Fish parasites: skin & gill flukes, skin trematodes and fish lice

What kind of fish parasites can you find on freshwater fish after a skin scraping and what do they look like under a microscope? In this third and last article and video we will show you skin flukes, gill flukes, skin trematodes and fish lice. Let’s dive into it!

The class of flatworms Monogenea (or monogenetic flukes) comprises of about 2.200 species, all of which are parasites of fish, amphibians, and other aquatic animals. Various representatives of this group can
be found in scrapings from the body and gills of fish. One important characteristic of these parasites is their high host-specificity, meaning that a monogenetic fluke species only parasitizes one species of fish or sometimes but rarely on a few closely related fish species.

Other fish species will have different species of worms. To the contrary, it is not uncommon that a fish species is a host for several different fluke species! However, these parasites share a common body
design that allows for their identification. Let’s look at some of them in more detail…

Skin flukes

The genus Gyrodactylus, or skin flukes, has over 500 species. The worm’s body is elongated, flattened, and up to 0.5 mm long. At the posterior end, there is an attachment disc equipped with two large central hooks and several smaller hooks arranged in a circular pattern around the disc. The anterior end of the body usually has two lobes. Skin flukes are viviparous hermaphrodites, so it is often possible to observe the development of a second-generation larva inside the body of one worm. Gyrodactylus can undergo a significant increase in population density, covering the entire body of the fish. These parasites pose a particular threat to young fish, often leading to mass mortality in aquariums and ponds. Macrogyrodactylus simentiensis is a representative of the genus Macrogyrodactylus that parasitizes on the body of Polypterus. It differs from the typical Gyrodactylus primarily in its size, reaching about 1 mm in length, which allows it to be seen even with the naked eye or using a magnifying glass.

The attachment disc also contains two large hooks, but 14 small hooks are arranged on one side rather than in a circular pattern. Macrogyrodactylus are also viviparous parasites, with up to three generations of larvae developing within their bodies. Fish infected with skin flukes become lethargic, display an abundance of mucus on their skin, often develop damage to the skin and fins and scrape against of decorative items in the aquarium. In addition, the blood vessels become visible on the light-colored body of the fish. This is usually characteristic of goldfish…

Gill flukes

The group of gill flukes, consisting of several hundred species, also belongs to the class Monogenea. As the name suggests, these worms parasitize on the gills of fish. However, gill flukes are often found not only in scrapings from fish gills but also in scrapings taken from the gill cover or pectoral fins of fish. When the parasites multiply in large numbers, they frequently extend beyond the gills. In contrast to viviparous skin flukes, gill flukes are egg-laying parasites. Gill flukes belonging to different genera, differ in size and differ in the structure of their attachment disc.

Dactylogyrus sp., Trianchoratus sp., Sciadicleithrum sp., Urocleidoides sp., Diclybothrium sp. are all gill flukes that cause respiratory problems in fish. Infected fish exhibit widely open gill covers, rapid breathing, and they become either lethargic or stressed. Gill flukes pose a significant danger to young fish and often lead to mass mortality.

Skin trematodes

Flatworms of the genus Transversotrema primarily parasitize marine fish species, although there are also species that affect freshwater fish. They are not commonly found in scrapings from the skin of the fish, but if you come across a similar butterfly-like worm with a patterned design, you will recognize it as this parasite.

Fish lice

The fish lice parasites of the genus Argulus can reach considerable sizes ranging from a few millimeters to 30 mm, making them noticeable even to the naked eye and removable with tweezers from the fish’s body.

However, in fish skin scrapings, you may find young Argulus that have not reached their adult size and are therefore not visible to the naked eye. In their juvenile stage, Argulus sp. feed on the mucus and skin cells of fish. As they mature, they puncture the host’s skin and feed on its blood. However, this is not the only danger posed by this parasite. Local inflammation and hemorrhage occur at the site of the puncture. The secretion of Argulus sp., which enters the wound through its proboscis, has a toxic effect on the fish. Additionally, it can act as a carrier of dangerous bacteria and protozoa, such as Aeromonas sp. and Trypanosoma sp., which are blood parasites.