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Rhodococcus equi Pneumonia: Clinical Findings, Diagnosis, Treatment and Prevention
Rhodococcus equi Pneumonia: Clinical Findings, Diagnosis, Treatment and Prevention Peter Heidmann, DVM, MPH, Dip. ACVIM,* John E. Madigan, DVM, MS, Dip. ACVIM,† and Johanna L Watson, DVM, PhD, Dip. ACVIM† Rhodococcus equi is a Gram-positive facultative intracellular bacterial pathogen and the most common cause of severe infectious pneumonia in foals. The pathogen is ubiquitous in the environment, but becomes concentrated in breeding farm situations due to its ability to reproduce in the gastrointestinal tract of herbivores. Foals may be exposed to the organism soon after birth; overt infection results in variable number of foals due to a combination of pathogen virulence, host susceptibility, and infective dose of the organism. Herd health programs using passive transfer of antibodies, especially with transfusions of equine hyperimmune plasma, can decrease incidence rates on affected premises, but rarely prevent infection in all susceptible individuals. An effective vaccine is not currently available. Combined therapy of clinically affected individuals using macrolide antibiotics combined with rifampin has improved outcome, including both morbidity and mortality. Clin Tech Equine Pract 5:203-210 © 2006 Elsevier Inc. All rights reserved. KEYWORDS Rhodococcus equi, pneumonia, herd health, virulence associated plasmid
Bacteriology and Pathogenesis
R
hodococcus equi is a Gram-positive facultative intracellular bacterial pathogen with the ability to replicate in infected equine macrophages.1 This organism was previously known as Corynebacterium equi, only being definitively included in the genus Rhodococcus in 1987 based on gene analysis.2 It most commonly causes pyogranulomatous bronchopneumonia in affected foals, often with extensive pulmonary abscesses (Fig. 1). Rhodococcus equi is present in the manure of many herbivores, and is found in higher concentrations in soil of equine environments.3 Virtually all foals are exposed to R. equi early in life through inhalation or ingestion, but the majority of foals develop protective lifelong immunity following exposure. Similar in many ways to Mycobacterium tuberculosis, R. equi is a nocardioform actinomycete with a cell wall containing mycolic acid. Also like M. tuberculosis, R. equi can infect and survive within alveolar macrophages, where the organisms can replicate within the phagosome and prevent phagolysosome fusion.4 Reports also indicate that virulent R. equi can destroy host cells, although it may not be able to enter pulmonary epithelial cells. The respiratory form of R. equi infection in foals, histori-
*Veterinary Medical Teaching Hospital, and the †Department of Medicine and Epidemiology, School of Veterinary Medicine, University of Califorina, Davis, CA. Address reprint requests to P. Heidmann, Montana Equine, 40 Buckskin Rd., Belgrade, MT 59714. E-mail: [email protected]
1534-7516/06/$-see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1053/j.ctep.2006.03.015
cally called “rattles,” is believed to infect foals within the first several days of life, but typically results in protective immunity. In affected individuals, clinical signs often remain unidentified for weeks or months following infection. Rhodococcus equi is considered to be the most common cause of severe pneumonia in foals between 3 weeks and 5 months of age.5,6 Less commonly, the bacteria can infect other organ systems, causing septic uveitis, ulcerative enterocolitis (Fig. 2), colonic and/or mesenteric lymphadenopathy, osteomyelitis, and septic physitis or arthritis (Fig. 3). Poorly characterized, immune-mediated conditions such as sterile uveitis (Fig. 4) are occasionally associated with R. equi infection,6 whereas immune-mediated polysynovitis with immunoglobulin deposition in synovium, more often affecting stifle or tibiotarsal joints (Fig. 5), has also been associated with R. equi infection.7 Although the organism is distributed worldwide, it has its greatest impact on large equine breeding farms due to increased prevalence and pathogenicity, causing increased morbidity and mortality.8 Case fatality rates of 12.5% to 42% have been reported in recent studies, although rates as high as 80% occurred before the advent of combined antimicrobial therapy using rifampin with a macrolide such as erythromycin.8-10 Rhodococcus equi is also a zoonotic agent, most frequently affecting people with compromised immune status, in whom it can cause pyogranulomatous pneumonic lesions resembling tuberculosis. Case fatality rates of approximately 50% have been reported in people with AIDS, although mortality may approach 11% in those rare cases where immunocompetent human patients are infected.11 To date, no specific risk 203
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Figure 1 Pathologic findings of a foal diagnosed with R. equi pneumonia showing multiple, well-defined pulmonary abscesses. (Color version of figure is available online.)
factors have been associated with infection in immunocompetent people. Rhodococcus equi is relatively resilient, and can not only be cultured from the soil on and around equine breeding facilities but also from elsewhere in the environment.12,13 It is more readily isolated from dusty soil than from wet ground, suggesting one possible risk factor for infection may be living in dry dusty environments which may enhance exposure to maximal infective doses. Similarly, R. equi has been shown to grow more readily in soils mixed with equine feces than in soil alone, suggesting that housing, paddock, and pasture environments may serve to amplify organism in the environment, increasing risk of exposure to maximal doses of the pathogen.14 Recent research has focused on explaining why a minority of exposed foals ultimately develop clinical infection with Rhodococcus equi. Are low case attack rates due to variations in pathogen virulence? Is there variability in foals innate or acquired immunity? Are some foals exposed to higher infective doses of organism due to combinations of farm managementbased risk factors? In reality, clinical infection occurs due to
Figure 2 Pathologic findings of a foal showing ulcerative, pyogranulomatous enterocolitis caused by R. equi. (Color version of figure is available online.)
Figure 3 Radiograph showing physitis and osteomyelitis associated with the distal physis of the left hind third metatarsal bone of a foal with R. equi pneumonia.
the complex interactions among host, pathogen, and environmental factors. We will discuss each of these factors in turn, all of which appear to play a role in the risk of developing clinical disease. Controlling each of these factors may be important in minimizing the risk of infection (herd-health preventative medicine programs), and in treating affected individuals.
Figure 4 Foal diagnosed with R. equi pneumonia showing bilateral immune-mediated uveitis. The picture shows epiphora, aqueous flare, and yellow-discolored iris of the left eye.
Rhodococcus equi pneumonia
205 cytotoxicity, and that no single-acting substance from culture causes its cytotoxic effect.18 Some workers have reported that no significant association exists between the R. equi disease status of a farm and the isolation of VapA positive or negative organisms in the soil.19 Other studies report that R. equi expressing VapA was present in higher concentrations on breeding farms with a history of R. equi infection than on farms without clinical cases.20 Although the organism itself may be ubiquitous, isolates from locations without horses are less likely to contain the virulence-associated plasmid. When more than 200 soil and sand samples were collected from nonequine related parks and yards in Japan, R. equi was isolated from almost 75% of the samples, but none of the more than 1000 microbiological isolates expressed VapA lipoprotein.21 Further elucidation of the precise mechanism of enhanced virulence associated with the VapA protein depends on ongoing and future research.
