Overview

• Anaerobic bacteria (i.e., bacteria requiring low oxygen tension) comprise a large portion of the normal flora, especially on mucosal surfaces.
• May be Gram-positive or Gram-negative cocci or rods.
• Most common genera-Bacteroides, Fusobacterium, Actinomyces, Propionibacterium, Peptostreptococcus (enteric Streptococcus), Porphyromonas, and Clostridium.
• Most anaerobic infections are polymicrobial and contain at least two different anaerobe species admixed with facultative anaerobes or aerobic bacteria (especially E. coli).
• Individual organisms vary in potential to withstand oxygen exposure.
• Injurious toxins and enzymes may be elaborated by the organisms, leading to extension of the infection into adjacent, healthy tissue.
• All body systems are at potential risk for anaerobic infection.

Signalment

Dog and cat

Signs

Causes & Risk Factors

• Usually caused by normal flora of the body; a break in protective barriers allows bacterial invasion.
• Infection in the proximity of a mucous membrane should raise one's index of suspicion for anaerobe involvement.
• Predisposing factors-immunosuppression, bite wounds, dental disease, open fractures, abdominal surgery, and foreign bodies.

Differential Diagnosis

• Wounds that fail to respond to appropriate medical therapy-if aerobic cultures are negative, suspect anaerobic organisms.
• Cats with non-healing wounds-test for FeLV and FIV.
• Middle-aged and old animals-tumor invasion (e.g., in the gastrointestinal tract).

CBC/Biochemistry/Urinalysis

• Neutrophilic leukocytosis and monocytosis common.
• Biochemical abnormalities depend on specific organ involvement.
• Systemic spread of infection suggested by leukocytosis, hypoglycemia, increased ALP, and hypoalbuminemia.

Other Laboratory Tests

• Culture of anaerobic bacteria is often unrewarding because of their fastidious nature and errors in sample handling and submission.
• Appropriate media and containers should be on-hand prior to sample collection; diagnostic laboratories can provide guidance.
• Samples should not be refrigerated prior to submission.
• Suitable samples for culture may include fluid (e.g., pleural, peritoneal, etc.) or tissue.

Imaging

As required for the circumstances of the individual patient (e.g., suspected bone infection, peritonitis, etc.).

Diagnostic Procedures

• Cytologic inspection reveals abundant degenerate neutrophils with morphologically diverse forms of intracellular and extracellular bacteria; presence of large filamentous bacteria is suggestive.
• If not performed in-house, Gram staining should be requested when the sample is submitted.

Surgery

• Should not be delayed when anaerobes are suspected.
• Combined with systemic antimicrobial therapy-the best chance of a positive outcome.
• Usually indicated when anaerobic organisms complicate pyometra, osteomyelitis, and peritonitis.
• Cleanse the wound of toxins and devitalized tissue.
• Enhance drainage of pus.
• Improve local blood flow.
• Increase oxygen tension.

Drug(s)

• Antimicrobial therapy alone-unlikely to be successful; poor drug penetration into exudates.
• Antibiotic selection-largely empiric, owing to the difficulty of isolating anaerobes and the delay in return of culture results.
• Because most anaerobic infections are polymicrobial, therapy targeted against both anaerobes and any aerobic components offers the greatest chance of success.
• Amoxicillin with clavulanate-in many cases, considered the antibiotic of choice; convenient and accessible; clavulanate improves activity against Bacteroides.
• Imipenem-beta lactam with significant activity against serious, resistant infections.
• Cefoxitin-a cephalosporin with reliable activity against anaerobes.
• Clindamycin-may be especially useful for respiratory tract infections; concentrated within leukocytes.
• Chloramphenicol-good tissue penetration but bacteriostatic and associated with adverse effects, especially in cats; concern for human exposure also limits use.
• Metronidazole-useful against all clinically significant anaerobes (except Actinomyces).
• Aminoglycosides-uniformly ineffective.
• Trimethoprim-sulfa combinations-ineffective; poor penetration into exudates.
• Quinolones-routinely ineffective, although newer expanded-spectrum quinolones do have activity against anaerobes (e.g., pradofloxacin).

Patient Monitoring

Monitoring parameters will vary with the circumstances of each patient.

Possible Complications

Localized infection may progress to systemic infection if not appropriately identified and treated.

Expected Course and Prognosis

Dependent upon identification and resolution of the underlying cause; long-term antibiotic therapy may be required.

Associated Conditions

See “Causes & Risk Factors”

Abbreviations

• ALP = alkaline phosphtase
• FeLV = feline leukemia virus
• FIV = feline immunodeficiency virus

Author Sharon Fooshee Grace

Consulting Editor Stephen C. Barr