Search

Acute hepatopancreatic necrosis disease (AHPND) has caused severe losses in farmed populations of marine shrimp Penaeus vannamei and P. monodon. The causative agents are unique strains of the bacteria Vibrio parahaemolyticus and related Vibrio species. The disease emerged in the People's Republic of China (China) and Vietnam in 2010 and spread throughout South-East Asia; it was later reported in countries in both North and South America. The disease has had significant economic impacts on the shrimp aquaculture industry. From 2010 to 2016, combined losses from China, Malaysia, Mexico, Thailand and Vietnam due primarily to outbreaks of AHPND, including losses at the farm gate and those resulting from a drop in feed sales and exports, were estimated at over US$ 44 billion. Other economic losses include those associated with processing facilities, decreased community revenues resulting from increased unemployment, financial investments, and the costs of implementing diagnostic and control measures. The reduced employment opportunities and increases in debt burden and investment risk have had sociological impacts. The responses to the disease have led to a gradual recovery of the shrimp industry in affected countries. These response efforts have included the implementation of changes in farming systems and management, including, among others, enhanced biosecurity and the use of AHPND-free and AHPND-resistant shrimp. This situation of losses and recovery illustrates the importance of having a multi-level response plan in place to prevent, or to reduce the risk of, outbreaks of disease.

Hepatopancreatic parvovirus (HPV) causes a common shrimp disease that occurs in many shrimp farming regions, especially in the Indo Pacific, and infects most of the cultured penaeid species. There are seven geographic HPV isolates known, so a method to detect different HPV types is needed. We developed a sensitive and generic real-time PCR assay for the detection of HPV. A pair of primers and TaqMan probe based on an HPV sequence obtained from samples of Fenneropenaeus chinensis from Korea were selected, and they were used to amplify a 92 bp DNA fragment. This real-time PCR was found to be specific to HPV and did not react with other shrimp viruses. A plasmid (pHPV-2) containing the target HPV sequence was constructed and used for determination of the sensitivity of this assay. The assay could detect a single copy of plasmid DNA, and it was used successfully in finding HPV in shrimp samples from the China-Yellow Sea region, Taiwan, Korea, Thailand, Madagascar, New Caledonia and Tanzania.

Since 1987, farmers in southern Taiwan have reported significant disease-related mortalities in freshwater prawns. Most mortalities have occurred during the winter, and usually almost all the adult prawns die within 4-5 days after clinical signs appear. Histopathological studies show that cells in hepatopancreatic ducts and tubules are vacuolized and degenerated. Large numbers of membrane-bound yeast aggregates are observed in the affected tissues. Two hundred and seventeen yeast isolates were obtained from diseased prawns, pond water and sediments from six areas. DNA restriction fragment length polymorphism (RFLP) was used to analyse the yeast genomes and to categorize them into five groups. Conventional biochemical and metabolic methods were then used to identify the yeasts of each group. The results showed that two strains of Candida sake (I and II), Pichia anomala, Endomyces fibuliger and Candida famata were present. In addition, the RAPD (random amplified polymorphic DNA) method was used to determine their genomic similarities. Two strains (I and II) of C. sake were the most similar (72%). C. sake I appears to be the primary causative agent of disease, based on frequencies of occurrence of the yeasts found in the diseased prawns.

Since 1992, mass mortalities among cultured giant tiger shrimp, Penaeus monodon (Fabricius), and kuruma shrimp, Penaeus japonicus (Bate), have been observed in Taiwan. The condition is known as ‘white spot disease’ (WSD), based on the characteristic white spots on the cuticle of diseased shrimp. With the scanning electron microscope, two sizes of white spots were observed. Each spot represented a protrusion on the inside surface of the carapace. The composition of white spots was similar to that of the cuticule, most calcium, as determined with an energy dispersive spectrometer. Histological studies of moribund, infected specimens revealed degenerated cells, characterized by hypertrophied nuclei, in various meso- and ectodermal tissues. Infected tissues included cuticular epidermis, connective tissue, lymphoid organ, antennal gland, and haematopoietic, gill and nervous tissue. Nuclei were Feulgen-positive and no occlusion body was found in the necrotic tissue. Transmission electron microscopy revealed the presence of rod-shaped and enveloped virions in the hypertrophied nuclei. The virions measured 298 ± 21 × 107 ± 8 nm in the giant tiger shrimp and 248 ± 12 × 104 ± 8 nm in the kuruma shrimp. In an experimental infection trial, cumulative mortality was 40% within 14 days under stress conditions. No mortality was observed in controls or in non-stressed infected shrimp. Experimental infections show that environmental stressors such as ammonia may enhance the severity of WSD virus infections in cultured shrimp.

