Fish Leech: Causes, Effects and Treatment

Leeches are common parasites found in many aquatic ecosystems, including freshwater environments where fish reside. Fish leeches can cause various health problems for their hosts, including decreased appetite, weakened immune system, and even death.

It is essential to understand the causes, effects, and treatment options available for fish leech infestations to prevent harm to aquatic life.

In this article, we will explore the basics of fish leeches and provide valuable information on how to identify, prevent, and treat this parasitic infection in fish.

Fish Leech

Piscicola geometra is a species of fish leech which is under the family named ‘Piscicolidae’. It is an ectoparasite on different fish species by sucking their blood.

Its body length is about 1 inch and the color of its body is pale brown with white transverse bands. It is mostly a temporary parasite on fish’s body which occurs in both moving and stagnant freshwater commonly found in Europe and in North America. It is also found in some marine environments such as the Baltic Sea. The classification of this annelid is given below.


  • Kingdom: Animalia (multicellular, eukaryotic)
  • Phylum: Annelida (segmented worms)
  • Class: Clitellata (having a clitellum that forms a reproductive cocoon)
  • Subclass: Hirudinea (segmented parasitic or predatory worms)
  • Order: Rhynchobdellida (the jawless leeches)
  • Family: Piscicolidae (parasitic on fish)
  • Genus: Piscicola
  • Species: Piscicola geometra

Morphology of Piscicola geometra

Appearance of the Piscicola leech
Figure: Piscicola geometra; (pp-pigmented pattern- es-eye like spot); Source:

As Piscicola geometra is a species of leech, its morphology is similar to a leech and its body structure is adapted for a parasitic lifestyle in aquatic environments.

Here are some key features of its morphology described below which help it for surviving in those types of environments:

Body: Piscicola geometra has a long and slender body which is segmented into numerous annuli or rings. Body is flattened and tapered at both ends, which allows it to move easily through narrow spaces.

Suckers: This leech has two suckers of which, one is situated at the anterior end and another one is at the posterior end. The anterior sucker is used for attachment to the host and the posterior sucker is used for locomotion. Leeches have a unique and specialized mouth that allows them to feed on the blood of their hosts.

Mouth: The mouth is located on the ventral side of its body and armed with three jaws with numerous teeth. Leeches have a unique and specialized mouth that allows them to feed on the blood of their hosts. The mouth of a leech contains several sharp teeth that the leech uses to make a small incision in the host’s skin.

The leech then secretes chemicals into the wound that prevent the host’s blood from clotting, allowing the leech to feed on the blood. Once the leech has finished feeding, it detaches from the host and falls off.

Eyes: Piscicola geometra has a pair of small eyes, which are located on the dorsal side of its anterior end. The eye structure of this species is relatively simple and consists of a single pigment cup with a lens.

The pigment cup contains photoreceptor cells that detect changes in light intensity and direction, allowing the leech to sense the presence of its prey and navigate its environment.

However, the visual acuity of leeches is not as well-developed as that of other animals with more complex eyes, such as vertebrates or insects.

Reproductive system: Piscicola is hermaphroditic animal, which means it has both male and female reproductive organs in the same body.

The male reproductive organs consist of testes and the female reproductive organs consist of ovaries. During copulation, two different individuals exchange their sperm packets, which fertilize their own eggs.

Midgut: The morphology of the midgut of Piscicola geometra can be described as follows:

Structure: The midgut of Piscicola geometra is a long, narrow tube-like structure that is lined with a single layer of epithelial cells. The epithelial cells are columnar in shape and are arranged in a regular pattern.

Histology: The midgut of Piscicola geometra has a simple histological structure. The epithelial cells are responsible for the absorption of nutrients and the secretion of digestive enzymes. The midgut does not have any goblet cells or other specialized cells.

Musculature: The midgut of Piscicola geometra is surrounded by a layer of circular and longitudinal muscle fibers. These muscle fibers are responsible for the peristaltic movements of the midgut, which help to mix the ingested food and facilitate its digestion.

Secretory Cells: The midgut of Piscicola geometra contains secretory cells that produce digestive enzymes such as proteases and lipases. These enzymes help to break down the ingested food into smaller molecules that can be absorbed by the epithelial cells.

Absorptive Cells: The epithelial cells of the midgut are responsible for the absorption of nutrients from the digested food. These cells have microvilli on their apical surface, which increase their surface area for nutrient absorption.

The morphology of Piscicola geometra is specialized for its parasitic lifestyle. Its flattened body and suckers allow it to attach to its host and move around easily. Its sharp jaws and reproductive system are adapted for feeding and reproduction.

