Liver flukes in dogs

Heterobilharzia americana (HA), a parasite that can cause infections in dogs, was recently found in snails in the Colorado River near the California-Arizona border, prompting numerous news stories. HA is a flatworm (fluke) that can cause clinical signs in dogs including diarrhea, vomiting, weight loss, lethary, and polyuria/polydipsia. Liver failure and gastrointestinal malabsorption have been reported in some severely affected dogs, when granulomas form around eggs and result in loss of normal tissue function. 

The majority of HA infections in the United States occur in Texas, Louisiana, and other southeastern states. This is the first proof that HA is present in snails in the Colorado River, but it is possible that it has been present in the region for years. In 2019, two dogs were diagnosed with HA infections in Los Angeles County after swimming in the Colorado River, and did not have a prior travel history to an endemic region. 

Raccoons are the definitive host of HA, though dogs can also act as a definitive host. They are infected with HA when they swim in natural bodies of freshwater where HA cercariae (larvae) are present. The cercariae can directly penetrate the skin and develop into adult worms, which migrate to the mesenteric and hepatic veins. Eggs then migrate to the gastrointestinal tract and are passed in the feces, although they can also circulate throughout the body, resulting in the formation of granulomas in other tissues. After the eggs are passed into the environment, they hatch and infect freshwater snails, which serve as the intermediate host. After additional development, cercariae are then released from snails back into the water. Dogs cannot directly transmit the infection to each other or to humans. Humans may be infected by swimming in water with HA cercariae, though the infection is typically limited to the skin. Infections can be prevented by avoiding swimming in bodies of water where the snail intermediate host lives. 

Standard fecal flotation techniques will not identify HA eggs, so fecal sedimentation testing must specifically be requested. In addition, shedding of HA eggs may be intermittent and repeat testing may be required. If you suspect that a dog is infected with HA, please contact the AZVDL to create a sampling and diagnostic plan.