Treatment of Corynebacterium equi pneumonia of foals: a review

Combined antimicrobial drug treatment was recommended for foals with Corynebacterium equi pneumonia. The preferred combination is orally administered erythromycin estolate (25 mg/kg of body weight, QID) plus rifampin (10 mg/kg, BID). Erythromycin estolate also can be combined for synergistic effect with sodium benzyl penicillin given IV (100,000 IU/kg, QID) or with ampicillin given IV (11 to 15 mg/kg, QID). A third choice is sodium benzyl penicillin IV with gentamicin IM (2.2 mg/kg, TID) or with kanamycin IM (10 mg/kg, QID). Gentamicin should be combined with penicillin G or ampicillin and not used for longer than one week without monitoring for nephrotoxicosis. Rifampin should be used only in combination with erythromycin or penicillin. Erythromycin or rifampin and gentamicin give antagonistic interactions in vitro. Chloramphenicol or trimethoprim-sulfamethoxazole may be effective if given in high doses but are not preferred drugs. Treatment response should be monitored clinically and radiographically and treatment should be continued for 2 weeks after the foal is clinically and radiographically normal.     

Risk of adverse effects in pneumonic foals treated with erythromycin versus other antibiotics: 143 cases (1986-1996)

OBJECTIVE: To determine whether foals with pneumonia that were treated with erythromycin, alone or in combination with rifampin or gentamicin, had a higher risk of developing adverse effects, compared with foals treated with trimethoprim-sulfamethoxazole (TMS), penicillin G procaine (PGP), or a combination of TMS and PGP (control foals). DESIGN: Retrospective study. ANIMALS: 143 foals < 240 days old. PROCEDURE: Information on age, sex, breed, primary drug treatment, total days of treatment with the primary drug, and whether the foal developed diarrhea, hyperthermia, or respiratory distress was obtained from the medical records. Relative risk (RR) and attributable risk (AR) were calculated to compare risk of adverse reactions between foals treated with erythromycin and control foals. RESULTS: Only 3 (4.3%) control foals developed diarrhea; none developed hyperthermia or respiratory distress. Foals treated with erythromycin had an 8-fold risk (RR, 8.3) of developing diarrhea, compared with control foals, and increased risks of hyperthermia (AR, 25%) and respiratory distress (AR, 15%). CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that use of erythromycin to treat foals with pneumonia was associated with an increased risk of diarrhea, hyperthermia, and respiratory distress, compared with use of TMS or PGP.     

The effects of combinations of selected antibiotics on the growth of Corynebacterium equi

The minimal inhibitory concentrations of penicillin G, ampicillin, gentamicin, erythromycin and rifampicin were determined for nine strains of Corynebacterium equi. The effect of combinations of any two of these antibiotics on the killing of these strains was determined at antibiotic concentrations achievable in horses using recommended drug dosages (ampicillin 4.0 μg/ml, gentamicin 1.0 μg/ml and erythromycin 0.25 μg/ml). Penicillin G was used at 4.0 μg/ml and rifampicin at 0.063 μg/ml. The combinations of gentamicin with erythromycin or rifampicin gave antagonistic effects on killing compared to either drug alone. Combinations of erythromycin with rifampicin or penicillin showed synergistic effects, as did penicillin-gentamicin. All other combinations, and a triple combination of penicillin-rifampicin-erythromycin, showed additive effects only.     

Diagnosis and therapy of Rhodococcus equi infection in the horse

Infection with Rhodococcus equi is an important cause of pneumonia in foals, but other organ systems may also be affected. The intracellular presence of R. equi and the formation of granulomatous and suppurative inflammatory tissue mean that prolonged treatment is needed. The pharmacological properties of the combination of erythromycin and rifampicin have improved the survival of foals infected with R. equi; however, erythromycin can cause adverse reactions in foals and mares, which has prompted the search for alternative therapies. The combination of azithromycin or clarithromycin with rifampicin seems to be a promising alternative. However these combinations are expensive and adverse effects remain to be determined, especially in the dams of treated foals. Thus correct diagnosis and appropriate use of drugs are essential for the treatment of R. equi infection in foals.     

