This document discusses major diseases affecting Asian seabass aquaculture. It describes several viral pathogens like infectious spleen and kidney necrosis disease caused by megalocytivirus, red sea bream iridovirus, and viral nervous necrosis. It also discusses bacterial diseases caused by pathogens like Tenacibaculum maritimum, pathogenic vibrios, Photobacterium damsela, Mycobacterium spp., and Streptococcus iniae. Control measures discussed include biosecurity, general management improvements, vaccination, and antibiotics when administered via feed.

1. Major reported diseases in Asian Sea bass
and recently known control measures
practiced in the world view
By:
B. Bhaskar
Dedicated to Aqua
farmers for preventing
losses by controlling
diseases

2. Introduction
• Seabass is considered as a euryhaline and catadromous species, it inhabits in
freshwater, brackish and marine habitats including streams, lakes, estuaries and,
coastal waters. Adults migrate to the sea for gonadal maturation and subsequent
spawning. It is distributed throughout the Indo-Paci c region.
• The countries that are popular for culture of seabass are Australia, Taiwan,Thailand,
Singapore, Indonesia, Philippines, and Malaysia. In Australia, seabass is locally
called as Barramundi and considered as an important food fish.
• It can be farmed in ponds and cages from freshwater to seawater environments. The
fish can grow to 1.0 kg size in 6-8 months by consuming pellet feed. Availability of
hatchery produced seed and formulated pellet feed ensures the year-round supply
and thus enables easy adoption of seabass farming by farmers. It can also be farmed
in net cages, floating cages and Recirculatory Aquaculture System (RAS).
• Seabass is carnivorous in nature. In the wild, they consume zooplanktons in their
very early stages and when they grow to 10-20 cm length; they prey upon small
fishes and shrimps. However, when this fish is farmed in the ponds and cages, it can
be adapted to consume formulated feed.
• Rotifer, Brachionus plicatilis can be supplied as first feed to seabass larvae from
2nd day (dph) and continued up to 15th dph. Meanwhile, Artemia nauplii can also
be included as feed from 9th dph along with rotifer. From 15 to 30 dph, Artemia
nauplii can be supplied along with artificial feed from 17 dph. The fry can be
completely weaned from the artificial diet between 30-35th dph.

3. List of reported Viral pathogens in Asian
seabass

4. Infectious spleen and kidney necrosis disease (ISKND):
• Causative agent: Infectious spleen and kidney necrosis virus (ISKNV) or
called Megalocytivirus ISKNV • Enveloped icosahedral viruses (130-150 nm in
diameter) with a linear double-stranded DNA genome of 111 kb.
• Clinical signs: darkening, pale gills, ascites, enlarged spleen (splenomegaly).
• Mortality: High mortality up to 85% of Asian sea bas.
• Susceptible stage: fingerling, juveniles, and grow-out of Asia sea bas.
• Transmission: horizontal & vertical.
• Distribution: Widely distributed • Cases in Asian sea bass = Vietnam, Thailand,
China.
• Histopathology: Basophilic hypertrophied cells (megalocytes) in spleen, kidney,
liver, and gills.
• Diagnosis: PCRs, qPCR, isothermal amplification, immunological based techniques.
• Cell culture: Grunt fin-1 (GF-1) =Vacuolization Mandarin fish fry cells (MFF-1) =>
Rounding cells & detachment
• Genotyping: MCP gene
• Prevention: Biosecurity + general managements

5. Red sea breamiridovirus(RSIV)
• 80-90% mortality of grow-out open-caged Asian sea bass in the
brackish water environment in India.
• RSIV positive using DNA polymerase PCR and sequence analysis.
• more closely related to Korean isolate of RSIV.
• examinations were investigated.
• reproduced 100% mortality in the healthy sea bass.
• Fig:Infected Asian seabass showing Pale gills and enlarged spleen

6. Turbot reddish body iridovirus(TRBIV):
Tsai et al, 2020. Viruses 12: 681–699.
• L. calcarifer imported to Taiwan die
up to 90%.
• body color, petechiae of gills,
severe anemia and an enlarged
abdomen.
• well as reddish livers.
• infection of TRBIV genotype II.
• -nested PCR was developed to
improve detection sensitivity.

