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Escherichia coli in Dogs & Cats

J. Scott Weese, DVM, DVSc, DACVIM, FCAHS, Ontario Veterinary College, Ontario, Canada

Infectious Disease

|October 2022|Peer Reviewed

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close-up of E coli


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.


Disease Examples that Can Involve Escherichia Coli

System Disease
Urogenital Cystitis Pyelonephritis Prostatitis Pyometra
Respiratory Pneumonia Pyothorax
Hepatobiliary Cholangiohepatitis
Skin and soft tissue Wound infections Cellulitis Necrotizing fasciitis
Blood Sepsis
GI Acute diarrhea Histiocytic ulcerative (granulomatous) colitis
Neurological Meningitis
Musculoskeletal Discospondylitis


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.



Antimicrobial Comment
Penicillins (amoxicillin/ampicillin) Can be effective, but resistance from beta-lactamase production is not uncommon. Amoxicillin remains a first-line treatment choice for bacterial cystitis because of high drug levels in urine.
Amoxicillin/clavulanic acid Can be effective against isolates producing narrow spectrum beta-lactamases, which likely account for the majority of E coli in most environments; however, efficacy against beta-lactamase–producing E coli in tissue (apart from bacterial cystitis) is controversial, and efficacy may be poorer than previously assumed
Cephalosporins Activity against E coli increases with later generation drugs. Third-generation cephalosporins are excellent against E coli but should be reserved for situations in which lower tier drugs cannot be used. Although cefovecin is a third-generation cephalosporin, its activity against E coli is limited.
Fluoroquinolones Excellent activity against E coli but should be reserved for situations in which lower tier drugs (eg, amoxicillin, amoxicillin/clavulanic acid, doxycycline, potentiated sulfonamides [eg, trimethoprim/sulfamethoxazole]) are not an option
Doxycycline Often overlooked but can be effective; resistance is not uncommon
Aminoglycosides Excellent activity against E coli, including most multidrug-resistant strains; typically reserved for isolates resistant to most other options (eg, ESBL-producing strains) due to parenteral administration and toxicity concerns
Carbapenems Similar to aminoglycosides, carbapenems have activity against E coli (including most multidrug-resistant strains) and should be reserved for exceptional circumstances in which isolates are resistant to most other options (eg, ESBL-producing strains).
Nitrofurantoin Can be useful for bacterial cystitis, as resistance is uncommon, even with multidrug-resistant strains; not effective for infections other than bacterial cystitis
Fosfomycin Dogs only; most often used for multidrug-resistant bacterial cystitis; resistance is rare; can be used for other infections (unlike nitrofurantoin)
Potentiated sulfonamides Potentially useful against E coli, especially for bacterial cystitis; resistance is not uncommon

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


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.


For global readers, a calculator to convert laboratory values, dosages, and other measurements to SI units can be found here.

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