Aquaculture and Animal Pathogens In the Marine Environment With Emphasis On Marine Shrimp Viruses

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Book Chapter

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Ocean Science and Engineering


The Oxford English Dictionary defines agriculture as "The science and art of cultivating the soil; including the allied pursuits of gathering in the crops and rearing live stock". Modifying the OED definition by replacing "soil" with "Earth's surface" makes aquatic agriculture-or in shorthand "aquaculture", the fastest growing sector of agriculture (FAO, 2002).Acharacteristic that sets the aquaculture industry apart from terrestrial livestock production is the rearing of species that are also natural resources. Although the spread of a terrestrial livestock pathogen serves as a primary concern to the livestock industry, dissemination of an aquaculture pathogen concerns commercial fisheries, recreational fisheries, and a wider environmental community. Aconverse concern is also felt by the aquaculture community; pathogens in natural populations may spread more easily into the aquaculture industry because of the shared host species. As human population size and wealth have grown, so has demand for seafood. Expansion of marine aquaculture meets much of the new demand. According to the data in the FAO database FishStat, the capture fishery sector of seafood production has been level since the 1980s; however, mariculture has continued an8%per annum exponential growth rate that began in the 1950s (Fig. 19.1). Further, the percentage of marine seafood coming from mariculture has increased steadily over that time and now approaches 25%. Given that a population (either wild or cultured) carries a pathogen, the probability of transferring that pathogen to the other population increases as the number of effective contacts between those two populations increases. An effective contact is one that carries with it some non-zero probability of transmission-of-infection. One correlate to the growth of marine aquaculture is increased contacts between wild and cultured host populations. More contacts presently occur because of an increase in cultured animals inhabiting the environment and therefore more opportunities for contact between wild and cultured animals. Effective contact can occur between cultured animals and local populations during fish rearing or more distant populations after harvest. Locally, the number and kind of contacts is a function of the specific rearing practice. Farming of marine organisms occurs in a variety of ways, but all the cases can be categorized into one of three broad groupings relating to proximity of wild counterparts; (1) in the natural habitat, (3) in the coastal zone, or (3) in the inland zone. In the natural habitat, organisms are cultured in situ and may be in the water column or on the bottom suspended in cages, trays, or nets, or attached to artificial substrate. In the case of ocean ranching, organisms are not contained at all, but young are released into the environment, allowed to range freely, grow, and be re-harvested-an interesting hybrid between aquaculture and fisheries.Wild organisms can make direct contact with free-ranging ranched organisms, with cultured animals that have escaped from their culture unit, or, if small enough, wild organisms may actually enter the rearing unit. Direct physical contact may not be required and effective contacts can occur with the rearing unit itself or with dispersing contaminated waste. Often, wild organisms will be attracted to in situ culture units because food is available from dietary commercial feed or prey organisms colonizing the rearing units. In the coastal zone, organisms are grown in earthen ponds or tanks located near the open ocean or estuary from which culturewater is obtained. The naturalwater is not only the source of newculturewater but is also the repository forwastewater discharge, and effective contacts may occur with the wastewater. Inland, as in the coastal zone, ponds or tanks are used; however, inland facilities are located some distance away from the open ocean or estuary. The water source inland usually consists of saline groundwater, and wastewater is often disposed into rivers and streams, or if the saline content is low enough, it is used for irrigation. Likelihood of effective contacts with wild marine or estuarine organisms is dependent on the distance inland. However, novel contacts may be made between non-marine or non-estuarine organisms and wastewater effluent and may increase the likelihood of an emerging disease in an unusual wild host. In addition to increasing the number of contacts between cultured and wild populations, the growth of mariculture also has changed the spatial distribution of contacts. Mariculture grows not only by an increase in the density of culture operations in locations where farming already exists, it also grows by expansion into areas where mariculture has not previously taken place. Pathogen range-extension can follow culture activities if the culture-species carries a pathogen into areas inhabited by uninfected susceptible populations of either conspecific or hetero-specific hosts. The converse remains equally true; pathogens that are unknown in aquaculture can be introduced from resident conspecific or hetero-specific populations, as culture operations expand to new localities. Although contacts between wild and cultured populations often occur during pre-harvest farming, significant contacts also are generated through post-harvest trade. Mariculture has developed into a global industry, with considerable international trade in products. The traded forms associated with pathogen dispersion are uncooked (frozen or raw) seafood and live animals for husbandry (industrial or hobbyist). Much seafood is exported by producing countries and imported by consumer countries, such as the USA, where the commodity product may be reprocessed, often generating waste that may carry infectious pathogens. Improper disposal of that waste may provide pathways through which marine pathogens contact geographically distant populations of susceptible host organisms. Marine shrimp constitute the most prominent seafood product in international trade, and aquaculture has been the major source for increased shrimp trading during the past decade. Marine shrimp ranks as the most traded seafood product internationally, and half of total shrimp production is derived from aquaculture (FAO, 2002). Durand et al. (2000), Lightner et al. (1997), Nunan et al. (1998), and our group together have provided convincing evidence of the potential threat that viruses in frozen shrimp product can pose to cultured as well as to wild shrimp. We will ultimately use four different penaeid shrimp viruses that have different life history strategies as examples of assessing disease transmission and spread. Many other examples could be used, but necessary data for a complete assessment for few of those exist. Although pathogens circulating between cultured and wild populations originally may have been pathogens of wild populations, the primary conduit for worldwide dissemination of a pathogen now is undoubtedly facilitated strongly by the movement of pathogens by the aquaculture industry. However, once a pathogen becomes established in wild populations, it then becomes a threat to culture activities since the flow of pathogens from wild to culture can now occur. The consequences of exchange of pathogens between cultured and wild populations can be severe for either population. Nevertheless, the introduction of a serious pathogen into aquaculture is more easily observed and often has a more immediately obvious economic significance than does the converse.Anoutbreak in a rearing unit resulting in mortality of80%is easily observed by the farmer whose money is being lost at a rate proportional to the fish rising to the surface. In some cases, losses can be severe enough to result in the loss of jobs. For example, Flegel and Alday-Sanz (1998) put the losses worldwide caused by the emergence of the shrimp pathogen white spot syndrome virus (WSSV) over the previous 6 years was in excess of several billion US dollars. In 1999, WSSV was introduced into Latin America and the production dropped precipitously. For example Ecuador, which was the leading producer of shrimp in the western hemisphere, dropped in production by 60% in a single year (Ormaza, 2001). Although the introduction of a pathogen into a wild population may be less often observed and serious outcomes less well documented, the spread of pathogens in natural populations has had clear economic as well as ecological effects. As an example in 1995 and again in 1998, a herpesvirus outbreak occurred in Australia in wild populations of the pilchard, Sardinops sagax. The outbreak affected the entire population of Australian pilchard, and during both outbreaks, it was estimated that 60% of the adult spawning stock was killed (Ward et al., 2001a,b). Ward et al. (2001b) suggested that the source was imported frozen food fish used to feed the southern bluefin tuna, Thunnus maccoyii. Not only the introduction of a new pathogen can occur, but there is also a possibility of enhancement of an existing pathogen when it occurs in both the populations. A pathogen in a cultured population could become amplified relative to the surrounding natural population due to the higher density in the culture enclosure, and enhancement of the pathogen population in the natural host population could increase even if it already exists in the wild population.

Publication Title

Oceans and Health: Pathogens in the Marine Environment

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