New study 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.

Common carp are among the world’s most widely farmed freshwater fish, providing food and livelihoods for millions of people. But across fish farms and freshwater ecosystems, they face a formidable threat: a highly contagious virus capable of causing devastating outbreaks and wiping out entire populations in a matter of days.

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.

Now, researchers from the Hebrew University of Jerusalem and Michigan State University have discovered an unexpected piece of 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.

Their study found 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.

"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 the Hebrew University of Jerusalem, senior author of the study. "Our results show that, although they can occasionally become infected, they are actually very poor at spreading the disease."

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.

The team then took the experiment one step further, injecting the virus directly into the fish to bypass their natural defences. 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, the researchers 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." In other words, they are unlikely to drive major disease outbreaks.

That matters because common carp, grass carp and silver carp remain among the world's most important farmed fish, supplying food to billions of people and supporting aquaculture industries across Asia, Europe, and the Middle East.

"The biggest risk still comes from infected common carp themselves," David said. "Our findings suggest that the presence of these other carp species is unlikely to be the main cause of disease outbreaks."

The findings offer a hopeful new perspective on managing one of the most damaging diseases in carp aquaculture. By revealing how closely related fish species resist infection and limit transmission, the study could help researchers and fish farmers better understand—and ultimately reduce—the risk of future outbreaks.