Resistance studies of erythromycin and rifampin for Rhodococcus equi over a 10-year period

This study sought to determine whether an increase in resistance of Rhodococcus equi to the antibiotics rifampin and erythromycin occurred over a 10-year period. This was carried out by the use of E test strips for rifampin and erythromycin to determine the MIC (minimum inhibitory concentration) values of Rhodococcus equi to this combination of antibiotics.The findings of this study indicated that there was an increase in resistance of Rhodococcus equi to rifampin and erythromycin over the 10-year period. The MIC for rifampin increased from 0.081 μg/ml in 1996 to 0.187 μg/ml in 2006 and from 0.258 μg/ml for erythromycin during the years prior to 2000 to 0.583 μg/ml in 2006.This finding suggests that there may be a problem in the treatment of Rhodococcus equi infections in foals in the future, particularly as the number of drugs available for treatment of Rhodococcus equi infection is limited because of the intracellular capabilities of this bacterium. Antibiotics used in its treatment have to be able to penetrate the polysaccharide cell wall of Rhodococcus equi as well as the alveolar macrophages in which the bacterium is capable of surviving.     

Characterisation of Rhodococcus equi strains isolated from foals and from immunocompromised human patients

The cultural, morphological, biochemical, serological characteristics and antibiotic susceptibility of 25 Rhodococcus equi strains isolated from lungs and lung abscesses of pneumonic foals and 5 R. equi strains isolated from immuno-compromised human patients were examined. All R. equi strains showed common cultural, morphological and biochemical characteristics both with conventional tests and on the basis of their enzyme profile. The R. equi strains examined were resistant to penicillins with the exception of ampicillin, to sulphamethazine and several strains also to sulphamethoxazole-trimethoprim. All strains were susceptible to erythromycin and rifampicin. The strains isolated from humans showed somewhat higher rate of antibiotic resistance to penicillin, cefotaxime, kanamycin, streptomycin, lincomycin, and oxytetracycline. The overwhelming majority (96%) of the equine isolates belonged to serotype 1 in Prescott's serotyping system, while the human isolates could not be serotyped.     

Use of erythromycin-rifampin combination in treatment of Rhodococcus equi pneumonia

The selection of lipid-soluble antibiotics capable of intracellular penetration is considered critical for the successful treatment of Rhodococcus equi pneumonia and lung abscesses in foals. Two such antibiotics: erythromycin (25 mg kg-1, three times daily) and rifampin (5 mg kg-1 twice daily) have been used in combination for this purpose at the University of Florida since 1981. Positive evidence of R. equi was present on culture of tracheal aspirates in 57 foals, most of which exhibited radiographic evidence of extensive lung abscessation. The duration of therapy ranged from 4 to 9 weeks. Mild diarrhea was sometimes noted, but was never severe enough to require the termination of therapy. No other adverse side effects were apparent. Judged by a return of chest radiographs and hematologic parameters to normal, 50 of the 57 foals were considered to have recovered from the disease; a success rate of 88%.     

Equine proliferative enteropathy: a cause of weight loss, colic, diarrhoea and hypoproteinaemia in foals on three breeding farms in Canada

Proliferative enteropathy (PE) is a transmissible enteric disease caused by Lawsonia intracellularis. An outbreak of equine PE was diagnosed in foals from 3 breeding farms. Most foals had been weaned prior to the appearance of clinical signs, which included depression, rapid and marked weight loss, subcutaneous oedema, diarrhoea and colic. Poor body condition with a rough haircoat and a potbellied appearance were common findings in affected foals. Respiratory tract infection, dermatitis and intestinal parasitism were also found in some foals. Haematological and plasma biochemical abnormalities included hypoproteinaemia, transient leucocytosis, anaemia and increased serum creatinine kinase concentration. Postmortem diagnosis of PE was confirmed on 4 foals based on the presence of characteristic intracellular bacteria within the apical cytoplasm of proliferating crypt epithelial cells of the intestinal mucosa, using silver stains, and by results of PCR analysis and immunohistochemistry. Antemortem diagnosis of equine PE was based on the clinical signs, hypoproteinaemia and the exclusion of common enteric infections. Faecal PCR analysis was positive for the presence of L. intracellularis in 6 of 18 foals tested while the serum of all 7 foals with PE serologically evaluated had antibodies against L. intracellularis. Most foals were treated with erythromycin estolate alone or combined with rifampin for a minimum of 21 days. Additional symptomatic treatments were administered when indicated. All but one foal treated with erythromycin survived the infection. This study indicates that equine PE should be included in the differential diagnosis of outbreaks of rapid weight loss, diarrhoea, colic and hypoproteinaemia in weanling foals.     

