Rhodococcus equi is an emerging human pathogen. It is mostfrequently associated with pulmonary infections; however,manifestations may be protean. It can be easily mistaken fora diptheroid-like contaminant or a mycobacterium. Therefore,a high suspicion of R equi infection and specialized testing areencouraged. Vancomycin-based therapy is recommended.Because human infection with this organism is uncommon,thorough reporting will help identify further characteristicsof infection and will help in devising treatment guidelines.[Infect Med. 2008;25:391-393
Rhodococcus equi was firstisolated from the lungs offoals in Sweden in 1923.1-3The first human infection was reportedin 1967.3 As the population ofimmunocompromised patients increases,more cases of R equi infectionare being reported. Infection inthe immunocompetent host is rare,with only 19 definitive cases reportedin the literature.4 We present acase of a woman with chronic pancreatitiswho required parenteralnutrition and whose condition ledto catheter-related bacteremia onmore than one occasion. R equi infectionwas the most recent causeof bacteremia in the patient, whowas successfully treated withoutcomplications.
A 41-year-old woman with chronicpancreatitis was admitted to ourhospital for worsening abdominalpain, chills, and leukocytosis. Thepain was located in the left upperand lower abdominal quadrants andwas associated with nausea andvomiting. The patient experiencedchills the night before presenting forcare but reported that she did nottake measures to confirm whetherfever was present. Her bowel habitswere unchanged, and a review ofsystems yielded negative results.
Her past medical history includedchronic pancreatitis of unknowncause. She underwent a laparoscopiccholecystectomy 4 years beforepresentation and underwent distalpancreatectomy, splenectomy, andpseudocyst drainage procedures 6months before presentation. She continuedto have persistent abdominalpain and inflammation and had lostmore than 100 lb in the past 4 years.
Because of the persistent pain andinflammation, the patient was unableto tolerate jejunal tube feedingand therefore had been receivingtotal parenteral nutrition for the past4 months. During that time, catheter relatedbloodstream infections developedtwice. Different organismswere implicated each time. The mostrecent infection, which occurred 2months before the current presentation,was diagnosed as Klebsiellapneumoniae bacteremia.
The patient lived with her husband,2 sons, and dogs. She reportedthat she had no exposure to other animalsor livestock.
The patient did not have fever orhypotension. Her heart rate was 129beats per minute. A left upper extremityperipherally inserted centralcatheter (PICC) line showed no signsof infection. She had upper leftquadrant tenderness on abdominalexamination without peritonealsigns.
The patient's white blood cell(WBC) count was 17,600/?L with aleft shift, indicative of infection.Blood samples for cultures were obtainedfrom peripheral sites as well as from the PICC line. The PICC linewas removed and the tip cultured.Findings on a CT scan of the abdomenand pelvis were unchangedfrom her previous imaging studies.
Given her history of catheterrelatedsepsis and the current clinicalpresentation, catheter-related bacteremiawas suspected. In light of recentsepticemia with K pneumoniae,empiric therapy with ampicillin/sulbactamwas started.
The preliminary results of theblood cultures revealed gram-negativebacilli in the blood drawn fromboth peripheral sites and the PICCline. Ampicillin/sulbactam therapywas continued, and the patient improvedclinically. Leukocytosis improved,with a decline in the WBCcount to 12,600/?L. After 4 days ofintravenous antibiotic therapy withampicillin/sulbactam, therapy wasswitched to a 10-day course of oralamoxicillin/clavulanate, and the patientwas discharged home.
A week after the patient had beendischarged, the blood culture resultswere received and were positive forRhodococcus species. Because the patientcontinued to show clinical improvement,her antibiotic regimenwas not changed. Since completingtherapy, she has not had a recurrenceof Rhodococcus infection.
DiscussionR equi is thought to be acquiredmainly via the respiratory tract;however, acquisition by oral routeand by traumatic inoculation or superinfectionof wounds has been reported.1-3 Exposure to domesticatedanimals, such as horses and pigs,may play a role. Rhodococcus specieswith biochemical properties similarto those of R equi inhabit the nasaltract of healthy adults, and so it alsohas been suggested that nasal colonizationplays a role in the acquisitionof infection.1
Pathogenesis in humans has beendifficult to establish because of thepaucity of human case reports andthe differences in human and animalisolates, which makes it difficult toextrapolate animal study results tohuman infections.
R equi, originally named Corynebacteriumequi, was found to bechemically and structurally relatedto Nocardia and Mycobacterium.1 It isa gram-positive pleomorphic organismthat ranges from rod to coccusforms depending on its growth environment.The organism can be acidfast,like Nocardia and Mycobacterium,but this is not a consistent feature.
Because of its easy growth andinitial morphological resemblance todiphtheroids, it can be easily dismissedas a contaminant, or on thebasis of acid-fast staining results, itcan be mistaken for Mycobacterium.Thus, it is crucial for the physicianand the laboratory personnel to accuratelyrecognize and differentiateit. It should not be dismissed as acontaminant in patients with clinicalpresentations consistent with septicemiaand cultures positive for diphtheroids.Similarly, clinical presentationshould be kept in mind tocorrectly interpret acid-fast stainingresults. R equi infection, in additionto Mycobacterium and Nocardia infection,must be included in the differentialdiagnosis.
Signs and symptoms of R equi infectionare site-specific. More than80% of immunocompromised patientsand 40% of immunocompetentpatients infected with R equi havepulmonary involvement.1 Patientswith R equi infection most commonlypresent with respiratory signs,such as a nodular infiltrate or apneumonic consolidation on imagingstudies. Pleural effusion or empyemamay be present. Pulmonarycavitation is a frequent occurrence.Bacteremia occurs more often in immunocompromisedpatients and isusually associated with concomitantpulmonary infection. However, bacteremiaunrelated to pulmonary infectionhas been reported as well,although these reports are isolatedand few.5-7
Treatment guidelines have notbeen clearly established because ofthe rarity of the infection and lackof standardization of in vitro susceptibilitytesting. Combination chemotherapyis recommended. Preferredantibiotics include vancomycin, aminoglycosides,rifampin, ciprofloxacin,and imipenem.1 There havebeen some reports of excellent susceptibilityto ampicillin/sulbactamand amoxicillin/clavulanate.8 Intravenouscombination therapy isrecommended for immunocompromisedpatients.1 Immunocompetentpatients with localized infectionsmay be treated with oral combinationtherapy.4
The duration of therapy is basedon clinical evidence of recovery. Twoweeks of treatment with intravenousagents is recommended for immunocompromisedpatients, followedby an oral regimen until there is evidenceof clearing of infection.1 Shorter,oral regimens may suffice for immunocompetentpatients with localizedinfections.4 Surgical resectionand debridement may be needed insome circumstances.1,4
Our patient did not have pulmonarysymptoms or the traditionalrisk factors, such as a history of HIV infection or exposure to livestock;however, her clinical presentation,which included a history of multipleincidents of catheter-related bacteremiaand a current indwelling PICCline put her at risk for infection. Highclinical suspicion for bacteremia anddiligent laboratory work to identifythe organism helped establish thecorrect diagnosis. The patient's responseto ampicillin/sulbactam and,later, to amoxicillin/clavulanate maysuggests a greater role for these antibiotics,as evidenced in a few previousstudies.8 Continued researchon treatment for R equi infection iswarranted.