Forty-nine typical and atypical enteropathogenic Escherichia coli (EPEC) strains belonging to different serotypes and isolated from humans, pets (cats and dogs), farm animals (bovines, sheep, and rabbits), and wild animals (monkeys) were investigated for virulence markers and clonal similarity by pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST). The virulence markers analyzed revealed that atypical EPEC strains isolated from animals have the potential to cause diarrhea in humans. A close clonal relationship between human and animal isolates was found by MLST and PFGE. These results indicate that these animals act as atypical EPEC reservoirs and may represent sources of infection for humans. Since humans also act as a reservoir of atypical EPEC strains, the cycle of mutual infection of atypical EPEC between animals and humans, mainly pets and their owners, cannot be ruled out since the transmission dynamics between the reservoirs are not yet clearly understood.Enteropathogenic Escherichia coli (EPEC) strains are among the major causes of infantile diarrhea in developing countries (71) and can be classified as typical and atypical, depending on the presence or absence of the E. coli adherence factor plasmid (pEAF), respectively (39).The pathogenesis of EPEC resides in the ability to cause the attaching and effacing (A/E) lesion in the gut mucosa of human or animal hosts, leading to diarrheal illness (40). The genes responsible for the A/E lesion formation are located in a chromosomal pathogenicity island of ϳ35 kb, known as the locus of enterocyte effacement (LEE) (23,47). LEE encodes an adhesin called intimin (38), its translocated receptor (Tir) (42), components of a type III secretion system (36), and effector molecules, named E. coli-secreted proteins (Esp proteins) (41). These virulence factors have a crucial role in A/E lesion formation, and their detection in EPEC strains is an indicator of their potential to produce these lesions (19,56).Atypical EPEC strains have been associated with diarrhea outbreaks in developed countries (31,73,77) and with sporadic cases of diarrhea in developing and developed countries (1,12,26,52,55). At present, the prevalence of atypical EPEC is higher than that of typical EPEC in several countries (1,12,26,52,55,65).Different from the situation in developed countries, where atypical EPEC outbreaks and sporadic infections are associated with children and adults, atypical EPEC infection in Brazil is mainly associated with children's illnesses (32, 71).Typical EPEC strains are rarely isolated from animals, and humans are the major natural reservoir for these pathogens (14,32,53,71). In contrast, atypical EPEC strains are present in both healthy and diseased animals (dog, monkey, cats, and bovines) and humans (4,6,18,28,71). Some studies have associated pets and farm and wild animals as reservoirs and infection sources of atypical EPEC strains for humans (32). However, these studies did not compare atypical EPEC strains isolated from humans and animals b...