Diagnostic Tests And Epidemiological Probes For Prawn Viruses In Nigeria
1.1 Background to the Study
Viral diseases have been seriously impacting the sustainability and economic success of prawn aquaculture as they have been resulting in heavy mortalities (Carr et al, 1996). About 20 different prawn viruses have been reported, which have been implicated in mass mortalities in cultured prawn, such as White spot syndrome virus (WSSV), yellow head virus (YHV), infectious hypodermic and hematopoietic necrosis virus (IHHNV) and Taura syndrome virus (TSV). IHHNV, Penaeus monodon densovirus (PmoDNV), formerly referred to as hepatopancreatic parvovirus (HPV) and lymphoidal parvovirus (LPV) associated with parvoviral infections in prawns (Felix & Devarag, 1993). Infectious hypodermal and hematopoietic necrosis virus (IHHNV) is one of the major viral pathogens of prawn worldwide, which has resulted in severe mortalities of up to 90 % in cultured prawn from Nigeria and hence designated Penaeus stylirostris densovirus (PstDNV). Infectious hypodermal and hematopoietic necrosis virus is distributed in prawn culture facilities worldwide. It causes large economic loss to the prawn farming industry (Anon, 2009). Our knowledge about the natural reservoirs of Infectious hypodermal and hematopoietic necrosis virus is still scarce. Infectious hypodermal and hematopoietic necrosis virus (IHHNV) of prawn, one of the major viral pathogens of prawns has recently been classified as Penaeus stylirostris densovirus (PstDNV) under the family Parvoviridae and is a DNA virus.
It has been reported to cause Runt deformity syndrome (RDS) in the two species of prawn namely P. (Litopenaeus) vannamei and P. monodon which is a chronic, non-lethal disease (Kalagayan, 1991). Like all other parvoviruses, Infectious hypodermal and hematopoietic necrosis virus do not encode a DNA polymerase and depend on host cells for DNA replication and multiplication (Bell et al, 1988). Therefore, they need rapidly proliferating cells of the host for their replication. The target organs for Infectious hypodermal and hematopoietic necrosis virus infection include tissues of ectodermal (cuticular epidermis, hypodermal epithelium of the fore and hind gut, nerve cord and nerve ganglia) and mesodermal (hematopoietic organs, antennal gland, tubule epithelium, gonads, lymphoid organ, connective tissue and striated muscles) origin. The virus does not infect organ systems of endodermal origin (i.e., hepatopancreas, mid-gut epithelium, anterior mid-gut caecum or posterior midgut caecum).
Infectious hypodermal and hematopoietic necrosis virus infection has been demonstrated in all life stages of prawn species including eggs, larvae, postlarvae (PL), juveniles and adults. PL and juvenile prawns are more susceptible to Infectious hypodermal and hematopoietic necrosis virus infection than adults due to the presence of actively dividing cells (Anon, 2009). Infectious hypodermal and hematopoietic necrosis virus-infected females fail to develop embryos and also fail to hatch the eggs (Anon, 2009). This virus is highly species-specific, particularly in terms of gross manifestations. Due to the importance of prawn culture, the availability of easy and rapid methods that allow early diagnosis is essential for routine monitoring of the animal health status and to restrain further disease outbreaks (Pantoja et al, 1999). Traditionally, the diagnosis of Infectious hypodermal and hematopoietic necrosis virus infection is by examining histological sections wherein prominent Cowdry type A, eosinophilic, intranuclear inclusion bodies surrounded by marginated chromatin in hypertrophied nuclei of cells is seen in tissues of ectodermal and mesodermal origin (Carr et al, 1996). These inclusion bodies of Infectious hypodermal and hematopoietic necrosis virus can be confused with the developing inclusion bodies.
1.2 Statement of the Problem
Viral pathogen of prawns has resulted in mass mortalities in prawn culture worldwide. In addition, it also causes abnormal deformities of cuticle, abdominal segments, tail fan and rostrum, wrinkled antennal flagella, bubble-heads along with wide size variations and reduced growth that finally affects the quality of the commodity prawn. Several important issues related to the process of infection, propagation and interaction of prawn viruses with hosts at the cellular and molecular level, remain to be elucidated. Studies are needed as well, to determine the true risks of the viral infections on wild populations of prawns. Since Infectious hypodermal and hematopoietic necrosis virus is widely distributed in prawn culture facilities throughout the world, this study will focus on its diagnosis and its epidemiological probes. Recent reports show that genetic diversity of Infectious hypodermal and hematopoietic necrosis virus is not uniform across geographic regions. For investigation on mutations within a geographic area or individual hosts, genome sequencing is the method of choice.
