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|Water Temperature and Early Mortality Syndrome (EMS)|
Michael Mogollon (email@example.com): Does anyone have information on the relationship between EMS and temperature? Is there a relationship between virulence/temperature and the strains of Vibrio parahaemolyticus (Vp) that cause EMS? Outdoor ponds range from 28° to 33° Celsius in most of most of the tropics, and, other than using deeper, cooler ponds, there are few options for controlling temperatures. Indoor systems, however, can manage temperature fairly well.
Loc Tran (firstname.lastname@example.org): I really enjoy these discussions on The Shrimp List. At the University of Arizona, we have done a number of EMS challenges and discovered that the pathogenicity of Vp might increase with the increase in temperature, probably because Vp prefers high temperatures to proliferate. Nonetheless, during a challenge in a greenhouse during the fall, when temperatures dropped to 20° C, Vp was still there. I think Vp can infect shrimp at a wide range of temperatures, but may prefer warmer temperatures to grow quickly. At our lab, we do our challenge tests with Vp at around 32° C.
Viruses have a narrow range of temperatures at which they replicate, making it easy to block their replications by using temperatures above or below their natural ranges for replication.
Bacteria, like Vp, on the other hand, are poikilotherm organism, meaning their internal temperatures vary considerably. Generally, when you increase their temperatures, you increase their metabolism. It’s important to consider the balance between the effect of temperature on the bacteria and the organism that it’s infecting and on the duration of the effect. Of particular significance are the organism’s heat shock proteins that are in charge of re-establishing the homeostasis of its cells disturbed by the temperature stress.
Martin Guerin (email@example.com): A few years ago, I believe the Center for Shrimp Molecular Biology and Biotechnology Centex (CENTEX) at Mahidol University in Thailand observed that it had difficulty infecting shrimp with Vp during the cold season. After discussions with them, we linked the problems we were having with low mortalities in Vp challenges to low water temperatures.
Daniel Gruenberg (firstname.lastname@example.org): I'd just like to chime in here from a scientific perspective. Experimental challenges in the lab are a far cry from actual infections in the pond. Sadly there has been little good science done regarding the presence of pathogens and their effects on morbidity and disease, and we don’t know what factors or co-factors affect transmission and pathogenicity. These things are very difficult to parse.
I have observed a general trend of less EMS at lower temperatures, but this may just be because the shrimp are feeding less or some other unknown factor.
In my opinion, much more research needs to be done. EMS continues and will continue to affect farming in affected regions for the foreseeable future.
Vijayan KK (email@example.com): In most of the rearing systems where aquaculture is done, pathogens (virus, bacteria, parasites and others) are opportunistic, lurking in the water or within the shrimp, waiting for the organism’s natural defenses to weaken, then they attack. Shrimp’s natural defenses are weakened during stress caused by a sudden shift in temperature, a drop in oxygen or even a change in the feeding regime. In temperate regions, a shift to higher temperatures can cause stress; while in tropical regions, a shift to lower temperatures can cause stress. A sudden rain that lowers temperatures often results in a whitespot outbreak, something that often happens here in India. Management strategies include: making sure your seedstock is free of pathogens, blocking other pathways for the entry of pathogens into your ponds, and not over feeding or under feeding. Men and terrestrial animals are surrounded with pathogens, but in water it gets much more complicated. So the discussion will go on. Often greed upsets the balance between our shrimp and its pathogens.
Alain Michel (firstname.lastname@example.org): What’s an opportunistic pathogen? A pathogen that’s lurking in your pond or within you animals waiting for the conditions to change so that it can attack and replicate. True pathogens don’t need conditions to change. Once they get into your pond they can immediately begin killing shrimp. The most devastating shrimp diseases like whitespot and EMS are true pathogens. For many years, shrimp farms in Mozambique and Madagascar were whitespot free, and then suddenly in 2011, whitespot arrived and started killing shrimp. The environment had not changed.
• Opportunistic pathogens have co-evolved with your shrimp. They can be controlled with biosecurity measures.
• True pathogens will replicate with no change in the environment and can result in 100% mortalities. They do not respond to biosecurity measures. They will eventually co-evolve, but it will take time. Shrimp farmers in Ecuador and Brazil are accelerating the co-evolution of whitespot by constantly challenging their shrimp with whitespot and then selecting the survivors. It’s an arms race between the pathogens and the shrimp. You can’t kill or eradicate true pathogens; you have to learn to live with them. Vaccines, when available, are a good way to deal with true pathogens, but currently, there are no vaccines that work with shrimp.
John Birkett (email@example.com): Low salinities help prevent EMS, but high temperatures and densities and low water exchange rates encourage it. An unbalanced ecosystem also encourages EMS problems. Antibiotics don’t help much with EMS, and developing resistance to EMS through selective breeding is not likely to work as well with EMS as it does with whitespot. Although in Ecuador in the 1980s, selection did work with the seagull syndrome, a toxic Vibrio parahaemolyticus disease. One of the reasons shrimp prices have remained high is the complexity of the EMS problem and its ability to limit production.
Michael Mogollon (firstname.lastname@example.org): John, this coincides with my observations in Central America during the dry season. When salinity increased dramatically and temperatures reached their maximum for the year, farmers, fearing the introduction of vectors, reduced water exchanges, creating a perfect storm for EMS, which wiped out entire ponds.
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