Antimicrobial susceptibility of Prototheca zopfii isolated from bovine intramammary infections

In vitro antimicrobial susceptibility tests were carried out on 48 strains of Prototheca zopfii, an achlorophyllous algae causing refractory mastitis in dairy cows; 27 antimicrobials were evaluated. All strains were susceptible to both myxin and nystatin. In addition, 22 strains were susceptible to amphotericin B, 21 to polymyxin B, and 18 to gentamicin. Only 1 strain was susceptible to kanamycin. All strains were resistant to ampicillin, bacitracin, carbenicillin, cephalothin, chloramphenicol, clotrimazole, cloxacillin, erythromycin, flucytosine, ketoconazole, lincomycin, miconazole, neomycin, nitrofurazone, novobiocin, oleandomycin, penicillin, rifampin, streptomycin, tetracycline, and vancomycin.     

Pharmacokinetics of erythromycin estolate and erythromycin phosphate after intragastric administration to healthy foals

OBJECTIVE: To determine pharmacokinetics and plasma concentrations of erythromycin and related compounds after intragastric administration of erythromycin phosphate and erythromycin estolate to healthy foals. ANIMALS: 11 healthy 2- to 6-month-old foals. PROCEDURE: Food was withheld from foals overnight before intragastric administration of erythromycin estolate (25 mg/kg of body weight; n = 8) and erythromycin phosphate (25 mg/kg; 7). Four foals received both drugs with 2 weeks between treatments. Plasma erythromycin concentrations were determined at various times after drug administration by use of high-performance liquid chromatography. Maximum plasma peak concentrations, time to maximum concentrations, area under plasma concentration versus time curves, half-life of elimination, and mean residence times were determined from concentration versus time curves. RESULTS: Maximum peak concentration of erythromycin A after administration of erythromycin phosphate was significantly greater than after administration of erythromycin estolate (2.9 +/- 1.1 microg/ml vs 1.0 +/- 0.82 microg/ml). Time to maximum concentration was shorter after administration of erythromycin phosphate than after erythromycin estolate (0.71 +/- 0.29 hours vs 1.7 +/- 1.2 hours). Concentrations of anhydroerythromycin A were significantly less 1 and 3 hours after administration of erythromycin estolate than after administration of erythromycin phosphate. CONCLUSIONS AND CLINICAL RELEVANCE: Plasma concentrations of erythromycin A remained > 0.25 microg/ml (reported minimum inhibitory concentration for Rhodococcus equi) for at least 4 hours after intragastric administration of erythromycin phosphate or erythromycin estolate, suggesting that the recommended dosage for either formulation (25 mg/kg, q 6 h) should be adequate for treatment of R equi infections in foals.     

Enzyme-linked immunosorbent assay for diagnosis of Corynebacterium (Rhodococcus) equi infection in foals

An enzyme-linked immunosorbent assay (ELISA) was used to diagnose Corynebacterium (Rhodococcus) equi infection in foals. In tests done with different antigen-extraction procedures (sodium dodecyl sulfate, sodium deoxycholate, polyoxy-ethylene [9] p-tert-octylphenol, polyoxy-ethylene [9-10] p-tert-octylphenol, sonification, homogenization, and heat treatment at 121 C), Tween 20 was a satisfactory reactive antigen. Using hyperimmune rabbit sera or infected foal sera, we investigated the specificity and the sensitivity of the ELISA with the Tween 20 antigen of the different serotypes or of the isolates. Corynebacterium equi strain ATCC 6939 antigen had the best activity for detecting antibodies to C equi in foals. Sera from 218 healthy horses, 11 healthy foals, 17 healthy newborn foals, a foal with suspected C equi infection, and 5 infected foals were evaluated for antibodies to C equi, using ELISA. The optical density values of 206 healthy horses, 17 healthy newborn foals, and 9 healthy foals were less than 0.1. Infected foal sera, except from foal 3, and serum from a foal with suspected C equi infection had higher optical density values. Using ELISA, specific antibodies against C equi were detected in a naturally infected 6-week-old foal after the foal had a rapid increase in the number of bacteria in the feces and after the initial development of clinical signs of illness at 5 weeks of age. Therefore, ELISA was useful for the early diagnosis of C equi infection in foals.     

