Escherichia coli in Dogs & Cats

Microbiology

Escherichia coli is a gram-negative bacterium in the Enterobacterales order and is commonly found in the GI tract and the environment. Strains are mostly nonpathogenic but can be opportunistic. Pathogenicity is largely related to a range of virulence genes, including those that influence the ability of the bacterium to adhere to tissue or produce toxins. E coli can be classified into groups (including enteropathogenic, enterotoxigenic, enterohemorrhagic, adherent invasive, and uropathogenic) based on the presence of various virulence mechanisms. Similar to other gram-negative bacteria, cell walls of E coli contain endotoxin, a pyrogenic toxin that can be associated with severe disease (eg, septic shock). 

Although many genetic lineages and strains of E coli can be found in dogs and cats,^1,2 some strains are shared among dogs, cats, and humans.^1,3 Clinically relevant transmission between species should thus not be ignored.

Diseases of Small Animals

E coli can cause opportunistic infections in any body system (see Table 1) but is most commonly involved in urinary tract and skin/soft tissue infections.

Table 1: Disease Examples that Can Involve Escherichia Coli

Diagnosis

Diagnosis requires detection of E coli at an infected site, primarily via culture. Definitive diagnosis is likely in cases in which E coli is isolated from a normally sterile site (eg, blood) or E coli is found at sites where it is not normally present and there are supportive clinical and cytologic findings (eg, isolation from the lower airways in a patient with septic changes on bronchoalveolar lavage cytology). Although E coli is the leading cause of bacterial cystitis, this bacterium can also be found in patients without classical clinical signs of lower urinary tract disease (ie, subclinical bacteriuria), making interpretation of culture and susceptibility results challenging.^4-9 Clinical signs and other urinalysis results are important for determining the clinical relevance of E coli isolation. 

Enteric disease is the most challenging to diagnose, as E coli is a common enteric organism found in many healthy dogs. Detecting specific virulence factors may be useful, but the range of potential virulence factors and diseases is not adequately understood, and E coli with disease-associated virulence genes can be found in healthy patients. Diagnosis of histiocytic ulcerative colitis (ie, granulomatous colitis) typically relies on identification of intracellular E coli via fluorescent in situ hybridization.¹⁰

Treatment & Antimicrobial Resistance

E coli is intrinsically susceptible to a wide range of antimicrobials (Table 2), but acquired resistance and resistance from narrow spectrum beta-lactamase production are common.^11,12 Extended spectrum beta-lactamase (ESBL)–producing strains are increasingly common and confer resistance to cephalosporins; however, these strains often acquire numerous additional resistance genes, making them resistant to most available antimicrobials.^13,14 Fluoroquinolone resistance is also increasingly common. Clinical observation suggests ESBL-producing E coli are typically susceptible to a limited range of drugs, particularly amikacin and meropenem. Fosfomycin (dogs only) and nitrofurantoin can be used to treat bacterial cystitis caused by multidrug-resistant E coli. Further development of resistance is a concern with E coli. In human medicine, E coli is increasingly resistant to most antimicrobials, including carbapenems.

Table 2: Common Antimicrobial Options For Escherichia Coli Infections

Treatment should ideally be based on culture and susceptibility results; however, empirical treatment may be indicated in lieu of or while waiting for culture and susceptibility results. Systemic antimicrobials can be withheld until culture results are available in some cases (eg, disease is very mild; anti-inflammatory drugs [eg, NSAIDs], topical treatment, or other supportive care might be effective). Culture importance depends on confidence in the diagnosis (ie, E coli is the likely pathogen), likelihood of antimicrobial resistance (potential for treatment failure), and implications of failed initial treatment (eg, prolonged mild disease vs life-threatening progression). Factors associated with increased risk for resistance include prior antimicrobial treatment, hospitalization, and feeding raw diets, including raw animal-based treats.^15-19

Prevention

There are no specific preventive measures for E coli, but some syndromes (eg, bacterial cystitis) are often associated with predisposing factors, treatment of which may reduce the risk for recurrent infection.

Zoonotic Risks

Although E coli is a common cause of infection in humans, zoonotic risks from companion animals are poorly understood. Overlap of strains in humans and companion animals is possible,^20-22 and presence of the same strain in humans and their pets has been reported.^23,24 Whether these overlaps reflect animal to human, human to animal, or common source infection is not well understood. Use of basic hygiene practices (eg, hand washing, avoiding contact with feces and infected sites) is prudent when handling infected patients.