Diagnosis and Control of Diseases of Fish and Shellfish

Brian Austin, Aweeda Newaj-Fyzul, Brian Austin, Aweeda Newaj-Fyzul

• Publisher: Wiley
• Year: 2017
• ISBN:
  9781119152132
• Edition: 1
• Topic: Tecnologia e ingegneria
• Subtopic: Industria ittica e acquacoltura

There has been a continual expansion in aquaculture, such that total production is fast approaching that of wild-caught fisheries. Yet the expansion is marred by continued problems of disease. New pathogens emerge, and others become associated with new conditions. Some of these pathogens become well established, and develop into major killers of aquatic species.

Diagnosis and Control of Diseases of Fish and Shellfish focuses on the diagnosis and control of diseases of fish and shellfish, notably those affecting aquaculture. Divided into 12 chapters, the book discusses the range of bacterial, viral and parasitic pathogens, their trends, emerging problems, and the relative significance to aquaculture. Developments in diagnostics and disease management, including the widespread use of serological and molecular methods, are presented. Application/dose and mode of action of prebiotics, probiotics and medicinal plant products used to control disease are examined, as well as the management and hygiene precautions that can be taken to prevent/control the spread of disease.

This book will be a valuable resource for researchers, students, diagnosticians, veterinarians, fish pathologists and microbiologists concerned with the management of diseases of fish and shellfish.

Chapter 1
Introduction

Brian Austin¹ and Aweeda Newaj-Fyzul²

¹Institute of Aquaculture, University of Stirling, Stirling, Scotland, UK

²University of the West Indies, Trinidad and Agriquatics, Chaguanas, Trinidad

There is confusion over the meaning of the term ‘disease’. A definition from an article in the British Medical Journal is as follows:

… a disease is the sum of the abnormal phenomena displayed by a group of living organisms in association with a specified common characteristic or set of characteristics by which they differ from the norm of their species in such a way as to place them at a biological disadvantage. (Campbell et al., 1979)

According to these authors, a disease is something that occurs to a group of organisms rather than to an individual. Also, the definition is far reaching and reflects the complex relationship between the disease-causing situation (not necessarily a micro-organism) and the host. However, there is more to disease than the interaction of a pathogen (dictionary definition is of a disease-causing organism) and the host. According to Kinne (1980), who was writing about diseases of marine animals, diseases could be caused by genetic disorders, nutritional imbalance, pathogens, physical injury and pollution. Thus, disease could be attributed to biological (biotic) as well as non-biological (abiotic) causes. Kinne (1980) described diseases in terms of epidemiology (epizootiology for animal diseases), as follows.

• Sporadic diseases, which occur sporadically in comparatively small numbers of individuals in a population.
• Epidemics/Epizootics, which are large-scale outbreaks of communicable disease occurring temporarily in limited areas.
• Pandemics/Panzootics, which are large-scale outbreaks of communicable disease occurring over large areas.
• Endemics/Enzootics, which are diseases persisting or reoccurring as low-level outbreaks in defined areas.

The interest in diseases of aquatic organisms is primarily directed towards aquaculture which, to paraphrase definitions, is the rearing of aquatic species in controlled conditions. Here, disease may be of sudden onset with rapid progression to high mortalities, with an equally quick decline (acute disease). Conversely, there may be cases where the disease develops more slowly, with less severity but longer persistence (chronic disease).

It is apparent that as society moves through the twenty-first century, aquatic animals continue to suffer the vagaries of disease, especially as new diseases continue to occur, e.g. acute hepatopancreatic necrosis disease syndrome (AHPND), which is attributed to infection of the shrimp with Vibrio parahaemolyticus. However, the study of aquatic pathobiology is largely an aerobic affair as most laboratories do not consider the possible role of anaerobes or microaerophiles. It would be interesting to determine if this reflects a lack of expertise, interest or suitable methods rather than the absence of occurrence of anaerobic or microaerophilic pathogens/parasites. The majority of literature points to a single species of pathogen as the main cause of disease situations but there are reports of sequential viral and bacterial as well as parasitic and opportunistic bacterial combinations. Certainly, it is appreciated that there have been important developments in disease diagnosis, with progression from traditional phenotyping to the use of newly developed molecular techniques. In many modern laboratories, identification is now routinely accomplished by sequencing of the 16S rRNA gene, a move that has led to greater confidence in the outputs although this will reflect the accuracy of the data in the databases.