Immunologic Factors
Figure 5 Foal with R. equi pneumonia showing distended right tibiotarsal joint due to immune-mediated synovitis. (Color version of figure is available online.)
Virulence Factors Beyond its ability to escape destruction within the phagolysosome in alveolar macrophages, early reports suggested enhanced cellular necrosis was caused by rhodococcal glycolipids. An 85- to 90-kb plasmid is consistently identified in isolates of R. equi from clinically affected foals, and is typically absent from strains cultured from nonequine environments. Virulent strains containing this plasmid express a family of seven virulence associated proteins (Vap), including the highly immunogenic protein VapA. The specific molecular function of these proteins remains undetermined. However, the VapA protein is critical for intracellular growth; experimental deletion of the VapA gene significantly diminishes virulence in mice macrophages.15 DNA sequencing of independent isolates from clinical cases revealed identical plasmid sequences, suggesting that the Vap proteins are pervasive in virulent strains.16 These in vitro studies of virulence appear to be consistent with evidence from experimental infection. When foals were experimentally inoculated intratracheally with R. equi, the minimum infective dose for the plasmid-containing strain was 104 bacteria, compared with ⬎109 bacteria for the cured (plasmid-free) partner strain.17 However, cytotoxicity is not seen in the absence of viable bacteria, suggesting that viable R. equi organism is required to produce
Normal adult horses and the majority of normal foals develop active immunity and clear the infection, even when exposed to highly virulent R. equi. However, a subset of foals does not develop protective immunity. Diminished host immune response may be due to a variety of factors. Therefore, most recent research has focused on elucidating the mechanism for effective immunity in the majority of exposed foals which do not develop clinical R. equi infection. The prevalence of R. equi infection in HIV-positive humans with low T-cell numbers provided early, if indirect, evidence of the protection imparted by T-lymphocytes.22 More recently, it has been shown that the style of immune response is critical to effective protection. Adult horses challenged with virulent strains of R. equi clear the organism from their lungs during increases in the number of pulmonary CD4⫹ and CD8⫹ lymphocytes.23 This clearance is associated with an increased number of CD4⫹ T-cells producing interferongamma (IFN-␥), and is characteristic of an appropriate Th-1 or cell-mediated immune response to intracellular infection. Evidence in mice suggests that the balance of Th-1 (cell mediated) to Th-2 (humoral) responses may be critical in successful clearance of R. equi infection, and the balance of Th-2 to Th-1 may also be relatively increased in clinically affected foals. In immunocompetent adult horses, challenge with R. equi results in proliferation of pulmonary T-cells and increased concentrations of IFN-␥. These horses did not show an increase in release of interleukin-4 (IL-4), which is typically associated with a Th-2 response and a less effective immune response to intracellular bacterial infection.24 Interferon-gamma appears to play a critical role in limiting R. equi infection. Production of IFN-␥ stimulates phagolysosome fusion and increases production of reactive oxygen species. Knockout mice defective in production of IFN-␥ or of inducible nitric oxide synthetase are particularly susceptible to R. equi infection.25 Mice lacking IFN-␥ have been shown to have decreased ability to clear R. equi infection, whereas stimulation of a CD4⫹ Th-1 response results in protective immunity and clearance of R. equi infection.26,27 Thus, the balance of Th-1 to Th-2 may be critical in pathogenesis of infection in foals, with increased Th-2 response predisposing to clinical infection.
206 The role of specific antibody subtypes has also been investigated. Following challenge of immunocompetent adult horses, the levels of R. equi and VapA-specific IgGa and IgGb were enhanced, suggesting that these antibody subtypes are important in functional immunity.24 Further investigation suggested that IgGa specific for Vap protein appears to be functionally protective, and was associated with a Th-1 response. IgGb and IgGT have been associated with a Th-2 style response, and may actually enhance susceptibility.28 IgGb synthesis does not begin until at least 2 months of age in normal foals, whereas IgGa may begin in the last trimester in utero and continue actively during the first 2 months of life in normal foals. Based on this information, it appears that promotion of active immunity to R. equi requires a robust Th-1 response characterized by the production of IFN-␥ by CD4⫹ and CD8⫹ T-lymphocytes. In turn, appropriate antigens (immunogens) must be identified to stimulate sufficient release of IFN-␥ from T-cells. Previous vaccination attempts have not produced reliable protection. Direct oral immunization of foals with a modified bacterin offered partial protection, but did not result in induction of a lasting anamnestic response.29 More recent work has emphasized subunit vaccines to minimize the risk of modified vaccine strains acquiring the virulence plasmid. However, these attempts are complicated by incomplete understanding of the interactions between Vap proteins. Effective vaccines rely on choosing the right antigen because not every immunogenic antigen produces protective immunity. Recent work has identified a group of immunogenic proteins in Vap-containing strains from infected foals. These proteins may be encoded or regulated by the R. equi virulence plasmid. T-lymphocytes from the lungs of adult horses following immune clearance of virulent R. equi proliferated in response to a combination of these immunogens and produced IFN-␥.30 It is not known which specific antigens promoted this response, nor whether a protective response (with release of IFN-␥) in naïve individuals can be generated by exposure to one or more of these proteins. To date, studies using various VapA vaccines have failed to produce immunity in mice.31 Successful vaccination of foals may be hampered by maternal interference. Although no work has been published specifically documenting maternal interference with R. equi, passive transfer of maternally antibodies can inhibit the ability of foals to develop specific antibody titers to other pathogens (eg, tetanus toxin, rabies) during the first 6 months of life. Equally important is the ability of R. equi to evade host immune mechanisms, allowing replication within macrophages. Previous attempts to promote immunity via passive transfer of maternal antibodies from vaccinated mares to their foals have produced inconsistent results, raising the dam’s titers and protecting foals in some studies but not establishing uniform immune protection in other studies.32,33 Meanwhile, colostral transfer of maternal antibodies produced in response to a VapA vaccine has shown some early promise.34 In comparison to prepartum vaccination programs, passive transfer by transfusion of hyperimmune plasma to foals has been shown to decrease morbidity and mortality. When given before exposure, the administration of equine hyperimmune plasma can prevent, or significantly reduce the se-
P. Heidmann, J.E. Madigan, and J.L. Watson verity of, experimental or naturally acquired pneumonia.35,36 However, some work has documented mixed results following plasma transfusion, possibly due to differences in sources and composition of the hyperimmune plasma used, as well as consumption of transfused antibodies and other factors.37 IgGb, associated with less significant protection against infection, is the predominant isotype in hyperimmune plasma, suggesting that the value of plasma administration may be due to neutralization of other virulence factors. For Rhodococcus-endemic farms, it is typically recommended that hyperimmune plasma is administered within 24 hours of birth, and again between 3 to 4 weeks of age due to waning protection from the initial transfusion. Ultimately, complete protection of at-risk foals may require a combination of active immunization with immunogens that stimulate a Th-1-specific response given immediately postpartum to avoid the effect of maternal blocking, followed by plasma transfusion to boost immunity in neonatal period. Although plasma can help provide immunoglobulins and other factors, the foal must still mount an appropriate immune response.
