* By Natan Wajsbrot
Tilapia Lake Virus (TiLV) is a serious viral fish disease that affects both farmed and wild tilapia and causes high mortality in infected fish. Its fast transmission is facilitated by various factors, including the movement of infected fish, contaminated water sources, and the global trade of tilapia fingerlings.
Tilapia farming has long been fundamental to global aquaculture, providing millions with an affordable and sustainable protein source. However, the industry has faced a significant threat in recent years: Tilapia Lake Virus (TiLV).
First reported in 2014, TiLV has since spread to numerous countries, causing mass mortalities in tilapia populations and posing a considerable challenge to aquaculturists worldwide. This article explores the epidemiology, clinical signs, diagnosis, and prevention strategies concerning TiLV.
Introduction
Tilapia tilapinevirus, belonging to the family Amnoonviridae, genus Tilapinevirus (TiLV) was first officially identified and reported in 2014 at the Sea of Galilee, Israel. Investigations linked the sharp decline of wild-caught tilapia in the Sea of Galilee since 2009 to an increasing presence of TiLV in wild fish populations.
Since then, TiLV outbreaks (Figure 1) have been reported in at least 17 countries, including Ecuador, Colombia, Peru, Egypt, India, Thailand, Malaysia, and the United States.
TiLV is a serious viral fish disease that affects both farmed and wild tilapia and causes high mortality in infected fish.
Its fast transmission is facilitated by various factors, including the movement of infected fish, contaminated water sources, and the global trade of tilapia fingerlings. Its ability to infect multiple tilapia species further exacerbates the challenge of containment and eradication efforts.
Transmission and Risk Factors
TiLV is transmitted both horizontally (among individuals of the same generation) and vertically (from broodstock to offspring). Under experimental conditions, the virus was detected in feces and contaminated water after a successful intragastric infection, suggesting an oral-fecal route of transmission. The virus can thus spread horizontally among conspecifics inhabiting the same water body.
Vertical transmission of TiLV was detected in infected broodstock and their offspring (as early as 2-day-old larvae). This means that it is technically possible to test larvae for the presence of TiLV before shipping them to farms, representing a significant opportunity to create TiLV pathogen-free larvae – a major biosecurity step.
It is also suggested that mollusks, aquatic insects, and other invertebrates are potential carriers of TiLV, though more research is needed to confirm these transmission pathways. Overall, these multiple transmission pathways make the containment of outbreaks particularly challenging.
Despite TiLV being a recently identified virus, some direct effects have already been identified as significant risk factors for its propagation, namely the presence and proximity of infected, wild, or farmed populations, and water temperatures ranging from 25°C to 31°C.
Other risk factors include any change that might affect the immune status of the fish, making them more vulnerable to the virus, either by directly altering their immunological competence or disrupting their homeostasis. The latter ones, which force the fish to energetically rebalance its physiological conditions, are referred to as immunosuppression drivers and include sub-optimal environmental parameters, increased stocking densities, and the presence of secondary bacterial and/or parasitic infection.
Clinical Signs of TiLV Infection
Identifying TiLV-infected fish is crucial for implementing timely control measures and minimizing economic losses. Clinical signs of TiLV infection vary depending on the fish’s age, size, and immune status. Common symptoms include:
1.Exophthalmia (Pop-Eye) and Unilateral or Bilateral Opacity of the Lens (Cataract): Infected fish may exhibit protruding eyes due to ocular swelling, a characteristic sign of TiLV infection (Figure 2).
2.Pale Gills: Discoloration of gills, ranging from pale toreddish-brown, is often observed in affected fish.
3.Skin Lesions: TiLV-infected fish may develop skin lesions, ulcers, or hemorrhages, particularly around the mouth and fins (Figure 3).
4.Abnormal Behavior: Affected fish may display abnormal swimming patterns, lethargy, or disorientation.
5.Reduced Feed Intake: Infected fish often show a decline in appetite, leading to reduced feed consumption and poor growth performance.