Chemoprophylactic effects of azithromycin against Rhodococcus equi-induced pneumonia among foals at equine breeding farms with endemic infections

OBJECTIVE: To determine the effect of azithromycin chemoprophylaxis on the cumulative incidence of pneumonia caused by Rhodococcus equi, age at onset of pneumonia, and minimum inhibitory concentration (MIC) of azithromycin for R equi isolates cultured from fecal and clinical samples. DESIGN: Controlled, randomized clinical trial. ANIMALS: 338 foals born and raised at 10 equine breeding farms; each farm had a history of endemic R equi infections. PROCEDURES: Group 1 foals were control foals, and group 2 foals were treated with azithromycin (10 mg/kg [4.5 mg/lb], PO, q 48 h) during the first 2 weeks after birth. Foals were monitored for development of pneumonia attributable to R equi infection and for adverse effects of azithromycin. Isolates of R equi were tested for susceptibility to azithromycin. RESULTS: The proportion of R equi-affected foals was significantly higher for control foals (20.8%) than for azithromycin-treated foals (5.3%). Adverse effects of azithromycin treatment were not detected, and there were no significant differences between groups for the MICs of azithromycin for R equi isolates cultured from fecal or clinical samples. CONCLUSIONS AND CLINICAL RELEVANCE: Azithromycin chemoprophylaxis effectively reduced the cumulative incidence of pneumonia attributable to R equi among foals at breeding farms with endemic R equi infections. There was no evidence of resistance to azithromycin. Nonetheless, caution must be used because it is possible that resistance could develop with widespread use of azithromycin as a preventative treatment. Further investigation is needed before azithromycin chemoprophylaxis can be recommended for control of R equi infections.     

Failure of antimicrobial therapy to accelerate spontaneous healing of subclinical pulmonary abscesses on a farm with endemic infections caused by Rhodococcus equi

Mass antimicrobial treatment of foals with small ultrasonographic pulmonary lesions is common on farms with endemic disease caused by Rhodococcus equi. The objectives of this study were to compare the relative efficacy of three antimicrobial protocols for the treatment of pulmonary abscesses on a farm with endemic infections caused by R. equi and to determine the frequency of spontaneous resolution of subclinical pulmonary lesions. Foals with ultrasonographic evidence of pulmonary abscesses ≥ 1.0 cm in diameter (n=128) were randomly allocated to one of four equal treatment groups. Animals with respiratory distress or severe pulmonary lesions were excluded from the study. Treatment groups consisted of: (1) tulathromycin intramuscularly; (2) azithromycin monotherapy orally; (3) azithromycin in combination with rifampin orally; and (4) glucose orally as a placebo. Physical examination, thoracic ultrasonography and white blood cell (WBC) counts were performed weekly by individuals unaware of treatment group assignment. Foals that worsened were removed from the study and treated with azithromycin and rifampin. Overall, 14/32 (43.8%) foals in the placebo group recovered without the need for therapy. The proportion of foals that responded to the initial therapy, the duration of therapy, the kinetics of ultrasonographic lesion resolution, the proportion of foals that had to be removed from the study and the number of days to removal from the study did not differ significantly between treatment groups. The study showed that many foals with pulmonary abscesses recover without antimicrobial therapy. Moreover, treatment of sub-clinically affected foals with antimicrobial agents did not significantly hasten recovery.     

Retrospective comparison of azithromycin, clarithromycin, and erythromycin for the treatment of foals with Rhodococcus equi pneumonia

The objective of this retrospective study was to compare the efficacy of azithromycin-rifampin, clarithromycin-rifampin, and erythromycin-rifampin for the treatment of pneumonia caused by Rhodococcus equi in foals. Eighty-one foals with naturally acquired pneumonia caused by R. equi were included in the study. Information on age, sex, breed, physical examination findings, laboratory testing, and thoracic radiography was abstracted from each medical record. Foals were divided in 3 groups based on the antimicrobial agent selected for therapy. Short-term (discharge from the hospital) and long-term (apparently healthy as a yearling) success rates, days of hospitalization, days with fever, days with tachypnea, and percentage of radiographic improvement were compared among groups. Foals treated with clarithromycin-rifampin had significantly (P = .02) higher odds of overall short-term (odds ratio [OR] = 12.2) and long-term (OR = 20.6) treatment success and significantly fewer days with fever than foals treated with erythromycin-rifampin. Foals treated with clarithromycin-rifampin had a significantly (P = .03) higher percentage of radiographic improvement and a tendency (P = .06) toward higher odds of overall short-term (OR = 8.1) and long-term (OR = 11.8) treatment success compared to foals treated with azithromycin-rifampin. Among foals with severe radiographic lesions, the success rates of foals treated with clarithromycin-rifampin both short-term (88%) and long-term (83%) were significantly (P = .02) higher than that of foals treated with azithromycin-rifampin (0%). For each treatment group, the only reported adverse effect was diarrhea that was mild and self-limiting in most cases. The combination clarithromycin-rifampin is superior to azithromycin-rifampin or erythromycin-rifampin for the treatment of pneumonia caused by R. equi in foals in a referral population.     

