Controlling Digenetic Trematode Parasite Infections in Tilapia: A Comprehensive Guide

By Dr. Ra'anan Ariav, D.V.M. - An internationally recognized Fish Health Management specialist with over 30 years of experience in the aquaculture industry, Co-Founder and CEO of The Aqua Consortium.

Tilapia aquaculture is a vital source of protein worldwide, but emerging diseases increasingly challenge its sustainability. Centrocestus spp.  poses a significant threat to Tilapia culture in, causing severe health problems and leading to substantial economic losses for farmers.

This article provides a comprehensive overview on control of Centrocestus spp. infection in Tilapia, focusing on its life cycle, health impacts, and integrated health management strategies for effective control.

Understanding the life cycle of Centrocestus spp. is crucial for developing effective control strategies.

This parasite has a complex life cycle that involves two intermediate hosts and a definitive host.

Microscopic larvae called miracidia hatch from Centrocestus spp. eggs and actively seek out specific red-rimmed melania snail species, (Melanoides tuberculata) via chemoreception,  penetrating their tissues to initiate infection.

The parasite is transmitted via a freshwater snail Red-rimmed melania - Melanoides tuberculata

Within the snail, the parasite multiplies asexually, producing numerous free-swimming cercariae that are released into the water.

These cercariae actively seek out Tilapia, attaching and penetrating to their gills tissue.

Inside the gill filaments, the parasite transforms into a metacercaria, encysting and maturing.

Birds become infected by eating weak and highly infected Tilapia, and the parasite matures in their gut.

The birds then release eggs back into the water through their droppings, restarting the cycle.

Microscopic view of fish gills showing parasites and distinctive black nodules, highlighting an aquatic ecosystem interaction.

Centrocestus spp.  infection will cause various health problems in Tilapia.

The parasite's encystment and migration within gill tissue cause extensive damage, including inflammation, hyperplasia, thickening and fusion of gill filaments.  

In severe cases, the normal cartilage support of the filaments may also be distorted and displaced, leading to severe deformities of filament and gill arch  structure. 

Centrocestus spp.  induces an unusual inflammatory response in  Tilapia, characterized by a proliferation of host fibroblasts. This host response forms a very unique and extremely strong elastic encapsulation around the parasite. The encapsulations continue to thicken, ultimately destroying the normal gill architecture, reducing the surface area of the respiratory epithelium.

These pathological changes severely impair respiration, leading to hypoxia, metabolic disorders, and ultimately reduced growth, increased susceptibility to secondary infections, and mortality. Consequently, infected fish often exhibit stunted growth due to impaired respiration and reduced nutrient absorption.

The stress of the parasite and the damage to the gills make infected fish more vulnerable to secondary infections by bacteria, viruses, and other parasites.

In severe cases, Centrocestus formosanus infection can lead to high mortality rates, especially in younger fish or those experiencing other stressors.

Fish struggle to breathe in a polluted pond as encapsulations damage their gills, degrading respiratory function.

Effective control of Centrocestus spp. infection in Tilapia requires an integrated health management approach that targets multiple stages of the parasite's life cycle and addresses the underlying risk factors.

As such, Tilapia producers implement several strategies in order to prevent and treat   Centrocestus spp.  infections in their operations:

• Direct treatment of the Infected fish with anti – parasitic medications:

• Eradication of Mollusks (1’st intermediate host) by use of chemicals.

• Eradication of Mollusks (1’st intermediate host) by use of Black Carp. (Biological control)

• Mechanical filtration of water carrying Parasite larvae and/or snails:

• Wide – spread use of nets for bird prevention.

Oral treatment of infected fish with specific anti-parasitic medications (such as praziquantel) can be effective in reducing the level of metacercarial infections in Tilapia.

Although the exact mode of action of these medications is not exactly known at present, there is experimental evidence that shows that praziquantel like medications increase the permeability of the   parasite membrane cells to calcium ions.  The drug thereby induces contraction of parasites, resulting in paralysis in the contracted state. The dying parasites are then destroyed by the host immune reaction through phagocytosis.

Treatment with anti - metacercarial medications are relatively expensive and will obviously not prevent concurrent infection and migration of the cercariae.

As such, this mode of treatment is restricted to valuable broodfish and/or high value ornamental fish in well isolated production systems where there is no risk of  re-infection of the parasite.

