Global Health Primer

What is Enterotoxigenic E. coli (ETEC)?

ETEC are bacteria that colonize the small intestine and cause severe diarrhea, dysentery, abdominal cramps, and fever. ETEC can be life threatening due to significant fluid loss and severe dehydration. Beyond its burden in endemic countries, ETEC is the leading cause of diarrhea in travelers from developed regions returning from vacation in low resource countries.

Global Burden


Countries at high risk for traveler’s diarrhea
(CDC, 2007)

Diarrheal disease is currently the most common illness in the world.1 There are many causes of diarrhea, including bacterial, viral, and parasitic pathogens. However, ETEC is estimated to cause 280-400 million diarrheal episodes per year in children under 5 years of age, resulting in 300,000 to 500,000 deaths.2 Due to difficulty in culturing the bacterium and the similarity of symptoms to other diarrheal diseases, these numbers are believed to be significantly underestimated.

ETEC is the second leading cause of death in children less than 5 years of age. Often ETEC is the first enteric infection experienced by infants in low resource countries, and in endemic areas almost all children will have had 1 ETEC diarrhea episode in their first year of life. In Bangladesh and Egypt, the majority of cases of ETEC occur in children less than 2 years of age, and between 15 and 18% of children 3 years and younger experience ETEC-associated diarrhea episodes. ETEC is less prevalent in children 5 years and older, as well as in adults, because of natural immunity that develops following several episodes of the disease.3 However, older adults are quite vulnerable and ETEC, along with cholera, is likely to be responsible for approximately half of the hospitalizations due to diarrheal disease in this age group in endemic areas.

As with other types of diarrheal disease, ETEC infection is associated with malnutrition, growth stunting, and cognitive deficits in children. It is not uncommon for children in the developing world to experience 10 severe episodes of diarrhea in a year, with several due to ETEC. The malnutrition and dehydration that translates into productivity loss of 15 to 20% in adult life.4

ETEC is also an important cause of disease in travelers. It is the leading cause of diarrhea in travelers from the developed world returning from vacation in low resource countries. The incidence of ETEC in travelers from industrialized regions to developing countries is projected to be in the 30 to 45% range.5 One out of every six travelers to endemic areas has been observed to be infected with ETEC.6

 

Causative Agent and Transmission


Escherichia coli (photo: CDC/Janice Carr)

E. coli is a bacterium with numerous serotypes, most of which normally inhabit the human intestinal tract with little ill effect. Several strains, however, secrete toxins that act on the intestinal lining and cause disease. E. coli that cause diarrheal illness can be broken down into four categories based on virulence mechanism: enterotoxigenic (ETEC),  enteropathogenic (EPEC), enteroinvasive (EIEC), shiga-toxic producing E. coli (STEC), and enteroaggregative (EAggEC). ETEC differs from the other E. coli serotypes in that it produces two toxins that induce extreme fluid secretion from the small intestine.

ETEC is transmitted when a person eats food, or drinks water or ice contaminated with ETEC bacteria. Human or animal wastes (e.g., feces) are the main source of ETEC contamination.

 

Pathogenesis

ETEC bacteria colonize the mucosal surface of the small intestine. The bacteria use fimbrial adhesins (projections from the bacterial cell surface) to bind enterocyte cells in the small intestine. There are 22 known fimbria types. Adhesion to the mucosal epithelial cells allows for transfer of enterotoxins produced by ETEC bacteria which stimulate the release of liquid from the cells lining the intestinal walls. ETEC make two toxins, heat-labile (LT) and heat-stable (ST), that cause intestinal epithelial cells to secrete excess fluid. Some strains produce only one of the toxins while others produce both.

ETEC infection results in the production of abundant watery diarrhea and abdominal cramping. Other symptoms such as fever, vomiting, chills, headache, muscle aches, and bloating can also occur but are less common. Illness usually lasts 3-4 days following exposure to the bacteria but can persist for up to 3 weeks. Supportive measures including rehydration tend to be sufficient for recovery, and hospitalization or antibiotics are usually not required.

 

Control Strategy

The primary control strategy for ETEC is prevention of oral-fecal transmission through building sanitation infrastructure. Other measures can be taken to control infection, these include cooking food and keeping it hot, peeling fruits and vegetables, and using water that has been boiled or chemically treated with iodine, chlorine, or another disinfectant. Bismuth subsalicylate preparations (essentially an antidiarrheal agent) can reduce ETEC and other common bacterial infections that cause diarrhea. Before taking bismuth subsalicylate, individuals with kidney disease should consult a physician. Secondary to prevention, management of the disease using oral rehydration therapy reduces morbidity and mortality associated with the disease.

 

Existing Products

Drugs

Individuals with diarrhea are treated with clear liquids to prevent dehydration and loss of electrolytes. Oral rehydration salts or premixed oral rehydration solutions are often used to treat dehydration. Zinc treatment can speed recovery time.

