Emerging dog parasite pushes veterinarians to rethink freshwater risk

A parasite once treated as a regional oddity is now prompting a broader rethink in small-animal practice. Heterobilharzia americana, the cause of canine schistosomiasis, has been tied to expanding geographic risk in the U.S., with veterinary researchers and clinicians highlighting confirmed snail hosts in California’s lower Colorado River region, prior outbreak activity in Utah, and positive samples from dogs across multiple states. The result is a growing sense that this is an underrecognized disease rather than a rare one. (todaysveterinarypractice.com)

Historically, H. americana was associated with the Gulf Coast and southern Atlantic states. But that picture has been changing for years. A 2021 One Health paper described a canine schistosomiasis outbreak in Utah and linked spread potential to acquisition of a new intermediate snail host, while a 2024 Pathogens study documented the parasite and two natural intermediate hosts along the Colorado River in California. In a recent clinical review, Texas A&M–linked authors said preliminary surveillance suggests unexpectedly high prevalence in parts of southern California and western Arizona bordering the Colorado River. (pubmed.ncbi.nlm.nih.gov)

The biology helps explain why the disease can be missed. Dogs become infected when cercariae in contaminated freshwater penetrate intact skin during swimming or wading. Adult flukes later settle in mesenteric vessels and eggs disseminate through tissues, driving granulomatous intestinal and hepatic disease. Illness may not appear until well after exposure; the prepatent period is about 84 days, and some dogs remain asymptomatic even while infected. When signs do emerge, they’re often nonspecific, including diarrhea, hematochezia, vomiting, weight loss, lethargy, PU/PD, and liver-related abnormalities. Fatal multiorgan disease can occur. (todaysveterinarypractice.com)

A major reason for renewed concern is diagnosis. Multiple sources note that routine fecal screening often won’t detect H. americana. In the 60-case retrospective study published in the Journal of Veterinary Internal Medicine, most diagnoses were established by fecal PCR, underscoring how much standard workflows can miss. Texas A&M’s GI Lab describes PCR as a sensitive, specific alternative to sedimentation, and the Companion Animal Parasite Council similarly warns that eggs are not found on routine fecal flotations, which can delay diagnosis and worsen tissue injury. (pmc.ncbi.nlm.nih.gov)

Expert commentary is converging on the same message: awareness has to improve before outcomes do. In Texas A&M’s recent public-facing explainer, researcher Lea Poellmann said clinicians still don’t know why some infected dogs become severely ill while others do not, and she stressed that specialized testing is required because routine fecal tests miss the parasite. A peer-reviewed review in Today’s Veterinary Practice goes further, calling H. americana an infection of “emerging importance” and noting that no preventive medications reliably protect dogs exposed in freshwater environments. (vetmed.tamu.edu)

Why it matters: For veterinary professionals, this is a reminder to update the differential list for chronic GI disease, hepatopathy, unexplained hypercalcemia, and mineralization patterns, especially when there’s any freshwater exposure history. It also has implications for client communication with pet parents: a negative routine fecal doesn’t rule this out, exposure may have happened months earlier, and treatment can be prolonged or repeated. With no labeled preventive and with infection risk tied to environmental exposure, case recognition depends heavily on history-taking, regional awareness, and willingness to order PCR or sedimentation testing. (capcvet.org)

Treatment remains imperfect. Current recommendations center on praziquantel plus fenbendazole, often with retesting and sometimes a second treatment round because juvenile schistosomes may persist after initial therapy. Texas A&M’s referral information notes there are no FDA-approved veterinary drugs specifically labeled for canine schistosomiasis, which leaves clinicians managing an emerging disease with off-label protocols and variable outcomes. (todaysveterinarypractice.com)

What to watch: The next phase is likely to be better mapping of endemic zones, broader use of PCR-based testing, and more local public health or veterinary advisories as surveillance data accumulate in Texas, the Southwest, and other freshwater corridors where competent snail hosts may be present. That may reveal that the parasite’s range, and case burden, have been underestimated for some time. (todaysveterinarypractice.com)