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Rhodococcus equi infections

Preferred citation: Anipedia, www.anipedia.org: JAW Coetzer and P Oberem (Directors) In: Infectious Diseases of Livestock, JAW Coetzer, GR Thomson,
NJ Maclachlan and M-L Penrith (Editors). L Huber and S Giguere, Rhodococcus equi infections, 2018.
Rhodococcus equi infections

Rhodococcus equi infections

Previous authors: J F PRESCOTT AND S GIGUERE

Current authors:
L HUBER - PhD Student, DVM, MS, Veterinary Medical Center, University of Georgia, 2200 College Station Road, Athens, Georgia, GA 30602, USA
S GIGUÈRE (Deceased) - Professor and Hodgson Research Chair in Equine Studies, DVM, PhD, Diplomate ACVIM, 2200 College Station Road Athens, Georgia, 30505, USA

GENERAL INTRODUCTION: ACTINOMYCETES

Introduction

Rhodococcus equi is one of the most important causes of disease in foals between one and six months of age with most foals showing clinical signs between five 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 polyarthritis, osteomyelitis, mesenteric lymphadenitis, uveitis, and ulcerative lymphangitis. Subclinical disease has been reported where foals have pulmonary abscesses identified by thoracic ultrasonography. Infection in adult horses is rare and may follow immunosuppression. 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 and systemic infections. Infection in other species is rare and sometimes associated with immunosuppression.

Rhodococcus equi has emerged as an important cause of pneumonia or systemic infections in humans infected with HIV or undergoing immunosuppressive therapy.

Rhodococcus equi has a world-wide distribution, but is most common on large farms in regions with long, hot summers.

Aetiology

Rhodococcus equi shares the characteristics of other actinomycete members of the Mycolata (e.g. Corynebacterium, Mycobacterium, Nocardia) including the presence of lipid-rich cell envelope components dominated by the presence of mycolic acids.141 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. 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. Semi-selective media have been developed for isolation of the organism from soil, air, or faeces.98, 173 These semi-selective media might also be useful for isolation of R. equi from biological samples heavily contaminated with other bacteria. Polymerase chain reaction (PCR) assays for rapid identification of R. equi have been described.8, 85

Epidemiology

Although R. equi is likely present in the environment of all horse farms, the clinical disease is endemic and devastating at some farms, sporadic at others, and unrecognized at many. Differences in the prevalence of the disease might reflect variation in environmental (temperature, dust) and managemental conditions, as well as differences in the number of virulent isolates in the environment.154 Rhodococcus equi is a soil saprophyte with simple growth requirements. The bacterium is found in greater numbers where horses are present since the volatile fatty acids in their manure enhance its growth.70 Rhodococcus equi pneumonia does not appear to be associated with lack of attention to routine preventive health practices.26 In contrast, breeding farms with a large acreage, a large number of mares and foals, a high foal density, and a large population of transient mares and foals have greater odds of being affected by R. equi pneumonia.25, 35

The highest numbers of R. equi are found in surface soil whereas the organism cannot be found at depths exceeding 30 cm.146 The bacterium can be isolated from the soil of virtually all horse breeding farms at numbers of 102 to 105 CFU per gram of soil.145 In the same study, the proportion of virulent plasmid encoded virulence-associated protein A (containing pVAPA) R. equi represented 1.7 to 23.3 per cent of all isolates.145 However, the presence or the concentration of virulent R. equi in soil are not positively associated with increased cumulative incidence of R. equi pneumonia at breeding farms.32, 105, 109 Rhodococcus equi can be cultured also from the soil of areas not inhabited by horses. In one study, it was isolated from approximately 74 per cent of soil samples collected from 115 parks and 49 household yards. The number of R. equi in those samples ranged from 10 to 105 colony forming units (CFU) per gram of soil.148 None of the 1,294 isolates recovered from those samples contained pVAPA or pVAPB.148 In contrast to soil concentration, airborne concentration of virulent R. equi is associated with disease incidence at breeding farms.33, 34, 84, 109 The fact that exposure to R. equi preceded development of pneumonia suggests that the higher concentration of airborne virulent R. equi is likely the cause of pneumonia rather than an effect.33

Currently, there is no compelling evidence that R. equi is contagious among foals and that the affected foals should be isolated from other foals. In one study, air samples from the breathing zone of pneumonic foals had higher concentrations of virulent R. equi than environmental air collected from lanes and pens at the same farms.110 However, concentrations of virulent R. equi from air samples collected from the breathing zone of pneumonic foals was not significantly different from that collected from healthy controls, indicating that affected foals do not represent a greater risk than healthy foals for aerosol transmission. In addition, there was no association between detection of virulent R. equi in the breathing zone of foals and subsequent development of pneumonia caused by R. equi.30

