Fluke Infestations in Waterfowl
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There are numerous parasites that can infect waterfowl. Among these are flukes, a type of parasitic flatworm that belongs to the scientific class Trematoda. Most people are at least remotely familiar with or have heard of liver flukes in sheep, cattle, and even humans. With some 24,000 species of flukes worldwide, there are fluke species that have the potential to invade just about any animal species imaginable, including mammals, fish, amphibians, reptiles, and birds.
Species of Flukes
There are several species of flukes that can affect waterfowl, but we’ll look at only three in this article. The first two species, Cyathocotyle bushiensis and Sphaeridiotrema globulus, are the most common. These species invade the lower intestines, caecum, liver, bile ducts, and gallbladders of the birds. They attach to the walls of the small intestine after being ingested. Eventually, some of these move through the bile ducts to the liver, where they cause inflammation and clog the ducts. They’re also known to invade the gallbladder. In the intestines, they suck blood, impede normal nutrient uptake, and weaken the host. Most cases result in death.
The third species, Philophthalmus gralli, commonly known as the avian eye fluke, invades and attaches to the eyelid membranes and conjunctival sacs of many species of birds, including domestic and wild waterfowl. It has been found in both North and South America.
Shapes and Habits
Most flukes have a flat body, leaf-shaped head, and a sucker mouth with a ring of hooks surrounding it. They use these hooks to attach to the gastrointestinal tract, as well as the tissues and organs of their hosts.
The likelihood of your flock of ducks or geese contracting these parasites is still somewhat low in North America. The heaviest concentrations of these parasitic infections seem to be found in Southeast Asia (including Myanmar, Bangladesh, and Vietnam) and tropical regions with waterways and wetlands that are perfect breeding and incubation grounds for these parasites. Nonetheless, species of flukes that infect waterfowl are also found in the northern United States, Canada, northern Europe, and other non-tropical regions of the world. Flukes aren’t something you’ll probably need to worry too much about, but they’re something to be aware of.
For an animal considered somewhat simple in design, flukes have a somewhat complex lifecycle. Flukes require an intermediary host during their larval stages. In most cases, these intermediary hosts are aquatic snails and slugs that inhabit waterways and damp, marshy pasture areas. In tropical regions, proliferation and parasitic infestations occur year-round, while in regions with colder winters, the proliferations often occur in late summer after the water has warmed. Waterfowl become infected after eating snails that have been parasitized by the developing larval flukes. As few as 100 flukes can be lethal to waterfowl. It’s common for one snail to contain more than 100 of these developing flukes. Death in a severely infected bird can occur within 3 to 8 days after contracting the parasites.
Signs and symptoms of parasitic infestation can include weight loss, lethargy, loss of appetite, decrease and cessation of egg production, increase in soft-shelled eggs, blood-stained vents, and, of course, an appearance of the worms or flukes.
The Intermediary Hosts
In most cases, aquatic snails become the intermediary hosts, in which the larval stages of flukes grow and develop. One species of snail, Bithynia tentaculata, or the European faucet snail, is probably the most common intermediary host and vector. Native to Europe, this gastropod has become invasive in many other regions of the world. However, it’s not the only snail able to become infected by the parasite and spread it. Field research done in the upper Mississippi River several years ago revealed four separate species of snails to be infected with both C. Bushiensis and S. globulus. In 1962, in Southwest Siberia, Russia, seven species of snails were infected with these flukes.
Geographic Distribution of Avian Flukes
Scientific studies and data on this topic have been inconsistent and irregular over the years. Research in Bangladesh in 2003 and 2004, during which 300 native ducks were examined, found that 69% had flukes in their livers and gallbladders. Studies from Korea, Vietnam, and other duck-raising regions of Asia have also shown abnormally high concentrations of these parasites.
The warm, tropical waterways and marshlands in many of these regions provide optimal conditions for the flukes and their intermediary hosts. Highly concentrated duck farming in these regions, where the ducks feed on aquatic snails, creates the perfect recipe for disastrous levels of infestation.
These parasites are by no means limited to Southeast Asia. Fatal, recurrent outbreaks were recorded in migratory waterfowl in southern Quebec between 1968 and 1988. Incidental findings have also been found in all major North American flyways. The potential danger for contraction of avian flukes in domestic waterfowl in North America is greatest in areas where domestic fowl and wildfowl may share a common pasture or pond.
Occurrences of P. gralli, the most common avian eye fluke, are also on the rise. There have been increased field findings in the Pacific Flyway in North America and increased occurrences in South America. This parasite may be spreading via common avian migration routes.
Lifecycles of the Flukes
New infestations of intestinal liver flukes often start when an infected waterfowl releases fluke eggs into the water or marsh via fecal discharge. Fluke eggs hatch almost immediately once in the water, and thus enter into the first larval stage. Known as miracidia, these free-swimming organisms then invade an aquatic snail or slug, where they take several months to develop and asexually reproduce numerous times into a second larval stage known as cercariae. These organisms eventually leave their host and invade a second aquatic snail or slug, encysting there in a form known as metacercariae. When a waterfowl consumes a snail infested with these metacercariae, the parasites latch onto the intestinal tract, where they mature and begin laying eggs. The newly infected birds then discharge the eggs into the water with their feces, and the cycle starts all over again.
In the case of eye flukes in the genus Philophthalmus, the life cycle is similar, but only one stage of the intermediary host is involved. The larval cercariae attach to aquatic plants, where they form a protective shell around themselves. When the bird ingests the aquatic plants, the cercariae break free of their protective shell and latch onto the mucous membranes and conjunctival sacs surrounding the bird’s eyes. There, the cercariae suck blood, reproduce, and cause extreme irritation to its host. Free-swimming cercariae may also attach directly to the conjunctiva when birds put their heads under the water.
While avoiding parasitic infestations and disease may not always be possible, regularly check your birds for any signs of infection or infestation, and if possible, keep wild waterfowl away from common areas. This can help avoid sporadic disease and parasitic outbreaks.
DOUG OTTINGER lives, works, and writes from his small hobby farm in
northwest Minnesota. Doug’s educational background is in agriculture
with an emphasis in poultry and avian science.
Originally published in the June/July 2023 issue of Backyard Poultry magazine and regularly vetted for accuracy.