Environmental Risk Factors Multiple studies have been published attempting to identify risk factors for subsequent development of R. equi pneumonia in foals. Effectively minimizing risk factors could limit exposure to infective doses insufficient to overwhelm the immune system of an otherwise healthy foal. Knowing that R. equi proliferates in the gastrointestinal tract of horses suggests that many risk factors may be associated with bioaccumulation on large breeding farms, including both management practices and the number of years a premise has held horses. Studies of farm-related factors have noted a higher risk of infection on farms with greater acreage, farms with large populations of mares and foals (defined as 160 horses or ⬎15 foals), farms with high density housing of foals, and in particular, farms with a transient population of mares.38,39 Previous work examining a farm’s age is divided. In one study, farms housing horses for over 30 years were shown to have higher concentrations of R. equi organism in the environment.40 Other work has not identified an increased risk of infection associated with longer established farms.38 However, the evidence is not always intuitive, for example, factors related to poor management, including a lack of preventative care practices, were not associated with an increased risk of infection. In fact, better run farms with more consistent preventative care may be more frequently affected.38 A combination of foal-related risk factors has been implicated in causing higher rates of disease in one study of two large breeding farms. Foals were found to be at higher risk of infection when their dam was multiparous (with parity greater than four) or had been at the farm for a minimum of 1 year, suggesting the possibility of bioaccumulation and increased shedding of organisms from these mares. Foals housed in a box stall up to 4 weeks of age were associated with decreased risk of infection, whereas early turnout (turnout at less than 2 weeks of age) was associated with increased incidence rates. Affected foals were less likely to have been housed in paddock turnout groups of 2 to 5 mare–foal pairs, and more likely to have been housed in groups of 6 to 10
Rhodococcus equi pneumonia pairs. In other words, this work suggested that maintaining relatively small groups of mare and foals appears to provide a measure of protection from clinical R. equi infection.38 In addition to minimizing overstocking and direct exposure to organisms shed into the environment (especially in feces), this strategy might also minimize environmental accumulation over time. Previous work has also indicated that maintaining foals in a stalled environment during early weeks may have a protective benefit, possibly because exposure to R. equi occurs very early in life, followed by active multiplication in the intestinal tract during the first 8 weeks of life.41 Increased rates of disease were also associated with less frequent use of supplemental immunoglobulins; when some form of concentrated immunoglobulins was administered, foals were found to have decreased rates of disease.38 However, prospective analysis of foal-related risk factors showed that many of the widely adopted strategies related to minimizing neonatal septicemia and other infections are not related to protection from clinical R. equi infection. For example, the specific gravity of mare’s colostrum, duration of time to stand and suckle, and foals’ IgG concentration were not related to risk of R. equi infection. Administration of antibiotics during the first day of life was not associated with risk or benefit.38 Of course, the goal of determining specific risk factors is to identify practical means to minimize the risk. In some farm situations, it may be practical to lengthen the duration of mare and foal living in a stall but not to keep mares and foals in groups of less than five pairs. It may be practical to provide supplemental immunoglobulins to all foals, but not to cull mares with parity greater than four. Moving a farm operation to a new premises simply to avoid endemic infection is a drastic step, and rarely practical. Therefore, veterinarians, farm managers, and owners must decide together which strategies to implement to minimize bioaccumulation and exposure to organism, and which are impractical, or unlikely to significantly impact herd health.
Diagnosis The techniques utilized to definitively diagnose respiratory R. equi infection are very similar to those for the other infectious causes of pneumonia discussed in this issue. Clinical signs of respiratory tract infection may be mild, but the disease progresses steadily, typically resulting in mild fever (or hyperthermia) and mild tachypnea. Since early clinical signs can be mild, some cases may be initially identified by physical examination of foals with poor growth, with additional diagnostics being prompted by detection of significant abnormalities. In more severe cases, adventitial lung sounds, including crackles and wheezes, may develop. Pleural friction rubs may be heard in advanced cases. Some foals succumb to severe infection without having been previously identified as sick presenting acutely febrile and in respiratory distress. Rarely, foals with R. equi may be discovered as sudden death without any history of illness. One practical screening technique has been recommended to farm managers (Wilson, WD, personal communication): (1) farm manager should evaluate all foals respiratory rate and effort in the morning; (2) in foals with increased respiratory rate (⬎ 24 breaths/min), the manager should measure rectal temperature; and (3) in foals with
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Figure 6 Smear from trans-tracheal wash fluid collected from a foal with R. equi pneumonia showing small, Gram-positive pleomorphic bacteria (Gram stain, 100⫻ objective). (Color version of figure is available online.)
increased rectal temperature (⬎102.5°F [39.2°C]), the manager should request complete blood count. In foals with increased total white cell count (⬎13,000/uL) or increased fibrinogen (⬎600 mg/dL), advanced diagnostics should be requested, including transtracheal wash, thoracic ultrasound, and/or thoracic radiography. Routine hematologic testing (CBC and plasma fibrinogen) provides valuable, if nonspecific, indicators of inflammation and/or infection. Especially by limiting examination to foals from farms with endemic infection, a CBC and fibrinogen may provide very effective screening tools. Elevations in total white cell count above 13,000/L in the presence of fever should prompt veterinary evaluation, whereas a total white count above 14,000/L is ground for further diagnostic evaluation, even in the absence of external clinical signs. Fibrinogen greater than 600 mg/dL should also prompt further examination or advanced diagnostics. Historically, serology has been used to assess the likelihood of clinical infection with R. equi. Acute and convalescent samples have been advocated for evidence of clinical infection, but interpretation is confounded by the widespread use of hyperimmune plasma and, more importantly, the fact that nearly all foals in endemic areas will develop rising titers to R. equi during the first 2 to 3 months of life. In fact, recent work indicated that serologic analysis by ELISA or AGID is not useful to definitively diagnose R. equi infection.42 Over time, however, serology may be useful as a farm-wide screening tool to determine the progression of foal-hood seroconversion to R. equi. Although there are multiple organisms which can cause suppurative bronchopneumonia in young foals, few are associated with abscess formation, which is a hallmark of advanced R. equi pneumonia. However, early infection with R. equi frequently lacks abscessation, requiring additional tests to obtain an etiologic diagnosis. The gold standard for diagnosis is the identification of R. equi from TTW samples via bacteriologic culture and/or molecular diagnostic techniques such as polymerase chain reaction (PCR). Identification of small, Gram-positive pleomorphic bacteria on Gram staining can facilitate early diagnosis, but is still not definitive (Fig. 6). PCR has been shown to have superior sensitivity and speci-
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Figure 7 Thoracic radiograph from a foal with R. equi pneumonia showing an alveolar pattern with ill-defined regional consolidation compatible with early abscess formation.