There is substantial genotypic variability among isolates of R. equi obtained from the same farm and among isolates from various countries or continents.21, 36, 108 In addition, foals can be infected with multiple strains of virulent R. equi and it is not possible to link infections to a given geographic site or region on the basis of analysis of isolate genotyping using current techniques. In one study, multiple different R. equi strains were isolated in five of the six cases in which more than one isolate from a single foal was examined.108 Similarly, six distinct genotypes were identified among nine R. equi isolates from one foal with pneumonia and concurrent abdominal lesions.14 None of the four pulmonary isolates were identical.14

Pathogenesis

Rhodococcus equi is a facultative intracellular pathogen. Its in vivo infectivity is limited to cells of the monocyte–macrophage lineage.66 Its ability to persist in, and eventually destroy, alveolar macrophages is the basis of its pathogenicity, but the mechanisms allowing R. equi to survive and even replicate inside macrophages are not fully understood.

Inhalation of virulent R. equi is the major route of pulmonary infection in foals. The incubation period under field conditions is unknown. After experimental intrabronchial infection, the incubation period ranges from 9 days after administration of a heavy inoculum to approximately two to four weeks when a lower infective dose is administered.6, 51 Lung consolidation can be observed as early as three days after intrabronchial challenge with a high inoculum.51

Young foals have the ability to mount protective immune responses against R. equi (see immunity). Ingestion of R. equi is an important route of exposure and likely of immunization but rarely leads to haematogenously acquired pneumonia unless a foal has multiple exposures to large numbers of bacteria.77 Indeed, oral administration of live virulent R. equi to foals during the first week of life is highly effective in preventing development of pneumonia after subsequent intrabronchial infection.67 Epidemiologic evidence suggests that most foals on endemic farms become infected early in life.68 Studies have shown that the median age at the time of diagnosis is approximately 35 to 50 days at most endemic farms.28, 48 In addition, experimental evidence suggest that new-born foals require a lower inoculum than older foals to induce pneumonia.103, 134, 136 Collectively, these findings indicate that many foals on endemic farms become infected at a young age when they are more susceptible than older foals to infection caused by R. equi. However, these findings do not indicate that foals are susceptible to R. equi only during the neonatal period. Older foals are susceptible to experimental infection with R. equi but they require a higher inoculum.

The study of the virulence factors of R. equi has been complicated by the fact that typical granulomatous lung lesions have not been consistently reproduced in any species other than horses. The normal murine lung can progressively clear a heavy inoculum of R. equi which would be sufficient to induce pneumonia in foals.177 Nevertheless, variation in virulence of R. equi has been observed between strains in experimentally infected mice and foals. Virulence in mice correlates with the ability of the organism to resist phagocytosis and intracellular killing by macrophages. Knowledge of the virulence mechanisms of R. equi was largely speculative until the discovery of a virulence plasmid in the early 1990s.156, 159 Sequencing and annotation of the virulence plasmid from equine isolates of R. equi indicated 73 coding sequences89, 149 divisible into four discrete areas based on open reading frame (ORF) amino acid sequence similarity and predicted protein function. The “backbone” area of the plasmid is very similar to that of a plasmid found in the environmental microorganism Rhodococcus erythropolis. It consists of a region for replication/partitioning, one for conjugation, and a region of unknown functions.89 The fourth plasmid region is a 21-kb pathogenicity island (PAI) that is indispensable for virulence.89, 149 The 26 coding sequences of the PAI include the virulence associated protein (Vap) family which is R. equi-specific.149 There are 6 full-length vap genes, (vapA, vapC, vapD, vapE, vapG, and vapH) and three truncated pseudogenes (vapF, vapI, and vapX).89 VapA, which encodes a surface-expressed, immunodominant, lipoprotein150, 158 is the only vap gene with a demonstrated role in virulence. VapA is required for intracellular growth in macrophages75 where it disrupts endolysosomes and contribute to preventing maturation of the phagosome to the stage of fusion of R. equi-containing vacuoles with lysosomes.168 The functions of the other Vap proteins are unknown. Although vapA is necessary for virulence, it is not sufficient.51 Two additional regulator genes of the PAI, virR and virS, are required for intracellular growth in macrophages and virulence.37, 78, 123, 129 Loss of either regulator results in decreased transcription, not only of vapA, but also of several chromosomal genes.37, 78, 123, 129 These findings suggest the presence of molecular “cross-talk” between the virulence plasmid and the R. equi chromosome.

