Escherichia coli O157:H7 in Ecuador: Animal Reservoirs,Yet No Human Disease

Introduction

E nterohemorrhagic Escherichia coli (EHEC) was first identified as a human pathogen in 1982 after a food-borne outbreak associated with the consumption of undercooked ground beef (Riley et al. 1983, Levine 1987, Taylor et al. 1998). From then on, this bacterium has been isolated from many human cases of bloody diarrhea, some of which were followed by the sometimes fatal hemolytic uremic syndrome (HUS) or thrombotic thrombocytopenic purpura (TTP). The main virulence factors of this pathotype are two Shiga toxins (Riley et al. 1983, Taylor et al. 1998). The main serotype of EHEC is O157:H7, which is found in the intestines of healthy cattle and other ruminants; it is the most important bovine E. coli strain that carries Shiga toxin genes (STECs) (Wells et al. 1991). Foods implicated in outbreaks involving this strain include undercooked beef, dairy products, and raw vegetables susceptible to cattle fecal contamination (Riley et al. 1983, Levine 1987, Taylor et al. 1998).

Most and more severe EHEC infections have been reported in industrialized countries, though the reasons for this distribution are still under debate (Palmeira et al. 2005). Developing nations such as Ecuador, despite having high rates of animal E. coli contamination, have had no human EHEC infections reported. The lack of the disease caused by this pathogen has led to the assumption that Ecuador has no circulating EHEC strains. The present study is the first report of EHEC O157:H7 from bovine fecal samples obtained from an Ecuadorian abattoir.

Material and Methods

Results and Discussion

We obtained 600 E. coli isolates, 1 for each collected fecal sample. Thirty-two isolates showed the typical E. coli O157:H7 phenotype (nonsorbitol fermenting and unable to cleave the X-glucuronide substrate) and carried stx genes, but only 6 isolates (1%) carried additional genes eaeA, λ-tir, hlyA, and rfbEO157 (Table 1). High-resolution genetic analysis via MLVA was attempted on the 32 isolates with the typical E. coli O157:H7 phenotype, but only 4 clustered with clinical strains of E. coli O157:H7 the rest of isolates did not amplify some of the VNTRs (Fig. 1).

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FIG. 1.

A maximum parsimony tree of MLVA patterns. The numbers on the tree represent the bootstrap values. Samples are from human clinical isolates (Manning et al. 2008) and publicly available whole-genome sequences, except for Ecuadorian isolates, which were obtained from cattle. The tree is rooted on E. coli O55 isolates. (Color image available online at www.liebertpub.com/vbz).

The present study demonstrates that Ecuadorian livestock are colonized with EHEC O157:H7 strains, which are genetically most closely related to EHEC clinical isolates from the United States. Human exposure to this pathotype in Ecuador is very likely due to the large amounts of E. coli found in Ecuadorian animal products. For instance, we have detected up to 110,000 cfu/gram of E. coli in soft cheese sold in Quito public markets (data not shown).

However, there have been no confirmed O157:H7 isolations reported from diarrheal cases in Ecuadorian hospitals. One of the main microbiology laboratories in Quito has conducted surveillance of diarrheal cases for 7 years using sorbitol–MaConkey agar and found only 1 suspicious isolate obtained from patient who was an American Embassy employee (a sorbitol-negative colony that reacted with anti-O157 and anti-H7 antisera) (Zurita 2011). In Peru, where EHEC O157:H7 has been found in many animal-derived foods (Mora 2007), there has been only 1 isolate reported from a clinical sample since 2001 (Huapaya et al. 2001).

Unexpectedly, Colombia, a neighboring country with higher food hygiene standards, has reported E. coli O157:H7 in children, with many cases of HUS in Bogota (Mattar et al. 1998).

Although deficient surveillance has been attributed to this lack of O157:H7 reports, there may be other factors explaining this phenomenon that deserve further consideration: (1) EPECs share many virulence-associated proteins with E. coli O157:H7 (with high amino acid sequence similarity among them) and constant exposure and subsequent immunity to them may produce cross protection to O157:H7 (Palmeira et al. 2005), (2) larger intakes of red meat in industrialized countries not only may increase exposure and the possibility of getting an infection, but also increases the affinity of the toxin to cell membranes by incorporating bovine sialic acid into host cell membranes (Lofling et al. 2009). The annual meat intake per capita in Ecuador is 46.5 kg and in the United Sates is 126.6 kg (Food and Agriculture Organization 2009); however, Japan (a nation with many reports of E. coli O157:H7 infections) has annual meat intake per capita of 45.4 kg (3) Finally, meat in developing countries often contains large amounts of microbial contaminants, which may outcompete EHEC (Vold et al. 2000). Understanding the factors involved in this apparent absence of developing country human morbidity will lead to a better understanding of the complex ecology, evolution, and pathogenesis of E. coli O157:H7 and potentially limit the impact of the disease in other locations. Finally, the detection scheme used in this study was directed to detect only E. coli O157:H7 in Ecuadorian cattle and the results do not reflect the prevalence of other EHECs.