Detection of intracellular bacteria in feline inflammatory cardiac, hepatic and renal diseases: a retrospective study using fluorescent in situ hybridisation

ObjectivesThe aims of the present study were to determine whether bacteria are present in feline cardiac, hepatic and renal tissues where inflammation has been identified and to compare the location of any bacteria with areas of inflammation within those tissues. Fluorescent in situ hybridisation (FISH) facilitates visualisation of intracellular bacteria in tissues. There is little research looking at the role of intracellular bacteria in inflammatory disease within feline medicine.MethodsStudy group (SG) cases were selected from Ross University School of Veterinary Medicine's pathology archive for 2012-2022. A total of 23 cases fulfilled the inclusion criteria. Three sequential sections were assessed with FISH (using eubacterial and non-eubacterial probes) and haematoxylin and eosin staining. Control group (CG) cases were selected from the same archive (n = 6) where death was trauma related; no other disease states were noted and the same three tissues were available for testing. Known bacteria-positive sections were included with each batch of slides processed to confirm successful hybridisation.ResultsOf the SG cases, 52.2% (95% confidence interval [CI] 30.6-73.2) demonstrated bacteria within some or all tissues tested, and 78.3% (95% CI 56.3-92.5) demonstrated the presence of inflammatory cells (ICs) in one or more tissues. Of the IC-positive SG cases, 61.1% (95% CI 35.7-82.7) demonstrated bacteria using FISH; the presence of bacteria in either the liver or kidney was frequently associated with the presence of ICs in 77.7% (95% CI 40.0-97.2) and 80% (95% CI 28.4-99.5) of cases, respectively. Among these, IC distribution did not match bacterial distribution. Of the CG cases, 83.3% (95% CI 35.9-99.6) were negative for ICs. Notably, in the IC-negative CG cases, two (40%) were positive for bacteria using FISH (95% CI 5.3-85.3). The Pearson χ² test demonstrated a χ² value of 0.71 (<i>P</i> = 0.40).Conclusions and relevanceDespite this pilot study being limited by a small sample size, bacteria were successfully detected within formalin-fixed paraffin-embedded samples of feline heart, liver and kidney. We demonstrated that bacteria may not co-locate with all instances of inflammation, suggesting the need for greater vigilance for the presence of fastidious bacteria and/or low-grade infection.