The morphogenesis of a highly organized cellular membrane structure, a membranous labyrinth (ML), in the cells of penaeid shrimp infected with Penaeus monodon baculovirus (MBV) is described. The ML possibly originates from dilated Golgi apparatus, endoplasmic reticulum (ER) or the outer nuclear membrane. The ML apparently grows by proliferation of the cellular membranes of these systems. In addition, its degree of development was correlated with stages of MBV reproduction.

The present study demonstrates the first discovery of a yellow head disease-like virus (YHDLV) in Penaeus japonicus that were diagnosed as white spot disease and describes the histological changes and the characters of the virus under electron microscopical observation. In histological studies, cells with the basophilic inclusion bodies and hypertrophied nuclei were found in various tissues, including the lymphoid organ, interstitial hepatopancreatic tissue, cardiac tissue, gill tissue, connective tissue, cuticular epidermis and hematopoietic tissue. Transmission electron micrographs revealed the presence of rod-shaped, enveloped virions in the cytoplasm of the necrotic cells. The virus was 150-200 nm in length and 40-50 nm in width, but extraordinarily long viruses were also detected in the cells. The features of the YHDLV were very similar to yellow head disease virus of Penaeus monodon in Thailand.

A digoxigenin-labelled DNA probe was used for in situ detection of the Penaeus monodon-type baculovirus (MBV) derived from cloned MBV polyhedrin genome in cultured Penaeus monodon Fabricius. First, the specificity of the probe against MBV DNA with dot blot hybridization analysis was verified. This probe indicated that cloned MBV polyhedrin fragment can be used as an MBV-specific probe. This was then used to microscopically examine sections of MBV-infected tissues for a blue-purple precipitate indicative of a positive reaction for MBV. MBV-positive cells were located only in the epithelium of the hepatopancreatic tubules and of the midgut. Furthermore, comparison of the susceptibility to MBV infection among several life-stages of the shrimp showed that the MBV genome was found in the zoea, mysis, post-larva, and adult stages, whereas MBV DNA was not detected in either eggs or nauplii. The results were quantified from in situ hybridization with an image analyser to compare the degree of cell infection among groups of cultured P. monodon collected from various farms in Taiwan.

The most important diseases of farmed penaeid shrimp have infectious aetiologies. Among these are diseases with viral, rickettsial, bacterial, fungal and parasitic aetiologies. Diagnostic methods for these pathogens include the traditional methods of gross pathology, histopathology, classical microbiology, animal bioassay, antibody-based methods, and molecular methods using DNA probes and DNA amplification. While methods using clinical chemistry and tissue culture are standard methods in veterinary and human diagnostic laboratories, the former has not been routinely applied to the diagnosis of penaeid shrimp diseases and the latter has yet to be developed, despite considerable research and development efforts that have spanned the past 40 years. No continuous shrimp cell lines, or lines from other crustaceans, have been developed. Hence, when molecular methods began to be routinely applied to the diagnosis of infectious diseases in humans and domestic animals in the mid- to late 1980s, the technology was applied to the diagnosis of certain important diseases of penaeid shrimp for which only classical diagnostic methods were previously available. A DNA hybridization assay for the parvovirus IHHNV was the first molecular test developed for a shrimp disease. This was followed within a year by the first PCR test for MBV, an important baculovirus disease of shrimp. Today, shrimp disease diagnostic laboratories routinely use molecular tests for diagnostic and surveillance purposes for most of the important penaeid shrimp diseases.