While virtually all isolates of R. equi cultured from infected foals contain the large circular plasmid described above (designated pVAPA because it encodes vapA), most isolates from swine with granulomatous lymphadenitis contain a similar large circular plasmid (designated pVAPB) that encodes a 20-kDa antigen (VapB) that is related to, but distinct from, VapA.147 Most isolates from cattle with granulomatous lymphadenitis and from goats with various infections contain a linear plasmid (designated pVAPN  with “N” standing for “no-A no-B”) unrelated to the circular virulence plasmids pVAPA and pVAPB.139, 162 In contrast, many environmental isolates of R. equi do not contain any of these three plasmids and are referred to as avirulent because they lack the ability to replicate in macrophages in experimentally infected mice, and they cannot induce disease in foals. While distinct plasmid types exist among R. equi isolates obtained from different species, host tropism is not determined exclusively by plasmid type. Isolates of R. equi from swine, carrying pVAPB, are capable of replication in macrophages obtained from a variety of species (murine, swine, and equine).171 Similarly, isolates from foals carrying pVAPA are capable of replication in pig macrophages.171 Plasmid swapping between equine and porcine strains by conjugation does not impair the intracellular replication capacity of the parental strain.171

The pathogenesis of R. equi infection in humans appears to be different from that in foals, in which pVAPA is always present. Analysis of isolates of R. equi from humans reveals that approximately 15 per cent of isolates contain pVAPA, approximately 17 per cent contain pVAPB and approximately 25 per cent contain pVAPN.17 Most isolates of R. equi cultured from humans do not contain pVAPA, pVAPB, or pVAPN.17, 151, 155 Similarly, isolates from cats and dogs can be avirulent or contain any of the three plasmids.18, 152 Therefore, the plasmid profile of R. equi isolated from other species is not as straightforward as it is in horses, swine and ruminants.

Sequencing and annotation of the genome of R. equi was another major advance in the understanding of R. equi virulence.88 The 5.04-Mb R. equi genome is most similar to that of the environmental bacterium Rhodococcus jostii and then to Nocardia farcinica and Mycobacterium tuberculosis.88A large number of the 4,598 genes of R. equi appear to be involved in lipid metabolism. Like M. tuberculosis, lipids are a key component of the outer cell envelope of R. equi.142 Several chromosomally encoded genes have also been proven to play essential roles in intracellular survival and virulence, notably those involved in various metabolic functions such as fatty acid metabolism, anaerobic metabolism, aromatic amino acid biosynthesis, regulation of secreted proteins, iron acquisition, hydrogen peroxide resistance, and cholesterol catabolic pathways.12, 13, 57, 88, 90, 95, 107, 111, 118, 144, 163, 170

Immunity

Phagocytic cells

Optimal binding of R. equi to macrophages in vitro requires complement and is mediated by Mac-1, a leukocyte complement receptor type 3 (CR3 or Mac-1).66 In addition, R. equi utilizes the macrophage mannose receptor for entry by recognizing lipoarabinomannan (LAM), an outer surface component of the bacterium, either directly or via mannose binding protein or surfactant molecules adhered to LAM.45 Once phagocytized by macrophages, virulent R. equi is capable of modifying the phagocytic vacuole thereby preventing its acidification and subsequent fusion with lysosomes.42, 160, 168, 178 The ability of R. equi to disrupt endolysosome maturation and function depends on VapA127 and addition of soluble recombinant VapA restores the ability of R. equi mutant lacking the vapA gene to survive and replicate in macrophages.133, 174 Excessive intracellular replication of R. equi leads to necrosis of the macrophage.94 Opsonisation of R. equi with specific antibody is associated with increased phagosome–lysosome fusion and significantly enhances killing of R. equi by equine macrophages, suggesting that the mechanism of cellular entry can mediate the fate of the bacteria.60 Killing of R. equi by mouse macrophages relies on the presence of interferon (IFN)-γ, which activates macrophages to produce nitrogen intermediates and reactive oxygen intermediates. These two radicals combine to form peroxynitrite, which efficiently kills R. equi.38 Additional cytokines such as tumor necrosis factor (TNF)-α might have similar effects on restricting intracellular growth of R. equi in macrophages, because both IFN-γ and TNF-α are required for clearance of virulent R. equi in mice.82 Priming of equine monocyte-derived macrophages with IFN-γ, TNF-α, or IL-6 decreases intracellular survival of R. equi whereas IL-1β or IL-10 enhance intracellular survival.9 Neutrophils play also an important role in early host defense against virulent R. equi.101 However, as opposed to macrophages, neutrophils from foals and adult horses appear fully able of killing R. equi.99, 153, 176 As also observed with macrophages, killing of R. equi by neutrophils is enhanced by specific opsonizing antibody.61, 175