Life Cycle of Piscicola geometra

Piscicola geometra has a complex life cycle that involves multiple developmental stages and hosts. Here are the key stages in its life cycle:

Life cycle of marine leech
Figure: Life cycle of Marine leech; Source: ScienceDirect

Eggs: Firstly, the adult leech lays eggs in the beginning of the life cycle of Piscicola geometra. They lay eggs typically in a cocoon or gelatinous mass, and these eggs can hatch in a few weeks to several months depending on environmental conditions.

Larvae: After hatching from the eggs, the larvae of Piscicola geometra are free-swimming and they find a suitable host to feed on. They are typically attracted to fish by their odor and will attach to them using their anterior sucker.

Juveniles: After larvae stage, they come to a stage called juvenile in which they act by feeding on their host for several weeks. The larvae of Piscicola geometra will detach and transform into juvenile leeches. Juveniles are similar in morphology to adults but are smaller in size.

Adults: The juvenile leeches continue their feeding and grow up. Then they will eventually reach adulthood. Adult Piscicola geometra can live for several years and will continue to feed on fish throughout their lives. They will mate and lay eggs, completing the life cycle.

Piscicola geometra has an indirect life cycle. Because it requires multiple hosts to complete its development. The larvae and juvenile stages feed on fish hosts, while the adults can also parasitize amphibians, reptiles, and mammals. This complex life cycle allows Piscicola geometra to adapt to a wide range of aquatic environments and hosts.

Geographical distribution of Piscicola geometra

Piscicola geometra has a wide geographical distribution. It can be found in freshwater habitats in Europe, Asia, and North America. In Europe, it is distributed from Scandinavia to the Mediterranean, including the British Isles. In Asia, it can be found in Russia, Japan, and Korea. In North America, it occurs from Alaska to the southern United States.

Piscicola geometra is typically found in shallow waters of rivers, streams, and ponds. It occurs as a parasite on a variety of fish species. It can also be found in other freshwater habitats like wetlands and swamps. Its distribution is influenced by factors such as water quality, temperature, and availability of suitable hosts.

Due to its parasitic nature, Piscicola geometra can have negative impacts on fish populations, particularly in aquaculture facilities where large numbers of fish are kept in close proximity. It is also an important model organism for studying host-parasite interactions. It has been used in research such as immune responses and genetic diversity.

Piscicola geometra is known as a parasite in a variety of fish species, including both freshwater and marine species. In freshwater habitats, it is commonly found on salmonids, such as brown trout, rainbow trout, and Atlantic salmon. It can also parasitize on other freshwater fish species, including perch, roach, pike, and eel.

In marine habitats, Piscicola geometra can be found on a variety of fish species including flounder, cod, herring, and mackerel. It is also known to parasitize some marine mammals, such as seals and sea lions, as well as sea turtles.

The impact of Piscicola geometra on fish populations can vary depending on the intensity of infestation and the health of the host.

Heavy infestations can lead to reduced growth rates, anemia, and even death in some cases. In aquaculture facilities, Piscicola geometra can be a significant problem to large numbers of fish which are kept in close proximity and can easily become infested.

Control measures, such as chemical treatments and physical removal, are often used to manage infestations in aquaculture settings.

What Causes Leech in Fish?

Fish leech can be introduced to a body of water through contaminated equipment, infected fish, or other means. Poor water quality, such as high levels of organic matter, can also increase the likelihood of a leech infestation.

Additionally, stress, poor nutrition, and a weakened immune system can make fish more susceptible to leech infestations. Once attached, leeches can feed on the blood of their host, causing harm and potentially leading to serious health problems.

Fish Leech Causing Signs and Symptoms

Piscicola geometra is a blood-sucking parasite which can cause a difference of signs and symptoms when it attacks its fish hosts.

The severity of these signs can vary depending on the intensity of infestation as well as the health of the host. Here are some of the common signs and symptoms caused by Piscicola geometra:

Anemia: Piscicola geometra feeds on the blood of its host, which can lead to anemia or low number of red blood cell counts. Anemic fish may appear pale or lethargic as well as may have reduced swimming ability.

Skin irritation: The blood feeding activity of Piscicola geometra can cause skin irritation and inflammation in the host. This can result in redness, swelling, and lesions etc. on the skin.

Reduced growth: Heavy infestations of Piscicola geometra can lead to diminish growth rates in fish. This can be particularly problematic in aquaculture facilities, where growth rates are one of the most important factors in profitability.

Behavioral changes: Fish infested with P. geometra may exhibit changes in behavior, such as reduced feeding activity, hiding, or swimming erratically.

Secondary infections: The skin injury caused by Piscicola geometra can make fish more vulnerable to secondary bacterial or fungal infections, which can be more dangerous or untreatable.

In addition to these symptoms, fish infested with Piscicola geometra may also have a weakened immune system as well as be more vulnerable to other stress, such as changes in water quality or temperature. It is important to monitor fish populations for signs of infestation and take measures to control infestations if they occur.

