Overview of Nocardiosis

Nocardia is a pleomorphic, gram-positive, facultative intracellular bacterium, nonmotile and non-spore-forming. In Gram smears, it appears as rods, cocci, or coccobacilli forms with characteristic long or branching filaments and a tendency to fragment into rods and cocci. When cultured, some species of Nocardia produce aerial filaments. Components of the cell wall, particularly mycolic acids, render Nocardia spp partially acid-fast. Pathogenic Nocardia spp are strictly aerobic, growing over a wide temperature range (10°–50°C [50°–122°F]).

In past decades, Nocardia species identification has been traditionally based on phenotypic methods, including hydrolysis of different substrates (casein, xanthine, hypoxanthine, and tyrosine), carbohydrate assimilation (glucose, glycerol, galactose, glucosamine, inositol, adonitol, and trehalose), and antimicrobial susceptibility profile. Based on this former classification, the most important pathogenic species for animals and people were represented by the Nocardia asteroides complex (N asteroides, N nova, N farcinica), N brasiliensis, N pseudobrasiliensis, N otitidiscaviarum, and N transvalensis. However, diagnosis based exclusively on phenotypic assays leads to misidentification or is insufficient to distinguish some species of Nocardia. More recently, studies in domestic animals and people have revealed that speciation of Nocardia requires confirmation by molecular methods. However, most of the literature about nocardiosis in animals and people was published before this new molecular speciation. Thus, because of the taxonomic transition of Nocardia speciation, for purposes of this chapter, the remaining text will use the former Nocardia spp (in reference to the old classification) or molecular species detection of Nocardia (in reference to the new taxonomy).

To date, >90 valid species of Nocardia are recognized. Of these, >30 have been described as causes of opportunistic human infections, and at least 30 as responsible for animal diseases. Recent use of molecular techniques has led to large taxonomic changes in the Nocardia classification, and new species continue to be described.

Nocardia spp are ubiquitous organisms, a component of normal soil microflora; they are commonly found in soil, organic material, freshwater and saltwater, dust, compost vegetation, and other environmental sources.

Nocardiosis is considered an uncommon disease in animals and people. However, reports of animal nocardiosis have increased worldwide.

Infections in livestock and companion animals caused by Nocardia spp are acquired by inhalation, traumatic percutaneous introduction of the microorganism, ingestion, or by the intramammary route. The occurrence of disease and infective Nocardia spp may vary geographically, influenced by animal management strategies as well as environmental factors, such as dry, dusty, or windy conditions.

Nocardia spp is considered an agent of mastitis of environmental origin in cattle and small ruminants. Nocardial mastitis usually affects herds with a history of inadequate milking management and/or poor hygiene conditions before and after milking. Mammary infections are predominantly caused by soil contamination of teat dips, udders, and milking equipment during washing procedures and by intramammary infusion therapy. Dairy herds affected by nocardial mastitis also have a history of inadequate concentrations of antiseptics in teat dips. Outbreaks of nocardial mastitis have been reported associated with dry-cow therapy (eg, neomycin) or improper intramammary therapy.

In companion animals, disease transmission is related to the inoculation of organisms through puncture wounds or foreign bodies, or secondary to bites, wounds, or scratches after cat fights. Canine infections have rarely been related to inhalation of the bacterium. The occurrence of nocardiosis in dogs and cats is intimately associated with underlying immunosuppressive disorders, particularly dogs infected by distemper virus and cats affected by leukemia or immunodeficiency virus. Canine nocardiosis occurs at any age group and in both sexes, although it appears to affect mainly males, particularly between 1–2 yr old.

Nocardiosis in horses is recognized as an opportunistic infection and is usually related to immune disorders. In most reported cases of equine clinical nocardiosis, there were underlying immunosuppressive problems, particularly pituitary pars intermedia dysfunction (see Hirsutism Associated with Adenomas of the Pars Intermedia) or severe combined immunodeficiency in Arabian foals (see Combined Immunodeficiency Diseases).

Pathogenicity of Nocardia in domestic animals is attributed to the virulence of the strain, the structure of the bacterial cell wall, host susceptibility, route of transmission, coinfection with immunosuppressive diseases, and development of pyogranulomatous lesions. However, the outcome of Nocardia infections is intimately linked to the ability of the strain to resist the initial neutrophil and activated macrophage attack and the cell-mediated immune response.