Antibody-mediated immunity

Immunity to R. equi pneumonia in foals remains to be fully understood but likely depends on both the antibody and cell-mediated components of the immune system. The partially protective effect of passively transferred anti-R. equi hyperimmune equine plasma (HIP) and the fact that maternal vaccination against Poly-N-acetyl glucosamine protects foals against intrabronchial challenge with R. equi are the strongest evidences for a role of antibody in protection against R. equi infection.126 The mechanisms by which antibody confers protection against facultative intracellular bacterial pathogens is an area of current investigation. As stated earlier, opsonization of R. equi with specific antibody has been shown to promote phagocytosis and killing of R. equi by alveolar macrophages and neutrophils.40

Cell-mediated immunity

Because R. equi is a facultative intracellular pathogen, cell-mediated immune mechanisms are thought to be of major importance in resistance to infection. Knowledge of cell-mediated immunity to R. equi infections comes mainly from infection of mice. Deficiencies in natural killer (NK) cells or complement component C5 in mice do not impair the pulmonary clearance of virulent R. equi.177 Conversely, T lymphocytes are essential for the clearance of virulent R. equi in mice.80, 96, 128 The two major mechanisms by which T lymphocytes mediate clearance of intracellular pathogens are secretion of cytokines and direct cytotoxicity. While both CD4+ (helper) and CD8+ (cytotoxic) T cells contribute to immunity against R. equi in mice, CD4+ T lymphocytes play the major role and are required for complete pulmonary clearance.79, 114, 128 Studies in mice have clearly shown that a type 1 response, characterized by IFN-γ production, is sufficient to effect pulmonary clearance of R. equi whereas a type 2 response, characterized by IL-4 production, is detrimental.80, 81

Because adult horses are typically resistant to R. equi infections, they have been used as a model to understand what type of immune responses are necessary for protection. Clearance of virulent R. equi in adult horses is associated with a significant increase in bronchoalveolar (BAL) fluid CD8+ and CD4+ T lymphocytes, lymphoproliferative responses to antigens of R. equi, development of R. equi-specific cytotoxic CD8+ T lymphocytes (CTL), and induction of IFN-g by CD4+ and CD8+ lymphocytes.65, 93, 117 Rhodococcus equi-specific CTL present in all immune adults are not  major histocompatibility complex (MHC) class I-restricted and they recognize unique bacterial lipids from the cell wall.58, 117 These lipid antigens might be presented to T lymphocytes via the CD1 system.116 How these findings in mice and adult horses relate to the foal is an area of active research.

Immunity to R. equi in foals

Neonates of multiple species demonstrate increased susceptibility to certain types of infection. Most new-born mammals are immunologically naive and have a variety of immunologic deficits when compared with older animals. In general, neonates have decreased antigen presenting cell function, diminished innate immune mechanisms, and are less able to mount type-1 immune responses.1 In addition to the lack of immunologic memory in new-borns, a number of immunologic deficits are postulated to account for the unique age-associated susceptibility of foals to pneumonia caused by R. equi. Deficiencies in R. equi-specific CTL activity has been documented in 3-week-old foals.117 Antigen-presenting cells from foals have significantly lower CD1 and MHC class II expression compared with adult horses.1, 116 In addition, multiple studies have demonstrated that the ability of equine lymphocytes, neutrophils, macrophages and dendritic cells to produce and/or upregulate various cytokines, is influenced by age.1, 15, 16, 44, 92, 112 The finding that young foals are deficient in their ability to produce IFN-g after stimulation with mitogens has led to dogma that an IFN-γ deficiency and type 2 bias might be at the basis of their peculiar susceptibility to R. equi infections.15, 44 However, foals also have impaired ability to produce IL-4 in response to stimulation with mitogens, stimulation with R. equi, and after vaccination with a killed adjuvanted vaccine, indicating that a pure polarization towards a type 2 response is unlikely in neonatal foals.91, 131, 132, 169 Consistent with these findings, experimental infection of young foals with virulent R. equi results in IFN-g induction and antibody responses similar to, or greater than, those measured in adult horses subjected to the same experimental challenge.72

Most foals at endemic farms do not develop disease or develop subclinical disease that resolves without therapy.164, 165, 167 Spontaneous resolution of pneumonia after experimental challenge with R. equi has been recognized.23, 104, 119 Furthermore, intragastric administration of live, virulent R. equi to new-born foals confers complete protection against subsequent heavy intrabronchial challenge.31, 67 Oral administration of live virulent R. equi induces accelerated development of R. equi-specific CTL,59 providing a possible mechanism for the protection conferred. Collectively, these findings prove that most foals have the ability to mount protective immune responses to R. equi. The basis for the susceptibility of foals to infection caused by R. equi is likely complex and multifactorial. The number of virulent R. equi in the environmental air, the age of the foal, the level of prior exposure to R. equi, and genetic susceptibility likely play a role. Genetic susceptibility to R. equi is likely a complex process with modest contributions from numerous genes involving various biological pathways and processes.106