Fluoroquinolones have been found to be effective in treating ETEC infection. Although antibiotics can shorten the length of diarrheal disease, especially if given early, ETEC is frequently resistant to common antibiotics, including trimethoprim-sulfamethoxazole and ampicillin. Since antibiotic resistance is on the increase worldwide, the decision to treat ETEC with an antibiotic should be carefully considered with regards to the severity of the illness.

Vaccines

No vaccines directed against ETEC bacteria are on market at this time. Dukoral, an oral whole-cell/recombinant B-subunit vaccine directed against cholera, has been found to provide short term efficacy against ETEC diarrhea. Protective efficacy against cholera is 85 percent, while protection against the heat-labile toxin of ETEC reaches 67 percent.

Diagnostics

ETEC detection relies on the identification of the two enterotoxins. Several immunoassays have been developed for detection of the heat-stable toxin (ST), including a radioimmunoassay and an enzyme-linked immunosorbent assay (ELISA). Two commercial agglutination tests are available for detecting ETEC heat-labile enterotoxin (LT).

ETEC strains were among the first pathogenic microorganisms for which molecular diagnostic techniques were developed. DNA probes are found to be useful in the detection of LT- and ST-encoding genes in stool and environmental samples. Several PCR assays exist for ETEC that are quite sensitive and specific when used directly on clinical samples or on isolated bacterial colonies.

 

References

  1. WHO (2008). The World Health Report 2008: Primary HealthCrae-Now More Than Ever
  2. WHO (2006) “Future directions for research on enterotoxigenic Escherichia coli vaccines for developing countries.” Weekly Epidemiological Record. 81:97-104. 
  3. Fisher-Walker CL and Black RE (2010) “Diarrheoa morbidity and mortality in older children, adolescents and adults.” Epidemiology and Infection 138:1215-1226.
  4. Qadri F et al. (2005) “Enterotoxigenic Escherichia coli in developing countries: epidemiology, microbiology, clinical features, treatment, and prevention.” Clinical Microbiology Reviews 18: 465-483.
  5. Subekti DS et al. (2003) “Prevalence of enterotoxigenic Escherichia coli (ETEC) in hospitalized acute diarrhea patients in Denpasar, Bali, Indonesia.” Diagnostic Microbiology and Infectious Disease 47: 399-405.
  6. Steffen R et al. (2005) “Vaccination against enterotoxigenic Escherichia coli, a cause of travelers’ diarrhea”. Journal of Travel Medicine 12: 102-107.

 

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Drugs

Although drug resistance to common antibiotics is a concern, numerous antibiotics are available for the treatment of ETEC. Vaccines are the primary focus of new product development for ETEC.

Vaccines

PIPELINE

Product/Research ProgramDevelopersDiscoveryPre-clinicalPhase IPhase IIPhase III
ACE527Johns Hopkins Bloomberg School of Public Health
PATH
Pierrel Research USA
TD Vaccines A/S

 

 

 

 

SBL 109Crucell
PATH

 

 

 

Peru-15 pCTB (ETEC)Celldex Therapeutics Inc.
National Institute of Allergy and Infectious Diseases

 

 

 

dmLTNational Institute of Allergy and Infectious Diseases
PATH
Tulane University

 

 

 

EtpA glycoproteinUniversity of Tennesee

 

 

FTL-LTB chimera proteinNaval Medical Research Center
Sanofi Pasteur
University of Colorado

 

 

LT/ST fusion proteinsInternational Enteric Vaccine Consortium
PATH
Research Council of Norway

 

 

ACE920TD Vaccines A/S

 

 

Typhetec (ETEC)Prokarium

 

Mimopath-based ETEC vaccineMucosis B.V.
PATH

 

Mucosal immunobiology and vaccines discoveryMucosal Immunobiology and Vaccine Center

 

Siderophore receptors and porin (SRP)-targetted vaccinesSyntiron

 

Traveler's diarrhea vaccineIntercell AG

 

 

 

 

On Hold

ANALYSIS

Vaccines for the prevention of ETEC are a key focus of research and development efforts for this disease. The only existing vaccine for ETEC is a cholera vaccine (Dukoral) that provides partial protection due to cross protection between the cholera B toxin and the ETEC toxins. Clinical stage vaccines include live attenuated, recombinant protein and inactivated whole cell vaccine technologies. Phase II clinical trial data for ACE527 (TD Vaccines), a live attenuated vaccine, showed reduction in diarrheal episodes but did not meet the primary endpoint of reduction of incidence of moderate to severe diarrhea.1

A recombinant protein vaccine for ETEC (traveler’s diarrhea vaccine, Intercell) was recently put on hold after it failed to meet efficacy endpoints in phase II and III trials.2 Interestingly, antibodies to the ETEC heat-labile toxin (LT) were detected in vaccinated individuals. This failure raises questions as to whether recombinant LT produces a sufficient immune response to protect against disease. Other recombinant protein vaccines in development are exploring LT and other protein antigens.

The discontinued recombinant protein vaccine was delivered by intradermal patch. The detection of antibodies to LT suggests this vaccine delivery technology was working, despite the endpoint failure of the study. This technology might have applications for other NTD vaccines.

