Infectious disease outbreaks are the key limiting factor to the sustainable expansion of the aquaculture industry. Many diseases exist in wild populations but only become problematic when crop species are held at high densities and under stressful conditions. These conditions are often associated with aquaculture. Attempts have been made to meet the challenge of disease by developing vaccines or prophylactic treatments to prevent disease or ‘boost’ the host immune system. Unfortunately, in the many cases in Asia these strategies have failed; vaccines have proved ineffective, too expensive, or are unrealistic to administer at farm scale. Immune stimulants and probiotics have often failed to provide consistent or sustained protection in shellfish. We argue that the approach to disease prevention must be radically rethought.

 

During this project we will study two diseases:

White spot syndrome (WSD) of crustaceans, caused by the globally-distributed White Spot Syndrome Virus (WSSV) is a disease of crustaceans and a major economic impact for shrimp farmers across the world www.oie.int/fileadmin/Home/eng/Health_standards/aahm/2009/2.2.05_WSD.pdf .

Global losses to WSSV have been estimated to cost between $8 – 15 bn [1]. In Bangladesh farming of tiger shrimp Penaeus monodon has continued, despite its susceptibility to WSSV. In 1996, in the Khulna region in Bangladesh, WSSV affected approximately 90% of shrimp farms resulting in a 20% decrease in production. As a consequence exports dropped from > 25k t to > 18k t in 1997–1998 [2]. Disease outbreaks on this scale have a very significant impact on the estimated 600000 people employed in the industry in Bangladesh [3]. In India, this virus represents the most severe economic loss to the industry [4], even though farmers have almost exclusively switched to the Pacific white shrimp Litopenaeus vannamei.

White spot disease in shrimp

The fungal-like oomycete Aphanomyces invadans (www.oie.int/fileadmin/Home/eng/Health…/chapitre_aphanomyces_invadans.pdf),  causes epizootic ulcerative syndromes (EUS) in fish, including major carps throughout Asia. At least 94 species of fish are affected by ulcerative syndromes (EUS) caused by the pathogenic oomycete Aphanomyces invadans [5]. In the period 1988–89 Bangladesh suffered a revenue loss of US$ 4.8 million due to EUS. In the period 2010-11, mortalities in the range of 20 to 50% of the total stock were reported in India, particularly in small-scale farms. The socioeconomic impacts of EUS have long been identified and are very serious [6].

Currently, there are no effective means of controlling these significant pathogens that have been shown to be cost effective at farm scale. We believe our current best option is to understand how the environment controls disease progression in ponds, as a means to reduce the risk of infectious outbreak. This knowledge, incorporated into guidelines for best management practice, will allow for the development of novel intervention strategies to be implemented in the future.

Of significance, there is an environmental component to the aetiology of both diseases. Farmers in South India have maintained a good harvest of shrimp in spite of apparent signs of WSSV infection [7] and it is known that WSSV infection outcome varies as a function of temperature acting on both the host and the virus [8]. The OIE have reported that low temperatures (typically < 22 C) can precipitate A. invadans infection [5].

[1] Stentiford GD et al. 2012 J. Invert. Path. 110: 141-157;  [2] Alam SMN et al. 2007. Aquacult. Int. 15: 363–370; [3] Karim M. et al. 2011. Aquacult. Res. 43: 1357–1371; [4] Remany C et al. 2012 J. Fish Dis. 35, 793–798; [5] OIE 2014. Chapter 2.3.2. Manual of Diagnostic Tests for Aquatic Animals 2014; [6] Kahn MH & Lilley JH 2002 pp. 27-39 FAO Fisheries Technical Paper 404, Rome 2002;[7] Stalinraj V et al. Aquacult. Res. 40: 129-138; [8] Granja CB et al. 2003. Dis. Aquat. Org. 54: 73-78.