A Unique of Rhizobium radiobacter Peritoneal Dialysis Catheter-related Peritonitis from a Cat Bite

Peritoneal dialysis catheter-related peritonitis is typically related to catheter insertion or hygiene. Organisms usually manifest as gram positive organisms typically Staphylococci or gram negative bacilli that typically colonize aqueous environments such as Pseudomonas spp. as well as other unusual mycobacterial or fungal pathogens. The authors present a unique case of peritoneal dialysis catheter-related peritonitis manifesting after the patient’s cat bit the proximal aspect of the catheter attachment tubing. After failure to clinically improve after outpatient treatment of Staphylococcus hemolyticus and Staphylococcus capitis catheter related-peritonitis with intraperitoneal Vancomycin and empiric Gentamicin, repeat peritoneal cultures were sent that demonstrated growth of Rhizobium radiobacter. The patient was admitted inpatient and treatment was tailored to intraperitoneal Cefepime with clearance of peritoneal cultures and rapid improvement in peritoneal white blood cell count and the patient’s clinical condition. The patient was subsequently discharged on oral Ciprofloxacin with complete recovery and catheter salvage.


Introduction
Rhizobium radiobacter, a formerly reclassified from Agrobacterium class by 16s RNA analysis [1], is an aerobic gram-negative, motile, non-spore forming oxidase positive bacillus. Natural habitats for the bacteria exist in plants and soil [2]. Initially regarded as a contaminant, the organism was first determined in 1980 to cause prosthetic aortic valve endocarditis [3]. Previous case series have demonstrated the bacteria to be an opportunistic pathogen in patients with chronic indwelling catheters and hematologic malignancies. To this date, there are no reported cases of transmission of R. radiobacter via cat exposure. Due to the infrequent incidence of R. radiobacter, large clinical trials assessing its impact and treatment options are lacking.

Case Presentation
A 38 year-old female with end-stage renal disease on intermittent nocturnal peritoneal dialysis via Tenkhoff catheter for over two years secondary to type-1 diabetes mellitus presented to Hahnemann University Hospital in Philadelphia, Pennsylvania in October 2016 upon her nephrologist's request due to positive IP cultures. Her symptoms began several days after her cat bit her peritoneal dialysis (PD) connection tubing just distal to the Tenkhoff catheter attachment (see Figure 1). She subsequently developed nausea, vomiting, and abdominal discomfort that lasted 1-2 days. Outpatient dialysis cultures were initially positive for Staphylococcus hemolyticus and Staphylococcus capitis with white blood cell count demonstrating 90 cells/uL and 27% neutrophils. The patient was subsequently treated with three doses of IP Vancomycin and one initial dose of IP Gentamicin over the course of nine days. Two days later, due to persistent fatigue, decreased energy, and decreased appetite, repeat outpatient peritoneal cultures were taken that were positive for gram-negative bacilli. The patient was contacted and referred to the hospital for further evaluation.
After admission, the patient initially underwent treatment with IP vancomycin and gentamicin (Dianeal PD-2 Intraperitoneal dialysis solution with 2L of 1.5% dextrose, 8meQ KCl additive, 4mg gentamicin and 40mg vancomycin) for 3 days and intravenous cefepime dosed at 2g q48 hours for 3 days. IP infusion dwell-time was set to 2.5 hours on a q3 hour basis. Initial inpatient peritoneal fluid analysis indicated WBC count of 274 cells/uL, 49% neutrophils, confirming the diagnosis of peritoneal catheter-related peritonitis. Peritoneal culture repeated while inpatient on 5% sheep's blood revealed gram-negative bacilli. Speciation by Vitek 2 (BioMérieux, Inc., Durham, NC) demonstrated Rhizobium radiobacter (see Figure 2, Figure 3). Subsequent peritoneal fluid analysis the next day revealed a WBC count of 9 cells/uL with 5% neutrophils with no demonstrated culture growth (see Table 1). Sensitivity analysis confirmed susceptibility to cefepime, ciprofloxacin, and piperacillin-tazobactam (see Table 2) and the patient's antibiotics were changed via infectious disease consult to IP cefepime 125mg/L (Dianeal PD-2 Intraperitoneal dialysis solution with 2L of 1.5% dextrose, 8meQ KCl additive). The patient's symptoms gradually improved with this regimen and was discharged on oral ciprofloxacin 500mg daily for a 3 week treatment course with PD catheter retention.

Discussion
Rhizobium radiobacter is an infrequent cause of opportunistic infection largely of indwelling catheters and foreign material. The largest case-series involved 13 Taiwanese patients with R. radiobacter infection, of which the majority had chronic indwelling catheters. All isolates were susceptible to cefepime, ciprofloxacin, piperacillin-tazobactam, and carbapenems [4]. Although treatment frequently required foreign device removal [5], recent reports indicate that early and appropriate antibiotic therapy may allow catheter salvage in catheter-related R. radiobacter peritonitis [6]. R. radiobacter isolates should be evaluated in the appropriate clinical context as pseudo-outbreaks have been reported due to nonsterile processing equipment and/or handling of laboratory specimens [7]. As two separate laboratories confirmed R. radiobacter by culture and reactive peritoneal cell counts in our patient, true infection was confirmed.
This case documents the first reported transmission of R. radiobacter via cat exposure. The patient's cat may have been colonized with R. radiobacter while being near plants and soil at or nearby the patient's home. Infections transmitted by cat bites tend to be polymicrobial, including Staphylococci, Streptococci, Corynebacterium, Capnocytophagia, Pasteurella, and anaerobic flora [8]. A 1997-2013 literature review by Poliquin et al. found 30 cases of PD catheter-related peritonitis resulting from Pasteurella with cats being the most common source animal. Appropriate treatment and catheter retention was generally obtained with two weeks of amoxicillinclavulinate or IP cefazolin [9].
The revised 2016 International Society of Peritoneal Dialysis lists pets as a modifiable risk factor for PD catheter-related peritonitis [10]. As zoonotic injuries constitute approximately 1% of emergency department visits and $30 million US dollars yearly, clinicians and patients alike should be aware of potential complications of pet-related interactions with indwelling medical devices [11].

Conclusion
Although uncommon, R. radiobacter should be considered in the differential diagnosis of potential pathogens in zoonotic peritoneal catheter-related peritonitis. The importance of patient pet owner education in taking measures to prevent zoonotic infections cannot be overstated.