested.

Washington had 945 confirmed STEC infections between 2005 and 2010, with 83 percent of them O157 and 17 percent non-O157. The percentage of those infected, however, rose each year during the six-year study period, from 6 percent of infections in 2005 to 41 percent in 2010. Again, these results correlated with the increase in non-O157 STEC testing. The authors predicted that non-O157 infections would have accounted for 60 percent of all STEC infections in 2010 if all labs had the infrastructure to test for them.

Four strains of STEC accounted for the large majority of all non-O157 infections in the state: O26 (48 percent), O103 (18 percent), O121 (12 percent) and O111 (5 percent).

Stigi said she believes more labs have implemented Shiga toxin testing procedures since the surveys were conducted in early 2011, and she expects the number to grow, as epidemiologists recognize that accurately tracking disease trends will rely on more complete data.

“These enhanced testing practices have likely contributed to the increased detection of STEC in Washington,” she said. “It’s important for public health professionals to assess laboratory STEC testing practices in order to correctly interpret disease trends.”

  • M. “Mike” Mychajlonka, Ph. D.

    The Shiga toxin “A chain” possesses enzymatic activity (N-glycosidase). The mode of action of this toxin is that its enzymatic activity is able to depurinate a specific region of the eukaryotic ribosomal RNA. Such a modified ribosome is no longer able to bind the elongation factor that is needed to create the next peptide link on the polypeptide the ribosome is in the process of making. The synthesis of that protein stops where it is because the affected ribosome is now no longer in the protein manufacturing business. Because Shiga toxin is an enzyme, it simply goes looking for new substrate, i.e., other ribosomes to inactivate. Before too long, there is no protein synthesis going on in the cell at all and the cell dies. If the appropriate receptors are present on the cells of the organism, the B-subunit of the Shiga toxin can facilitate entry into a new cell containing more functioning for the Shiga toxin to inactivate and so on. In cattle, such cell to cell transmission is relatively ineffective. In humans, it tends to go like gang-busters.
    Cattle are subject to infection by bovine leukemia virus. Unchecked, this can be a devastating disease ending in death. However, it has been known for some years now that, in bovines, Shiga toxin is able to protect the animal against bovine leukemia virus because the enzymatic activity is able to inactivate the RNA-based genetic information that this retrovirus needs for its own replication. This situation leads to an intriguing speculation. Most of the bovines we humans feast upon spend their last days before slaughter in feedlots where they are quite stressed in consequence of being so crowded together, a condition known to favor infections of all kinds. It is quite conceivable [as has been suggested in the literature – Infection and Immunity (2003) 71(1):327] that under these conditions, evolution is at work spreading (with the help of the ever helpful E. coli) the Shiga toxin that helps keep bovines alive under such conditions. In other words, in the presence of Shiga toxin the bovine is able to live and grow with bovine leukemia virus as a chronic disease rather than as an acutely lethal infection.
    The CDC has already come up with a Shiga-toxin protocol using a synthetic substrate, which allows quantitation of minute amounts of Shiga toxin. This protocol is said to take some two hours to a final result. Since it is a functional assay (measuring N-glycosidase activity itself) it will work equally well with any Shiga toxin. The only real problem with this assay is that it requires a mass spectrometer (MALDI MS/MS). These instruments are not cheap, neither to buy nor to keep. I would welcome comments on this scheme from those in the meat industry who might be interested in a rapid and extremely sensitive method that might make for a far more effective CCP than the other tests out there right now (microbial isolation or antibody). If the guardians of this discussion forum object to inclusion of my email address (mikem@foodsafetyanalysis.com), just Google my name. I’m in the book and am the only one with that name in the State of Michigan.

  • In Canada, and particularly in Alberta, the current practice for cattle finishing is to confine these animals into high density feedlots. It is now expected that the majority of cattle will test positive for E. coli O175:H7. We have confirmed this in our own testing analysis of 1 particular site in Alberta. Of particular concern is their location and proximity to irrigation canals. With such heaving loading of manure into these canals during the course of the irrigation season we expect to see high incidents of vegetable product contamination. The presence of E. coli, E. coli O157:H7, and Salmonella was found in in our testing group of 60 sites located in Southern Alberta along existing irrigation Canals. Alberta Health Services (AHS) has conducted and provided a mapping of intestinal food related illnesses based on patient residence location (home address). We find there to be a high correlation of that map and the location of production sites which also suggests that workers within these facilities are also carrying / transmitting infection to their communities and family members. Unfortunately, unlike CDC’s primary recommendations patients in Canada are not routinely tested for the presence of Shiga toxin in case of preliminary diagnosis of E. coli / Salmonella or Campylobactor infection. Rather this can only be done by direct request of the family physician and / or pathologist upon death. Therefore, AHS is likely to be vastly under reporting these outbreaks in Canada. Despite that, Alberta ranks the highest province in Canada for intestinal related illness and also ranks the highest province for the number of livestock animals in production. This is despite the Canadian Cattleman’s Association (CCA)adopting a zero tolerance position of E. coli O157:H7 to existing within cattle populations in the province in the summer of 2000. Note also that the existing of an existing cattle vaccination to eliminate / control the continual shedding of E. coli O157:H7 within existing populations has not been adopted within Canada, despite the $10 vaccine being produced and readily by Bioniche located in Belleville Ontario. Please also note that in Canada the governmental gap that exists between the Canadian Food Inspection Agency (CFIA), Health Services Canada, Provincial Health Agencies, Livestock Producer Associations, Federal Animal Health Act administrators (including CFIA and the Canadian Federal Departement of Agriculture), Canadian Veterinarians Associations, and Provincial Natural Resource Conservation Boards has assured the breakdown of communication and inability to adequately respond to a massive outbreak as was experienced in Europe last summer.