Clinical signs and pathology

The most common clinical manifestation of disease caused by R. equi infections in foals is a chronic suppurative bronchopneumonia with extensive abscessation (Figure 1). The slow spread of the lung infection coupled with the ability of foals to compensate for the progressive loss of functional lung make early clinical diagnosis difficult.46 Early clinical findings of a slight increase in respiratory rate and a mild fever may be either missed or ignored, allowing the condition to progress. Therefore, even in the chronic form of the disease, respiratory signs are often of apparently acute onset. If pneumonia progresses, clinical signs may include decreased appetite, lethargy, fever, cough, tachypnea, and labored breathing characterized by nostril flaring and increased abdominal effort. Bilateral nasal discharge is an inconsistent finding. In a report of 161 foals affected with R. equi pneumonia, the most common clinical signs were cough (71 per cent), fever (68 per cent), lethargy (53 per cent), and increased respiratory effort (43 per cent).28 A smaller percentage of affected foals present with a more devastating subacute form. These foals may be found dead or, more commonly, present in acute respiratory distress with a high fever and no previous history of clinical respiratory disease, and often die within several days despite aggressive treatment. At necropsy, this subacute form of the disease is characterized by the presence of a diffuse miliary pyogranulomatous pneumonia and the nature of the lesions suggests extensive lung involvement before recognition of the clinical signs.102 On auscultation, the lung sounds of foals affected by both the chronic or subacute form of the disease vary considerably and the findings can be confused by referred sounds from the upper airway.

Rhodococcus equi infections

Figure 1 Multiple confluent caseonecrotic foci and accompanying atelectasis in the lungs of a foal that died of Rhodococcus equi infection

Because ultrasonographic screening for early detection of pneumonia caused by R. equi has become routine practice at some endemic farms, the most frequently recognized manifestation of R. equi infection at those farms is a subclinical form in which foals develop peripheral pulmonary consolidation or abscessation without manifesting clinical signs.166 At those farms, the cumulative frequency of pulmonary lesions considerably exceeds the historical frequency of clinical pneumonia attributed to R. equi, indicating that many foals with mild subclinical disease recover spontaneously without therapy. In adult horses, the disease caused by R. equi is rare but there are sporadic reports of infection involving primarily the lungs or abdominal lymph nodes (similar to infection observed in foals) and, less common, of wound infection.

Extrapulmonary manifestations of rhodococcal infections are common. In a retrospective study of 150 foals with R. equi infections, at least 1 of 39 different extrapulmonary disorders (EPDs) were recognized in 74 per cent of foals.124 Survival was significantly lower in foals with EPDs (43 per cent; 48/111) relative to foals without EPDs (82 per cent; 32/39).124 This must be interpreted with caution because some EPDs, particularly abdominal lesions, can be difficult to recognize antemortem and were recognized only during necropsy.124 Some EPDs (e.g. polysynovitis, uveitis) might be the first clinical manifestation of R. equi-associated disease detected prior to clinical signs of pneumonia.  

Intestinal lesions are present in approximately 50 per cent of foals with R. equi pneumonia presented for necropsy.179 Clinical signs associated with the abdominal form of the disease may include fever, depression, anorexia, weight loss, colic, and diarrhea.124 Diarrhea might occur in foals infected with R. equi either as an EPD caused by pyogranulomatous typhlocolitis or as a result of antimicrobial treatment. However, the majority of foals with R. equi pneumonia do not show clinical signs of intestinal disease. The intestinal form of R. equi infection is characterized by a multifocal ulcerative enterocolitis (Figures 2 and 3) and typhlitis over the area of the Peyer’s patches with granulomatous or suppurative inflammation of the mesenteric and/or colonic lymph nodes.179 In some cases, the only abdominal lesion noted may be a single large abscess (usually in a mesenteric lymph node) that often adheres to the large or small bowel.

Rhodococcus equi infections

Figure 2 Multiple ulcerative colitis in a foal that suffered from Rhodococcus equi infection

