August 27, 2013

Controlling Freshwater Fish Disease

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In recent years, fish in the Great Lakes region have been affected by viral hemorrhagic septicemia virus, a disease that attacks the blood vessels of certain freshwater fish.

While mass die-offs have not occurred for some time, researchers at Cornell, including Emily Cornwell grad, have been closely monitoring the disease in the area to ensure it does not spread.

“I was really interested in finding a way to continue our surveillance without lethally sampling a large number of fish,”  Cornwell said.

Whereas previous tests required samples from the internal organs of fish and therefore proved lethal to the sampled individuals, a new non-lethal method of sampling from fins or gills has allowed for a more humane approach for the surveillance of VHSV.

VHSV was first prevalent in European waters, with noted effects on rainbow trout populations. While this non-lethal testing has been available in Europe for some time, the Great Lakes strain of VHSV has not been thoroughly studied until recently.

VHSV causes internal bleeding in freshwater fish and, in some cases, can also affect their brains, causing abnormal behavior in swimming patterns. Some external symptoms can include bloating, protruding eyes and skin lesions.

The virus is generally lethal to fish, although there have been cases where fish have recovered on their own. According to Cornwell, these fish can become carriers, making it dangerous in terms of spreading the virus. Although there is a vaccine available for fish, it is neither legal in the U.S. nor is it practical to try to inoculate entire fish populations. There is currently no known cure for VHSV.

According to Cornwell, this virus has no known effects on humans due to its temperature sensitivity. It cannot survive in warm temperatures, making it ineffective in both birds and mammals that ingest fish. Fish, however, if inhabiting waters between 37-54° F, are susceptible to the virus. The virus has been known to be transmitted between fish through ingestion of infected live fish and through infected water entering the gills or skin.

Although the virus is not transferred through warm-blooded animals or through high-temperature regions, it has the potential to infect any fish it encounters.

A strange characteristic of this disease is that it can stay dormant until the fish becomes stressed, at which time it manifests itself full-force, Cornwell said. This is especially worrisome for fish farmers.

“In addition to affecting populations of fish in the wild, it’s become a concern for fisherman as well as people who are raising fish in aquaculture,” Cornwell said.

Because aquaculture fish are often enclosed and are more inclined to stress, VHSV can heavily affect these populations. U.S. hatcheries and farms have yet to find the virus within its populations. However, all five Great Lakes have reported the strain of the virus within almost 30 different fish populations.

“The species that we do know are susceptible represent a wide taxonomic range. However, there are many species that we do not know if they are affected or not,” Cornwell said.

Cornwell and other Cornell researchers have been concerned with keeping VHSV in the Great Lakes region under close surveillance. However, to do so, many fish would previously have to be killed in order to provide a relatively accurate assessment of the water and fish population health.

The new non-lethal test for VHSV takes tissues from either the fins or gills. This makes testing more humane and easier to practice on protected species. The new test also helps preserve fish populations used for sport in the region.

“It allows us to track the virus much more closely,” Cornwell said. “We can sample a much larger sample of fish because we’re only taking a small biopsy of their fin or gill and can release them as soon as we do that.”

“You can never be a hundred percent sure unless you tested every individual, but a general rule of thumb is that if you want to be 95 percent confident, then you need to test 60 individuals,” Cornwell said.

Although there have not been many mass die-offs due to VHSV in the Great Lakes region since 2003, around the time when the virus was first discovered, Cornell researchers are still keeping tabs on the virus.

“It is important to know what the baseline prevalence is in case there is an additional outbreak and also continue to make sure people are aware and continue to take the precautions,” Cornwell said.

Farmers and fishermen have taken precautions in making sure the disease does not spread to inland waters and aquaculture.

Due to its threat to fish populations, VHSV is also a reportable disease.

“If the disease is found in samples of a previously unaffected region, the test must be reported to the state veterinarian,” Cornwell said.

The government keeps close tabs on this virus due to its potential risk to aquaculture and the economy. According to Cornwell, however, it is through the education of the community and those who would be in contact with potentially infected fish that the disease can remain controlled and only in the wild fish populations.

Original Author: Yvonne Huang

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