However, whereas the use of new technologies is to be encouraged, an ongoing dilemma remains about the authenticity of isolates studied. Also, many studies are based on the examination of single isolates, the relevance of which to fish pathology or science in general is not always clear. Certainly, too many conclusions result from the examination of too few isolates. Nevertheless, the study of pathogenicity mechanisms, diagnostics and disease control by means of vaccines has benefited from molecular approaches. Yet it is appreciated that many laboratories still rely on conventional methods for achieving disease diagnoses, and it is unlikely that this situation will change in the foreseeable future. However, it is pertinent to enquire what diagnosis is supposed to achieve. If the underlying aim is to underpin efforts at disease control, then it is unclear whether detailed and time-consuming work resulting in identifying a pathogen to species level would necessarily help disease control strategies.

The use of pathological specimens taken from advanced cases of disease would be unlikely to reveal species succession, which could occur throughout a disease cycle. Also, we focus on pure cultures, and are generally uncomfortable with the notion that two or more organisms could be associated with a single disease condition. Diagnostic microbiology aims to isolate the dominant organism, as a single pure culture, from pathological material. It is speculative how often the wrong organism may be chosen, as the true pathogen becomes overwhelmed by contaminants. Yet laboratory cultures are used extensively for associated studies of pathogenicity mechanism and disease control. Unfortunately, all too often, cultures lose some of their characteristics in the laboratory which may reflect loss of DNA – and therefore ensuing data need not reflect the true role of the culture with its host.

Histological examination of diseased tissue may be invaluable in recognizing cases where organisms, presumed to the pathogen, may be observed but culturing not achieved. The uncultured Candidatus have become associated with some diseases. It is unclear if such organisms are incapable of growing in vitro or if suitable media have not been developed. It is unknown how many more of these unculturable organisms remain to be recognized. Could such uncultured organisms be dormant, damaged or senescent, a concept which has been put forward for some water-borne organisms by Stevenson (1978)? Then there are situations where there is pathology, for example red mark syndrome of rainbow trout, for which the cause is uncertain but may reflect infection with a rickettsia (Metselaar et al., 2010).

It is interesting to note that Al Gore, when Vice President of the USA, suggested that diseases (of humans) would be controlled within our lifetimes. Since then, human society has suffered from the emergence of new diseases (e.g. Middle East respiratory syndrome (MERS) coronavirus) and the recurrence of others (e.g. Ebola, H1N1 and Zika). The use of medically important antibiotics in any non-medical situation, including aquaculture, is fraught with problems, of which the development and spread of resistance, and issues with tissue level, top the list of concerns. The result is that there is a deliberate move away from the use of antibiotics in many countries. Certainly, control of diseases of aquatic organisms has undergone dramatic improvements from the initial emphasis on control (therapy) to prevention (prophylaxis). Unfortunately, the dramatic progress in human vaccinology has not been reflected in the number of commercial products available to aquaculture. However, there is no shortage of ingenuity in the development of disease control strategies, as illustrated by the growing interest in non-specific immunostimulants, prebiotics, probiotics and plant products.

This text will focus on the diagnosis and control of infectious diseases of farmed aquatic animals.

Conclusion

• The list of parasitic, bacterial and viral pathogens continues to grow, although the significance of some organisms to pathology is difficult to ascertain – are they truly parasite/pathogens, secondary invaders or contaminants?
• There has been considerable improvement in the taxonomy and hence diagnosis (accuracy and sensitivity) of many pathogens, particularly involving the sequencing of the 16S rRNA gene.
• Disease control has progressed from therapeutic to prophylactic, and now involves a wide range of approaches including vaccines, non-specific immunostimulants, prebiotics, probiotics and plant products.

References

1. Campbell, E.J.M., Scadding, J.G. and Roberts, M.S. (1979) The concept of disease. British Medical Journal, 2, 757–762.
2. Kinne, O. (1980) Diseases of Marine Animals, vol 1. General Aspects, Protozoa to Gastropoda, John Wiley & Sons, Chichester.
3. Metselaar, M., Thompson, K.D., Gratacap, R.M.L., et al. (2010) Association of red-mark syndrome with a Rickettsia-like organism and its connection with strawberry disease in the USA. Journal of Fish Diseases, 33, 849–858.
4. Stevenson, L.H. (1978) A case for bacterial dormancy in aquatic systems. Microbial Ecology, 4, 127–133.