Now that many wild fish populations are too depleted to harvest, aquaculture offers a way to provide an ever-expanding global population with fish to eat. This rising demand has made fish farming the fastest growing food industry on Earth.

But it’s not easy to keep farmed fish healthy. Infectious diseases can run rampant in the confined spaces of fish production facilities. That is the challenge J. Oriol Sunyer, a professor in Penn Vet’s Department of Pathobiology, is addressing with paradigm-shifting discoveries about the immune system of fish.

While the vast majority of immunologists study mice, Sunyer’s study organism of choice is a species most people associate with mountain streams or a tasty dinner entrée: rainbow trout. Working with these evolutionarily primitive animals (ancestors of rainbow trout swam Earth’s waters 400 million years ago) Sunyer has identified features of immune system function that mice researchers have missed.

“We study the fish immune system so that we can learn from a so-called ‘primitive’ organism,” says Sunyer. “Because the immune system of mammals is so complex, sometimes you find things in fish that escaped notice in mice.”

In 2006, Sunyer and colleagues made a surprising discovery related to B cells, a type of white blood cell that helps fight infection. The scientists found that fish have a subset of B cells with the ability to phagocytose, or “eat,” microorganisms. Before, scientists believed that only other kinds of immune cells, such as macrophages and neutrophils, possessed this capacity.

Sunyer’s work may lead to better vaccines to protect fish from infection. Because insights into fish biology have led to a greater understanding of the immune systems of other organisms, this research may also improve not only fish health, but perhaps human health as well.

Earlier this year, Sunyer teamed with other researchers at Penn Vet to show that these microbe-consuming B cells are also present in mammals. “We broke the dogma,” says Sunyer, “because people thought that B cells were not phagocytic in fish or in vertebrates in general.”

This finding recasts how researchers seeking therapies and vaccines think about the various roles of cells in the human immune system. Currently, some experimental cancer treatments extract phagocytic cells from a patient, prime the cells to detect cancer cells, and then re-inject them to attack the cancer. Sunyer says that it’s possible phagocytic B cells could be “trained” to do the same thing.

Sunyer, the only fish researcher at Penn Vet, is already reshaping the field of fish vaccinology. Since the 1980s, many farmed fish have been receiving vaccines to protect them against common infections like columnaris disease and strep. But the current best method of vaccinating fish—individually, by injection—is time-consuming and stressful to the animals.

A 2010 discovery by Sunyer and his colleagues is introducing new ways of protecting fish from disease. His research identified a type of antibody that patrols the mucous membranes of fish. Unlike human skin, all fish skin is considered a mucous membrane and is quite permeable to pathogens. Finding an antibody specific to this kind of tissue could help researchers find ways to fend off diseases before they make fish sick.

The ideal inoculation, Sunyer says, would provide mucosal immunity and wouldn’t have to be injected. Instead, he’s working to find vaccines that fish could be bathed in, or could eat, keeping in mind that the vaccines also have to be safe for humans to eventually consume as well.

To take his research from the lab to the dinner table, Sunyer is partnering with vaccine manufacturers, including the largest rainbow trout aquaculture facility in the United States, to work on developing a better vaccine.

“The last 20 years have been huge in increasing the use of fish vaccines, but we are way behind the mammalian field,” he says. “We need to do so much more research to create good, safe, and lasting vaccines.”