22 Disease Management
Tilapia lake virus: Clinical signs, disease diagnosis and prevention By Raanan Ariav and Natan Wajsbrot
T
ilapia is the second most important finfish cultured worldwide. Among the secrets for tilapia’s success are its low production costs while maintaining high product quality, particularly its nutritional content. Additionally, tilapia tolerates high stocking densities and is highly resistant to diseases. Indeed, tilapia farmers were able to effectively deal with bacterial disease agents, such as Streptococcus spp. and Aeromonas spp., through different approaches, including the use of feed additives and antimicrobials, coupled with improved management and strict biosecurity strategies. However, the tilapia industry has recently witnessed the emergence of highly virulent infectious diseases such as the tilapia lake virus (TiLV) that has been affecting wild and farmed tilapia for over a decade. TiLV was first officially identified and reported in 2014 at the Sea of Galilee, Israel. After learning more about the disease’s origin and progression, the sharp decline of wild-caught tilapia in the Sea of Galilee since 2009 was linked to an increasing presence of TiLV in the wild fish populations. Awareness on this novel virus spreads rapidly and by May 2017 the Food and Agriculture Organization of the United Nations (FAO) triggered a global alarm. The FAO warned that the virus can adversely impact global food security and nutrition and advised countries importing tilapia to take appropriate risk management strategies such as diagnostic testing, health certification, and imposition of quarantine measures and contingency plans to contain the outbreaks.
Transmission and risk factors
Tilapia is the main host of TiLV but other important warm water aquaculture species can also be infected. Some fish species such as snakeskin gourami (Trichogaster pectoralis), iridescent shark (Pangasianodon hypophthalmus), walking catfish (Clarias macrocephalus), striped snakehead fish (Channa striata), climbing perch (Anabas testudineus), common carp (Cyprinus carpio), silver barb (Barbodes gonionotus), Asian sea bass (Lates calcarifer) and Indian major carp (Labeo rohita) have been found to be resistant to this emerging aquatic disease. This apparent resistance to TiLV might be due to the lack of viral receptors or other mechanisms that enable replication in the hosts. Nevertheless, other factors, such as stress, co-infection and environmental conditions also play important roles in the vulnerability to TiLV.
old larvae). This also means that it is theoretically possible to test larvae for the presence of TiLV before shipping them to farms; this represents a significant opportunity to create TiLV pathogen-free larvae – a major biosecurity step. It is also hypothesised that molluscs, aquatic insects and invertebrates are potential carriers of TiLV and thus the virus could also be transmitted through these organisms. However, more research needs to be performed to confirm transmission pathways via these other vectors. 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 and make them more vulnerable to the presence of the virus, either directly altering the fish immunological competence or disrupting their homeostasis. The latter, which forces the fish to energetically rebalance its physiological conditions, are referred to as immunosuppression drivers and include suboptimal environmental parameters, increased stocking densities and presence of secondary bacterial and/or parasitic infection in the background.
Clinical signs and diagnosis
Clinical signs of TiLV include lethargic movements and changes in fish behaviour, such as swimming near the water surface, swirling motion, reduced schooling, imbalanced movements and loss of appetite (Figure 1). Ocular and skin lesions, discolouration and abdominal swelling can also be present. However, TiLV infection is not necessarily limited to these symptoms and behavioural changes. Since these clinical signs are similar to those associated with various other tilapia diseases, the identification of the disease often relies on more specific diagnostic methods, such as histological observations or more recent molecular tools, involving polymerase chain reaction (PCR).
TiLV is transmitted both horizontally (among and between individuals of the same generation) and vertically from broodstock to offspring. Under experimental conditions, the virus was detected in faeces and contaminated water after a successful intragastric infection, suggesting an oral–faecal route of transmission. The virus can thus spread horizontally amongst conspecifics inhabiting the same water body. The vertical transmission of TiLV was detected in infected broodstock and their offspring (early 2-day March/April 2021 AQUA Culture Asia Pacific
Figure 1. Clinical signs of TiLV and additional methods to complement the diagnosis.