Immune response against nocardial infections is primarily cell-mediated. These intracellular organisms are able to inhibit phagosome-lysosome fusion in neutrophils and macrophages because of the presence of mycolic acids in their bacterial cell wall. Nocardia is also resistant to acids, oxidative enzymes (catalase and superoxide dismutase), and other enzymatic mechanisms of phagocytic cells. In addition, some toxins have been identified in Nocardia spp that appear to contribute to the virulence of pathogenic strains.

Nocardia does not induce an effective humoral immune response mediated by the action of B lymphocytes.

Mastitis, cutaneous/subcutaneous lesions, abscesses in organs, and pneumonia are the most common clinical signs of nocardiosis in livestock and companion animals.

Routine diagnosis is based on epidemiologic findings, clinical signs, and microbiologic examination. Samples of abscesses, skin, tracheobronchial lavage fluid, milk, aspirates, organs, or other tissues should be cultured on sheep blood and/or Sabouraud agar and incubated aerobically for 2–7 days at 37°C and 25°C [98.6°F and 77°F], respectively. However, growth of some Nocardia spp in culture media is slow, and incubation should be extended for at least 2 wk. Colonies are circular, convex, smooth or rough, firmly adherent to agar surface, odorless, with various carotenoid-like pigments (cream, white, orange, pink, or red), and present aerial hyphae and typical powdery and dry surface, like fungal organisms. Because Nocardia spp are ubiquitous in the environment, microbiologic isolation of a small number of organisms from clinical specimens must be evaluated together with clinical signs and other diagnostic methods.

Microscopically, gram-positive, typically filamentous organisms are seen, with a tendency to fragmentation. Modified Ziehl-Neelsen stain shows partially acid-fast organisms. Fine-needle aspiration has been used in the diagnosis of skin nocardiosis in companion animals. Gram, Giemsa, and panoptic stains show filamentous organisms in aspirated specimens. The leukogram reveals mainly leukocytosis with neutrophilia and monocytosis, whereas the erythrogram shows moderate anemia.

Radiographic images of dogs with pulmonary nocardiosis show diffuse inflammation, nodules, abscesses, and lobar consolidation. Differential diagnoses in domestic animals include infections with Actinomyces or Streptomyces spp because of the similarities in microbiologic appearance and clinical signs. Face and jaw enlargement in cattle and horses caused by oral nocardiosis should be differentiated from Actinomyces bovis (actinomycosis, see Actinomycosis), Actinobacillus lignieresii (actinobacillosis, see Actinobacillosis), and Staphylococcus aureus (botryomycosis).

Postmortem examination of internal organs and tissues reveals abscesses and/or numerous small to large nodules, discrete to coalescing, of white to gray color. Histologic findings of nocardiosis are characterized by pyogranulomatous to suppurative lesions with areas of necrosis. The lesions or nodules show a suppurative, necrotic center containing filamentous organisms surrounded by macrophages, lymphocytes, and plasma cells. Occasionally, epithelioid and multinucleated giant cells are found. Histolopathology sometimes reveals small, soft granules in discharges of lesions, formed by microcolonies of the organism. Lymph nodes are enlarged.

Different serologic (immunodifusion, complement fixation, and ELISA) and cutaneous hypersensitivity tests have been proposed to diagnose nocardiosis in animals. However, host animals commonly develop a nonspecific antibody response against Nocardia, limiting the use of serologic tests in routine diagnosis.

Currently, phenotypic diagnoses of Nocardia spp have been confirmed using molecular methods, including PCR, restriction endonuclease analysis of amplified 65-kDa heat shock protein gene (hsp65), 16S rRNA gene sequence, essential secretory protein A (secA1), gyrase B (gyrB), and DNA-DNA hybridization. These molecular techniques offer a timesaving and reliable means of speciation and have led to a number of taxonomic changes and the identification of new species of Nocardia.

Animal and human nocardiosis is usually refractory to conventional therapy because of the intracellular location of the bacterium, development of pyogranulomatous lesions, and antimicrobial resistance patterns.

The antimicrobial susceptibility profile varies dramatically between Nocardia spp and geographic areas. The National Committee for Clinical Laboratory Standards approved an in vitro standardized susceptibility test for Nocardia by broth microdilution. The modified disc diffusion method has been used as well.