1.3 Objectives of the Study
The following are the objectives of this study:
- To identify the diagnostic tests for Infectious hypodermal and hematopoietic necrosis virus of prawns.
- To examine the epidemiology of Infectious hypodermal and hematopoietic necrosis virus of prawns.
- To examine the mortality rates of prawns infected with Infectious hypodermal and hematopoietic necrosis virus.
1.4 Research Questions
- What are the diagnostic tests for Infectious hypodermal and hematopoietic necrosis virus of prawns?
- What is the epidemiology of Infectious hypodermal and hematopoietic necrosis virus of prawns?
- What is the mortality rates of prawns infected with Infectious hypodermal and hematopoietic necrosis virus?
1.5 Significance of the Study
The following are the significance of this study:
- The outcome of this study will reveal various approaches to the diagnosis of Infectious hypodermal and hematopoietic necrosis virus in infected prawn. It will also educate on the epidemiology and mortality rate occasioned by Infectious hypodermal and hematopoietic necrosis virus in prawns.
- This research will be a contribution to the body of literature in the area of the effect of personality trait on student’s academic performance, thereby constituting the empirical literature for future research in the subject area.
1.6 Scope / Limitations of the Study
This study will cover procedures and investigations into the diagnostic tests and epidemiological probes for viruses in prawn in Nigeria with specific emphasis on Infectious hypodermal and hematopoietic necrosis virus.
Limitation of Study
Insufficient fund tends to impede the efficiency of the researcher in sourcing for the relevant materials, literature or information and in the process of data collection (internet, questionnaire and interview).
The researcher will simultaneously engage in this study with other academic work. This consequently will cut down on the time devoted for the research work.
Conclusion and Recommendations
A variety of environmental and best management practice strategies have been attempted for the control of viral diseases in penaeid shrimp aquaculture. These strategies range from the use of improved culture practices (i.e. where sources of virus contamination are reduced or eliminated, source water is treated, filtered, and aged to remove potential vectors, culture ponds fallowed and treated between crops, routine sanitation practices are improved, stocking densities are reduced, etc.) to stocking “specific pathogen-free” (SPF) or “specific pathogen resistant” (SPR) species or stocks.
Clues to aetiology come from comparing disease rates in groups with differing levels of exposure – for example, the incidence of congenital defects before and after a rubella epidemic or the rate of mesothelioma in people with or without exposure to asbestos. Clues will be missed, or false clues created, if comparisons are biased by unequal ascertainment of cases or exposure levels. Of course, if everyone is equally exposed there will not be any clues – epidemiology thrives on heterogeneity. If everyone smoked 20 cigarettes a day the link with lung cancer would have been undetectable. Lung cancer might then have been considered a “genetic disease”, because its distribution depended on susceptibility to the effects of smoking.
There is a growing need to standardize and validate the DNA-based diagnostic methods and the laboratories that use them (Walker and Subasinghe, 1999). Standardization of DNA-based diagnostic methods is almost inherent in the nature of the tests. That is, a specific DNA probe, or a specific set of primers, that is used to demonstrate the presence or absence of a unique DNA or RNA sequence does not vary from batch to batch. Hence, with proper controls, these DNA-based methods are readily standardized (Reddington and Lightner, 1994). However, despite the growing dependence of the shrimp culture industry on DNA-based diagnostic methods, none of the tests that are available from commercial sources nor from the literature have been validated using controlled field tests. Likewise, there are no formal accreditation or certification programs yet in place to assure that test results from technicians and laboratories running the tests are indeed accurate and properly controlled (Lightner and Redman, 1998; Lotz and Lightner, 1999; Lightner, 1999b). The implementation of a formal program by appropriate international agencies or professional societies is needed to validate new diagnostic methods and to periodically review the accreditation and certification of diagnosticians and diagnostic laboratories. The establishment of regional reference laboratories for DNA-based diagnostic methods of penaeid shrimp/prawn pathogens would fit well into such a program with the goal of making these methods uniform, reliable, and readily applicable to disease control and management strategies for viral diseases of cultured penaeids.
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