Diagnosis

DIAGNOSIS
  • Specimens to be collected
    • Specimens should include exudates, aspirates, mastitic milk samples, tissue from granulomas and thin sections from granulomas in 10% formalin for histopathology.
  • Based on direct microscopy
    • Soft granules are not common in exudates from N. asteroides infections. Smears made from exudates, aspirates, granulomatous tissue and from centrifuged deposits of bovine mastitic milk are stained by Gram's and MZN stain.
    • Gram-stained smears reveal Gram-positive branching filaments that often show some fragmentation into coccobacillary elements.
    • The MZN –stained smears exhibit a similar morphology but most of the filaments retain the carbol fuchsin dye and stain red.
  • Based on isolation and identification
    • Characteristic colonial morphology on blood and SDA agar and microscopic appearance.
  • Based on biochemical reactions
  • Differential diagnosis with Actinomyces
  • Actinomyces infections respond well to penicillin and other commonly used antibiotics, nocardial infections are often refractory to treatment and N. asteroides is susceptible only to limited range of antimicrobial agents such as Trimethoprim-sulphamethoxazole or erythromycin.

Characters
Nocardiosis
Actinomycosis
Granules in exudates
Not common
Usually present
Filaments MZN positive
+
-
Fragmentation of filaments
+
-
Growth on SDA
+
-
Powdery, white colonies ( aerial hyphae)
+
-
Susceptibility to penicillin
-
+

Note

  • Infection with Nocardia is by inhalation from the environment, while Actinomyces infection begins in the host as a normal flora invading damaged tissues.
  • Disseminated disease caused by Nocardia is more common in the dogs, while granuloma formation is the rule with Actinomyces infection and spreads by local extension.
  • When there is a doubt as to whether an animal has actinomycosis or nocardiosis, precautionary measures to preserve the anaerobic Actinomyces should be instituted, including prompt delivery to the laboratory under anaerobic condition, culturing on brain heart infusion agar, blood plates and incubating under anaerobic, microaerophilic, and aerobic conditions.
  • While Actinomyces fails to grow on Sabouraud agar, Nocardia grows uninhibited.
  • Acid-fast staining procedure of the sample exudate before culturing will also be helpful in the presumptive identification of the infecting agent.
Last modified: Monday, 4 June 2012, 5:06 AM