ficity for diagnosing R. equi when compared with standard bacteriologic culture43 but is not universally available. With experience, TTW collection can readily be performed under field conditions under light sedation, even in dyspneic foals. Radiography is considered the principal imaging technique for evaluation of the thorax. Thoracic radiographs of foals with R. equi vary depending on the progression of clinical disease. Digital and computed systems are becoming more prevalent, and may increase the diagnostic sensitivity of thoracic radiography. Typically, earlier lesions include a structured interstitial pattern and/or consolidation, whereas advanced infection results in pulmonary abscessation revealed as variably sized nodules on thoracic radiographs (Fig. 7). Because other organisms, such as Streptococcus spp. and anaerobes, may uncommonly cause pulmonary abscesses, a definitive diagnosis cannot be obtained without bacteriologic culture. Radiographic evaluation may be limited by patient size or due to poor accessibility to hospital equipment. Thoracic ultrasonography has been used with increasing frequency, especially in on-farm situations. Ultrasound has been shown to correlate highly with thoracic radiographs, although it may not be able to evaluate lesions deep within the pulmonary parenchyma. Ultrasonographic examination of the chest should include all lung fields bilaterally, beginning caudally at the 17th intercostal space and continuing cranially to the 3rd intercostal space. Normal lung surfaces are highly echogenic due to the presence of air in well-aerated fields. Mild pulmonary changes are characterized by roughening of the pleural surface, resulting in comet-tail artifacts
(Fig. 8). Consolidated lung fields may be difficult to distinguish from pulmonary abscessation, but are typically hypoechoic to muscle; areas of consolidation are frequently surrounded by hyperechoic margins at the interface of
Figure 8 Thoracic ultrasonography of the left cranioventral lung field from a foal with R. equi pneumonia showing multiple comet-tail artifacts due to pleural roughening.
Rhodococcus equi pneumonia consolidated and aerated lung. Abscesses may or may not be surrounded by an echogenic capsule, whereas the center may be variable echogenicity, and may be septated, depending on the type of fluid contained within the capsule. If gas is present within the abscess, irregular hyperechoic foci may also be identified. In addition to portability, ultrasound may provide more accurate localization of abnormalities, allowing more accurate monitoring of lesions during treatment.
Treatment Combined antimicrobial therapy is accepted as the treatment modality of choice for cases of R. equi pneumonia. Erythromycin and rifampin have been shown to be superior to penicillin and gentamicin, even when treating R. equi pneumonia with in vitro susceptibility to gentamicin,44 in part due to the ability of the organism to escape host defenses and replicate within macrophages. Both erythromycin and rifampin are concentrated in granulocytes and macrophages by an active mechanism. However, erythromycin has variable absorption in foals when given orally, and is associated with adverse effects, including colitis, and idiosyncratic hyperthermia and tachypnea in hot weather.45 A recent report indicated better outcomes with clarithromycin/rifampin than other combination therapies, based on quicker discharge from the hospital, and better long-term recovery (defined as being clinically healthy at 1 year of age46). Clarithromycin and azithromycin have better stability, bioavailability, and concentration in phagocytic cells and tissues in people when compared with erythromycin. The concentration of azithromycin in pulmonary epithelial cells and bronchoalveolar cells of foals following oral dosages has been documented, but no work has yet been published documenting the pharmacokinetics of clarithromycin in equids. However, previous work has shown a better in vitro activity of clarithromycin against R. equi,46 and it is superior to azithromycin and erythromycin in humans with pulmonary Mycobacterium avium infection, a facultative intracellular organism closely related to R. equi. This research indicates that the combination of clarithromycin (7.5 mg/kg PO twice per day) with rifampin (5-10 mg/kg PO twice per day) was superior to azithromycin (10 mg/kg PO q24h) with rifampin. The combination of clarithromycin/rifampin was also superior to erythromycin (37.5 mg/kg PO twice per day to 25 mg/kg PO q6h) with rifampin. Antibiotic-associated colitis/diarrhea was found in all treatment groups, and immediate discontinuation of all antimicrobials is warranted if diarrhea develops during treatment with any of the above-mentioned drug combinations. In our hospital, foals are most often treated with azithromycin (10 mg/kg PO q24h for 5 days, then 10 mg/kg PO every other day for the duration of the treatment interval) and rifampin (5 mg/kg PO twice per day).