Rhodococcus equi infections

Figure 3 Closer view of a single ulcer in the colonic wall

Polysynovitis, characterized by effusion of multiple synovial structures and absence of lameness, occurs in approximately one fourth to one third of foals with naturally acquired R. equi infections.124, 143 Uveitis has also been described in foals naturally infected with R. equi.124 Foals with uveitis might display epiphora, photophobia, aqueous flare, hypopyon, iris discoloration, miosis, and other ophthalmic abnormalities. Experimental infection with virulent R. equi consistently results in polysynovitis and uveitis and development of these conditions is associated with the severity of pulmonary disease.51, 69 Culture of synovial fluid and aqueous humor within days of the onset of clinical signs in experimentally infected foals often yields R. equi and histologic examination of the synovial membrane reveals suppurative inflammation with bacteria.51, 69 These findings demonstrate that polysynovitis and uveitis are typically septic processes resulting from bacteraemia. However, the infection is eventually cleared from the synovial structures and the eye, resulting in nonseptic inflammation at these sites at the time of diagnosis or necropsy in chronic cases. Bacteraemic spread of the organism from the lungs or gastrointestinal tract may occasionally result in severe septic arthritis or vertebral osteomyelitis and/or discospondylitis.29, 53, 115 Other less common EPDs associated with metastatic spread of R. equi include endocarditis, pericarditis, pleuritis, sinusitis, cellulitis, dermatitis, subcutaneous abscesses, peripheral lymphadenopathy, guttural pouch empyema, ulcerative lymphangitis, myositis, stomatitis, omphalitis, and pyometra.76, 124

Diagnosis

It is important not to assume that every coughing foal with an elevated temperature is suffering from R. equi infection. Streptococcus zooepidemicus infection may cause bronchopneumonia in young foals and is far more readily treated than R. equi infection. However, the distinction between lower respiratory tract infections caused by R. equi and those caused by other pathogens is problematic especially at farms without previous history of R. equi infections. Thoracic radiography and ultrasonography are useful in evaluating the severity of pneumonia and in assessing response to therapy. Ultrasonography is also a useful tool for detection of some abdominal abscesses and for screening for R. equi-infected foals on farms where the disease is endemic (see Control). Ultrasonographic or radiographic detection of lung abscesses raises the degree of suspicion that pneumonia in a given foal is caused by R. equi. However, detection of pulmonary abscesses, despite being commonly used as a screening test, is not a definitive diagnostic test.

The definitive diagnosis of bronchopneumonia caused by R. equi should be based on bacteriologic culture or amplification of the vapA gene by polymerase chain reaction (PCR) from a tracheobronchial aspirate obtained from a foal meeting at least one of the following criteria:

  1. clinical signs of pneumonia,
  2. septic airway inflammation on cytologic examination, or
  3. radiographic or ultrasonographic evidence of bronchopneumonia.

If performed, amplification of vapA by PCR should be done in conjunction with bacterial culture because PCR does not permit identification of other bacterial pathogens and in vitro antimicrobial susceptibility testing of R. equi isolates.47 The definitive diagnosis of extrapulmonary infections (e.g. abdominal abscess, osteomyelitis) caused by R. equi must rely on bacteriologic culture or PCR amplification of vapA from samples obtained from the site of infection. The diagnosis of extrapulmonary disorders from sites at which R. equi cannot be detected (e.g. uveitis or polysynovitis) should be based on isolation of R. equi from a TBA or other primary sites of infection. The diagnosis of enterocolitis caused by R. equi is problematic because isolation of R. equi from faeces cannot be taken as evidence of enterocolitis caused by R. equi.47

Currently available serologic tests should not be used for the diagnosis of pneumonia caused by R. equi. Independent studies evaluating the performance of serologic tests available for diagnosis of infection caused by R. equi at endemic farms have demonstrated that these tests have either low sensitivity, low specificity, or both.49, 50, 100, 120

Control

Rhodococcus equi pneumonia is often not recognized until it is well advanced and, therefore, difficult to treat. Even severely affected foals might appear normal to a casual observer. Thus, in the absence of an effective commercially available vaccine or other effective methods for preventing this devastating disease, screening is a reasonable approach to control R. equi at endemic farms.  Implementing screening testing at farms that are only sporadically affected by R. equi pneumonia may not be warranted. Over the past decade, control of R. equi infections at many farms where the disease is endemic has relied on early detection of pulmonary disease using thoracic ultrasonography and initiation of therapy before development of clinical disease.166 Thoracic ultrasonography has been reported to decrease mortality due to R. equi pneumonia at some farms relative to historical controls but controlled studies are lacking.166 Historically, antimicrobial treatment was recommended for all foals with pulmonary lesions ≥ 1 cm in diameter.166 The temporal association between widespread use of macrolides and rifampin resulting from ultrasonographic screening and a perceived increase in the frequency of detection of isolates of R. equi resistant to macrolides and rifampin suggest that this practice might not be innocuous. Emergence of widespread macrolide- and rifampin-resistance at a farm after widespread use of these drugs was implemented on the basis of an ultrasonographic screening program has been documented.21 Blinded and  randomized, placebo-controlled studies indicate that 80 to 90 per cent of foals with lesion scores (defined as the sum of the largest diameter of all pulmonary lesions ≥ 1cm) between 1 and 10 cm recover without antimicrobial therapy. In addition, antimicrobial treatment does not significantly hasten lesion resolution in these foals compared to administration of a placebo.164, 167 By being more selective and treating only foals with larger lesion scores combined with weekly or twice weekly clinical and ultrasonographic monitoring of the foals, one farm was able to decrease antimicrobial drug use considerably without an increase in mortality.63, 130