Early detection of these clinical signs is crucial for implementing quarantine measures and preventing the spread of TiLV within aquaculture facilities.
Diagnosis of TiLV
Accurate diagnosis is essential for effectively managing TiLV outbreaks and preventing further dissemination. Various diagnostic methods are employed to confirm TiLV infection, including:
1.Molecular Techniques: Polymerase Chain Reaction (PCR) assays targeting specific regions of the TiLV genome are commonly used for rapid and sensitive detection of the virus in fish tissues or water samples.
2.Virus Isolation: TiLV isolation can be performed using cell culture techniques, enabling the visualization and characterization of the virus under laboratory conditions.
3.Serological Tests: Enzymelinked immunosorbent Assay (ELISA) and Indirect Fluorescent Antibody Test (IFAT) are employed to detect TiLV-specific antibodies in tilapia serum, providing valuable information on the fish’s immune response to the virus.
4.Histopathology: The most common histopathological feature found in TiLV outbreaks is syncytial hepatitis of tilapia.
Combining these diagnostic methods enhances the accuracy and reliability of TiLV detection, enabling prompt intervention measures to limit its spread.
Prevention Strategies
Preventing the introduction and spread of TiLV in tilapia farms is paramount for ensuring the sustainability of the aquaculture industry. Key prevention strategies include:
1.Biosecurity Measures: Implementing strict biosecurity protocols, such as controlling the movement of fish, disinfecting equipment, and monitoring water sources, helps minimize the risk of TiLV introduction into aquaculture facilities.
2.Health Screening: Regular health monitoring and screening of tilapia populations enable early detection of TiLV and other pathogens, allowing for prompt intervention and containment.
3.Genetic Resistance: Breeding programs aimed at developing tilapia strains with increased resistance to TiLV can reduce susceptibility to infection and mitigate the impact of outbreaks.
4.Education and Training: Educating aquaculturists about the risks associated with TiLV and providing training on disease management practices promote proactive measures and rapid response to outbreaks. By adopting a multifaceted approach that integrates biosecurity, surveillance, and genetic improvement, stakeholders can effectively mitigate the impact of TiLV on tilapia farming operations.
5.Vaccination Against TiLV: A promising approach for disease management.
As the aquaculture industry grapples with the challenges posed by TiLV, the development of effective vaccination strategies emerges as a promising avenue for disease management. Vaccination offers a proactive approach to bolstering tilapia health, reducing susceptibility to TiLV infection, and minimizing economic losses associated with disease outbreaks.
Efforts to develop TiLV vaccines have gained momentum in recent years, driven by the urgent need to safeguard tilapia populations against this viral pathogen. Researchers have explored various vaccine formulations and delivery methods to elicit protective immune responses in tilapia. These include:
- Inactivated Vaccines: Inactivated TiLV vaccines contain whole or fragmented viral particles that are rendered non-infectious but immunogenic. Tilapia are vaccinated with inactivated virus preparations to stimulate the production of specific antibodies, providing immunity against subsequent TiLV exposure.
- Recombinant Vaccines: Recombinant TiLV vaccines employ genetically engineered viral proteins or antigenic fragments to induce protective immune responses in tilapia. Recombinant protein subunits or viral vectors carrying TiLV antigens are administered to tilapia, triggering the production of antibodies and cell-mediated immune responses.
- DNA Vaccines: DNA vaccines encode TiLV-specific antigens within plasmid DNA molecules, which are delivered into tilapia cells to stimulate immune responses. DNA vaccination offers advantages in terms of stability, scalability, and cost-effectiveness, making it an attractive option for TiLV vaccine development. Despite progress in TiLV vaccine development, several challenges persist, including regulatory approval, vaccine delivery logistics, and cost-effectiveness. Collaborative efforts among researchers, industry stakeholders, and regulatory agencies are crucial to overcoming these obstacles and advancing TiLV vaccine technologies.
This article is sponsored by: PHIBRO ANIMAL HEALTH CORPORATION
* Natan Wajsbrot Fish Pathologist and Health Management Expert. Phibro Animal Health Corp.
