Quantitative histopathology and epidemiology of prawn viral diseases

Littik, Semuel Ayub Mathias (2003) Quantitative histopathology and epidemiology of prawn viral diseases. PhD thesis, James Cook University.

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The interaction of prawns, viruses and the environment in disease development at individual and population levels was studied using histological and statistical analyses based on quantitative models and simple quantitative methods. At the individual level, quantitative histopathology was utilized to differentiate monodon baculovirus (MBV) in the digestive tissues (the midgut or MG and hepatopancreas or HP) of Australian Penaeus monodon and Penaeus merguiensis. It seems MBV in P. monodon was less damaging than in P. merguiensis. Indeed, P. monodon sustained significantly lower severity of MBV infection (mean±SE of severity index (SI) = 1.6 ± 0.1) and proportion of abnormal cells (Rac = 34 ± 3%) with a significantly higher proportion of infected nuclei (Rin = 64 ± 3%); while P. merguiensis had significantly higher SI (2.1 ± 0.1) and Rac (76 ± 5%) with significantly lower Rin (46 ± 3%). Besides, the diameter of MBV occlusion bodies (OBs) in P. monodon (5.4 ± 0.3 μm) was significantly larger than that in P. merguiensis (3.5 ± 0.4 μm). Also, the number of MBV OBs per nuclei in P. monodon was moderately correlated to the OB diameter (R2 = 0.34, p << 0.05), while in P. merguiensis it was weakly correlated (R2 = 0.12, p << 0.05). Perhaps, P. monodon and MBV mutually determined the size of the OBs, whereas P. merguiensis mostly influenced the size of the OBs. Tissue tropism of Australia-type MBV in the AMG of P. merguiensis was reported for the first time based on MBV prevalence (91± 6%), SI (2.4± 0.1), Rcc (91±3%) and significant dominance of the final stage of nuclear change. Overall, the HP of both penaeid species was more tolerant to MBV than the AMG since the HP contained significantly larger OBs and higher proportion of nuclei with the early stage of MBV infection compared to that in the AMG. For the first time a transect method was applied to analyse lymphoidal changes in P. monodon. This method was simple and cost effective since it detected the changes of the length of lymphoidal tubules and spheroids using a micrometer and a low-power light microscope. Two novel quantitative models were developed and tested here, i.e. the spheroid-total length ratio (STLR) and morphotype (MT) score. The results show that the ratio of spheroid length and morphotypes of the spheroids continually changed in a successive fashion from an early infection type (morphotype A) to a terminal type (morphotype C) in chronic or acute experimental infection by an insect Autographa californica nuclear polyhydrosis virus (AcNPV). Overall, this simple technique was sensitive and reliable to detect lymphoidal changes in the prawns. At the population level, risk factor analysis was applied relative to a declining prawn production in a farm with no written records of prawn diseases. Statistics were employed to investigate any possible correlation between the food budget (proxy measure of prawn health) and 10 parameters of daily water quality (265 working days from 18 crops); the biomass (proxy measure of prawn health) to 5 parameters of prawn biology, 6 parameters of farming management and 1 parameter of production (29 crops within 9 month data span); and 2 parameters of histology (35 sub-adult P. monodon). Coherence, analogy and histology show that high salinity (33 ppt), low temperature (26 oC), young age (< 107 days), high stocking density (> 20 individuals/m2), systemic bacterial infection (46% prevalence) and muscle necrosis (91% prevalence) were the potential risk factors of declining prawn production in the farm. Non-systemic and minor cellular changes associated with suspected viruses suggest that viral disease was very likely not a risk to the prawn production. Since molecular techniques can be used to confirm the diagnosis by histology, the histological diagnosis of MBV was compared to non-isotope in situ hybridization (NISH) and polymerase chain reaction (PCR) assays. Molecular diagnosis of MBV appears to encounter technical difficulties, which were clarified in terms of possible causes and procedural modifications. Quantitative histopathology, statistical epidemiology and simple, low-costs, time-efficient and robust quantitative methods in this research may be suitable for places like Indonesia lacking sophisticated diagnostic facilities.

Item ID: 1245
Item Type: Thesis (PhD)
Keywords: prawns, baculoviruses, disease, Penaeus monodon, Penaeus merguiensis, lymphoid organ, aquaculture, histopathology, epidemiology
Date Deposited: 23 Jul 2007
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0707 Veterinary Sciences > 070704 Veterinary Epidemiology @ 0%
07 AGRICULTURAL AND VETERINARY SCIENCES > 0707 Veterinary Sciences > 070709 Veterinary Pathology @ 0%
07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070401 Aquaculture @ 0%
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