Epizootic Ulcerative Fish Disease Syndrome

291 Pages·2015·15.562 MB·English

by  Kar, Devashish

Epizootic Ulcerative Fish
Disease Syndrome
Devashish Kar
Department of Life Science
Assam (Central) University
Silchar, India
AMSTERDAM lBOSTON lHEIDELBERGlLONDON lNEWYORK lOXFORD lPARIS
SANDIEGO lSANFRANCISCO lSINGAPORE lSYDNEY lTOKYO
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Dedication
Dedicated to the memory of a person who was a highly talented,
generous soul, but had left this mundane world prematurely
without seeing the fruit of my humble efforts. He is the late
Devapriya Kar, FCA, LLB, my respected elder brother.
Also dedicated to the loving memory of my respected
parents, teachers,
and to my beloved students and fishermen.
About the Author
MPEDA. These events have been attended by preeminent
scientific personalities, notably Prof. Asis Datta, Prof.
Samir Bhattachryya, Dr K.C. Jayaram, and others. Dr Kar
has presented papers and chaired scientific sessions at a
largenumberofnationalandinternationalsymposiabothin
Indiaandabroad,notablytheGordonResearchConference
intheUnitedStates;2ndInternationalSymposiumonGIS/
Spatial Analysis in Fisheries and Aquatic Sciences in
England during 2002; Lake Symposium at IISc, Bangalore
(2000, 2002, 2010, 2012, and 2014, including chairing
sessions); and Indian Science Congress (2012 and 2013,
including chairing sessions), to name a few. He also has
published in more than 190 national and international
Dr Devashish Kar is a pioneer and preeminent researcher journals.Ofparticularnote,16researchscholarshavebeen
in India in the fields of wetlands, rivers, fisheries, and awarded MPhil and PhD degrees under his supervision.
aquaculture. He completed his master’s program at the Professor Kar has authored 34 books (14 books single-
University of Gauhati with specializations in Fishery authored by him), including one published by Springer
Science and Aquaculture. He was awarded a PhD by the (London) and one in press with Elsevier (USA). Also, the
UniversityofGauhatiforhisoutstandingworkinthe“beel” book Community Based Fisheries Management is in press
(wetlands) fisheries of Assam. On a prestigious British with Apple Academic Press (USA).
Council Study Fellowship Award, Dr Kar was in King’s As President of Conservation Forum, Dr Kar has made
College London for its nine-month Advanced Training a profound contribution to the Society in Environment-
in Science Education program. Dr Kar was awarded the related works in collaboration with Prof. Madhav
prestigious Biotechnology National Associateship by the GadgiloftheIndianInstituteofScience(Retd.),Bangalore.
Indian government’s Department of Biotechnology (DBT) In addition to being Editor of the Conservation Forum
for his pioneering research in the field of fish disease, Journal, Dr Kar is a Scientific Fellow of the Zoological
particularly in tackling the dreadful epizootic ulcerative Society of London and a Fellow of the Linnean Society of
syndrome (EUS) fish disease in India and for conducting London;FellowoftheZoologicalSociety,Calcutta;Fellow
further research in defining EUS in collaboration with the of theAppliedZoologists Research Association;Fellow of
National Institute of Virology, Pune. As convener, Dr Kar the Society of Environmental Biologists; Fellow of the
has organized a number of national and international Inland Fisheries Society of India; and others.
symposiaandworkshopsinthefieldsofwetlands,fisheries, At the moment, Dr Devashish Kar is seniormost
and aquaculture, including ornamental fishes, in collabora- Professor and the Dean of the School of Life Sciences in
tion with DBT, DST, CSIR, ICAR, MOEF, UGC, and Assam (Central) University at Silchar, India.
xiii
Foreword
EUS, is causing severe mortality among our fishes. This is
an exotic disease that entered our country in July 1988.
Sincethen,manyfisheryenterpriseshavebeenclosed.Itis
inthiscontextthatthestudyofProfessorDevashishKarof
the Assam (Central) University at Silchar assumes great
importance. The present book on epizootic ulcerative fish
disease syndrome, which is being published by Elsevier
Inc., USA, is a timely contribution. It can help to revive
many fishery enterprises.
I therefore hope that the book will be widely read, and
usedtocontrolEUSontheonehandandrevivecaptureand
culturefisheriesontheother.Ifthisisdone,itwillbeavery
important contribution to strengthening nutrition security
among children, women, and men.
