A risk in the ripple: Understanding Heterobilharzia americana in dogs

For many dogs, there's nothing better than splashing in a lake, pond, or river. However, in certain regions of the United States, freshwater fun can carry a little-known risk—a parasitic flatworm called Heterobilharzia americana.

Once considered rare and geographically limited, new research suggests that H. americana may be more common—and more widespread—than previously thought.

With the weather warming and owners beginning to take their dogs on more outdoor adventures, Dr. Kathleen Aicher, an assistant professor of internal medicine in the Texas A&M College of Veterinary Medicine & Biomedical Sciences' (VMBS), and Ph.D. student Dr. Lea Poellmann, both members of the VMBS Gastrointestinal Lab (GI Lab), share what owners need to know to protect their dogs against H. americana as the research duo works to better understand the parasite and its growing impact on dogs.

What is H. americana?

H. americana is a trematode, or flatworm, that lives in the blood vessels that drain the gastrointestinal tract.

Unlike more familiar intestinal worms that live in the GI tract, H. americana resides in blood vessels and lays eggs that migrate through the intestinal wall. Some eggs exit the body in feces, but others can become trapped in organs such as the liver or pancreas, where they trigger inflammation.

Dogs become infected through contact with freshwater, where the parasite must first develop inside certain freshwater snails before becoming capable of infecting animals. Once in its infectious stage, microscopic larvae can penetrate a dog's intact skin while swimming or wading—no bite or wound required.

"It's really just unseen danger," Poellmann said. "It just goes through the skin."

Wildlife also carry the parasite, helping maintain it in the environment.

More common than once thought

Historically, H. americana was associated primarily with Gulf Coast and Southern Atlantic states. However, recent studies conducted by the VMBS team found unexpectedly high infection rates in certain dog populations.

In parts of the Southwestern United States, including communities along the lower Colorado River basin, researchers identified prevalence rates around 24%. In some Texas water-exposed dog groups, rates approached 28%.

"This prevalence is unexpectedly high for a parasite that was basically unheard of in the Southwestern United States," Poellmann said.

Importantly, infection is not limited to traditional "water breeds," a finding that challenged earlier assumptions about which dogs are most at risk.

"We've documented this in breeds you just wouldn't expect," Aicher said. "French bulldogs, Chihuahuas, Boxers, mixed breeds—little dogs, puppies, adult dogs."

Signs can be subtle—or severe

One of the challenges with H. americana is how differently it can affect dogs.

Some infected dogs show no outward symptoms and may carry the parasite quietly. Others develop significant illness.

When symptoms do occur, they may include:

• Chronic diarrhea
• Weight loss
• Vomiting
• Lethargy
• Changes in appetite

"We don't really know what the deciding factor is between a dog getting really sick from this and a dog being OK," Poellmann said.

In some cases, inflammation caused by migrating eggs can affect multiple organ systems. Severe complications can become life-threatening if not addressed.

Diagnosis and treatment

Routine fecal tests performed at many veterinary clinics do not detect H. americana. Specialized testing, including PCR testing and fecal sedimentation techniques, is required.

Treatment typically involves two antiparasitic medications—praziquantel and fenbendazole.

"The problem is that we need really high doses of praziquantel, much higher than those used for more common parasites," Poellmann said. "That can make dogs that are already feeling sick feel even worse."

Administering multiple pills several times a day can also be challenging, especially when dogs are already feeling unwell.

Because treatment can be intensive, early detection is critical.

What owners can do

Currently, there is no approved preventive medication that reliably protects dogs against H. americana.

Limiting exposure to freshwater sources known or suspected to harbor the parasite can reduce risk. While avoiding freshwater entirely may not be realistic, understanding the risk allows owners to make informed decisions for their dogs.

For dogs that frequently swim or wade in freshwater—especially in regions where the parasite has been documented—regular screening may be the safest approach.

"If you do have a dog that goes in the water, we would recommend testing your dog," Poellmann said. "Detecting infection early is the safest way right now to protect our dogs."

Testing every four to six months may be appropriate for high-risk dogs, depending on geographic location and water exposure.

The VMBS team continues to study how H. americana spreads, why some dogs become severely ill while others remain symptom-free, and how better preventive strategies can be developed.

Aicher said the team has made progress in recognizing the parasite as a growing concern and is now focused on prevention, improved treatment strategies, and education for both owners and veterinarians.

With increased awareness and early testing, dog owners can continue to enjoy time near the water while better protecting their pets' long-term health.

Provided by Texas A&M University