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isotype responses of pneumonic and healthy, exposed foals and adult horses to Rhodococcus equi virulence-associated proteins. Clin Diagn Lab Immunol 10:345-351, 2003 Chirino-Trejo JM, Prescott JF, Yager JA: Protection of foals against experimental Rhodococcus equi pneumonia by oral immunization. Can J Vet Res 51:444-447, 1987 Kohler AK, Stone DM, Hines MT, et al: Rhodococcus equi secreted antigens are immunogenic and stimulate a Type 1 recall response in the lungs of horses immune to R. equi infection. Infect Immun 71:63296337, 2003 Vanniasinkam T, Barton MD, Heuzenroeder MW: Immune response to vaccines based upon the VapA protein of the horse pathogen, Rhodococcus equi, in a murine model. Int J Med Microbiol 294:437-445, 2005 Becú T, Polledo G, Gaskin JM: Immunoprophylaxis of Rhodococcus equi pneumonia in foals. Vet Microbiol 56:193-204, 1997 Varga J, Fodor L, Rusvai M, et al: Prevention of Rhodococcus equi pneumonia of foals using two different inactivated vaccines. Vet Microbiol 56:205-212, 1997 Cauchard J, Sevin C, Ballet JJ, et al: Foal IgG and opsonizing antiRhodococcus equi antibodies after immunization of pregnant mares with a protective VapA candidate vaccine. Vet Microbiol 104:73-81, 2004 Madigan JE, Hietala S, Muller N: Protection against naturally acquired Rhodococcus equi pneumonia in foals by administration of hyperimmune plasma. J Reprod Fert 44:571-578, 1991 Martens RJ, Martens JG, Fiske RA: Rhodococcus equi foal pneumonia: Protective effects of immune plasma in experimentally infected foals. Equine Vet J 21:249-255, 1989 Giguere S, Hernandez J, Gaskin J, et al: Performance of five serological assays for diagnosis of Rhodococcus equi pneumonia in foals. Clin Diagn Lab Immunol 10:241-245, 2003
38. Chaffin MK, Cohen ND, Martins RJ: Evaluation of equine breeding farm characteristics and preventative health practices as risk factors for development of Rhodococcus equi pneumonia in foals. J Am Vet Med Assoc 222:476-485, 2003 39. Cohen ND, O’Conor MS, Chaffin MK, et al: Farm characteristics and management practices associated with development of Rhodococcus equi pneumonia in foals. J Am Vet Med Assoc 226:404-413, 2005 40. Prescott JF: Epidemiology of Rhodococcus equi infection in horses. Vet Microbiol 14:211-214, 1987 41. Horowitz ML, Cohen ND, Takai S, et al: Application of Sartwell’s model (lognormal distribution of incubation periods) to age at onset and age at death of foals with Rhodococcus equi pneumonia as evidence of perinatal infection. J Vet Intern Med 15:171-175, 2001 42. Giguere S, Hernandez J, Gaskin J, et al: Evaluation of white blood cell concentration, plasma fibrinogen concentration, and an agar gel immunodiffusion test for early identification of foals with Rhodococcus equi pneumonia. J Am Vet Med Assoc 222:775-781, 2003 43. Sellon DC, Besser TE, Vivrette SL, et al: Comparison of nucleic acid amplification, serology, and microbiologic culture for diagnosis of Rhodococcus equi pneumonia in foals. J Clin Microbiol 39:1289-1293, 2001 44. Sweeney CR, Sweeney RW, Divers TJ: Rhodococcus equi pneumonia in 48 foals: response to antimicrobial therapy. Vet Microbiol 14:329-336, 1987 45. Stratton-Phelps M, Wilson WD, Gardner IA: Risk of adverse effects in pneumonic foals treated with erythromycin versus other antibiotics: 143 cases (1986-1996). J Am Vet Med Assoc 217:68-73, 2000 46. Giguère S, Jacks S, Roberts GD, et al: Retrospective comparison of azithromycin, clarithromycin and erythromycin for the treatment of foals with Rhodococcus equi pneumonia. J Vet Intern Med 18:568-573, 2004
Bacteriology and Pathogenesis
R
hodococcus equi is a Gram-positive facultative intracellular bacterial pathogen with the ability to replicate in infected equine macrophages.1 This organism was previously known as Corynebacterium equi, only being definitively included in the genus Rhodococcus in 1987 based on gene analysis.2 It most commonly causes pyogranulomatous bronchopneumonia in affected foals, often with extensive pulmonary abscesses (Fig. 1). Rhodococcus equi is present in the manure of many herbivores, and is found in higher concentrations in soil of equine environments.3 Virtually all foals are exposed to R. equi early in life through inhalation or ingestion, but the majority of foals develop protective lifelong immunity following exposure. Similar in many ways to Mycobacterium tuberculosis, R. equi is a nocardioform actinomycete with a cell wall containing mycolic acid. Also like M. tuberculosis, R. equi can infect and survive within alveolar macrophages, where the organisms can replicate within the phagosome and prevent phagolysosome fusion.4 Reports also indicate that virulent R. equi can destroy host cells, although it may not be able to enter pulmonary epithelial cells. The respiratory form of R. equi infection in foals, histori-
*Veterinary Medical Teaching Hospital, and the †Department of Medicine and Epidemiology, School of Veterinary Medicine, University of Califorina, Davis, CA. Address reprint requests to P. Heidmann, Montana Equine, 40 Buckskin Rd., Belgrade, MT 59714. E-mail: [email protected]
1534-7516/06/$-see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1053/j.ctep.2006.03.015
cally called “rattles,” is believed to infect foals within the first several days of life, but typically results in protective immunity. In affected individuals, clinical signs often remain unidentified for weeks or months following infection. Rhodococcus equi is considered to be the most common cause of severe pneumonia in foals between 3 weeks and 5 months of age.5,6 Less commonly, the bacteria can infect other organ systems, causing septic uveitis, ulcerative enterocolitis (Fig. 2), colonic and/or mesenteric lymphadenopathy, osteomyelitis, and septic physitis or arthritis (Fig. 3). Poorly characterized, immune-mediated conditions such as sterile uveitis (Fig. 4) are occasionally associated with R. equi infection,6 whereas immune-mediated polysynovitis with immunoglobulin deposition in synovium, more often affecting stifle or tibiotarsal joints (Fig. 5), has also been associated with R. equi infection.7 Although the organism is distributed worldwide, it has its greatest impact on large equine breeding farms due to increased prevalence and pathogenicity, causing increased morbidity and mortality.8 Case fatality rates of 12.5% to 42% have been reported in recent studies, although rates as high as 80% occurred before the advent of combined antimicrobial therapy using rifampin with a macrolide such as erythromycin.8-10 Rhodococcus equi is also a zoonotic agent, most frequently affecting people with compromised immune status, in whom it can cause pyogranulomatous pneumonic lesions resembling tuberculosis. Case fatality rates of approximately 50% have been reported in people with AIDS, although mortality may approach 11% in those rare cases where immunocompetent human patients are infected.11 To date, no specific risk 203
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P. Heidmann, J.E. Madigan, and J.L. Watson
Figure 1 Pathologic findings of a foal diagnosed with R. equi pneumonia showing multiple, well-defined pulmonary abscesses. (Color version of figure is available online.)
factors have been associated with infection in immunocompetent people. Rhodococcus equi is relatively resilient, and can not only be cultured from the soil on and around equine breeding facilities but also from elsewhere in the environment.12,13 It is more readily isolated from dusty soil than from wet ground, suggesting one possible risk factor for infection may be living in dry dusty environments which may enhance exposure to maximal infective doses. Similarly, R. equi has been shown to grow more readily in soils mixed with equine feces than in soil alone, suggesting that housing, paddock, and pasture environments may serve to amplify organism in the environment, increasing risk of exposure to maximal doses of the pathogen.14 Recent research has focused on explaining why a minority of exposed foals ultimately develop clinical infection with Rhodococcus equi. Are low case attack rates due to variations in pathogen virulence? Is there variability in foals innate or acquired immunity? Are some foals exposed to higher infective doses of organism due to combinations of farm managementbased risk factors? In reality, clinical infection occurs due to
Figure 2 Pathologic findings of a foal showing ulcerative, pyogranulomatous enterocolitis caused by R. equi. (Color version of figure is available online.)