Thereisnowincreasingemphasisonnutritionsecurity,
which covers the qualitative aspects of food. It is in this Prof. M.S. Swaminathan
contextthatfishesoccupyanimportantpositioninthefood Founder Chairman, M.S. Swaminathan
chain. Like all living organisms, fishes are also subject to Research Foundation
infection by diseases. Epizootic ulcerative syndrome, or Chennai, India
xv
Preface
‘Water’ is ‘life’ and must be protected and conserved. sub-set of the environment could be variable; and, disease
Having its origin in water, life has evolved itself into an outbreak(e.g.,EUS,inthiscase)mayoccur,onlyifthereis
enthralling world of coveted and bewilderingly diverse sufficient relationship among them. Although a filterable
flora and fauna. The dependence of the living world, biological infectious agent is thought to be the primary
notably humankind, on the biological wealth of rivers, causeofEUSoutbreak,itisgenerallyacceptedthat,certain
lakes, seas and oceans, cannot be overemphasized and, abioticfactorswhichresultinsub-lethalstressofthefish,are
possibly,doesnotneedanyelucidation.Therefore,thereis, also important in initiation of disease outbreaks. Snieszko
perhaps, a requirement to broaden and deepen our com- cited temperature, eutrophication, sewage, metabolic prod-
prehension about the aquatic ecosystem with regard to its uctsoffishes,industrialpollutionandpesticidesaspotential
physical,chemicalandbiologicalfeaturesandinteractions. sourcesofstressfulenvironmentalconditions.
The water, as an ecosystem, performs a number of sig- Lifeanddiseaseprocessesoffishesaresimilarinmany
nificant environmental functions, notably, re-cycling of ways to those of other vertebrates, in that, most animals
nutrients, re-charging of ground water, augmentation and have muscles, skeletons, skins and internal organs which
maintenance of stream flow and recreation of people, to function in approximately similar ways. However, there is
exemplify a few. one major difference: ‘fish live in water’. As such, all of
Notwithstanding the above, fresh water is one of the their physiological structures and functions are influenced
mostimportantnaturalresources crucialforthesurvivalof bythisfact.Thus,tounderstandwhatwatermeanstofishis
all living beings. It is even more important for humans as to lay a foundation for a more complete understanding of
they depend on it also for food production, industrial what fish need for health and how disease processes are
growth, hydropower generation and waste disposal as well related to physiological and environmental requirements.
as for cultural requirements. Limnology is the science In order to understand the influence of environmental
which deals with the freshwater environments, their phys- factors on fish disease, one has to realize that, in aquatic
ico-chemical characteristics, their biota and the ecosystem environment,thereisamuchgreaterchemicalandphysical
processes therein. Limnology is, therefore, universal in its variabilitythanexistsintheterrestrialenvironment.Invery
significance. large bodies of water, such as oceans, conditions may be
Fishes are significant living components of both lotic relatively stable; but, in the coastal and estuarine areas,
and lentic systems. They constitute almost half the total there is a greater magnitude of environmental change(s).
number of vertebrates in the world. Of the c 39,900 ver- In small bodies of water, this variability is even greater;
tebrate species known to exist so far in the world, 21,723 while,infishhatcheryoperations,manyman-madestresses
are living species of fishes; of which, c 8411 are of fresh- are added. Therefore, in the aquatic environment, life goes
waterandtherest,11,650aremarinespecies.IntheIndian on under dynamic and unstable circumstances and fishes
region alone, of the c 2500 species, 930 are freshwater must continually adapt to changes in population density
inhabitants and 1570 are marine. pressure, temperature, dissolved gases, light, pH, etc.
A living body is prone to suffer from a disease being However, it may be noted here that, the effects of these
attackedbypathogen(s)orparasite(s).Thisistruealsowith parameters could be far more severe than those ever faced
a fish body. Often, a disease may be so virulent, that it by the terrestrial animals. Such environmental changes
could sweep unabated in an epidemic dimension. In this could impose stresses of considerable magnitude on the
connection, it may be said that, patterns and long-term somewhat limited homeostatic mechanisms of fishes.