Survival of foals with experimentally induced Rhodococcus equi infection given either hyperimmune plasma containing R. equi antibody or normal equine plasma

The purpose of this study was to determine if colostrum-deprived foals with experimentally induced Rhodococcus equi pneumonia have a decreased severity of the disease and decreased mortality rate when given hyperimmune (HI) R. equi antibody plasma (R. equi titer at least 100 % and virulence-associated protein A [VapA] at least 10000) prophylactically versus when given normal equine plasma (R. equi titer less than 20 % and VapA less than 160). Sixteen colostrum-deprived foals (R. equi titer less than 5 %) each received normal equine plasma in the first 24 hours of life (R. equi titer less than 20 %). At 14 days of age, six foals were given normal equine plasma and 10 foals were given HI plasma. All foals were subsequently infected intrabronchially with a pathogenic strain of R. equi (2.5 x 10 sup 8; organisms) at 21 days of age. Repeated physical examinations, weight measurements, complete blood cell counts, fibrinogen measurements, and thoracic radiographs (ventrodorsal and lateral) were performed to help determine the severity of the disease. Foals given HI plasma had significantly higher R. equi ELISA titers (42.4 %) than those given normal plasma (20.9 %) on the day of experimental infection. Mortality rates and severity of disease were statistically similar (P >.05) for the groups. Although none of the foals was treated with antibiotics, several with severe R. equi pneumonia recovered. Either HI or normal equine plasma administered to foals in the first few weeks of life caused no adverse effects and may be protective against R. equi, although the exact constituent responsible for protection is undetermined and requires further investigation.     

Presumed immune‐mediated hemolytic anemia in two foals with Rhodococcus equi infection

Objective – To describe the clinical presentation, case management, and outcome in 2 foals with Rhodococcus equi infection associated with presumptive severe immune‐mediated hemolytic anemia. Series Summary – Two foals diagnosed with R. equi pneumonia on the basis of tracheal wash cultures, thoracic radiographs, and thoracic ultrasonography were concurrently diagnosed with hemolytic anemia. Both foals required whole blood transfusions, and were treated with the antimicrobial combination of rifampin and a macrolide (eg, clarithromycin, erythromycin, or azithromycin). Dexamethasone was used to prevent further hemolysis in both foals, and to treat acute lung injury/acute respiratory distress syndrome in 1 of the foals. Both foals survived, and required prolonged antimicrobial therapy. New or Unique Information Provided – Although extra‐pulmonary disorders are commonly diagnosed in foals infected with R. equi, hemolytic anemia is rarely described. Dexamethasone is considered the treatment of choice for immune‐mediated hemolytic anemia, but may be contra‐indicated in foals with severe bacterial infections. In these foals, a relatively low dose and short duration of dexamethasone was utilized in an attempt to minimize immune suppression, although early discontinuation in 1 foal precipitated a second hemolytic crisis.     

Quantitative fecal culture for early diagnosis of Corynebacterium (Rhodococcus) equi enteritis in foals.

Quantitative culture of Corynebacterium (Rhodococcus) equi from feces of 17 foals on a farm (A) with an endemic C. equi infection problem and 26 foals on a farm (B) without the disease in the past decade was done with a selective medium at weekly or monthly intervals from April to August of 1984. Corynebacterium equi was observed in the feces of 16 of 17 foals on farm A, and 19 of 26 foals on farm B. The mean viable count of C. equi in one gram of feces was 4.1 +/- 3.7 (log10) on farm A, and 3.9 +/- 3.4 (log10) on farm B. Corynebacterium equi was recovered from feces of foals as young as two weeks old. Almost all foals at an age between two to four weeks shed the bacteria in the feces. During the observation period two foals showed clinical signs: fever, diarrhea, and cough, at four or five weeks old. At the same time the bacterial count per gram of feces increased from 4 to 7 or 8 (log10). They shed large number of bacteria in the feces and continued to show the clinical signs until death at 10 or 11 weeks old. One of the foals was diagnosed as having had C. equi enteritis and pneumonia by the postmortem recognition of lesions with bacteriological confirmation. The quantitative culture of the feces of foals at weekly intervals after birth on farm A was found to be very useful as an aid in early diagnosis of C. equi enteritis in foals.     

