|
Nocardiosis: Introduction |  |
|
The nocardioform actinomycetes comprise a suprageneric group of bacteria that cause a chronic, noncontagious disease (nocardiosis) in humans and domestic animals. Nocardiae are gram-positive, strictly aerobic, nonmotile, pleomorphic, and nonsporeforming; organisms in this genus may take the form of rods, cocci, or diphtheroids, and they sometimes produce branching filaments and aerial hyphae. Most reduce nitrate, produce catalase, and oxidize
sugars. Some are partially acid-fast. |
| Epidemiology and Pathogenesis: |
| Nocardiae are found commonly in soil, decaying vegetation, compost, and other environmental sources. They enter the body through contamination of wounds or by inhalation. Species in this genus include
N
amarae
,
N
brevicatena
,
N
carnea
,
N
pinensis
,
N
seriolae
,
N
transvalensis
,
N
vaccini
,
N
brasiliensis
, and
N
otitidiscaviarum (N caviae)
;
N
asteroides
, the type species, together with
N
farcinica
, and
N
nova
, constitute the
N
asteroides
complex.
N
asteroides
is found frequently in temperate regions, while
N
brasiliensis
is more common in tropical and subtropical areas. |
| Cell wall mycolic acids, including trehalose 6,6′-dimycolate, contribute to virulence of nocardiae, perhaps by inhibiting phagosome-lysosome fusion; membrane-bound catalase and superoxide dismutase probably mediate resistance to killing by neutrophils. Filamentous forms prominent in log phase are more virulent than the coccoid forms of stationary phase. A mouse-toxic secreted product of
N
otitidis caviarum
may be involved in pathogenesis. Resistance to nocardiosis is likely primarily cell-mediated. |
|  |
| Clinical Findings: |
| Anorexia, fever, lethargy, and weight loss are common nonspecific signs associated with all infection sites. The first clinical sign of
N
asteroides
infection is the appearance of an indurated nodule or pustule, which ruptures and suppurates. Discrete lesions may become joined by sinuses, with frequent development of chronic, progressive disease. Infection of the bovine mammary gland causes the mammary tissue to become enlarged and firm, often with draining tracts. Milk from infected glands contains a viscid exudate, with discrete blood clots and microcolonies of
N
asteroides
. The organism can disseminate from the mammary gland to other organs, where it causes suppurative granulomatous lesions. Canine and feline infections are often localized, with subcutaneous lesions, mycetomas, and lymphadenitis. Nocardial stomatitis manifests as gingivitis and ulceration of the oral cavity, with severe halitosis. Canine thoracic nocardiosis often involves suppurative pleuritis or peritonitis; abscessation of heart, liver, kidneys, and brain are common.
Occasionally, young dogs experience a disseminated form that begins in the lower respiratory tract following inhalation of the organism. Skin infection and lymph node abscessation are common presentations in horses, with respiratory or disseminated disease in the immunosuppressed. Nocardial abortion may occur in horses and pigs, and respiratory infection of monkeys is common in research colonies. |
|
N
brasiliensis
has been isolated from suppurative wounds in various animal species on rare occasions.
N
brasiliensis
is isolated from horses with pneumonia and pleuritis.
N
otitidiscaviarum
was named for its occurrence in guinea pig ear infections, but it is perhaps more important as a cause of bovine mastitis and pneumonia and disseminated infections in other animals.
N
farcinica
is associated with
bovine farcy, and its importance has increased in recent years.
N
salmonicida
and
N
seriolae
cause granulomatous lesions in salmonid fish and other fresh and saltwater fish, respectively. |
| Nocardioform actinomycetes from placentitis and abortion in horses in some areas of the USA have been placed in the genus
Crossiella
, eg,
C
equi
. |
| |
| Diagnosis: |
| Presumptive diagnosis can be based on pathology and the presence of gram-positive, acid-fast, branching, beaded filaments in smears of affected tissues. Specimens should be plated on blood or Sabouraud’s agar, incubated at 25°C and at 37°C for 4-5 days. The resulting cultures have the odor of wet dirt. Microscopic morphology is best determined by observation of undisturbed colonies in slide cultures on tap water agar or corn meal agar without dextrose, incubated at 25°C for 2-3
wk. Finely filamentous subsurface hyphae are dichotomously branched at right angles. The presence of aerial hyphae differentiates nocardiae from related genera. Colonies adhere to the surface of blood or chocolate agar, with the leading edge embedded in the agar. Nocardiae may be hemolytic, but
N
asteroides
is usually nonhemolytic. Aerial hyphae of
N
otitidiscaviarum
are sparse and off-white; colonies are usually pale tan, but may vary from cream-colored, to gray, peach, or purplish.
N
salmonicida
substrate mycelium is extensively branched and fragments into rod- to coccoid-shaped elements. Identification of species is based on phenotypic properties, including decomposition of casein, xanthine, hypoxanthine, and tyrosine. |
| |
| Treatment: |
| β-Lactam antimicrobial agents are not therapeutically effective for any nocardial infection, due to antimicrobial resistance. Udder infusions of novobiocin, combined with nitrofurazone, for 3-5 days has been successful for treatment of bovine nocardial mastitis. Nonmastitic forms can be treated with sulfamethoxazole-trimethoprim, sulfonamides, novobiocin, ampicillin, or tetracyclines. Therapy must often be continued for >3 mo. The prognosis is guarded, due to the long
treatment time and the likelihood of relapse. |
| |
;)
;)