Figure 3 Radiograph showing physitis and osteomyelitis associated with the distal physis of the left hind third metatarsal bone of a foal with R. equi pneumonia.
the complex interactions among host, pathogen, and environmental factors. We will discuss each of these factors in turn, all of which appear to play a role in the risk of developing clinical disease. Controlling each of these factors may be important in minimizing the risk of infection (herd-health preventative medicine programs), and in treating affected individuals.
Figure 4 Foal diagnosed with R. equi pneumonia showing bilateral immune-mediated uveitis. The picture shows epiphora, aqueous flare, and yellow-discolored iris of the left eye.
Rhodococcus equi pneumonia
205 cytotoxicity, and that no single-acting substance from culture causes its cytotoxic effect.18 Some workers have reported that no significant association exists between the R. equi disease status of a farm and the isolation of VapA positive or negative organisms in the soil.19 Other studies report that R. equi expressing VapA was present in higher concentrations on breeding farms with a history of R. equi infection than on farms without clinical cases.20 Although the organism itself may be ubiquitous, isolates from locations without horses are less likely to contain the virulence-associated plasmid. When more than 200 soil and sand samples were collected from nonequine related parks and yards in Japan, R. equi was isolated from almost 75% of the samples, but none of the more than 1000 microbiological isolates expressed VapA lipoprotein.21 Further elucidation of the precise mechanism of enhanced virulence associated with the VapA protein depends on ongoing and future research.
Immunologic Factors
Figure 5 Foal with R. equi pneumonia showing distended right tibiotarsal joint due to immune-mediated synovitis. (Color version of figure is available online.)
Virulence Factors Beyond its ability to escape destruction within the phagolysosome in alveolar macrophages, early reports suggested enhanced cellular necrosis was caused by rhodococcal glycolipids. An 85- to 90-kb plasmid is consistently identified in isolates of R. equi from clinically affected foals, and is typically absent from strains cultured from nonequine environments. Virulent strains containing this plasmid express a family of seven virulence associated proteins (Vap), including the highly immunogenic protein VapA. The specific molecular function of these proteins remains undetermined. However, the VapA protein is critical for intracellular growth; experimental deletion of the VapA gene significantly diminishes virulence in mice macrophages.15 DNA sequencing of independent isolates from clinical cases revealed identical plasmid sequences, suggesting that the Vap proteins are pervasive in virulent strains.16 These in vitro studies of virulence appear to be consistent with evidence from experimental infection. When foals were experimentally inoculated intratracheally with R. equi, the minimum infective dose for the plasmid-containing strain was 104 bacteria, compared with ⬎109 bacteria for the cured (plasmid-free) partner strain.17 However, cytotoxicity is not seen in the absence of viable bacteria, suggesting that viable R. equi organism is required to produce
Normal adult horses and the majority of normal foals develop active immunity and clear the infection, even when exposed to highly virulent R. equi. However, a subset of foals does not develop protective immunity. Diminished host immune response may be due to a variety of factors. Therefore, most recent research has focused on elucidating the mechanism for effective immunity in the majority of exposed foals which do not develop clinical R. equi infection. The prevalence of R. equi infection in HIV-positive humans with low T-cell numbers provided early, if indirect, evidence of the protection imparted by T-lymphocytes.22 More recently, it has been shown that the style of immune response is critical to effective protection. Adult horses challenged with virulent strains of R. equi clear the organism from their lungs during increases in the number of pulmonary CD4⫹ and CD8⫹ lymphocytes.23 This clearance is associated with an increased number of CD4⫹ T-cells producing interferongamma (IFN-␥), and is characteristic of an appropriate Th-1 or cell-mediated immune response to intracellular infection. Evidence in mice suggests that the balance of Th-1 (cell mediated) to Th-2 (humoral) responses may be critical in successful clearance of R. equi infection, and the balance of Th-2 to Th-1 may also be relatively increased in clinically affected foals. In immunocompetent adult horses, challenge with R. equi results in proliferation of pulmonary T-cells and increased concentrations of IFN-␥. These horses did not show an increase in release of interleukin-4 (IL-4), which is typically associated with a Th-2 response and a less effective immune response to intracellular bacterial infection.24 Interferon-gamma appears to play a critical role in limiting R. equi infection. Production of IFN-␥ stimulates phagolysosome fusion and increases production of reactive oxygen species. Knockout mice defective in production of IFN-␥ or of inducible nitric oxide synthetase are particularly susceptible to R. equi infection.25 Mice lacking IFN-␥ have been shown to have decreased ability to clear R. equi infection, whereas stimulation of a CD4⫹ Th-1 response results in protective immunity and clearance of R. equi infection.26,27 Thus, the balance of Th-1 to Th-2 may be critical in pathogenesis of infection in foals, with increased Th-2 response predisposing to clinical infection.