  Strengths Weaknesses Opportunities Risks
Live attenuated
Most advanced program:  ACE527, Phase II; Peru-15 pCTB (ETEC), Phase I ACE527 is a live, deletion attenuated strain that mimics natural infection Difficult to achieve balance between immunogenicity and attenuation of virulence Combination with vaccines that cause other diarrheal diseases by expression of additional protein antigens in the modified live vaccine strain May cause symptoms if attenuation is incomplete
Recombinant/purified protein
Most advanced program:  dmLT, Phase I

Product formerly in phase III on hold as of December 2010

Additional products in pre-clinical development
As it is the ETEC secreted toxins that cause the majority of disease, protection against the toxin is likely to be important

Based on concept that cross protection against ETEC with Dukoral (cholera vaccine) is due to cross protection between cholera and ETEC toxin proteins
Previous phase III study failed to meet endpoints of preventing ETEC/diarrhea

Previous phase II vaccine and dmLT vaccine target the heat labile toxin (LT) but not the heat stable toxin (ST)
Although on hold at the phase III development stage, transdermal patch technology may be applicable to other recombinant protein vaccines in development Unlikely to be delivered via the oral route of natural infection which may affect efficacy
Inactivated
Most advanced program: SBL 109, Phase I Can use virulent disease strain to generate vaccine

Can be delivered orally via the natural route of infection
Inactivated whole cell vaccine for cholera not fully effective, clinical data for ETEC not yet available Potential for combination with recombinant protein vaccines as was done for Dukoral for cholera May not generate as strong an immune response as the live attenuated vaccine in phase II development

 

Diagnostics

As the treatment for all forms of diarrhea focuses on supportive therapy, including rehydration, diagnostics are not necessarily essential for diarrheal management. The key needs for diarrheal disease diagnosis are point of care tests that can determine the origin of the illness (i.e., viral, bacterial, or protozoan) thus directing patient treatment with antibacterial or anti-parasitic medications in conjunction with rehydration.

 

References

  1. Intercell press release

 

Get Involved

To learn how you can get involved in neglected disease drug, vaccine or diagnostic research and development, or to provide updates, changes, or corrections to the Global Health Primer website, please view our FAQs or contact us at globalhealthprimer@bvgh.org.

The following series of tables describe the availability of tools for research, discovery, and development of novel drugs, vaccines, and diagnostics for ETEC. The tools listed in the following tables are not intended to be an all-inclusive list but rather capture the most common tools used for drug, vaccine, and diagnostic development. The tools for ETEC are generally well developed.

Drug Development Tools

Basic Research: Target Identification Target Validation Screening: Hit/Lead Identification Optimization Pre-clinical Validation Clinical Validation
Genome:  ETEC strain H10407 sequenced and annotated; ETEC strain E1392/75 sequenced  


Key databases:
 

EB-Eye Search: Strain H10407
Accession no. FN649414

EB-Eye Search: Strain E1392/75
Plasmid accession nos. FN822745, FN822746, FN822747, FN822748, FN822749

In vitro culture: Yes
Gene knock-outs: Yes

Conditional gene knock-outs: Yes

Transposon mutagenesis:
Yes

RNAi: No

Other antisense technology:   Yes

Transcription microarrays:
Yes

Proteomics:
Yes

Crystal structures:
  Yes, several crystal structures of ETEC proteins exist including fimbrial adhesion F17-G, heat-labile enterotoxin, and type 2 secretion system GspK-GspI-Gsp-J complex.
Whole-cell screening assays: Yes, can be done in liquid culture and on culture plates, heat-labile toxin is often used as the assay readout

Enzymatic screening assays:  Yes
Animal models: Several models exist, including Balb/c mice; infant mouse strain Swiss 0F1; infant rabbit; infant piglet; rabbit ileal loop; and dog-jejunal Thiry-Vella loops Monitoring treatment efficacy:  Immunoassays to detect ST toxin and agglutination tests for detection of LT produced by bacterial cultures can be used for disease monitoring PCR to detect ST and LT from clinical samples or isolated cultures

Availability of endpoints:
  Yes

Availability of surrogate endpoints:
  Yes, watery diarrhea

Access to clinical trial patients/sites:
  Yes

 

Vaccine Development Tools

 

Basic Research: Antigen Identification Immune Response Characterization Clinical Validation
See drug development tools above Predictive animal models:  Yes, most commonly mouse and pig

Detection of endogenous antigen specific response in clinical samples:
Yes to LT not to ST; serum titers to LT do not last long

Natural immunity well characterized:
  Yes
Surrogate markers of protection:  Yes

Challenge studies possible:
  Yes

 

Diagnostic Development Tools

Basic Research: Biomarker Identification Biomarker Validation Clinical Validation
See drug development tools above Biomarkers known: Yes

Access to clinical samples:
 Yes

Possible sample types: Stool
Access to clinical trial patients/sites: Yes

Treatment available if diagnosed: Yes

 

Get Involved

To learn how you can get involved in neglected disease drug, vaccine or diagnostic research and development, or to provide updates, changes, or corrections to the Global Health Primer website, please view our FAQs or contact us at globalhealthprimer@bvgh.org.

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