Another widely – used strategy involves the use of chemical Molluscicides.

Molluscicides are specialized chemical substances designed to control mollusk populations, such as snails, which can be pests in various settings, including   aquaculture.

Molluscicides disrupt the biological processes of mollusks, often by affecting their water balance or respiratory functions, ultimately leading to their elimination.

In aquaculture, molluscicides are employed to manage snail populations that can harbor parasites harmful to fish. However, their use requires careful consideration due to potential impacts on water quality and non-target organisms.

Copper sulfate is commonly used as a molluscicide in fishponds. However, the effectiveness of copper sulfate for reducing snail densities is temperature dependent and its toxicity to fish is a function of a complex combination of proper pH, total alkalinity, and total hardness of the water.

Copper sulfate, shown in powder form here, is effective in reducing snail populations but poses a risk to fish.

Copper sulfate concentrations needed to facilitate elimination or severe reductions of a snail population are lethal to fish when applied to treat the entire pond.

As such, Copper products (copper sulfate and copper carbonates or chelates) can be used effectively to control mollusks in open water systems prior to stocking of fish at a dose range of 25Kg. /1000  Square meters.

Effective control with copper sulfate will usually require multiple treatments (2 – 3 consecutive treatments) several times per year, often resulting  in delayed toxicity to fish and algae population.

Treatment of pond before and/or after the production cycle will not prevent the build – up of the snail populations during the growth period.

An alternative substance for  chemical eradication of snails includes the use of Saponins.

Saponins seem to hold the greatest promise for chemical control of snail populations due to their advantage of being more readily available, less expensive  and less polluting than the synthetic molluscicides.

Saponins will degrade in the aquatic environment to non-toxic compounds within 10 days, and may also allow treatment in the presence of the fish. (Depending on the dose)

Black carp (Mylopharyngodon piceus)  is widely used for snail control in water reservoirs since the early 70’s.

Black carp (Mylopharyngodon piceus) are native to East Asia and are known for their unique diet, primarily consisting of snails and other mollusks. This feeding behavior has led to their use as a biological control agent for snails in aquaculture. Snails serve as intermediate hosts for various parasites that can infect fish, and by reducing snail populations, black carp can help disrupt the life cycle of these parasites. However, the effectiveness of black carp for mollusk control in fish ponds can vary depending on factors such as the size and density of the black carp population, the availability of alternative food sources, and the specific snail species present. In some cases, black carp may show a preference for supplemental feed over snails, limiting their effectiveness in controlling snail populations.

Bird control is also important, as preventing bird predation and reducing contamination from bird droppings can help minimize the introduction and spread of Centrocestus spp.

Measures like nets and bird scarers can deter birds from aquaculture facilities.

Filtration is another useful tool, as segregating production units using mesh or disc filters can help prevent the spread of parasites within the farm.

Advanced disc filters in operation, ensuring the segregation and isolation of production units to maintain biosecurity and prevent the spread of Centrocestus spp. and other pathogens.

Segregation and Isolation of specific production units through Filtration – Disc Filters

Implementing strict biosecurity measures is essential for preventing the introduction and spread of Centrocestus spp. and other pathogens. These measures include quarantine for new fish, disinfection of equipment and facilities, and control of water sources.

A heron captures a Tilapia, highlighting the importance of balance in fish farming practices for sustainable operations.

Maintaining good water quality is crucial for reducing stress on fish and making them less susceptible to infections. Regular monitoring of water quality parameters is essential. Early detection of Centrocestus spp. infection can be achieved through regular monitoring of fish health and gill condition. Microscopic examination of gill biopsies can confirm the presence of the parasite.

Providing a balanced and nutritious diet can help strengthen the immune system of fish and minimizing stress factors can improve fish health and reduce their susceptibility to infections.

In conclusion, Centrocestus spp. infection is a significant threat to Tilapia production, but its impact can be minimized through an integrated and well planned health management approach.

By understanding the parasite's life cycle and implementing a combination of control strategies, Tilapia farmers can protect their fish stocks and ensure the sustainability of their operations.

Regular monitoring, early detection, and proactive management are key to success.

Prevention is always better than cure and investing in fish health management is an investment in the future of Tilapia culture.