Differences in virulence between white spot syndrome virus (WSSV) isolates and testing of some control strategies in WSSV infected shrimp

White spot syndrome virus (WSSV) is a fatal pathogen of shrimp. In this thesis, differences in virulence of WSSV isolates and control strategies against WSSV infection in Penaeus vannamei juvenile, including treatment with cidofovir an antiviral, feeding a diet supplemented with Spirulina platensis and exposure to high water temperature (33°C), were evaluated. In all experiments shrimp were inoculated intramuscularly or orally with a standardized protocol using a fixed WSSV infectious titre. Virulence was compared between three WSSV isolates based on time of onset of mortality and time to reach a cumulative mortality of 50 and 100%. Quantification of WSSV infected cells (VP28 positive) in tissues with an important role in shrimp physiology including gills, stomach cuticular epithelium, cuticular epithelium of the body wall, hematopoietic tissues and antennal glands was done to find the basis of differences in virulence. Mortality started at 36 h post inoculation (hpi) in shrimp inoculated with two Thai isolates and 36-60 hpi with the Viet isolate. Cumulative mortalities reached 100% at 72-84, 108 and 204-348 hpi with WSSV Thai-1, WSSV Thai-2 and WSSV Viet, respectively. The mean numbers of infected cells were significantly higher in each examined tissue of naturally deceased shrimp inoculated with a highly virulent WSSV Thai-1 than with the low virulent WSSV Viet isolate except in hematopoietic tissues. The difference was prominent in gills (6x). Cidofovir was not effective in blocking WSSV infection in shrimp, which was in contrast with the results obtained with DNA viruses in man. This could be due to (i) limited cellular uptake, (ii) disability of shrimp cells to metabolize cidofovir in its active form, and/or (iii) differences between WSSV and other DNA viruses. A diet supplemented with S. platensis did not reduce mortality. In this thesis, three water temperatures (27°C, 30°C and 33°C) were tested on the clinical and virological outcome of WSSV infection in shrimp. Two temperatures, 27°C and 30°C were permissive for virus replication, which resulted in disease and mortality of shrimp. At 33°C continuously before and after inoculation or after inoculation only, shrimp did not show clinical signs, mortality was almost absent and virus replication was inhibited. The effect of 33°C was independent of inoculation route and infectious titre used. To explore the potential of 33°C to control white spot syndrome, the impact of 33°C for 6 h, 12 h, 18 h and 24 h per day in shrimp infected with a high dose of WSSV Thai-1 or WSSV Viet isolate was investigated. A duration of 33°C for 6 h per day had no beneficial effect. Longer durations (12-24h) were effective to delay and reduce mortality in shrimp inoculated with both isolates. The results with 18 h per day at 33°C broaden the application potential of 33°C to prevent disease and reduce mortality by minimizing the negative effects on shrimp physiology. The effect of increasing temperature from 27°C to 33°C at different stages of WSSV infection (0, 12, 24, 48 and 96 hpi) in shrimp inoculated with a low or a high dose of WSSV Thai-1 or WSSV Viet was examined. Increasing temperature to 33°C was effective in inducing protection only at an early stage of infection (≤ 24 hpi) in shrimp inoculated with each isolate. At a later stage of infection, it had no or even an adverse effect, a sharp rise of mortality. The cause of this sharp rise of mortality could be a combination of progression of infection in tissues and negative effects of 33°C on shrimp physiology such as increased requirement of oxygen and reduced oxygen exchange in gills

A Model Innovation : Improving Disease Management for Meeting the Challenges of Bangladesh’s Aquaculture Hatchery Sector

Reducing disease within shrimp and prawn production is a key policy aim for Bangladesh’s export aquaculture. Hatcheries that supply the farms with seed — or larvae — are potential hotspots for disease and the production of antimicrobial resistance traits. Disease pathogens and antibiotic resistant bacteria or genes can easily be transferred to farms via infected larvae. Efforts to reduce disease and transmission have focused on testing all hatchery output, improving hatchery production techniques and management practices, and generating markets for pathogen-free seed. Whilst the intrinsic value of the innovations for reducing disease and improving quality appeared evident, uptake of improvements in the hatcheries has been low. Disease remains a key production challenge, and despite some evidence of reduction in antibiotic use, antibiotics remain a necessary component of disease control. To test the viability of the new technologies and management practices we have developed a sociotechnical method of analysis, inspired by Actor Network Theory. The method utilizes interessement to analyse the role different actors/actants play in determining the destiny of the hatchery production innovations. Our approach has highlighted how the multifaceted socioeconomic and biological elements of hatchery production combine to create a weak innovation and investment environment. We therefore advocate the development of models that combine social and technical analysis for the purposes of assessing the viability of an innovation and improving the prospects of successful implementationPeer reviewe

Implementing aquaculture technology and innovation platforms in Asia

First paragraph: Aquaculture has emerged as one of the fastest growing agri-food systems, playing an increasingly important role in global nutrition security and contributing economic welfare to rural and coastal regions (Beveridge et al., 2013; Béné et al., 2016). At the same time the sector in globally important regions of the world like Southeast Asia remains vulnerable to a range of production risks, related to farming conditions, disease, processing and trade issues related to environmental and social sustainability (Little et al., 2016, 2018; Ahmed and Thompson, 2019; Bush et al., 2019). For the aquaculture sector to maintain its important role as a globally important sector, continual innovation of farming systems and sector level systems of response to key issues like disease are therefore required (Lebel et al., 2010; Asche and Smith, 2018; Joffre et al., 2018)

Innovation at the Margins: The Challenges for Bangladesh’s Aquaculture Hatchery Sector

