"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

August 16, 2021

Heterobilharzia americana

Heterobilharzia americana is a species of blood fluke which is native to North America. It is mainly a parasite of raccoons, but is also capable of infecting a wide range of other mammals. Its broad taste for different hosts brings them into contact with various domesticated animals, in particular, dogs. Indeed, it is more commonly known as the canine schistosome.

Left: Cercaria of H. americana, Right: Adults (left = male+female pair, right = single female fluke) 
Photos from Fig. 2 and Fig. 4 of the paper

When the aquatic larvae of these flukes come into contact with humans, much like those of bird schistosomes,  they get intercepted and killed by the immune system as they burrow into the skin, and the death throes of these larval parasites manifest themselves as an itchy rash. But in dogs, not only is the parasite able to establish itself and grow to sexual maturity, it also causes far more severe symptoms than merely an itchy rash.

Dogs infected with H. americana exhibit a host of serious pathologies including vomiting, bloody diarrhoea, weight loss, and lethargy. Additionally, the vast number of microscopic eggs produced by the adult flukes are transported via the circulatory system to various internal organs where they can cause inflammation, and form pockets of mineralization. While the parasite can be treated with prazinquantel and fenbendazole, their presence often go under-diagnosed as the disease they manifest are rather nonspecific, and the existence of this parasite is not as commonly known. 

Some of those eggs that are carried in the circulatory system eventually make their way to the outside world via the host's faeces. If an egg reaches a water body such as a pond, it hatches into a ciliated larva which seeks out a suitable snail host. Snails play an important role in the life cycle of digenean flukes like H. americana, for this is where asexual reproduction takes place. Through commandeering much of the snail's internal organs, the parasite raises an entire clone army of free-swimming larval stages called cercariae. A single infected snail can produce and release hundreds of infective cercariae into the surrounding waters on a regular basis.

The usual snail host for H. americana is Galba cubensis, a pond snail mostly found in the warmer parts of the Americas including Mexico, South America, and south-eastern parts of the United States. But a new study indicates that it has recruited a new snail host for the asexual stage of its life cycle, one which would allow it to spread further across North America. 

This study was based on a two-year long investigation into a small outbreak of H. americana in east-central Moab, Utah, where two severely ill dogs were euthanised after exhibiting symptoms associated with canine schistosomiasis. A necropsy revealed many of their internal organs were riddled with inflammation and mineralization caused by the presence of blood fluke eggs. Examination of faecal samples from other dogs in the neighbourhood found that some of them also contained the parasite's eggs.

Given the life cycle of H. americana, the researchers determined that the most likely source of the infection was a nearby irrigation pond which was regularly visited by dogs living in the neighbourhood, including the two deceased dogs. The pond was filled with many different species of aquatic snails, but there was just one species that was shedding schistosome cercariae - an amphibious snail called Galba humilis, which lived along the banks and waterline of the pond.

Galba humilis is widely known to serve as a host for liver flukes (Fasciola hepatica),  but this is the first time it has been recorded to host the canine schistosome as well. While these tiny snails are barely a centimetre in length, each can produce thousands of infective cercariae over its lifetime. The researchers found that on average, an infected snail can release about 800 cercariae during each shedding period. Furthermore, to increase their chances of encountering a host, they mostly come out of the snail at night between 6:00 pm and 7:30 pm, which overlaps with the active period of their main mammalian hosts - raccoons. 

Based on the faecal samples the researchers found in the area, the animals which introduced H. americana to the snails in that pond in Moab were most likely to be the aforementioned raccoons. And they were able to confirm this via experimental infection of snails from a captive-raised colony.

The most worrying implications from this study is that by acquiring a snail like G. humilis as a host, H. americana would be able to spread to more temperate regions. Galba humilis is widely found across the United States and is a common snail in human-built habitats like irrigation dams and ponds. These habitats also attract various animals like raccoons and other mammals which are viable hosts for this parasite. Thus these water bodies can bring together everything H. americana needs to complete its life cycle and reproduce.

With landscape changes due to agriculture, urbanisation and climate change, raccoons have become more abundant and are expanding their range across North America. In addition to raccoons, dogs are also common in urban areas and can serve as key reservoirs and means of dispersal for the parasite. Whenever they visit a pond with snails, infected dogs can introduce and establish a new H. americana hotspot for the local mammal population.

All these factors contribute to further the spread of this parasite across North America, and possibly elsewhere too. Raccoons have become a prolific invasive species in many parts of the world, and since H. americana has already switched its snail host once, it may do it again to whatever amphibious or aquatic lymnaeid snail it encounters. For the canine schistosome and other parasites, what played out at that pond in Moab is a sign of things to come in our changing world.