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Steelhead rainbow trout. Credit: U.S. Fish and Wildlife Service.

University of Pennsylvania’s School of Veterinary Medicine Scientists Unravel Mystery of How Fish Develop Immunity to Disease

Nifa Authors
Lori Tyler Gula, Senior Public Affairs Specialist

Jawed vertebrates comprise more than 99% of living vertebrate species, including humans. And when the immune systems of these animals are stimulated by an infection or immunization, they generate proteins called antibodies that fight disease. In warm-blooded vertebrates such as humans, these antibodies are produced in a germinal center, which is a specialized structure that forms in secondary lymphoid tissues.  

However, until recently, scientists believed that cold-blooded jawed vertebrates such as fish did not have these specialized structures. This led them to wonder how these fish, called teleost fish, such as cod, salmon and rainbow trout, were able to mount an immune response.  

With support from USDA’s National Institute of Food and Agriculture, University of Pennsylvania’s School of Veterinary Medicine scientists have discovered, contrary to earlier belief, that fish develop an antibody response in similar structures located in the spleen. J. Oriol Sunyer, professor of immunology at Penn Vet, said this finding is significant because scientists now know how and where antibody responses are produced in these economically important fish. This fundamental understanding will result in vaccines that better activate the fish immune system, hopefully providing stronger protection against a myriad of diseases. These new findings were reported in the journal Science Immunology, and features on the cover of the December issue. 

A rainbow trout M-LA. Immunofluorescence microscopy shows a representative M-LA induced in the spleen from Ichthyophthirius multifiliis-infected trout. Cryosections were stained for IgM (green) and CD4 (red). Scale bar, 100 μm. Groups of dark cells are MMCs.
A rainbow trout M-LA. Immunofluorescence microscopy shows a representative M-LA induced in the spleen from Ichthyophthirius multifiliis-infected trout. Cryosections were stained for IgM (green) and CD4 (red). Scale bar, 100 μm. Groups of dark cells are MMCs.

“We predict that our findings will lead to the development of more effective adjuvants and vaccines for teleost fish,” Sunyer said. “In turn, this will minimize disease-related losses in fish farms, thus making aquaculture a significantly more economically robust and profitable industry. Thus, the return in investment of this research is potentially enormous.” 

Only a handful of fish vaccines produce highly protective responses against diseases. Most vaccines currently used for aquaculture are not so effective in protection fish against disease. Additionally, there are many old and emerging new fish pathogens for which vaccines currently do not exist.   

A major problem that has thwarted the generation of more effective fish vaccines has been the lack of knowledge on how and where antibody responses are induced in fish. Thus, these findings significantly close this gap of knowledge, and will now allow the development of more effective vaccines for fish. 

“In turn, this will reduce fish mortalities in fish farms, which will make fish farms more cost-effective, and this will lead to reduced prices of farmed fish, making farmed fish more affordable for the general population,” Sunyer said. “This will positively impact the overall health of the population as fish represent a healthier and sometimes less expensive source of protein when compared to other protein sources, including red meat.”  

“Fish farming generally has a smaller carbon footprint and requires less land and fresh water than terrestrial livestock farming,” he said. “And fish are far more efficient at converting feed into protein for human consumption than beef, pork and poultry. Americans should care about our findings, as they pave the way to a healthier fish farming industry, which in turn will produce a cheaper and greener source of protein, thus protecting the pocket of Americans as well as their health and that of the planet.” 

Finfish aquaculture is a growing industry in the United States, according to Timothy Sullivan, Ph.D., NIFA national program leader. In the 2018 USDA Census of Aquaculture, the two most profitable freshwater finfish species, trout and catfish, accounted for $117 million and $367 million in farm gate sales, respectively. Its growth has the potential to improve food security, vastly reduce trade deficits and protect national security by increasing U.S. economic and nutritional sovereignty.  

“The greatest threat to yearly aquaculture production and the growth and viability of farms is disease,” Sullivan said. “Because of this, efforts to understand fish immunity and produce more successful vaccines are of paramount importance.”  

“If this project serves to improve teleost vaccine efficacy by even a small percentage, it will return millions of dollars in value for producers around the country and result in greater domestic production of healthy protein for U.S. consumers,” he said. 

Farm Bill Priority Areas
Animal health and production and animal products
U.S. States and Territories

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