Intravenous administration of HIP obtained from horses vaccinated against R. equi using various antigens has generally proved effective in significantly reducing the severity of R. equi pneumonia in foals following experimental challenge.67, 104, 122, 135 However, studies evaluating the efficacy of various HIP preparations under field conditions have given equivocal results.  Although the data are conflicting and not all trials have shown a statistically significant reduction in the cumulative incidence of R. equi pneumonia, five of seven studies demonstrated reduction of relative risk, suggesting some benefit of HIP.27, 28, 48, 62, 71, 97 The optimal amount of plasma to be transfused and the optimal age at which transfusion should occur remain to be determined. Because of evidence that many foals become infected early in life,68 it is commonly recommended that foals receive transfusion of at least one litre of commercially produced and licensed HIP no later than the second day of life. Because early administration may result in the decline of passively transferred antibody to a non-protective level at a time when foals might still be susceptible to R. equi and when environmental challenge is high, it is common practice to administer a second litre of HIP at two to four weeks of age. Transfusion of HIP is not completely effective and therefore does not eliminate the need for careful monitoring of foals at risk. Thus, the cost-effectiveness of transfusion depends on the value of the foals and the prevalence of disease at a given farm.

Treatment

Treatment of even severely affected foals is worthwhile, since studies have established that foals that recover from R. equi pneumonia perform in races as well as expected, although foals that contract infection might be slightly less likely to race.2, 3  It is important to consider treatment of subclinically affected foals and foals with clinical disease separately because ultrasonographic lesions in many foals that are clinically healthy will resolve without treatment (see Screening).

Treatment of foals with severe clinical pneumonia

A wide variety of antimicrobial agents are active against R. equi in vitro. However, many of these drugs are reported to be ineffective in vivo, possibly because of poor cellular uptake and resulting low intracellular concentrations. The combination of a macrolide antimicrobial agent with rifampin has been the mainstay of therapy in foals with clinical pneumonia caused by R. equi although randomized clinical trials comparing efficacy of various therapies are lacking. The combination of a macrolide and rifampin is synergistic both in vitro and in vivo, and the use of the two classes of drugs in combination reduces the likelihood of development of resistance to either drug.55, 113, 121 In addition, studies in immunodeficient mice indicate that the combination of a macrolide with rifampin is superior to either drug used in monotherapy.20, 21, 113 The antimicrobial agents commonly used for the treatment of R. equi infections in foals are listed in Table 1. Macrolides and rifampin are highly active against R. equi in vitro but only exert bacteriostatic activity.55, 113 The three macrolides that have been used for the treatment of pneumonia caused by R. equi are erythromycin, clarithromycin and azithromycin. Clarithromycin is the most active against R. equi in vitro and against intracellular R. equi in cell culture.11, 55 The minimum inhibitory concentration at which 90 per cent of R. equi isolates are inhibited (MIC90) is 0.06, 0.5 and 1.0 µg/ml, for clarithromycin, erythromycin, and azithromycin, respectively.74, 125 The combination clarithromycin-rifampin was significantly more effective than erythromycin-rifampin or azithromycin-rifampin in a retrospective study, especially in foals with severe pulmonary lesions.52 Despite data analysis being adjusted for disease severity and age, these results must be interpreted cautiously because of potential biases inherent to retrospective data.

Table 1 Recommended dosages, oral bioavailability and serum half-lives of antimicrobial agents commonly used for the treatment of R. equi infections in foals.

Class

Drug

Dose (mg/kg)

Route

Frequency
(h)

References

Macrolides

Erythromycin*

25

POa

6-8

86, 87

 

Clarithromycin

7.5

PO

12

172

 

Azithromycin

10

PO

24-48b

39, 73

Rifamycins

Rifampin

5

PO

12

19, 24, 83

* Erythromycin base, stearate, phosphate, estolate, or ethylsuccinate.
a Oral bioavailability is low unless the foal is fasted
b Administration q 24 h for 5 days and q 48 h thereafter

The most common adverse effect in foals treated with macrolides alone or in combination with rifampin is diarrhoea. Hyperthermia and tachypnoea have been described in foals treated with macrolides during periods of very hot or humid weather.140 Recent studies demonstrate that azithromycin, clarithromycin and erythromycin suppress sweating in foals, with erythromycin inducing more severe sweating dysfunction.137, 138 Another rare complication of macrolides is severe enterocolitis in mares whose foals are treated with erythromycin, presumably by disrupting the mare's normal colonic microflora following ingestion of small amounts of active drug during coprophagia or by contamination of feeders or water buckets with drug present on the foal's muzzle.7

