Aquatic animals and their products are among the most widely traded commodities where some 40 percent of global production enters the international market. The international trade in aquatic animals and their products are carried out for various reasons. A number of aquatic animal health threats and risks in the international movement of live aquatic animals include the emergence of new pathogens due to the rapid development of aquaculture, limitations in control options for aquatic animal diseases, occurrence of multi-factorial disease syndromes, frequent sub-clinical infections in aquatic animals, undomesticated status of aquatic animals and little information available on biological requirements and health status, etc. In addition to these, because of the volume of live aquatic animals traded internationally, the diversity of species being moved, the many known and potential pathogens that infect aquatic species, the lack and/or weak enforcement of regulations and in other cases, lack of sector regulations itself– it has been a difficult task to find ways that will reduce the risks of spreading transboundary pathogens. This paper looks at the lessons learned and future challenges in managing the risks of disease incursion associated with the international trade of live aquatic animals.
Describing diseases in populations is the literal aim of epidemiology. From that point of view, molluscs and mollusc diseases put epidemiologists and other related “ogists” under serious challenges. Two recent assessments conducted by the Animal Health and Welfare (AHAW) Panel of the European Food Safety Authority (EFSA) have highlighted some of these challenges. In an attempt to establish a list of species susceptible to certain diseases, a set of objective criteria pertaining to pathogen replication, viability, host response and pathology, were established. Screening the peer-reviewed literature to document these criteria revealed that studies on diseases of molluscs and techniques applied to case investigation usually provide a great deal of information. However, thorough identification of the pathogen is frequently lacking. In addition, application of the criteria has led to unexpected outcomes of the assessment when species usually regarded as susceptible do not fulfil the criteria. If not susceptibility, the presence of a pathogen in a host leaves very little space for interpretation. From the viewpoint of transfer of pathogens, susceptible species and biological vectors may present equivalent risks. The concept of vector has itself proved to be a subject for controversy and definitions in the international standards for health management have shown to be potentially misleading in assessing the risk of transferring pathogens via transfers of live animals. A second assessment of the AHAW panel to list potential vector species highlighted a range of situations from mechanical carriage to actual infection. Interestingly, some of the conclusions of this risk assessment were later comforted by experimental data. Both the lack of accurate identification of mollusc pathogens and inconsistency in host species categorization result in poor understanding of epidemiology of diseases in molluscs. These issues are illustrated by selected examples.
White spot syndrome virus (WSSV: a synonym of penaeid rod-shaped DNA virus, PRDV) is the causative agent of white spot disease (WSD: penaeid acute viremia, PAV), one of the most serious diseases affecting decapod crustaceans around the world. Recently, “quasi-immune response” was found in kuruma shrimp Penaeus japonicus, wherein individuals that naturally survived from WSD showed protection against a rechallenge with WSSV. The phylaxis against WSSV was also inducible by oral vaccination with recombinant WSSV proteins, rVP26 and rVP28. In the present study, kuruma shrimp orally vaccinated with rVPs were sequentially challenged with WSSV to evaluate onset and duration of phylactic response and booster effect. The phylactic response of shrimp against WSSV-challenge peaked at day 45 after the vaccination with rVP26 (RPS: 100%) and at day 55 with rVP28 (RPS: 93%), and decreased within 10-20 days. The phylaxis against WSSV-challenge was boosted by the secondary vaccination with homologous rVPs, but not by those with heterologous rVPs. The peaks of phylactic responses appeared at day 22 after the secondary vaccination more rapidly than those after the primary vaccination. These results demonstrated that the duration of phylaxis induced by oral vaccination with rVPs was relatively short, but could be extended by booster vaccination with homologous rVPs.
This study compared the histopathology of young striped jack Pseudocaranx dentex experimentally infected with the dematiaceous fungus Ochroconis humicola NJM 0472 with that of spontaneously infected fish. Moribund and freshly dead fish from both groups showed similar histopathology, and appeared to have been killed due to hyphae penetrating the visceral organs. Fish that survived the infection appeared to be able to suppress the fungal growth by well-established inflammatory reaction involving mycotic granulomas and granulation tissues. The results suggested that two types of O. humicola infection occur in young striped jack: an acute type infection, which is characterized by penetrating hyphae that cause direct tissue destruction and a chronic type infection, which is characterized by severe inflammatory reaction that causes functional disorders of the affected organs.
KDM-2 is a medium widely used for isolation of Renibacterium salmoninarum (R.s.), the causative agent of bacterial kidney disease (BKD). KDM-2 still has a problem for colonization of R.s. at low concentration levels. In the present study, we modified KDM by supplementation of the culture-spent medium of R.s. (SMRs) in substitution of FBS. No difference was observed in the growth rate of R.s. at ≥ 103 cells/mL in KDM broth with 1% FBS regardless of SMRs supplementation. Growth rate of R.s. decreased at 101 cells/mL of inoculation into KDM with 1% FBS, but it was recovered by supplementation of ≥ 1% SMRs into the medium. The activity of SMRs supporting bacterial growth was stable to treatment at 60ºC for 30 min and freezing at -20ºC for 7 days. At inoculation of ≤ 300 CFU of R.s., expected colony counts were obtained on the agar plates containing SMRs, while non or less than half of bacteria colonized on the agar plates without SMRs. It was thus considered that the modified KDM by supplementation of SMRs instead of FBS was convenient and inexpensive for isolation of R.s., especially at low concentration levels.
In December 2006, a Halioticida infection was found in wild mantis shrimp Oratosquilla oratoria in Tokyo Bay, Japan. Fungi were found in the gills of mantis shrimp, isolated from lesions using PYGS agar, and identified by morphological observation and molecular analysis. The fungi formed fragments in the hyphae and several discharge tubes developed from each fragment. Zoospores were formed within the fragments and released into the seawater through the tops of discharge tubes. Based on the characteristics of zoospore production mode, the fungi were classified into the genus Halioticida. Fungal isolates NJM 0642 and NJM 0643, isolated from mantis shrimp, were compared by molecular analysis of the D1/D2 region of the large subunit ribosomal RNA gene (LSU rDNA) with other fungi belonging to Peronosporomycetes, isolated from various marine crustaceans and abalones Haliotis spp. As a result, both isolates were identified as Halioticida noduliformans, which has been isolated from abalone. Moreover, experimental infection demonstrated that the fungus was pathogenic to mantis shrimp. This is the first report of fungal disease caused by Peronosporomycetes in mantis shrimp.
Examination via light and electron microscopy of juvenile summer flounder Paralichthys dentatus experiencing a subacute to chronic mortality revealed severe necrotizing hepatitis characterized by necrotic multinucleated giant cells (syncytia). The cytoplasms of syncytia contained paracrystalline arrays of reovirus-like particles, strongly suggesting that the epidemic was caused by the virus. This is the first report of a putative viral infection in summer flounder.