Aquaculture and Fisheries News => Aquaculture News => ข้อความที่เริ่มโดย: Nicaonline ที่ พฤศจิกายน 27, 2006, 12:35:26 PM

หัวข้อ: Vaccines That Keep Farmed Salmon Safe to Eat May Help Humans
เริ่มหัวข้อโดย: Nicaonline ที่ พฤศจิกายน 27, 2006, 12:35:26 PM
Vaccines That Keep Farmed Salmon Safe to Eat May Help Humans

September 23, 2005; Page B1

Two DNA-based vaccines for animals have become the first to make it from the lab into commercial use, buoying hopes for similar vaccines for human diseases, such as Ebola, HIV/AIDS and SARS.

One already has protected some salmon that could be headed for your dinner table soon. Fisheries scientists have known since the early 1990s that some wild salmon returning from the open Pacific Ocean to spawn in British Columbia carry a virus that wreaks havoc on the fish farms that dot the coastal inlets in Canada's westernmost province.

The domesticated fish are Atlantic salmon, favored by farmers for their docile temperament and fast growth -- qualities that make them the Hereford cattle of aquaculture. But unlike indigenous types, the transplanted species have little natural resistance to the local virus, which causes fatal hemorrhaging, and can contract it from the wild fish swimming past their pens.

In mid-2001, a destructive epizootic, the animal equivalent of an epidemic, struck, sickening Atlantic salmon in 36 farms over a two-year period. In some pens, more than 90% of the young fish succumbed. "It's devastating to watch your animals die and not be able to do anything to stop it," says Linda Sams, environmental manager at Marine Harvest Canada, a fish-farming concern in Campbell River, British Columbia.

The large loss of fish spurred new efforts to develop a vaccine against the virus, which causes the untreatable disease called Infectious Hematopoietic Necrosis, or IHN. This time, scientists tried arming the immune system using a snippet of viral genetic code translated into DNA, rather than a traditional approach, such as culturing a weakened IHN virus.

The unorthodox approach sprang from a surprising discovery more than a decade earlier: An enzyme that fireflies use to glow could be detected in the muscles of mice months after they were injected with the gene that makes it. No one had thought that injecting DNA in a muscle would do anything. Researchers of human diseases then reasoned they could use DNA to train immune systems to resist foreign invaders, such as HIV, that had eluded conventional vaccines.

Scientists at Aqua Health, a unit of Novartis in Prince Edward Island, Canada, solved the problems of potency and mass production. They took advantage of work by Ottawa scientists, who put a gene for a protein that covers the IHN virus in a ring of DNA, or plasmid, which some bacteria use to share genetic code. A single shot of vaccine behind the salmon's dorsal fin contains 10 micrograms of these DNA rings.

Plasmids make their way into muscle cells, much as infecting viruses do, where they spur the cells' protein-making machinery to pump out copies of the viral protein. By tricking these cells to make telltale proteins of a virus, the DNA-based vaccines better mimic infections and so can confer greater protection.

Like salmon swimming upstream, the viral protein produced by the fish cells migrates into the bloodstream. The fish gird for battle by producing antibodies and preparing white blood cells to fight the virus, a response traditional vaccines barely stimulate. After a few months, the muscle cells containing the plasmids die -- as normal muscle cells would -- so fish vaccinated as youngsters carry virtually no traces of vaccine except their immunological armor.

More than six million salmon were inoculated while the vaccine was evaluated by Canadian regulators. Tests showed the vaccine, called Apex-IHN, protected the fish without adverse effects. In July, Canada licensed the product for sale.

Also in July, the U.S. Department of Agriculture gave its OK to a DNA-based vaccine from drug maker Wyeth to protect horses from West Nile virus. Until now, DNA vaccines had worked in experiments with mice, but were disappointing in humans and some large animals, which required large doses of the DNA plasmids to work.

Wyeth improved production of plasmids by bacterial colonies, and found ways to purify and stabilize the DNA on a commercial scale, said Hsien-Jue "Steve" Chu, head of research at the company's Fort Dodge Animal Health unit. Adding proprietary chemical boosters to the vaccine also helped. "The improvements were the result not of any single button that we pushed but a very systematic approach to take the whole process apart," he says.

Enhancements in production and potency could make a DNA-based vaccine viable in human research. "We've gone from six or seven years ago being bitterly disappointed by the low responses to expecting at this point that DNA vaccines will yield pretty good immune responses" in humans, says John Eldridge, vice president for vaccine discovery at Wyeth.

Recent human tests of vaccines against HIV and Ebola being developed by the National Institutes of Health have led to measurable antibodies and responses in white blood cells. The NIH has been working on several vaccines with Vical Inc., San Diego, whose technology aided Novartis on the salmon vaccine. This year, an NIH-run pilot plant in Maryland will begin producing DNA for use in vaccines.

Write to Scott Hensley at

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