Trimethoprim-sulfonamides, aminoglycosides (amikacin, gentamicin), linezolid, amoxicillin-clavulanate, imipenem, and some cephalosporins (cefotaxime, ceftriaxone) are considered drugs of choice for nocardiosis therapy in animals and people. Ampicillin, clarithromycin, doxycycline, erythromycin, and minocycline are described as treatment alternatives for animals. Combined therapy using amikacin with sulfonamides, and amikacin with imipenem or cephalosporins (cefotaxime, ceftriaxone) also have been proposed. Intramammary infusions of trimethoprim-sulfonamides, cephalosporins or aminoglycosides (gentamicin) have been used for 5–7 days to treat clinical mastitis in cattle and goats.

Long-term therapy (1–6 mo in domestic animals and 6–12 mo in people) is required because of clinical relapses after short-term protocols. In companion animals, surgical procedures (debridement, drainage, extirpation of foreign bodies, and washing of lesions with antiseptic solutions) are indicated in cutaneous/subcutaneous lesions and osteomyelitis. However, antimicrobial therapy is successful in only 30%–50% of cases of mastitis in cattle and goats, as well as in pulmonary or extrapulmonary (disseminated or systemic) infections in companion animals and horses. The mortality of animal nocardiosis is attributed mainly to underlying conditions, delayed diagnosis, and improper therapy.

There are no specific or effective measures to control animal and human nocardiosis, probably because of the wide distribution of the microorganism in the environment.

In companion animals, immunosuppressive pathogens or debilitating conditions should be investigated as predisposing factors to development of nocardiosis.

Control and prevention of nocardial mastitis is based on measures recommended for environmental agents. Thus, the best measures to control and/or prevent nocardial mastitis remain the early microbiologic diagnosis of mastitis, proper hygiene conditions, cleaning the environment during milking, correct antiseptic concentrations in post- and mainly in pre-dipping solutions, high-quality water to wash the animals and milking equipment, removal of organic material from the milking area, and appropriate intramammary therapy procedures. Because of poor success rates in treatment of mammary infections, segregation of infected animals, chemical drying of affected quarters, or culling of animals are also recommended in control of nocardial mastitis in dairy herds.

Human nocardiosis is an opportunistic disease. In some countries, the clinical impact of nocardiosis is fragmentary, indicating that diagnosis of disease may be neglected or underestimated. Curiously, reports of human nocardiosis have recently increased all over the world. Although the disease typically occurs in immunocompromised patients, nocardiosis has been described in immunocompetent people as well. Former N asteroides complex is the main species described in human nocardiosis. Currently, based on molecular methodologies and rearranged taxonomy, N cyriacigeorgica, N brasiliensis, N asteroides, N nova, N farcinica, N transvalensis, N pseudobrasiliensis, and N otitidiscaviarum have most frequently been detected in human patients.

Pneumonia, cutaneous-subcutaneous lesions, mycetoma, and neurologic manifestations are the most common clinical signs. However, clinical cases of human nocardiosis are intimately associated with immunosuppressive or debilitating disorders, such as AIDS, organ transplants, cirrhosis, diabetes, alcoholism, rheumatic and malignancy diseases (lymphosarcoma, lymphoma), or prolonged use of corticosteroids.

The environment is the natural reservoir of Nocardia spp for human and animal infections. Most cases of transmission to people probably occur by inhalation of the organism in dry and warm climate regions (aerosolization). Trauma with skin inoculation is another form of transmission of the bacterium to people. Cases of cutaneous/subcutaneous nocardiosis have been reported in some patients, secondary to bites or scratches of clinically ill dogs and cats. However, human nocardiosis is apparently not directly transmitted person-to-person or by nosocomial infections. Interestingly, studies have shown great similarity between Nocardia spp involved in human and animal infections. Experimental studies regarding temperature resistance using former N asteroides and N brasiliensis isolated from bovine milk and submitted to time/temperature conditions used in usual pasteurization procedures indicated a potential risk of Nocardia transmission by milk.

Precautions should be taken among human patients who have immune dysfunctions or debilitating diseases, with special reference to contact with soil or organic material from environments contaminated by domestic animals, contamination of traumatic cutaneous lesions, or close contact with animals suspected of having nocardiosis.