trends are important when deciding whether or not an Epizootic Ulcerative Syndrome or EUS is a hitherto
epidemic exists in the present period and in predicting unknown, dreadful, virulent and enigmatic disease among
future epidemics. the freshwater fishes which had been sweeping the water
Snieszko (1974) had stated that, an overt infectious bodies in an epidemic dimension almost semi-globally
disease could occur when a susceptible host is exposed to causing large-scale mortality among the freshwater fishes;
a virulent pathogen under stress. The influence of each thus,renderingmanyofthemendangeredandthrowingthe
xvii
xviii Preface
life of the fishermen out of gear through loss of avocation on this challenging fish disease problem (sacrificing his
andcausingdifficultiestothefisheatersthroughscarcityof remuneration leave, and with the help of many generous
fishfleshasaprotein-richsourceofnutrition.Thisdreaded souls), as part of discharging his social responsibility, at
fish disease, thus, has been a major concern in several thiscrucialjunctureoftheSocietyandthePeople.InIndia,
countriesoftheworld,particularly,theAsia-Pacificregion. theoutbreakofthevirulentEUSfishdiseasehadbeenfirst
As a historical resume, in Queensland, Australia, an encountered during July, 1988 among the fishes in the
epizootic of marine and estuarine fishes, characterized by FreshwaterbodiesofNorthEast(NE)India.In1989,Nepal
shallow hemorrhagic ulcers, had occurred in 1972 with had also been affected by EUS (Das and Das, 1993;
recurrence(s) in subsequent years. The disease had been Kar, 2007, 2013).
named as ‘red spot disease’. A similar type of disease, EUS had been a hitherto unknown enigmatic virulent,
characterized by dermal ulcers, had been reported from epidemicamongthefishessinceabout4decades.Ithadbeen
Papua New Guinea: (a) from theriversofthe south during designatedbyvariousnames,someofwhichareoftencol-
1975e76; and, (b) from the north during 1982e88. Con- loquial. Egusa and Masuda (1971) had, perhaps, at first,
comitantly, Indonesia had also reported similar type of describeditinJapanasAphnomycesinfection.Theinfection
disease in Bogor during 1980, which, subsequently, had had also, possibly, occurred among other fishes, being
spread to West, Central and East Java. This disease was named as Mycotic Granulomatesis (MG). Such a nomen-
named as infectious dropsy or ‘hemorrhagic septicemia’. clature was mainly based on histopathological (HP) results
Later, the disease was reported from Malaysia during (Miyazaki and Eugusa, 1972). Concomitantly, a hitherto
1981e83. The affected fishes had red or necrotic areas of unknown EUS condition had been encountered, since
ulceration all over their bodies and were called ‘Webak 1972, mainly on the skin of estuarine fishes in Australia.
Kudes’. The disease was, subsequently, reported from Such an element had been named as Red Spot Disease
fishingareasofKampuchea,duringearly1984,alongwith (RSD) (McKenzie and Hall, 1976). Subsequently, almost
a significant decrease in the natural fish stock. Also, a similarconditionshadoccurredamongthefreshwaterfishes
similartypeofdiseasehadbeenreportedfromthesouthern throughout South-East (SE) and South Asia since the late
and central parts of Lao PDR during 1984. Myanmar had 70’s. The main symptoms included dermal ulcerations
experiencedtheoutbreakofEUSduring1984e85affecting andtherehadbeenlargescalemortalitiesamongthefishes.
bothwildandculturedfishstocks.InThailand,thedisease The condition was designated as ‘Epizootic Ulcerative
epizootic was first reported in 1980 in the natural water Syndrome’ (EUS) at the consultation of Experts on Ulcer-
systems and the disease had recurred somewhat regularly ative Fish Diseases held at Bangkok during 1986 (FAO,
almost every year from 1980 to 1985 in different water 1986). In addition to the above, almost similar ulcerative
bodies.InSriLanka,thediseasewasfirstreported in1988 condition had also been reported among the estuarine
in the Kelani river, Dandugan Oya, and in the nearby fishes in the east coast of USA since around 1978. Such a
streamscausingextensivefishmortality.InBangladesh,the condition was termed as Ulcerative Mycosis (UM) (Noga
firstoutbreakofEUShadoccurredduringFebruary-March andDykstra,1986).
