16       Corynebacterium species and Rhodococcus equi

 

Corynebacterium species

Corynebacterium species are small, pleomorphic, Gram-positive bacteria which occur in coccoid, club and rod forms. In stained smears they may occur in angular clusters resembling Chinese letters. Most corynebacteria are catalase-positive, oxidase-negative, non-spore-forming facultative anaerobes which require enriched media for growth. Colonies are small with variable haemolysis, depending on species. Many Corynebacterium species are commensals on mucous membranes. Most pathogenic corynebacteria are relatively host-

specific and produce identifiable clinical syndromes. The host species and the nature of the disease may suggest the causal agent. Identification critieria include bacterial cell morphology, colonial appearance and biochemical reactions. Molecular detection and typing methods have been developed for some corynebacterial species, particularly for C. pseudotuberculosis. Two biotypes of C. pseudotuberculosis are recognized. The ovine/caprine strains lack nitrate-reducing capacity, while the equine/bovine strains usually reduce nitrate. The biotypes differ in their geographical distribution.

Many corynebacteria are opportunistic pathogens. With the exception of C. bovis, these organisms are pyogenic and cause a variety of suppurative conditions in domestic animals. Urease is produced by all pathogenic corynebacteria except C. bovis and fimbriae are important virulence attributes of the uropathogenic corynebacteria. The main diseases caused by infections with Corynebacterium species are summarized in Table 16.1.

Table 16.1 The pathogenic corynebacteria, their hosts, usual habitats and the disease conditions which they produce.

Pathogen	Host	Disease condition	Usual habitat
Corynebacterium bovis	Cattle	Subclinical mastitis	Teat cistern
C. kutscheri	Laboratory rodents	Superficial abscesses, caseopurulent foci in liver, lungs and lymph nodes	Mucous membranes, environment
C. pseudotuberculosis
Non-nitrate-reducing biotype	Sheep, goats	Caseous lymphadenitis	Skin, mucous membranes, environment
Nitrate-reducing biotype	Horses, cattle	Ulcerative lymphangitis, abscesses	Environment
C. renale group
C. renale (type I)	Cattle	Cystitis, pyelonephritis	Lower urogenital tracts of cows and bulls
	Sheep and goats	Ulcerative (enzootic) balanoposthitis	Prepuce
C. pilosum (type II)	Cattle	Cystitis, pyelonephritis	Bovine urogenital tract
C. cystitidis (type III)	Cattle	Severe cystitis, rarely pyelonephritis	Bovine urogenital tract
C. ulcerans	Cattle	Mastitis	Human pharyngeal mucosa
	Cats	Rare cases of upper respiratory tract infections

Caseous lymphadenitis is a chronic suppurative condition of sheep and goats caused by C. pseudotuberculosis; cattle are rarely affected. Sheep become infected through contamination of shearing wounds, by arthropod bites or from contaminated dips or shearing equipment. Infection results in abscessation and enlargement of superficial or internal lymph nodes. The incubation period is about three months. The organism is a facultative intracellular pathogen, which can survive and replicate in phagocytes. Its major virulence factors include its cell wall lipid and an exotoxin, phospholipase D. The disease, which is prevalent in Australia, New Zealand, the Middle East, Asia, Africa and parts of North and South America, is being reported with greater frequency in Britain and other European countries. Ill-thrift may be evident in affected animals and the disease invariably results in condemnation of carcasses and devaluation of hides. Infection is spread by pus from ruptured abscesses and from nasal and oral secretions. The organism can survive in the environment for several months. Affected lymph nodes are enlarged and exhibit characteristic encapsulated abscesses containing greenish pus. They often have an ‘onion ring’ appearance on their cut surfaces. The visceral form of the disease may not be detectable antemortem. Goats usually develop the superficial form of the disease. The disease may be suspected on clinical grounds or at postmortem examination. Smears from lesions may reveal Gram-positive coryneform bacteria. Isolation and identification of C. pseudotuberculosis from abscess material is confirmatory. Because of the chronic nature of lesions and the ability of the organisms to survive intracellularly, therapy is usually ineffective. Appropriate control measures for individual countries are determined by the prevalence of the disease. Owing to the difficulties of detecting infection in the live animal, a number of different ELISA methods have been developed. Serologically positive animals may be culled. Vaccines are available in some countries.

The nitrate-reducing biotype of C. pseudotuberculosis causes sporadic cases of ulcerative lymphangitis in horses and cattle. This disease occurs in Africa, the Americas, the Middle East and India. Infection occurs through skin wounds, arthropod bites or by contact with contaminated harness. The condition presents as either lymphangitis of the lower limbs or abscessation in the pectoral region. Organisms belonging to the C. renale group can be isolated from the vulva, vagina and prepuce of apparently normal cattle. The stress of parturition and the shortness of the urethra in the cow predispose to infection of the urinary tract. Ascending infection from the bladder through the ureters can result in pyelonephritis. Clinical signs of pyelonephritis include fever, anorexia, restlessness due to pain and decreased milk production. Dysuria, an arched back and blood-tinged urine are invariably present. Culture of C. renale from urinary deposits, in association with the presence of characteristic clinical signs, is confirmatory.

Rhodococcus equi

Rhodococcus equi is a Gram-positive, aerobic soil saprophyte which occurs worldwide. It is an opportunisitic pathogen of foals under six months of age. Rhodococcus equi grows on non-enriched media and produces characteristic mucoid salmon-pink colonies. Some strains of R. equi appear as cocci, and others as rods up to 5 μm in length.

Suppurative bronchopneumonia of foals is the major disease caused by this pyogenic organism. Infection is generally acquired by inhalation of dust contaminated with R. equi, either from dusty pastures in warmer climates or indoors from dust in poorly ventilated, dusty stables in temperate regions. The organism is often present in large numbers in the faeces of healthy foals and also in the faeces of older horses. A build-up of R. equi can occur on pastures heavily stocked with horses. Only virulent R. equi, which possess a large plasmid containing the vapA gene, are associated with disease. Virulent organisms can survive and multiply within macrophages. Acute disease often occurs in one-month-old foals, with sudden onset of fever, anorexia and signs of bronchopneumonia. In older foals, the disease tends to be insidious and lesions can be well advanced before animals exhibit coughing, dyspnoea, weight loss and characteristic loud, moist rales on auscultation of the lungs. A history of the disease on the farm, the age of the affected foal and clinical signs may suggest infection with R. equi. Culture or PCR-based detection of R. equi from tracheal aspirates and pus from lesions, in association with clinical signs, is confirmatory.