Diagnostic Tests and Epidemiological Probes for Prawn Viruses in Nigeria
Chapter One of Diagnostic Tests and Epidemiological Probes for Prawn Viruses in Nigeria
INTRODUCTION
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.
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.
OBJECTIVES OF THE STUDY
The following are the objectives of this study:
1. To identify the diagnostic tests for Infectious hypodermal and hematopoietic necrosis virus of prawns.
2. To examine the epidemiology of Infectious hypodermal and hematopoietic necrosis virus of prawns.
3. To examine the mortality rates of prawns infected with Infectious hypodermal and hematopoietic necrosis virus.
RESEARCH QUESTIONS
1. What are the diagnostic tests for Infectious hypodermal and hematopoietic necrosis virus of prawns?
2. What is the epidemiology of Infectious hypodermal and hematopoietic necrosis virus of prawns?
3. What is the mortality rates of prawns infected with Infectious hypodermal and hematopoietic necrosis virus?