1988intheriversMeghna,PadmaandJamuna;aswellas, Lack of a comprehensive treatise on ‘Epizootic Ulcer-
in the adjoining water bodies with colossal loss of the ative Fish Disease Syndrome’ on a global scenario, has
commercial fish stock. After sweeping semi-globally in prompted this humble piece of work and the author will
epidemic dimension, this hitherto unknown enigmatic and consider himself amply rewarded if this humble piece of
virulent fish disease entered India, during July 1988, work proves to be useful to those for whom it is meant.
through Barak valley region of Assam and started causing
large-scale mortality among the FW fishes. It had caused Devashish Kar
panicamongthefishconsumersandhadthrownthelifeof Professor, Department of Life Science
thepoorfishermenoutofgearandrenderingthemstarving School of Life Sciences
fordaystogetherinviewofnosaleoffish.And,theauthor Assam (Central) University
of the present Treatise had decided to start researching Silchar, Assam, India
Acknowledgments
“Today’spansyistomorrow’shawthorn.”Withthefervent Dyutiman Kar (son) had not been with me like shadows. I
prayer that this humble piece of work will benefit the must say here that I am fortunate to have the intellectual
people of this land, the undersigned offers his heartfelt stimulation,companionship,andassociationwithabandof
gratitude to the numerous benevolent souls who never excellent,intelligent,andobedientstudentswhohavebeen
grudged him any help in his strenuous and hazardous with me at different stages of preparation of this humble
fieldwork spread overmorethan 40years. The village folk piece of work. In particular, Ratnabir, Romen Singh, and
who kept a vigil with him on many a stormy night can Binkuhelpedinsometypingwork.Sulata,Papia,andUma
neither be named nor their help fully acknowledged. also helped me much in typing the arrangement of refer-
May the undersigned utilize the privilege of this ences. But it is Biplab Das who has been with me like a
opportunity to express his profound regards and dutiful shadow and helped me at almost every step since I started
reverence to his respected Late Parents, Dr Himangshu writing the manuscript. Biplab helped me substantially in
Jyoti Kar (Father) and Mrs Sunity Kar (Mother), without arrangingthetextandnumberingtheillustrations,andalso
whose blessings this humble piece of work would not helped me much in computational work. My very dear
possibly have seen the light of day. student, Mr Satyajit Das, enormously helped in arranging
The undersigned utilizes the privilege of this opportu- thechaptersandreferences.Theundersignedalsowishesto
nity to pay his very humble profound regards to Professor acknowledge the help and cooperation received from his
M.S.Swaminathan,FRS,Ex-DirectorGeneraloftheIndian esteemed colleagues, friends, well-wishers, and all those
Council of Agricultural Research, for his very kind benevolentsoulsconnectedwiththishumblepieceofwork.
Foreword. Particularly, I would like to mention Mr Govinda Datta,
Theundersignedalsorecordshisdeepsenseofgratitude Proprietor of Jayanti Press, Silchar, who not only gave me
and profound regards to his respected teachers, notably encouragements and blessings as my brother-in-law, but
Dr Subhas Chandra Dey, Retired Professor of Gauhati alsoguidedmeinmanyaspectsofpreparationofthispiece
University, Dr Naresh Chandra Datta, Retired Professor of of work. Further, contemporary literature has been con-
Calcutta University, and Dr Martin Monk and Dr R.G. sulted with gratitude.
Bailey of King’s College London for their constant The author utilizes this opportunity to express his deep
encouragement, guidance, and blessings. sense of gratitude to Elsevier Inc. for acting as a pillar of
The undersigned also expresses his deep sense of logistic support, for its excellent manuscript delivery
appreciation and gratitude to his family members, notably system, and for its kind efforts in the publication of this
Professor Mrs Radha Rani Dev, Late Mr Devapriya Kar, humble piece of work.
Mr Devajyoti Kar, and Dr Deva Prasad Kar for their help
andencouragement.Lastbutnotleast,thishumblepieceof Devashish Kar
work would perhaps have been nipped in the bud if my Silchar
wife, Doctor and Professor Mrs Swarupa Kar, and my June 1, 2015
children, Miss Devarati Kar (daughter) and Master
xix
Chapter 1
Introduction
1.1 ABOUT FISH characterizationandgenebankingisastepforwardtoward
further confirmation of the species at the molecular level.
“Fishes” are primarily adapted cold-blooded aquatic ver-
tebrates with paired fins for locomotion and gills for
1.2.1 An Account of the Ichthyospecies
respiration.
