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The History of the Taura
At Aquaculture America 2005 (New Orleans, Louisiana, USA, January 17-20, 2005), Dr. Jeff Lotz, a shrimp disease specialist with the Gulf Coast Research Laboratory in Mississippi, reviewed the history of the Taura virus in the United States.
I’m going to present a very general review of the Taura Syndrome Virus (TSV) in the United States. The original outbreaks in 1994 in Hawaii and Florida were traced to imported seed from Hawaii and to wild broodstock from Florida. The first time Taura hit any large farms in the USA was 1995, when it hit the mid-coast and south coast of Texas, causing great losses. The source was never determined. The outbreak in 1996, which occurred in South Carolina, mid-Texas and south Texas, was probably caused by contaminated seed. If you look at 1997, 1998 and 1999, there were outbreaks on the central Texas coast, but no outbreaks in South Texas. The sources were never determined for these outbreaks. From 2000 to 2003, there was no TSV in Texas. In 2004, there was an outbreak in southern Texas, but not in central Texas.
So what you see is a spotty pattern that looks like the virus is being reintroduced each year. What has been ruled out, except in 1996, is the seed. We have the best grasp on the seed because the seed going to most places in the United States is certified disease free.
When TSV hits a shrimp pond, acutely infected animals show all the gross signs and a high mortality rate. The shrimp that survive the initial onslaught become chronically infected and don’t necessarily show any signs of the disease. Over time, some of them may recover.
Taura is transmitted to new animals by acute, chronic and dead animals. The infection rate from dead animals is six times greater than it is from chronically infected animals. That’s because dead animals are consumed, leading to direct infection. Acutely infected animals die within two and a half days or become chronically infected. Dead animals lose their infectivity as they decompose and are eaten. They are usually eaten in a day, or if all of the shrimp have died, they decompose over five days. Dead animals lose about 20% of their infectivity every day. If you look at how quickly dead animals are eaten by live animals, you can see that the decrease in infectivity is about 6% per hour. The decomposition of dead animals is really accelerated by consumption.
The highest probability of infection is from chronically infected animals, and they can be around until you harvest. An epidemic on a shrimp farm is around five or ten days, so during that period the chronic animals are not very important; the dead animals at that stage are more important from an infection point of view. At the end of an outbreak on a farm, there are no susceptible animals left; they are either dead or chronically infected. Chronically infected animals can start new outbreaks. Over time, however, chronically infected animals, which can live for up to two years, or longer, lose some of their infectivity. Chronically infected animals are not as healthy as uninfected animals. If they’re stressed, or if it rains, they could start a new epidemic.
The other source of vertical transmission that we looked at is broodstock. Several years ago we had six broodstock animals that were infected with Taura that we spawned. Their offspring did not show any Taura. Taura is not passed through the eggs; that’s not to say that larvae will not get Taura from the water as soon as they hatch.
Other animals that transmit the infection are mechanical carriers, carrying it in their gut, on their body surfaces, or some other place, but they are not infected, they are carriers. Birds can carry the infection in their gut and it can stick to equipment and shoes. Another source is processing plants, where imported shrimp come into the United States. We do know there are significant levels of viruses on imported shrimp.
It’s difficult to disinfect for Taura. It will stay in the water at least a month and possibly longer. It’s heat tolerant. When boiled for ten minutes and injected into shrimp, you can still produce infections. When hit by Taura, probably the best thing for a shrimp farmer to do is remove all shrimp tissue from the farm and dry out.
We’re seeing lots of variation in the Taura genome, at least three strains so far. Variation can come from a number of sources, but the one I want to concentrate on is the mutation rate. RNA viruses like Taura have very high mutation rates, and Taura has a big genome, over 10,000 base pairs, so you get more mutations than you would in a smaller genome. If you have 10,000 base pairs, over 60% of the progeny will have one or more mutations with each replication.
There is a very large potential for viral evolution, which is going to make the Taura virus and other RNA viruses moving targets. I think there is a good probability that new strains of Taura will show up. We’re always worried about the next virus. Well the next virus to show up is very likely to be a new strain of Taura.
Sources: 1. Aquaculture America 2005 (a World Aquaculture Society Meeting). Review of the epidemiology of TSV. J.M. Lotz, R. Adams, Y.S. Juang, A.L. Lawrence, D.V. Lightner, A.C. Ostrowski and M.A. Soto. January 20, 2005. 2. Bob Rosenberry, Shrimp News International, March 13, 2017. Updated.
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