Heterobilharzia americana in Dogs: Characterizing Clinical Infection, Evaluating Diagnostic Test Performance, and Exploring Novel Methods of Diagnosis
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Heterobilharzia americana is a waterborne trematode parasite (Family: Schistosomatidae) of dogs. More complete information regarding clinical, geographic, and diagnostic aspects of this parasite is needed to aid in more effective awareness and diagnosis. A total of 238 cases diagnosed through the Texas A&M Veterinary Medical Diagnostic Laboratory, Texas A&M Veterinary Medical Teaching Hospital, Texas A&M Diagnostic Parasitology Service, and Texas A&M Gastrointestinal Laboratory were reviewed. Cases were distributed primarily in the eastern region of Texas. Clinical signs were diarrhea (67%), weight loss (38%), anorexia/hyporexia (27%), vomiting (22%), hematochezia (20%), lethargy (17%), and polyuria/polydipsia (6%). H. americana was attributed to death in 20 of 39 necropsy cases. Trematode eggs were identified histologically in the small intestine (84%), liver (84%), large intestine (39%), pancreas (35%), lung (9%), lymph node (8%), and spleen (4%). A total of 69 dogs were enrolled in a diagnostic methods comparison study. Relative test sensitivities were 50% (29.1-70.9) for fecal saline sedimentation, 58.3% (36.6-77.9) for PCR of fresh feces, and 95.8% (78.9-99.9) for PCR of fecal sediment. PCR of fresh feces was no more sensitive than fecal saline sedimentation. Circulating anodic antigen was detected in the serum of 8 dogs using the Schistosoma mansoni point-of-care assay (POC-CAA). Circulating cathodic antigen was detected in urine of 7 dogs using the POC-CCA test. Next generation sequencing technology and the Galaxy-based RepeatExplorer computation pipeline were used to discover highly repetitive DNA sequences in the H. americana genome. A novel probe-based real-time PCR diagnostic assay targeting these highly repetitive sequences was developed. No DNA amplification was detected when testing DNA of common parasites indicating that the assay is highly specific. The real-time assay detected 9 samples as positive that were negative by conventional PCR targeting a segment of the 18S ribosomal DNA. Increased awareness of H. americana by veterinarians is crucial for a timely diagnosis. Promising methods to increase test sensitivity include sample concentration before DNA extraction, and using highly repetitive DNA targets in a real-time PCR assay. Circulating antigens were detected in some dogs; however, more sensitive test modalities should be developed in order to make circulating antigens accurate diagnostic targets.
Rodriguez, Jessica Yvonne (2017). Heterobilharzia americana in Dogs: Characterizing Clinical Infection, Evaluating Diagnostic Test Performance, and Exploring Novel Methods of Diagnosis. Doctoral dissertation, Texas A&M University. Available electronically from