Fish Leech Diagnosis

Diagnosis of Piscicola geometra infestation can be challenging as the Piscicola geometra is often hidden in the gills or other internal organs of the fish host. Here are some procedure used for diagnosing Piscicola geometra infestation:

Visual inspection: External symptoms of infestation, such as skin lesions or discoloration, may be visible on the surface of the fish. However, Piscicola geometra is often hidden in the gills or other internal organs, making it very difficult to diagnose based on external signs alone.

Microscopic examination: The presence of Piscicola geometra can be confirmed by examining a sample of gill tissue or blood under a microscope. The leeches can be identified based on their characteristic morphology,includes a cylindrical body with a sucker at each end and a row of teeth along the mouth.

Polymerase chain reaction (PCR): PCR can be used to detect DNA sequences specific to Piscicola geometra. This method is highly sensitive and can detect low-level infestations, but requires specialized equipment.

Serological testing: Antibodies specific to Piscicola geometra can be detected in the blood of fish hosts using serological testing. However, this method is rarely used for diagnosis due to its expense and limited availability.

Monitoring fish populations regularly should be done for signs of infestation and take steps to control infestations if they occur. Early detection and treatment can help prevent the spread of infestations and minimize the impact on fish populations loss.

Fish Leech Prevention

Prevention of P. geometra infestation is important to minimize the impact of the parasite on fish populations. Here are some procedures that can be taken to prevent Piscicola geometra infestations:

Quarantine: New fish need to be quarantined before being introduced to existing populations to prevent the introduction of Piscicola geometra or other parasites or diseases.

Regular monitoring: Regular monitoring of fish populations can help detect Piscicola geometra infestations early as well as prevent their spread. Monitoring can include visual inspections, microscopic examination of gill tissue, and PCR testing.

Water quality management: Maintaining good water quality is important for controlling Piscicola geometra infestations. This includes maintaining appropriate oxygen levels, pH, and temperature, as well as minimizing stressors such as overcrowding and poor nutrition.

Biosecurity measures: Biosecurity measures such as disinfection of equipment and facilities, and preventing the introduction of potential carriers of Piscicola geometra (such as birds, mammals, or other fish species) can also be helpful to prevent infestations.

Chemical treatments: Chemical treatments such as formalin or hydrogen peroxide can be used to control Piscicola geometra infestations. However, these treatments should be used judiciously and according to label instructions to minimize their impact on fish health and the environment.

Regular communication as well as collaboration between fish producers, fisheries managers, and researchers can help improve prevention strategies and reduce the impact of Piscicola geometra infestations on fish populations.

Fish Leech Treatment

There are several treatment options available for P. geometra infestations. Treatment effectiveness can vary depending on the severity of the infestation, the type of fish host, and environmental factors. Here are some common treatment options:

Chemical treatments: Formalin and hydrogen peroxide are most commonly used to treat Piscicola geometra infestations. These chemicals are effective in killing the parasites, but can also be detrimental to fish and the environment if not used correctly. It is important to follow label instructions carefully when using chemical treatments.

Biological treatments: Biological treatments, such as the use of parasitic wasps or predatory fish, have shown some promise in controlling Piscicola geometra infestations. However, more research is needed to determine their effectiveness and also potential impact on non-target species.

Physical removal: Piscicola geometra can be physically removed from the gills or skin of the host using a fine-meshed net or tweezers. This method can be much time-consuming and labor-intensive, but can be effective in removing individual parasites.

Improving fish health: Improving the overall health of the fish host through good nutrition and management practices can help reduce the impact of Piscicola geometra infestations. Healthy fish are seen better able to withstand the effects of parasite feeding and are less vulnerable to secondary infections.

Integrated pest management: Integrated pest management (IPM) strategies that combine multiple treatment methods may be most effective in preventing Piscicola geometra infestations. IPM strategies can include regular monitoring, early detection, and a combination of chemical, biological, and physical control methods.

It is very important to work with a veterinarian or fisheries expert to determine the most appropriate treatment method for a specific infestation. Otherwise, it reduces the production of fishes and occurs tremendous loss. Treatment effectiveness should be regularly monitored to ensure that infestations are being adequately controlled.


Can leeches harm fish? | Is leech harmful?

Yes, leeches can harm fish by attaching themselves to the fish and feeding on their blood. While some species of leeches feed exclusively on fish, others may feed on a variety of animals, including fish. When a leech attaches to a fish, it can cause wounds that may become infected, and repeated feeding by large numbers of leeches can weaken the fish and make it more susceptible to disease.

However, it’s worth noting that not all species of leeches are harmful to fish, and some can even be beneficial. For example, some species of leeches can help to control populations of snails and other potential hosts for fish parasites, thereby reducing the risk of disease transmission.

So, whether a leech is harmful to fish depends on the species of the leech and the circumstances in which it is found. If you notice a large number of leeches on your fish, it’s best to take action to remove them to prevent harm to your fish.


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