Nocardiae are aerobic, saprophytic, gram-positive, partially acid-fast filamentous bacteria. Currently, there are 30 species included within the genus. Members of the genus Nocardia were originally classified as fungi, but these are now considered bacteria based on the presence of cell wall muramic acid, lack of membrane-bound nucleus and mitochondria, and sensitivity to antibacterials. Nocardiae were first described by Edmond Nocard, a French veterinarian who isolated the organism in 1888 from cattle with farcy. Nocardiosis is a chronic, granulomatous, suppurativeinfectious disease caused principally by N. asteroides which occurs as saprophyte in soil. Other species of Nocardia such as N. caviae and N. brasiliensis have also been incriminated in the aetiology of nocardiosis. The pathogen has a predilection for central nervous system which is manifested by brain abscess and other pyogenic lesions of meninges. The disease has been observed in human, dog, cattle, horse and monkey. The infection is acquired through the inhalation of infectious organisms from the soil. Though the primary focus of infection is in lung, infection may be disseminated to other parts of the body through haematogenous route.
Symptoms: A dog exhibits reduced appetite, sub-laryngeal swelling and fever. Very rarely a dog may attack and bite unprovoked and show neurological signs suggestive of rabies. However, the examination of brain will exclude the possibility of nigri bodies which is pathognomonic for rabies. Madura foot may also occur in canines which is manifested by swelling of the foot, pain and oozing out pus from sinus. The pus contains whitish yellow to red or black granules. In cows and goats, this organism is a frequent case of mastitis. The affected animals show swelling of mammary gland, reduced milk yield, anorexia and mild temperature. The mammary exudate is whitish and viscous in consistency and contains small whitish granules and blood clot. The organism is also associated with corneal ulcer in cattle. The diseased bullock shows photophobia and oedematous swelling of the eye.
Diagnosis: The organism can be easily isolated on Sabouraud’s agar without antibiotics from the infected material such as pus, milk, corneal scraping and other tissues. Gram-positive, acid-fast fimaments, along with shorter cocco-bacillary forms are demonstrated in the smear prepared from clinical specimens. Specimens should not be chilled or frozen. Specimens for diagnosis of nocardiosis should be cultivated on blood or Sabouraud dextrose agar and incubated at 25°C and 37°C for 4 to 5 days. The cultures have an odor like wet dirt. The presence of aerial hyphae differentiates the genus Nocardia from others.
Histological examination of autopsy or biopsy material may also be helpful in confirming the diagnosis of nocardiosis. Tissue sections stained with modified Brown- Brenn stain or methanamine silver nitrate are examined for the presence of thin- branched hyphae. The pathogenicity of the isolate is tested in mice, guinea-pig and rabbit. The disease may be differentiated from tuberculosis and rabies.
Diagnostic serology offers little value due to antigenic heterogeneity among nocardial pathogens. Detection of antibodies in patients can be done using indirect immunofluorescent microscopy and enzyme-linked immunosorbent assay (ELISA) or Western blot analysis for specific antigens.Treatment and prevention: Antimicrobial susceptibility should be done by a broth microdilution method rather than by use of antimicrobial-impregented disks in a conventional Kirby-Bauer test. Penicillin is not effective therapeutic agent for nocardial infection. There is no effective antimicrobial treatment for noardial mastitis. Since the species of Nocardia are sensitive to sulpha drugs and antibiotics, treatment with any of the chemotherapeutic agents may be helpful. Trimethoprim-sulfonamide, sulfamethoxazole, ampicillin or tetracycline therapy yield fruitful results. Nocardiae are fairly resistant to the fluoroquinlones. Abscesses, empyemas and serosa effusions are treated by drainage and lavage. Other drugs include doxycycline and minocycline.