Rhodococcus equi infections

Rhodococcus equi infections

J F PRESCOTT AND S GIGUERE

Introduction

Rhodococcus (Corynebacterium) equi is one of the most important causes of disease in foals between one and six months of age; most foals show clinical signs between 5 and 12 weeks of age. Infection is characterized by a subacute to chronic abscessating bronchopneumonia, sometimes accompanied by ulcerative typhlocolitis, but other manifestations in foals include reactive arthritis, purulent arthritis, osteomyelitis, mesenteric lymphadenitis, and ulcerative lymphangitis. Infection in adult horses is rare and may follow immunosuppression of various origins.78 In cattle and pigs, tuberculosis-like lesions caused by R. equi may occur in the submandibular and other lymph nodes, whereas in goats the organism may cause granulomatous lesions in the liver associated with wasting and death in young animals. Infection in other species is rare and associated with immunosuppression. In recent years, R. equi has emerged as an important cause of AIDS-associated pneumonia in HIV-infected humans. Rhodococcus equi has a world-wide distribution, but is most common in regions with long, hot summers. In Africa, infection has been described in foals in South Africa and Zimbabwe, and in goats in Botswana.26, 39, 55, 86

Aetiology

Rhodococcus equi shares the characteristics of other actinomycete members of the Mycolata (e.g. Corynebacterium, Mycobacterium, Nocardia) of the presence of lipid-rich cell envelope components dominated by the presence of mycolic acids.88 It is a Gram-positive, obligately aerobic coccus to coccobacillus. The organism grows well on nonselective media, achieving its characteristic flowing, mucoid colonies after 48 hours of culture at 37 °C.78 Characteristic colonial variants occur. The organism is usually recognized in clinical bacteriology laboratories by its microscopic and colonial appearance, its strong urease activity, and its production of synergistic haemolysis (‘CAMP reaction’) with Corynebacterium pseudotuberculosis or Staphylococcus aureus. Salmon-pink or darker red colonies may develop after a week or longer of incubation, or during storage, but are often difficult to recognize in younger colonies.

Selective media have been developed for isolation of the organism from soil or faeces, and have been used with advantage in isolation from broncho-alveolar lavage fluids in foals concurrently infected with Streptococcus zooepidemicus infection.

Epidemiology

Although all horse farms are likely to be infected with R. equi, the clinical disease is endemic and devastating on some farms, sporadic on others, and unrecognized on most. These differences reflect variation in environmental (temperature, dust, soil pH) and managemental conditions, as well as differences in virulence of isolates.102 The bacterium is found in greater numbers where horses are present since the volatile fatty acids found in their manure enhance its growth.48 Although intestinal carriage in adult horses is probably passive, R. equi can replicate in the intestine of foals up to about three months of age.103 Inhalation of contaminated dust particles is the most important route of pneumonic infection in foals. There is a progressive build-up of infection on horse farms that have been used for rearing foals for prolonged periods. Farms on which R. equi infections are endemic are therefore likely to be those used for breeding horses for many years, those with heavy concentrations of mares and foals, and those located where summer temperatures are high, where the soil type is sandy, and where dust is extensive. Endemically affected farms are often easily recognized as those where the loafing paddocks have become grassless and dusty. Large numbers of foals kept on bare, dusty, manure-containing paddocks will result in heavy challenge, with clinical disease enhancing the environmental presence of virulent bacteria. Foal-virulent R. equi are those isolates that contain large, 85 to 90 kb plasmids which encode plasmid-associated, virulence-associated proteins, notably VapA (see below).

In a survey of the prevalence of virulent R. equi on horsebreeding farms in Japan, the organism was isolated from almost all soil samples, at numbers of 102 to 105 colony forming units (CFUs) per gram of soil. The vast majority of these isolates did not contain plasmids.91 Virulent R. equi isolates containing either an 85 or 90 kb plasmid and expressing VapA were cultured from 24 of the 31 farms examined. On those farms, virulent R. equi represented 1,7 to 23,3 per cent of isolates.104 It had earlier been shown that, although the total numbers of R. equi in the environment may be similar on farms with and without a history of R. equi infections, farms with endemic disease are more heavily infected with virulent R. equi than those where the disease is absent.102

The study of the virulence of R. equi has been complicated by the fact that typical granulomatous lung lesions have not been consistently reproduced in any species other than the horse. The normal murine lung can progressively clear a heavy inoculum of R. equi which would be sufficient to induce pneumonia in a foal.115 Nevertheless, variation in virulence of R. equi has been observed between strains in experimentally infected mice and foals.64, 100 More recently, progressive lung lesions have been observed in athymic (nude) mice, mice with severe...

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