206 The role of specific antibody subtypes has also been investigated. Following challenge of immunocompetent adult horses, the levels of R. equi and VapA-specific IgGa and IgGb were enhanced, suggesting that these antibody subtypes are important in functional immunity.24 Further investigation suggested that IgGa specific for Vap protein appears to be functionally protective, and was associated with a Th-1 response. IgGb and IgGT have been associated with a Th-2 style response, and may actually enhance susceptibility.28 IgGb synthesis does not begin until at least 2 months of age in normal foals, whereas IgGa may begin in the last trimester in utero and continue actively during the first 2 months of life in normal foals. Based on this information, it appears that promotion of active immunity to R. equi requires a robust Th-1 response characterized by the production of IFN-␥ by CD4⫹ and CD8⫹ T-lymphocytes. In turn, appropriate antigens (immunogens) must be identified to stimulate sufficient release of IFN-␥ from T-cells. Previous vaccination attempts have not produced reliable protection. Direct oral immunization of foals with a modified bacterin offered partial protection, but did not result in induction of a lasting anamnestic response.29 More recent work has emphasized subunit vaccines to minimize the risk of modified vaccine strains acquiring the virulence plasmid. However, these attempts are complicated by incomplete understanding of the interactions between Vap proteins. Effective vaccines rely on choosing the right antigen because not every immunogenic antigen produces protective immunity. Recent work has identified a group of immunogenic proteins in Vap-containing strains from infected foals. These proteins may be encoded or regulated by the R. equi virulence plasmid. T-lymphocytes from the lungs of adult horses following immune clearance of virulent R. equi proliferated in response to a combination of these immunogens and produced IFN-␥.30 It is not known which specific antigens promoted this response, nor whether a protective response (with release of IFN-␥) in naïve individuals can be generated by exposure to one or more of these proteins. To date, studies using various VapA vaccines have failed to produce immunity in mice.31 Successful vaccination of foals may be hampered by maternal interference. Although no work has been published specifically documenting maternal interference with R. equi, passive transfer of maternally antibodies can inhibit the ability of foals to develop specific antibody titers to other pathogens (eg, tetanus toxin, rabies) during the first 6 months of life. Equally important is the ability of R. equi to evade host immune mechanisms, allowing replication within macrophages. Previous attempts to promote immunity via passive transfer of maternal antibodies from vaccinated mares to their foals have produced inconsistent results, raising the dam’s titers and protecting foals in some studies but not establishing uniform immune protection in other studies.32,33 Meanwhile, colostral transfer of maternal antibodies produced in response to a VapA vaccine has shown some early promise.34 In comparison to prepartum vaccination programs, passive transfer by transfusion of hyperimmune plasma to foals has been shown to decrease morbidity and mortality. When given before exposure, the administration of equine hyperimmune plasma can prevent, or significantly reduce the se-
P. Heidmann, J.E. Madigan, and J.L. Watson verity of, experimental or naturally acquired pneumonia.35,36 However, some work has documented mixed results following plasma transfusion, possibly due to differences in sources and composition of the hyperimmune plasma used, as well as consumption of transfused antibodies and other factors.37 IgGb, associated with less significant protection against infection, is the predominant isotype in hyperimmune plasma, suggesting that the value of plasma administration may be due to neutralization of other virulence factors. For Rhodococcus-endemic farms, it is typically recommended that hyperimmune plasma is administered within 24 hours of birth, and again between 3 to 4 weeks of age due to waning protection from the initial transfusion. Ultimately, complete protection of at-risk foals may require a combination of active immunization with immunogens that stimulate a Th-1-specific response given immediately postpartum to avoid the effect of maternal blocking, followed by plasma transfusion to boost immunity in neonatal period. Although plasma can help provide immunoglobulins and other factors, the foal must still mount an appropriate immune response.
Environmental Risk Factors Multiple studies have been published attempting to identify risk factors for subsequent development of R. equi pneumonia in foals. Effectively minimizing risk factors could limit exposure to infective doses insufficient to overwhelm the immune system of an otherwise healthy foal. Knowing that R. equi proliferates in the gastrointestinal tract of horses suggests that many risk factors may be associated with bioaccumulation on large breeding farms, including both management practices and the number of years a premise has held horses. Studies of farm-related factors have noted a higher risk of infection on farms with greater acreage, farms with large populations of mares and foals (defined as 160 horses or ⬎15 foals), farms with high density housing of foals, and in particular, farms with a transient population of mares.38,39 Previous work examining a farm’s age is divided. In one study, farms housing horses for over 30 years were shown to have higher concentrations of R. equi organism in the environment.40 Other work has not identified an increased risk of infection associated with longer established farms.38 However, the evidence is not always intuitive, for example, factors related to poor management, including a lack of preventative care practices, were not associated with an increased risk of infection. In fact, better run farms with more consistent preventative care may be more frequently affected.38 A combination of foal-related risk factors has been implicated in causing higher rates of disease in one study of two large breeding farms. Foals were found to be at higher risk of infection when their dam was multiparous (with parity greater than four) or had been at the farm for a minimum of 1 year, suggesting the possibility of bioaccumulation and increased shedding of organisms from these mares. Foals housed in a box stall up to 4 weeks of age were associated with decreased risk of infection, whereas early turnout (turnout at less than 2 weeks of age) was associated with increased incidence rates. Affected foals were less likely to have been housed in paddock turnout groups of 2 to 5 mare–foal pairs, and more likely to have been housed in groups of 6 to 10
Rhodococcus equi pneumonia pairs. In other words, this work suggested that maintaining relatively small groups of mare and foals appears to provide a measure of protection from clinical R. equi infection.38 In addition to minimizing overstocking and direct exposure to organisms shed into the environment (especially in feces), this strategy might also minimize environmental accumulation over time. Previous work has also indicated that maintaining foals in a stalled environment during early weeks may have a protective benefit, possibly because exposure to R. equi occurs very early in life, followed by active multiplication in the intestinal tract during the first 8 weeks of life.41 Increased rates of disease were also associated with less frequent use of supplemental immunoglobulins; when some form of concentrated immunoglobulins was administered, foals were found to have decreased rates of disease.38 However, prospective analysis of foal-related risk factors showed that many of the widely adopted strategies related to minimizing neonatal septicemia and other infections are not related to protection from clinical R. equi infection. For example, the specific gravity of mare’s colostrum, duration of time to stand and suckle, and foals’ IgG concentration were not related to risk of R. equi infection. Administration of antibiotics during the first day of life was not associated with risk or benefit.38 Of course, the goal of determining specific risk factors is to identify practical means to minimize the risk. In some farm situations, it may be practical to lengthen the duration of mare and foal living in a stall but not to keep mares and foals in groups of less than five pairs. It may be practical to provide supplemental immunoglobulins to all foals, but not to cull mares with parity greater than four. Moving a farm operation to a new premises simply to avoid endemic infection is a drastic step, and rarely practical. Therefore, veterinarians, farm managers, and owners must decide together which strategies to implement to minimize bioaccumulation and exposure to organism, and which are impractical, or unlikely to significantly impact herd health.
Diagnosis The techniques utilized to definitively diagnose respiratory R. equi infection are very similar to those for the other infectious causes of pneumonia discussed in this issue. Clinical signs of respiratory tract infection may be mild, but the disease progresses steadily, typically resulting in mild fever (or hyperthermia) and mild tachypnea. Since early clinical signs can be mild, some cases may be initially identified by physical examination of foals with poor growth, with additional diagnostics being prompted by detection of significant abnormalities. In more severe cases, adventitial lung sounds, including crackles and wheezes, may develop. Pleural friction rubs may be heard in advanced cases. Some foals succumb to severe infection without having been previously identified as sick presenting acutely febrile and in respiratory distress. Rarely, foals with R. equi may be discovered as sudden death without any history of illness. One practical screening technique has been recommended to farm managers (Wilson, WD, personal communication): (1) farm manager should evaluate all foals respiratory rate and effort in the morning; (2) in foals with increased respiratory rate (⬎ 24 breaths/min), the manager should measure rectal temperature; and (3) in foals with
207
Figure 6 Smear from trans-tracheal wash fluid collected from a foal with R. equi pneumonia showing small, Gram-positive pleomorphic bacteria (Gram stain, 100⫻ objective). (Color version of figure is available online.)
