Monday, 20 January 2014

Shrimp Farm Logistics Responsible for Virus Infections?

In the last lectures we talked about viral and bacterial diseases in marine animals, and this paper in particular focuses on the occurrence of the white spot syndrome virus (WSSV) which was mentioned briefly in the lecture, and infectious hypodermal and hematopoidetic necrosis virus (IHHNV) in wild organisms which live close to shrimp farms around the Pacific coast of Mexico. The WSSV has a broad range of host animals, and especially crustaceans appear to be most affected. Since the start of shrimp farming in Mexico, WSSV and IHHNV have been identified as major threat to the industry and much effort has been put into studying the dynamics of these pathogens in order to prevent disease reoccurring disease outbreaks.

The viruses spread horizontally (food/water intake by animals) or vertically (infected brood stock to developing larvae), and as the use of non-infected larvae did not solve the problem, it is now generally accepted that wild animals in the surrounding environment of the farms are the sources and carriers of the viruses. In this study, the authors wanted to address the question of how prevalent WSSV and IHHNV are in wild populations of animals around these shrimp farms.
Methods involved the collection of 1736 organisms (11 species) in total over a period of 10 months, before, during and after the shrimp cultivation cycle. The total DNA was extracted and a nested PCR was conducted with the DNA samples with primer sets for WSSV and IHHNV. The amplicon sequences obtained from this were compared to WSSV and IHHNV sequences from databanks in order to establish a similarity profile.

The viruses were found in various species, including fish, shrimps and crabs throughout sample period with a prevalence of 19.5% for IHHNV and 3.6% for WSSV. Moreover, the occurrence of these viruses before cultivation was discovered in 1.2% of animals (WSSV) and 2.3% (IHHNV). After cultivation, IHHNV had a prevalence of over 30% (5.7% for WSSV) and it becomes obvious that the cultivation has a major influence on the spread of the viruses. During cultivation, the water used in the farms is constantly exchanged with water from the surrounding environment, which could potentially be the cause of an increase in virus prevalence in wild organisms. 

Typical hosts of these viruses have been previously considered to be crustaceans, however this study could detect the viruses in fish as well, and they are now considered as virus vectors; susceptibility of fish to these viruses is yet to be established. Persistence of WSSV and IHHNV in animals surrounding the farms has been revealed which explains repeated disease outbreaks when wild fish enter the farm ponds. The solution to this problem seems to be fairly logical to me; moving and isolating the farm ponds away from the natural environment could prevent future outbreaks, however I guess this would be generally difficult to realise. I think this was a very neat study and the results fill important gaps about the dynamics of the viruses affecting the aquaculture industry so greatly.

Macías-Rodríguez, et al., 2014. Prevalence of viral pathogens WSSV and IHHNV in wild organisms at the Pacific Coast of Mexico. Journal of Invertebrate Pathology, Vol. 116, pp. 8-12.

PS.: I hope the authors noticed their several spelling mistakes, it was very irritating to read some parts. They also managed to calculate their percentages wrong. Nevermind, still nice work ;-).



  1. The post I just did literally recommends the opposite to this study; it showed that hatching salmon in enclosed river channels instead of hatcheries improved their ability to resist disease. This post makes me think that this was mainly due to salmon sexual selection rather than just microbe exposure. This highlights how the control of disease between different aquaculture species and groups can vary massively.

  2. I guess salmon that are grown in river channels rather than hatcheries are given the chance to establish immunity to diseases when exposed to sub-lethal levels of a pathogen, this would improve their ability to resist bacterial disease at least.
    Invertebrates like shrimp do not have an adaptive immune system with immunological memory, so sub-lethal exposure in the wild will not improve their immunity. As there is no advantage then to exposing them to the wild environment, it is probably best then to keep them in isolated ponds.

    1. But given that invertebrates apparently have no adaptive immune systems, then surely they would be more dependent on beneficial bacterial associations as a pathogen-excluding defense? This is no reason to expose them to the wild, but bacteria from there could have probiotic applications. Also, the invertebrate immune system is assumed to have no adaptive capacity because it lacks the T receptor, Ig immunoglobin and major histocompatibility complex; but yet they do exhibit adaptive immunity...

  3. I agree with you Dave, the exposure to the two viral pathogen has been proven to be a constant issue, and the shrimp populations will always reinfect at some point under these conditions. If the viruses are carried by fish as well which are able to swim greater distances and can spread the virus even further, I question the locations of shrimp farms in general. This study gave evidence that more wild organisms are infected by the viruses during the cultivation cycles which means that the cultivation causes diseases in these animals, and therefore shrimp farms are an actual threat to wild populations. However, the authors stated that this was the first study of virus prevalence in wild organisms around these farms in Mexico, so may be more studies are needed to confirm this, and hopefully the aquaculture industry will find a solution for the spread of the viruses.