Reducing disease and improving quality control for shrimp and prawn production are key policy aims for Bangladesh’s export aquaculture with its hatchery sector, the foundation of aquaculture production, targeted for technical innovation. Hatchery reared postlarvae are carriage for the transferral of pathogens and potentially antibiotic resistant genes onto farms, compelling the Department of Fisheries to introduce the Hatchery Act in 2011, stipulating that all hatchery reared PL should be tested before onward sale. Policy implementation is achieved in collaboration with externally funded, locally managed development programmes for upgrading production management practices, delivering genetically enhanced breeds, and improving business margins by providing opportunities for pathogen-free guarantees and branding. However, uptake of improvements in the hatcheries has been low. Disease remains a key production challenge, and despite a reduction in antibiotic use, antibiotics remain a necessary component of disease control. In this paper, we develop a sociotechnical diagram, inspired by Actor Network Theory, that combines analytical models for innovation and disease outbreaks with a sociology of translation. Our approach highlights how vernacular trading practices, farmer financial and adaptive capacities, poor quality materials, and limited laboratory facilities tighten business margins and increase production risks, leaving hatcheries with little to no extra investment capacity or incentive to implement seed testing and improved management practices. We demonstrate the kinds of alliances to be made, interests to be negotiated, and adjustments to be considered for seed innovations to gain purchase in Bangladesh’s food production ecology

Production without medicalisation : Risk practices and disease in Bangladesh aquaculture

Improved biosecurity and livestock disease control measures in low resource settings are often regarded as beneficial for agricultural productivity, rural incomes, global health, and sustainability. In this paper we present data from a study of shrimp and prawn aquaculture in Bangladesh to argue that this relationship is not as straightforward as it would seem. Analysing quantitative and qualitative data from a multi‐method field study involving 300 “missing middle” farmers, we demonstrate the importance of socio‐economic and ecological conditions to any disease management strategy. We describe how a technical programme to introduce “disease‐free” seed faltered partly as a result of the farmers' tendency to offset disease and livelihood risks by frequently re‐stocking their ponds. Changes to seed provision were accompanied by calls to alter farmers' livestock production practices. Paradoxically, these changes exposed farmers to more intense risks, potentially locking them into unsustainable disease management practices. The analysis emphasises that vernacular farming practices should be considered as key assets rather than barriers to disease management strategies, and that closer attention be paid to value chain and other risks as drivers of unsustainable practices.Peer reviewe

An assessment of health management practices and occupational health hazards in tiger shrimp (Penaeus monodon) and freshwater prawn (Macrobrachium rosenbergii) aquaculture in Bangladesh

Diseases have been recognized as the major obstacle to the shrimp (Penaeus monodon) and prawn (Macrobrachium rosenbergii) aquaculture production in Bangladesh. This study provides an assessment of shrimp and prawn diseases/syndromes, health management practices, and occupational health hazards associated with the handling of chemical and biological products to prevent and treat shrimp and prawn diseases. A survey was conducted using a semi-structured questionnaire with 380 shrimp and prawn farmers in the southwest of Bangladesh during February and June of 2016. The farms were categorized on the basis of the three cropping patterns: shrimp polyculture, prawn polyculture, and shrimp and prawn polyculture. Eight different diseases and/or symptoms were reported by the surveyed farmers. The white spot disease and the broken antenna and rostrum symptom were the most common in shrimp and prawn species, respectively. In total, 35 chemical and biological products (4 antibiotics, 15 disinfectants, 13 pesticides, 2 feed additives and probiotics) were used to treat and/or prevent diseases in the all farm categories. The major constraints for disease management were limited access to disease diagnostic service, inadequate product application information and lack of knowledge on better management practices. Handling chemicals and preparation of medicated feed with bare hands was identified as a potential occupational health hazard. This study suggests improvements in farmers’ knowledge and skill in disease diagnostics and health management practices, and appropriate handling of potentially hazardous chemicals. Keywords: Penaeus monodon, Macrobrachium rosenbergii, Disease, Antimicrobials, Occupational health hazard

Benefits of stocking white spot syndrome virus infection free shrimp (Penaeus monodon) post larvae in extensive ghers of Bangladesh

Shrimp farming is an important income generating activity for hundreds of thousands of small scale farmers operating extensive ghers in Bangladesh. White spot syndrome virus (WSSV) has caused major shrimp production losses globally including Bangladesh. This study investigated the benefits of stocking WSSV infection free (WSSVF) Penaeus monodon post larvae (PL) to extensive gher holders of southwest Bangladesh and considered different parameters of the farming system. The longitudinal observational study included 636 gher holders comprising two distinct PL user types; 312 gher holders had stocked polymerase chain reaction (PCR) tested WSSVF and 324non-tested (NT )PL.Data on explanatory and outcome variables were collected fortnightly during April–October 2015. The major findings were: (i) gher holders who stocked WSSVF PL had significantly higher survival, less disease, higher shrimp production and increased profitability compared to NT PL users, (ii) WSSVF PL stocking had an influence in strengthening the resilience, overall management and economic performance of the extensive gher holders. The study showed that WSSVF PL users had significantly small ghers, higher levels of better management practices (BMP) compliance such as nursery rearing of PL, double netting at inlet, periodic sampling, net fencing and bird netting, significantly higher application rates and usage amount of inputs (lime, inorganic fertilizer, feed) compared to NT PL users. It also found that gher size, water depth, stocking density, stocking frequency, cropping patterns affected levels of shrimp survival, production and disease incidence of the overall population. The results suggested that improving the availability of WSSVF PL through a PCR testing service, production of specific pathogen free PL and limit/stop the sale of WSSV infected PLs could significantly increase shrimp production of extensive gher holders