Zoogeographically, FW fishes have been classified
1.2 ABOUT FISH DIVERSITY differently by different workers. Although the classifica-
tion made by Myers (1949) has proved to be the most
Fish constitutes almost half of the total number of verte- useful and widely accepted one, FW fishes of marine
brates in the world. They live in almost all conceivable origin had been further classified as “peripheral FW
aquatic habitats. Approximately 21,723 living species of forms” by Nichols (1928) and Darlington (1957), which
fishes have been recorded out of 39,900 species of verte- has also been accepted by many recent fish geographers.
brates(Jayaram,1999).Ofthese,8411arefreshwater(FW)
Incidentally, the ichthyofaunas of this region have by and
and11,650aremarine (Jayaram,1981).Indiaisoneofthe
large been found to belong to the following categories
megabiodiverse countries in the world and occupies ninth
(Kar, 1990):
positionintermsofFWmegabiodiversity(Mittermeierand
Mittermeier, 1997). In India, there are about 2500 species
1.2.1.1 Primary Freshwater Fishes
offishes(Jayaram,2003),ofwhichc930liveinFWandc
1570 are marine (Kar, 2003). The bewildering ichthyodi- Genera-wise, the breakup of species under this group in-
versity of this region has been attracting many ichthyolo- cludes, among others, Notopterus, Chitala, Labeo, Cir-
gists from both India and abroad. Concomitantly, the rhinus, Catla, Cyprinus, Puntius, Rasbora, Aspidoparia,
Northeastern region of India has been identified as a Amblypharyngodon, Barilius, Danio, Esomus, Salmo-
“Hotspot” of biodiversity by the World Conservation phasia, Botia, Lepidocephalichthys, Acanthocobotis, Rita,
Monitoring Centre (WCMC, 1998). The rich diversity of Mystus, Wallago, Ompok, Ailia, Eutropiichthys, Clupi-
this region could be assigned to certain causes, most soma, Pangasius, Gagata, Glyptothorax, Clarias, Hetero-
notably the geomorphology and tectonics of this zone pneustes, Badis, Nandus, Anabas, Trichogaster,
(Jayaram, 2010). The hills and the undulating valleys of Mastacembelus, Macrognathus, and Tetraodon.
this area give rise to a large number of torrential hill
streams,whichleadtobigriversandfinallybecomepartof 1.2.1.2 Peripheral Freshwater Fishes
the GangaeBrahmaputraeBarakeChindwineKolodynee
Genera-wise,thebreakupunderthisgroupincludes,among
GomatieMeghna system (Kar, 2000, 2013).
others, Gudusia, Tenualosa, Pisodonophis, Chanda,
Based on IUCN categories, the CAMP Workshop
Xenentodon, Aplocheilus, Monopterus, Sicamugil, Rhino-
(CAMP, 1998) for FW fishes has identified certain fish
mugil, and Glossogobius.
species that have attained threatened/endangered status. At
In addition to the above, on the basis of Indian and
the same time, there has been little study with regard to
extra-Indian fish distribution (Motwani et al., 1962), the
details of endemism and species richness in Northeast
ichthyospecies of this region could be significantly incor-
India. As such, a detailed study related to Germplasm
porated under the following two groups:
inventory, evaluation, and gene banking, along with the
healthanddiseaseofFWfishes, wouldnotonly helpusto
landataconcretedecisionontheabove-mentionedaspects, Widely Distributed Species
but also contribute to fulfilling India’s obligation to the Genera-wise,thebreakupunderthisgroupincludes,among
Convention onBiological Diversity, with special emphasis others, Esomus, Puntius, Rasbora, Ompok, Wallago,
on Articles 6 and 8 (UNEP, 1992). Further, development Clarias, Xenentodon, Channa, Glossogobius, Anabas,
of the database of biological parameters is a prerequisite Macrognathus, and Mastacembelus. Incidentally, these
for preparation of a detailed fish inventory. Genetic ichthyospecies, in addition to being from this region, also
1
EpizooticUlcerativeFishDiseaseSyndrome.http://dx.doi.org/10.1016/B978-0-12-802504-8.00001-8
Copyright©2016ElsevierInc.Allrightsreserved.
2 EpizooticUlcerativeFishDiseaseSyndrome
occur in many other parts of India, Pakistan, Bangladesh, marine and brackish waters. Under suitable environmental
Sri Lanka, and Malaya. conditions, they grow to considerable cell densities, called
“blooms”or“tides”;duringorafterblooms,toxinsmaybe
Species of Northern India produced that are lethal to fish.