increased rectal temperature (⬎102.5°F [39.2°C]), the manager should request complete blood count. In foals with increased total white cell count (⬎13,000/uL) or increased fibrinogen (⬎600 mg/dL), advanced diagnostics should be requested, including transtracheal wash, thoracic ultrasound, and/or thoracic radiography. Routine hematologic testing (CBC and plasma fibrinogen) provides valuable, if nonspecific, indicators of inflammation and/or infection. Especially by limiting examination to foals from farms with endemic infection, a CBC and fibrinogen may provide very effective screening tools. Elevations in total white cell count above 13,000/L in the presence of fever should prompt veterinary evaluation, whereas a total white count above 14,000/L is ground for further diagnostic evaluation, even in the absence of external clinical signs. Fibrinogen greater than 600 mg/dL should also prompt further examination or advanced diagnostics. Historically, serology has been used to assess the likelihood of clinical infection with R. equi. Acute and convalescent samples have been advocated for evidence of clinical infection, but interpretation is confounded by the widespread use of hyperimmune plasma and, more importantly, the fact that nearly all foals in endemic areas will develop rising titers to R. equi during the first 2 to 3 months of life. In fact, recent work indicated that serologic analysis by ELISA or AGID is not useful to definitively diagnose R. equi infection.42 Over time, however, serology may be useful as a farm-wide screening tool to determine the progression of foal-hood seroconversion to R. equi. Although there are multiple organisms which can cause suppurative bronchopneumonia in young foals, few are associated with abscess formation, which is a hallmark of advanced R. equi pneumonia. However, early infection with R. equi frequently lacks abscessation, requiring additional tests to obtain an etiologic diagnosis. The gold standard for diagnosis is the identification of R. equi from TTW samples via bacteriologic culture and/or molecular diagnostic techniques such as polymerase chain reaction (PCR). Identification of small, Gram-positive pleomorphic bacteria on Gram staining can facilitate early diagnosis, but is still not definitive (Fig. 6). PCR has been shown to have superior sensitivity and speci-
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Figure 7 Thoracic radiograph from a foal with R. equi pneumonia showing an alveolar pattern with ill-defined regional consolidation compatible with early abscess formation.
ficity for diagnosing R. equi when compared with standard bacteriologic culture43 but is not universally available. With experience, TTW collection can readily be performed under field conditions under light sedation, even in dyspneic foals. Radiography is considered the principal imaging technique for evaluation of the thorax. Thoracic radiographs of foals with R. equi vary depending on the progression of clinical disease. Digital and computed systems are becoming more prevalent, and may increase the diagnostic sensitivity of thoracic radiography. Typically, earlier lesions include a structured interstitial pattern and/or consolidation, whereas advanced infection results in pulmonary abscessation revealed as variably sized nodules on thoracic radiographs (Fig. 7). Because other organisms, such as Streptococcus spp. and anaerobes, may uncommonly cause pulmonary abscesses, a definitive diagnosis cannot be obtained without bacteriologic culture. Radiographic evaluation may be limited by patient size or due to poor accessibility to hospital equipment. Thoracic ultrasonography has been used with increasing frequency, especially in on-farm situations. Ultrasound has been shown to correlate highly with thoracic radiographs, although it may not be able to evaluate lesions deep within the pulmonary parenchyma. Ultrasonographic examination of the chest should include all lung fields bilaterally, beginning caudally at the 17th intercostal space and continuing cranially to the 3rd intercostal space. Normal lung surfaces are highly echogenic due to the presence of air in well-aerated fields. Mild pulmonary changes are characterized by roughening of the pleural surface, resulting in comet-tail artifacts
(Fig. 8). Consolidated lung fields may be difficult to distinguish from pulmonary abscessation, but are typically hypoechoic to muscle; areas of consolidation are frequently surrounded by hyperechoic margins at the interface of
Figure 8 Thoracic ultrasonography of the left cranioventral lung field from a foal with R. equi pneumonia showing multiple comet-tail artifacts due to pleural roughening.
Rhodococcus equi pneumonia consolidated and aerated lung. Abscesses may or may not be surrounded by an echogenic capsule, whereas the center may be variable echogenicity, and may be septated, depending on the type of fluid contained within the capsule. If gas is present within the abscess, irregular hyperechoic foci may also be identified. In addition to portability, ultrasound may provide more accurate localization of abnormalities, allowing more accurate monitoring of lesions during treatment.
Treatment Combined antimicrobial therapy is accepted as the treatment modality of choice for cases of R. equi pneumonia. Erythromycin and rifampin have been shown to be superior to penicillin and gentamicin, even when treating R. equi pneumonia with in vitro susceptibility to gentamicin,44 in part due to the ability of the organism to escape host defenses and replicate within macrophages. Both erythromycin and rifampin are concentrated in granulocytes and macrophages by an active mechanism. However, erythromycin has variable absorption in foals when given orally, and is associated with adverse effects, including colitis, and idiosyncratic hyperthermia and tachypnea in hot weather.45 A recent report indicated better outcomes with clarithromycin/rifampin than other combination therapies, based on quicker discharge from the hospital, and better long-term recovery (defined as being clinically healthy at 1 year of age46). Clarithromycin and azithromycin have better stability, bioavailability, and concentration in phagocytic cells and tissues in people when compared with erythromycin. The concentration of azithromycin in pulmonary epithelial cells and bronchoalveolar cells of foals following oral dosages has been documented, but no work has yet been published documenting the pharmacokinetics of clarithromycin in equids. However, previous work has shown a better in vitro activity of clarithromycin against R. equi,46 and it is superior to azithromycin and erythromycin in humans with pulmonary Mycobacterium avium infection, a facultative intracellular organism closely related to R. equi. This research indicates that the combination of clarithromycin (7.5 mg/kg PO twice per day) with rifampin (5-10 mg/kg PO twice per day) was superior to azithromycin (10 mg/kg PO q24h) with rifampin. The combination of clarithromycin/rifampin was also superior to erythromycin (37.5 mg/kg PO twice per day to 25 mg/kg PO q6h) with rifampin. Antibiotic-associated colitis/diarrhea was found in all treatment groups, and immediate discontinuation of all antimicrobials is warranted if diarrhea develops during treatment with any of the above-mentioned drug combinations. In our hospital, foals are most often treated with azithromycin (10 mg/kg PO q24h for 5 days, then 10 mg/kg PO every other day for the duration of the treatment interval) and rifampin (5 mg/kg PO twice per day).
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