Foals with mild to moderate subclinical lesions

Only studies enrolling foals with lesions scores ≥ 10 cm (see Screening) have documented a significant difference in the rate of recovery between foals treated with antimicrobials and a placebo group.63, 165 In two separate, blinded, placebo-controlled studies, azithromycin alone (10 mg/kg PO q 24 h) or gamithromycin alone (6 mg/kg IM q 7 days) were non-inferior to azithromycin with rifampin. 63, 165 In another study, therapy with tulathromycin was more effective than a placebo but not as effective as the combination of azithromycin with rifampin.130 It is important to note that up to 78 per cent of the foals receiving the placebo in the aforementioned studies recovered without the need for antimicrobial treatment.63 Therefore, the apparent efficacy of macrolide monotherapy in foals with mild lesions should not be interpreted as evidence of similar efficacy in foals with severe pneumonia.  It is also unknown if macrolide monotherapy is more likely to select for resistance in vivo than combination therapy.

Emergence of resistance to macrolides and rifampin

Although resistance has increased in recent years, the vast majority of R. equi isolates from foals are still highly susceptible to macrolides and rifampin in vitro. Failure to respond to antimicrobial therapy does not provide evidence that a given foal is infected with an isolate resistant to antimicrobial agents. Therefore, the presence of an isolate resistant to macrolides or rifampin can only be confirmed by in vitro susceptibility testing done by a laboratory following the criteria established by the Clinical and Laboratory Standards Institute. In a recent study, the overall prevalence of macrolide and rifampin-resistant isolates in Texas and Florida over a 10-year-period was 4 per cent and the odds of death were seven times higher in foals infected with resistant isolates compared with foals infected with susceptible isolates.54 More recently, it was documented that mass antimicrobial treatment of subclinically affected foals selected for antimicrobial resistance over time with isolates of R. equi resistant to all macrolides and rifampin being cultured from as many as 40 per cent of the foals at some farms.21 Macrolide resistance in R. equi is conferred by acquisition of a rRNA methylase gene designated erm(46). This gene confers resistance to all macrolides, lincosamides, and streptogramins type B, but not to other classes of antimicrobial agents. The gene can be transferred by conjugation from resistant to susceptible isolates of R. equi.4 Most macrolide resistant isolates of R. equi identified to date are also resistant to rifampin. Resistance to rifampin in R. equi is the result of mutations in the rpoB gene.5, 43, 125, 157 Therapy of foals infected with isolates confirmed to be resistant to macrolides, rifampin, or both is problematic because of the limited range of effective alternatives. Macrolide- and rifampin-resistant isolates of R. equi are susceptible in vitro to fluoroquinolones, gentamicin, linezolid, and vancomycin.10, 22, 54 In one study, 18 of 24 isolates were also susceptible to chloramphenicol, minocycline, and trimethoprim-sulfamethoxazole.54 Currently, there are no data to indicate the preferred antimicrobial agent(s) for the treatment of foals infected with resistant isolates. Vancomycin, imipenem or linezolid in foals should be used only for the treatment of life threatening R. equi infections caused by isolates confirmed to be resistant to all other possible alternatives. The use of systemic and/or nebulized gentamicin is a reasonable consideration for foals infected with macrolide- and rifampin-resistant isolates.56

Duration of therapy and prognosis

Resolution of clinical signs and radiographic or ultrasonographic resolution of lung lesions are commonly used to guide the duration of therapy which generally ranges between two and eight weeks, depending on the severity of the initial lesions and response to therapy.  Foals treated based on subclinical lesions identified during ultrasonographic screening typically require a shorter treatment duration. In fact, as addressed below, many foals with subclinical ultrasonographic lesions clear the infection without therapy.164, 167

The prognosis of R. equi-infected foals was poor prior to the introduction of the combination of erythromycin and rifampin as the treatment of choice in the early 1980s, with survival rates as low as 20 per cent.41 Using erythromycin and rifampin, Hillidge reported a successful outcome in 50 (88 per cent) of 57 foals with confirmed R. equi pneumonia.64 Studies at various referral centres have shown survival proportions ranging between 59 and 72 per cent.3, 52, 124 The prognosis for foals with abdominal abscesses is poor, although rare cases have responded to prolonged antimicrobial therapy.124, 161

Currently, there is inadequate evidence to recommend active immunization of mares or foals to control or prevent R. equi pneumonia but there are promising vaccine candidates under development.

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