7. SDDV (Scale drop disease virus)
• SDDV (Scale drop disease virus) farmers also called red belly disease
• => Scale drop disease
• oLCHV (Lates calcarifer herpesvirus)
• =>SDD-like symptom
• oA unique strain of Vibrio harveyi
• => scale drop and muscle necrosis
• Causative agent:Scale drop disease virus (SDDV)
• •Enveloped hexagonal virions (100-180 nm) with double-stranded DNA genome of 131 kb (135 ORFs). & SDDVin
freshwatercultured Asian sea bass co-infected with Flavobacterium
• clinical signs: Asian sea bass: Scale loss, some with “red belly”
• Mortality: Asian sea bass: 40-50% in marine, brackish, and freshwater culture. susceptible stage- Juveniles, subadut, adult.
• Histopathology: Basophilic hypertrophied cells (megalocytes)
• Multifocal necrosis, pyknosis and karyorrhexis
• Dermal inflammation and severe infiltration of lymphocytic inflammatory cells
• Diagnosis: PCRs,qPCR, isothermal amplification
• Genotyping: Major capsid protein (MCP) and adenosine triphosphatase (ATPase) genes .
• Cell culture: Seabass kidney (SK) SK21 cells Grunt fin (GF-1) cells MFF-1 cell.
• Prevention: Biosecurity + general managements
• Ref: Gibson-Kuehet al. 2012 J Fish Dis 35:19-27 de Groof et al. 2015 PlosPathog11(8): e1005074

8. LCHV, Lates calcarifer herpesvirus
• Diseased fish with SDD-like symptom
• Singapore and Vietnam, 2015
• Mortality 30-70% (natural)
• 77% mortality in co-habitation or i.p.challenges
• Genome 130 kb

9. Scale Drop and Muscle Necrosis Disease (SDMND)
• Dong et al. Aquaculture (2017) 473:89-96

10. Viral Nervous Necrosis (VNN)
or Viral encephalopathy and retinopathy (VER) disease
• Causative agent: Betanodavirus, Nervous necrosis virus (NNV)
• Non-enveloped icosahedral virus (appx 25 nm in diameter) with two positive-strand
RNA segmentsof 3.1 and 1.4 kb
• Transmission: Horizontal & vertical
• Clinical signs: Darkening of the skin & erratic swimming. Some with pop-eye. Viral
Nervous Necrosis (VNN) is acute in larvae that are ten days old and oftenresults in 100%
mortality. The transparent larvae (due to the contraction ofchromatophores) will
display corkscrew or whirling swimming patterns and havehyper-inflated swim
bladders.
• Mortality:upto 100% in Marine water host fishes & Susceptible all stages, mainly larvae
and juveniles .
• Diagnosis: RT-PCRs,qPCR, isothermal amplification, Immunological based techniques
• Histopathology: Vacuolation in brainandretina
• Genotyping: RNA 2 segment encoding a viral capsid protein
• Cell culture: Striped snakehead cell line, SSN-1E-11, a clone of SSN-1 & others
• Prevention: Biosecurity + improvement of host immunity + water management +
reduce stress
• Ref: Ziaratiet al. Current Microbiology (2020) 77:3919–3926

11. Lates calcarifer Birnavirus(LCBV)
• ExperimentalchallengeofnaïvefishwithLCBVsho
wing“whitepatch”lesionsbutmortalitywasnotd
ifferentwiththecontrolgroup.
• Ref: Chen et al. Virology Journal (2019) 16:71

12. Red eye', an often-typical clinical sign of Iridovirus in
barramundijuveniles because of massive internal
haemorrhaging
• In barramundi the disease mainly occurs in fish of 10 to50g and
causes acute peaks of mortality of up to 80 - 90% (Picture 5). The
fish willturn black and lose appetite. On closer clinical examination
the gills will appear verypale and may bleed when handled and
blood may also leak into the iris giving a ‘redeye' appearance (
Picture 6). Internally, a pale spleen is characteristic of this disease.

13. Bacterial diseases in Asian seabass
• Bacteria Tenacibaculum maritimum (T. mar)Tenacibaculum maritimum, also described as T. mar, is a rod shaped
gram negative,filamentous bacterium. In Singapore, the team has been working with this pathogensince 2003.
There are 95 isolates from China, Indonesia, Malaysia,Singapore, Korea, Japan and Greece stored in the
laboratory.
• T. mar can be especially severe when combined with skin parasites and outbreaks ofthe disease often occur after
a stressful event. Typically, a T. mar outbreak will startas small lesions on gills and the ventral side of the fish,
resulting in chronic scale lossand spreading to other cartilaginous body parts including the face and jaw.
• pathogenic vibrios which have been isolated from seabass include Vibrio parahaemolyticus, V.
anguillarum and V. alginolyticus.
• Pasteurellosis – Photobacterium damsela.
• Gliding bacterial disease/tail rot disease (Flexibacter sp.):Ampicillin and florfenicol have been reported
to be effective when administered in feed. Tenacibaculum maritimum (formerly Flexibacter maritimus
• Mycobacteriosis
• Recent report sindicate that this disease is becoming more severe with devastating losses of nurseryfish after
stocking in pre-grow out cages (Note 2009).
• Late stage progressive ‘saddleback' skin lesions due to
T.maritimum& Isolation of the rod shaped bacterium on selective media illustrating
therusty colouration, Presumptive diagnosis on the farm is possible using a gram stained
wet mount of askin scrape from the edge of an infected lesion.
• coupled with rapid onset and transmission make therapeuticcontrol extremely difficult.
Prevention of the disease through vaccination prior toexposure is the strategy for the near
future.

