I left off yesterday with the initial discovery of “Vero toxin,” a toxin produced by E. coli (also called “Shiga toxin” or “Shiga-like toxin”). Though this may initially seem unconnected to hemolytic uremic syndrome (HUS), the discovery of this cytotoxin paved the way for a clearer understanding of the etiology of this syndrome, as well as the mechanisms by which disease progressed. By the early 1980s, several lines of research pointed toward E. coli, and particularly O157:H7, as the main cause of HUS.

A 1982 Centers for Disease Control and Prevention MMWR report found a rare E. coli serotype, O157:H7, associated with hemorrhagic colitis following consumption of hamburgers. Similar results were reported in a 1983 Lancet paper, which found serotype O157 among their collection of verotoxin-producing strains. Another paper that same year from a Canadian group showed that O157:H7 was the second most common cytotoxic strain in their collection of over 2,000 E. coli isolates. The most common was serotype O26–more on that below. This paper also discussed an outbreak of hemorrhagic colitis that had occurred at a nursing home, with O157 identified as the cause. The evidence was mounting, but these were small studies and not always associated with HUS. Still, these papers collectively were suggestive of a connection between E. coli infection (especially with strains that produced the shiga/vero toxin), hemorrhagic colitis, and HUS.

In 1985, a new study came out which really helped to seal the deal. Rather than look only at cases in isolation, the authors designed a case-control study looking at patients with “idiopathic HUS” (in other words, HUS of unknown origin which started with diarrhea, rather than the other variant lacking this symptom). They ended up with 40 patients who qualified. They then picked a single control for each patient, matching them on age, sex, and season of the year. The controls were children either diagnosed with Campylobacter enterocolitis (and therefore, enterocolitis of a known cause) or were healthy children either from a local daycare center, or kids coming in for elective surgeries. Stools were collected from each group and tested for a variety of organisms, including vero toxin-producing E. coli (VTEC, also known as STEC for the shiga-like toxin nomenclature). They also tested for activity of the toxin itself in fecal samples. Finally, in the case patients, attempts were made to collect what are called “acute” and “convalescent” blood samples. These are samples taken when the patient is actually sick (“acute”), and then ones taken a few weeks later (“convalescent), to look at the presence of antibodies in the blood. If it was an infection by the suspected organism (in this case, STEC/VTEC), you should see a rise in antibodies the host produces that target the organism–for these kids, they were looking for antibodies to the shiga/vero toxin.

They found either vero toxin or VTEC in 60% of the case patients, but in none of the controls. Of the VTEC isolated, serotypes included O26, O111, O113, O121, and O157. For the latter, it was the most common type isolated (25% of the VTEC found). Of the patients who were negative for both VTEC and vero toxin, from those who had paired blood samples (12/16 of the remaining cases), 6 did show a rise in antibody titer against the vero toxin–suggesting they had been exposed and were producing antibodies to neutralize the toxin. So, for those keeping score, 75% of the cases had evidence of VTEC infection either by culture or serological techniques. It may not have been the nail in the coffin and there are certainly some flaws (the diversity of controls and lack of analysis of blood titers for the controls being two that pop out at me), but this paper went a long way toward establishing VTEC/STEC as the cause of HUS, which has been subsequently confirmed by many, many studies worldwide.

The most common vehicles of transmission of these organisms have also come into clearer focus since the 1950s, with almost all HUS/STEC outbreaks associated with food products; most common is still the O157:H7 serotype. O157 is a bit unique, in that this strain typically doesn’t ferment sorbitol–as such, this is often used as a diagnostic feature that sets it apart from “normal” E. coli. However, as I mentioned above (and as the current outbreak has shown), a number of other serotypes besides O157:H7 can also cause HUS. Most of these don’t appear to be as commonly associated with outbreaks–rather, they may more commonly cause sporadic disease where fewer people may become sick. Because these don’t have the unique sorbitol-non-fermenting feature, these may be overlooked at a diagnostic lab. There are assays that can detect the Shiga-like toxin directly (actually, we now know there are multiple families of related toxins), but not all labs use these routinely, so it’s likely that the incidence of infection due to non-O157 STEC is higher than we currently know.

HUS was once a mysterious, “complex” disease whose perceived etiology shifted almost overnight, as scientific advances go. What implications does this have for other diseases whose etiology is similarly described as HUS was 50 years ago? More on that tomorrow.

Comments

  1. #1 Anonymous
    June 22, 2011

    Thanks for the concise HUS history. FYI, interesting comments from CDC’s Robert Tauxe on some of the unresolved questions about the German outbreak: http://www.foodsafetynews.com/2011/06/outbreak-an-interview-with-dr-robert-v-tauxe/
    Are you aware of any papers other than Cassar 2004 Vet Record 154: 237-239 indicating that the original EAEC strain would have no animal reservoir?

  2. #2 Tara C. Smith
    June 22, 2011

    Thanks! That’s a good interview. I can’t access the Vet Record paper from here (currently at a conference) so not sure what it claims.

  3. #3 Anonymous
    June 23, 2011

    Cassar et al 2004 failed to detect EAEC in approx 1200 samples drawn from cattle, sheep and pigs at slaughter in Great Britain (~400/spp). This was the paper cited by EFSA’s statement that “EAEC have rarely been identified in animals, suggesting that they are not zoonotic, but exclusive to humans as a pathogen.” I presume there are other lines of evidence that support the notion that the outbreak strain most likely comes from a human reservoir.