Species under this group include, among others, Mega- The anatomy and physiology of fish are modified prin-
rasbora elanga, Botia dario, Lepidocephalichthys guntea, cipallytowardthetwomajorecologicalfactorsthatcontrol
Glyptothorax telchitta, Parambassis baculis, Rhinomugil their existence, viz., the aquatic environment and the poi-
corsula, Sicamugil cascasia, and Tetraodon cutcutia. kilotherm’sinabilitytocontrolitstemperature.Thesefactors
In addition to the foregoing analyses, ecomorphologi- areofoverridingsignificanceindictatingthechainofevents
cally (Dey, 1973) the fishes of this region could further be following any pathological attack such as microbial infec-
categorized into four distinct groups as follows: tion,traumaticdamage,ornutritionaldeficiency.
Infishdisease,morereadilythanindiseaseofanyother
True Hill-Stream or Rheophilic Forms Fishes with farmed or wild species, it is possible to recognize the sig-
strong body musculature and adapted to torrential abodes: nificanceof“stress”factors.Theword“stress”hasdifferent
Garra, Psilorhynchus, Balitora, Glyptothorax, etc. meanings for different groups of workers. As originally
defined by Selye (1950), it was the sum of all the physio-
Semitorrential Forms Fishes with less body modifica- logical responses by which an animal tries to maintain or
tions compared with rheophilic forms: Botia, Lep- reestablishanormalmetabolisminthefaceofaphysicalor
idocephalichthys, Acanthocobitis, Schistura, Canthophrys, chemical force. Brett (1958) gave a definition that is
Gagata, etc. correlatedmorereadilywiththefishdiseasesituationwhen
he suggested that “stress” is a stage produced by environ-
Migratory Forms Well-built fish having the power of mental or other factors that extends the adaptive responses
overcoming adverse ecological conditions: Tenualosa, of an animal beyond the normal range, or that disturbs
Barilius, Channa, Badis, etc. normal functioning to such an extent that chances of sur-
vival are significantly reduced. The changes that occur in
Plain Water Forms Fishes having minimum body modi- response to environmental stress are termed general adap-
fications and insignificant migratory habits: Pisodonophis, tation syndrome (GAS). They are said to be nonspecific
Gudusia, Notopterus, Chitala, Amblypharyngodon, Aspi- physiological and biochemical changes that take place in
doparia, Catla, Cirrhinus, Cyprinus, Danio, Esomus, three phases:
Labeo, Puntius, Rasbora, Salmophasia, Mystus, Sperata,
1. The alarm reaction
Ompok,Wallago,Rita,Clupisoma,Eutropiichthys,Silonia,
2. The stage of resistance, during which adaptation to
Pangasius, Clarias, Heteropneustes, Chaca, Xenentodon,
achieve homeostasis under the changed circumstances
Aplocheilus, Monopterus, Chanda, Nandus, Sicamugil,
takes place
Rhinomugil, Anabas, Trichogaster, Glossogobius, Macro-
3. Thestageofexhaustion,whenadaptationhasceasedto
gnathus, Mastacembelus, and Tetraodon.
be adequate and homeostasis is not achieved
Inadditionto theabove, the specific immune systemis
1.3 ABOUT FISH DISEASE
one component of the protective system of all vertebrates
The teleosts are said to inhabit an alien world, and their that enables the individual to survive and maintain its ho-
utilization by humans for food or sport has until recently meostasis in an environment that is innately hostile. The
been virtually dependent on the hunting of wild species. It property that distinguishes the immune defense system
isonlyinthelast100yearsthatintensiveculturalmethods from other defense systems is its specificity and ability to
for a few species have evolved. As such, the diseases that “remember” a particular infectious agent and thus confer
result from the intensive cultural techniques have been a subsequent resistance to an individual who has recovered
major constraint on the economic development of aqua- from an infection. Thus, a salmon that has recovered from
culture (Hatai,1994). furunculosismayberefractorytoasubsequentinfectionby
Theaquaticenvironmentencompassesawidevarietyof Aeromonas salmonicida, but this immunity is irrelevant to
parameters, virtually all of which influence the stability of subsequent infection by Mycobacterium piscium. In fish,
the homeostasis that is essential for the growth and repro- generally the thymic tissue develops embryologically from
duction of fishes. If altered beyond acceptable limits,these primordial cells associated with the epithelium of the
may predispose to or actually cause disease. pharyngealpouches.Lelemadeacomparative studyofthe
Allsurfacewatersmaycontainspeciesofwildfishthat morphology of thymus in fishes and found the organ to
couldactasreservoirsofinfectiousdisease.Further,toxin- have a variable origin and variable duration in different
producing algae are found in freshwaters in addition to species of fishes.