14. Streptococcus iniae
• The most important bacterial species affecting the culture of
barramundi throughout the tropics is Streptococcus iniae.
Indeed, S. iniae is considered to be one of the most serious
bacterial diseases of all warm water fish.
• Typical clinical signs associated with S. iniae of exophthalmia
and internal septicaemia.
• In barramundi, the disease is systemic and characterised by
massive acute mortality peaks with cumulative mortality of
70% being very common.
• Infections can occur throughout the entire growth cycle but
are more often seen in large and harvest size fish. S. iniae is
therefore considered to be an ‘expensive' disease for farmers
who will experience a loss of production efficiency and
marketable product but more importantly a reduction in
overall feed conversion rates.

15. Norvax® Strep Si, a water-based inactivated vaccine,
registered for use against S. iniae infections in farmed fish
• Since 2005, Intervet/Schering-Plough Animal Health has Norvax® Strep Si,
a water-based inactivated vaccine, registered for use against S. iniae
infections in farmed fish.The vaccine was designed with two applications in
mind; the first of which is as animmersion vaccination in which fish are
immersed for 30 seconds at approximately2g. In the laboratory, protection
from immersion vaccination has been confirmed for 5weeks. The second
application method is by intraperitoneal injection (IP) and isconducted when
fish reach 15g. Laboratory studies indicate that when 15gbarramundi are
vaccinated using a single intraperitoneal (IP) injection with 0.1ml
ofNorvax® Strep Si, maximum protection is achieved as soon as one week
aftervaccinationStudies under field conditions after IPvaccination at 15g
have demonstrated protection up to 18 months post vaccination when fish
are >2kg.

16. Reported few parasitic diseases in Asian seabass
• ParasitesIte is almost impossible to avoid parasites because usually they are present as part
ofthe aquatic ecosystem.
• Helminthes parasites which have been found in seabass include monogenetic trematodes,
digenetic trematodes, nematodes and acanthocephala.
• Crustaceans belonging to the Branchiura, Copepoda, Isopoda and Amphipoda are
frequently found on the body surface and/or gills of caged marine fish.
• Parasitic copepods:Argulus sp.& Caligus sp. has caused big problems in cultured
seabass&Lernanthropus sp. are found attached to the gill of seabass especially in cage
cultured fish.
• Parasitic Isopods: Nerocila sp. and Gnathia sp. have also been reported in seabass.
• Health monitoring and early diagnosis is key for control ofparasitic diseases. Protozoa
(particularly Cryptocaryon irritans and Trichodina spp.)and capsalid monogeans on the
body surface (particularly Neobenedinia spp.) most commonly affect newly stocked
barramundi in open ocean cages. Current observations and reports from South East Asia
indicate that capsalid monogean infections are the most serious and pathogenic amongst all
of the parasitic diseases. If left untreated barramundi quickly develop skin and tail rot and
mortality rates of 30-40% are common. Neobenedinia most commonly affects younger fish
but if a population has been compromised in anyway (even from the nursery stage) then
they will always be susceptible. Typically, a proportion of the fish will be ‘off feed' and
lethargic. The parasite also irritates the eyes causing opacity and exophthalmia (‘pop eye')
& gradually the caudal and pectoral fins will become frayed (white appearance in the
water) and hemorrhagic when handled.

17. Dorsal fin of a barramundi illustrating
opaque Neobenedinia parasites after
immersion in freshwater
• Neobenedinia is particularly easy to spot at the farm because after the
whole fish is immersed in freshwater for some minutes, the parasites
will turn opaque
• A specific parasite prevention program with routine freshwater
immersion, skinscrapes and gill clips should be considered an integral
part of the health management protocols.

18. Rational
• In Addition to diseases and abnormalities due to
environmental contaminants and nutritional
deficiencies have been recognized as important
problems in fish culture whenever diets as well as
control or water quality become inadequate.
Malnourishment or undernourishment of seabass
under culture can result in slow growth,
susceptibility to diseases or death.
• Strategies for aquacultured Asian sea bass.
• No treatment for viral diseases in aquaculture, so
‘Prevention is better than cure’.
• Vaccination and selective breeding are envisioned as
future disease management