Jews eat fish and fish heads on Rosh Hashanah to symbolize leadership, abundance, and the hope of being “the head and not the tail” in the coming year.
The word Rosh means “head” in Hebrew, and so eating a fish head or serving a whole fish is a physical metaphor paired with this traditional blessing, which expresses a desire to be proactive leaders and changemakers rather than followers.
Fish also reproduce in large numbers, symbolizing a wish to be fruitful, multiply, and merit an abundance of good deeds, blessings, and prosperity in the Jewish New Year. These denizens of the deep keep their eyes open at all times; their vigilance represents the idea that God is constantly watching over the world with open, protective care.
For many Israeli families, no Rosh Hashanah table is complete without carp. Whether served as traditional gefilte fish or baked whole, this fish has long symbolized abundance and prosperity for the year ahead.
For carp farmers around the world, the holiday favorite has a serious enemy. Unfortunately, they are susceptible to several serious viral, parasitic, and bacterial diseases that can reduce their numbers in fish ponds and fisheries dedicated to capturing, processing, and managing fish.
Koi herpesvirus (CyHV-3) is one of the most destructive diseases in carp aquaculture, capable of wiping out entire populations in just days. Every outbreak can mean enormous economic losses and lasting damage to wild fish populations.
But now a new international study that Israeli and American researchers have been working on for the last 15 years finds that while common carp are highly vulnerable to a devastating virus, their close relatives help stop its spread, offering hope for fish farmers and freshwater ecosystems.
It has just been published in the journal Aquaculture under the title “Differences in CyHV-3 infection and infectivity among common carp and Asian carp species.”
The popular fish in Israel is the “mirror carp,” a type of domesticated fish commonly found in Europe but widely introduced or cultivated elsewhere. They are a variety of the common carp (Cyprinus carpio) developed through selective breeding. The name “mirror carp” originates from their scales’ resemblance to mirrors.
They taste like other kinds of carp, but they are popular here because of their impressive appearance.
Just in time for the Jewish New Year, researchers from the Hebrew University of Jerusalem (HUJI) and Michigan State University have discovered unexpected good news: while common carp are highly vulnerable to the virus, several of their closest relatives are surprisingly good at stopping it in its tracks.
They discovered that goldfish, grass carp, silver carp, and black carp rarely become infected naturally. Even when scientists deliberately infected them, these fish fought off the virus quickly and were far less likely to pass it on to others.
The team said their research offers a timely reminder that protecting one of the world’s oldest farmed fish depends not only on understanding the virus, but also on understanding how some of its closest relatives have learned to resist it.
“Because these fish often live alongside common carp in ponds and lakes, there has been concern that they might act as silent carriers of the virus,” said Prof. Lior David of HUJI’s Robert Smith Faculty of Agriculture, Food and Environment in Rehovot and the study’s senior author. He was assisted by Ph.D. candidate Batya Dorfman.
“Our results show that, although they can occasionally become infected, they are actually very poor at spreading the disease,” he said.
To find out, the researchers recreated how infections occur in the real world. Healthy fish were placed in tanks with infected common carp. While the virus spread rapidly among common carp, only a small fraction of the other carp species became infected, and almost none became ill, he told The Jerusalem Post in a long interview.
The team injected the virus into the fish to bypass their natural defenses; even then, the differences remained striking. Unlike common carp, which developed severe disease, the other species rapidly cleared the virus from their bodies and showed little sign of illness.
“Finally, we asked the most important question: Could these healthy-looking fish spread the virus back to common carp? The answer was ‘yes,’ but only inefficiently.”
Compared with infected common carp, the other species transmitted the virus much less effectively, making them what scientists describe as “inefficient carrier hosts” that are unlikely to drive major disease outbreaks, said David, who is academic curator of over 30,000 different species of preserved fish that lived in the Mediterranean Sea and Kinneret, ponds, streams, and rivers of Israel the preserved fish collection at the National Natural History Collections of HUJI’s Givat Ram Campus.
David, a postdoctoral fellow at Stanford University’s School of Medicine and Department of Biological Sciences in California, has been on the faculty of HUJI’s Animal Sciences department since 2006.
Common carp, grass carp, and silver carp remain among the world’s most essential farmed fish, supplying food to billions of people and supporting aquaculture industries across the Middle East, Asia, and Europe.
“The biggest risk still comes from infected common carp themselves,” David asserted. “Our research found that the presence of these other carp species is unlikely to be the main cause of disease outbreaks.”
Asked what the most surprising finding of the research was, David said, “It was that from an evolutionary point of view, these four carp relatives are close to common carp but respond very differently to the virus.” The study took so long because “it takes two to five years for these fish to reach puberty and reproduce, so we had to be patient.”
Fish have different immune systems compared to animals living on land, including humans, continued David, who said he likes eating baked fish, “but I’m not a fan of traditional boiled gefilte fish.” When the virus enters the fish, the immune system identifies it and responds. There are several lines of defense.
“Common carp are different. The virus enters them efficiently, and nothing prevents it from reaching tissues. The resistant types have a much more efficient immune system that doesn’t allow the pathogen to replicate.”
There’s a huge difference between disease outbreaks in the wild and in fish farms.
“There are pathogens in the seas, but the fish are at a distance from one another – like humans were told to observe social distancing during the COVID-19 pandemic. In fish farms, there are many fish in a small volume of water. If one is infected, it can easily infect others. It can spread like wildfire,” he explained.
“CyHV-3 exists everywhere there’s common carp. It can devastate carp populations very rapidly, mostly in spring and autumn when the temperatures change. Animals on land, including humans, maintain a steady body temperature, so their immune systems are always on.
“But fish, reptiles, and amphibians can’t regulate their own heat, so they are more affected by their environment.”
Fortunately, the carp sold for the Israeli holiday market does not pose a food-safety concern for people who eat infected fish: “Veterinary regulations are very strict, and the fish are checked regularly. And of course, cooking kills pathogens,” he declared.
He added that he and his team are now working with AI to determine the condition of fish in farms.
When queried why evolution apparently gave these closely related fish such different responses to the same virus, the researcher said: “We don’t know what caused it. Perhaps everything started from a better point with more resistance to the virus, which then adapted in common carp and not the other species.
“It’s likely that they all started being not susceptible. First reports of this virus causing the disease in carp were only around 1980.”
If they succeed in identifying the genes, David thinks they could eventually be used to breed CyHV-3-resistant common carp.