Pharmacokinetics of amikacin in critically ill neonatal foals treated for presumed or confirmed sepsis

Fourteen foals less than four days of age were treated with the aminoglycoside, amikacin sulphate, and either penicillin or ampicillin for septicaemia, pneumonia, and/or failure of passive immunoglobulin transfer. Serum amikacin concentrations were determined at three times during an 8 or 12 h dosing interval. A 7.0 mg/kg bodyweight dose of amikacin every 8 h was appropriate. Prematurity did not influence mortality. All seven premature foals survived, whereas four of the seven full term foals died. Uraemia in three foals was caused by urinary bladder rupture; amikacin-induced nephrotoxicity was not recognised by clinical chemistries (elevations in serum creatinine or blood urea nitrogen concentrations) or post-mortem findings.     

Genotypic characterization of VapA positive Rhodococcus equi in foals with pulmonary affection and their soil environment on a warmblood horse breeding farm in Germany

Pulsotypes of VapA positive Rhodococcus equi isolated from foals and soil on a farm in Germany were characterized on the basis of nasal and tracheal samples simultaneously collected in 2003 from 217 foals with sonographic evidence of pneumonia or pulmonary abscesses. Of the 217 double samples, R. equi was isolated in 118 (54%) of the tracheal samples and in 52 of the nasal swab samples (24%) (P<0.001). Furthermore, 37 and 55 isolates were also randomly selected from nasal swabs and the tracheal samples, respectively, and further processed to determine the presence of VapA by colony blot enzyme-linked immunosorbent assay. This method showed that 26 (68%) of the nasal swabs and 40 (73%) of the tracheal samples were VapA-positive. R. equi was isolated from 56 (87%) of the 64 soil samples taken from the paddocks and stables in March and from 17 (68%) of the 25 samples taken in July of 2003. Three (21%) of these randomly selected 14 isolates from March and 13 (81%) of the 16 from July were VapA-positive. The VapA positive isolates from foals and soil were genotyped by plasmid profiling, restriction fragment length polymorphism analysis and pulsed-field gel electrophoresis. Of the 83 isolates, 80 contained an 85-kb type I plasmid and three contained an 87-kb type I plasmid. Pulsed-field gel electrophoresis yielded four distinct VspI profiles dividing 83 isolates into three major (A1, 51; D, 14; and 11 isolates) and three minor (C, 4; A3, 2; and A2, 1 isolates) groups. These results suggest that the majority of foals were exposed to and infected with three pulsotypes of VapA positive R. equi containing an 85-kb type I plasmid.     

Septic pleuritis and abdominal abscess formation caused by Rhodococcus equi in a foal

A 3-month-old female Arabian horse was evaluated because of fever, respiratory distress, lethargy, and decreased appetite of 5 days' duration. Pleural effusion was diagnosed on the basis of ultrasonographic and radiographic examinations. Cytologic examination of pleural fluid collected via thoracocentesis revealed septic inflammation; bacteriologic culture of a sample of that fluid yielded Rhodococcus equi. A large intra-abdominal mass adjacent to the body wall was identified ultrasonographically. A specimen of the mass was collected via aspiration; the specimen was identified cytologically as purulent exudate that contained large numbers of rod-shaped bacteria, which confirmed abdominal abscess formation. Bacteriologic culture of a sample of the exudate also yielded R. equi. The foal was treated with azithromycin (10 mg/kg [4.5 mg/lb], PO, q 24 h for 5 days then q 48 h) and rifampin (5 mg/kg [2.3 mg/lb], PO, q 12 h) for 8 weeks and metronidazole (15 mg/kg [6.8 mg/lb], PO, q 8 h) for 3 weeks. Clinically, the foal responded to antimicrobial treatment within 2 weeks. At 8 weeks after the initial evaluation, ultrasonographic examination of the foal revealed resolution of the pleural effusion and abdominal abscess. In foals, R. equi infection typically results in pyogranulomatous pneumonia, and pleural effusion is an uncommon clinical sign. The combination of azithromycin and rifampin appears to be an effective treatment for R. equi infection in foals.