Do Any Ocean Animals Spread Diseases to Humans?

Question: COVID-19 is spread by a coronavirus that is thought to have jumped from bats to humans. Are there any ocean animals that can spread diseases to humans? Can sharks spread diseases to humans? – Kate O., Charleston, SC

Answer: There are indeed zoonotic diseases that can be spread from ocean animals to humans, and while I don’t know of any cases of sharks getting humans sick, all kinds of nasty microbes have been found in sharks. It’s perhaps worth noting that when I asked colleagues who study this stuff, opinion was split over whether foodborne illnesses that only infect the person who eats them and can’t be spread to others count as a zoonotic disease.

Perhaps the most widespread zoonotic disease from ocean critters is vibriosis, which you can get from eating raw oysters infected with Vibrio bacteria. This illness affects tens of thousands of Americans every year, according to the CDC, and kills dozens.

There’s also something called fish-handler’s disease, which you can get by, um, handling fish. Specifically, if you have tiny cuts on your hand while handling a dead fish, a bacteria commonly found on fish skin can infect you, which can result in a nasty, painful rash. If you’re handling marine mammal meat with an open wound (or are bitten by a marine mammal), you can get a similar condition known colloquially as “seal finger.”

As for sharks, their teeth famously contain all sorts of nastiness that can result in bite victims getting severe medical conditions—shark bite victims who survive the initial trauma of the bite can still get very ill from infection. In addition to flesh-eating bacteria, other bacteria found on a shark’s tooth can cause respiratory tract infections, bloodstream infections and more—and the bacteria from shark’s mouths seem to be unusually resistant to antibiotics. Other shark tissues have also been found to contain a variety of other disease-causing microorganisms. Shark muscle, blood and digestive tracks have been sampled, and they were found to contain pathogens including several kinds of Vibrio, E. coli, C. botulinum, and a dozen other disease-causing microorganisms I had never heard of but seem to result in extremely unpleasant symptoms. I’ve been unable to find a documented case of a human getting severely ill as a result of handling shark meat or shark tissue, but this definitely could happen and probably already has.

Coronaviruses have not been found in marine organisms, but lots of other disease-causing microorganisms have—and the scourge of zoonotic diseases infecting humans seems likely to be a problem for many years to come.

Ask a Marine Biologist is a monthly column where Dr. David Shiffman answers your questions about the underwater world. Topics are chosen from reader-submitted queries as well as data from common internet searches. If you have a question you’d like answered in a future Ask a Marine Biologist column, or if you have a question about the answer given in this column, email Shiffman at WhySharksMatter@gmail.com with subject line “Ask a marine biologist.”

Dr. David Shiffman is a marine conservation biologist specializing in the ecology and conservation of sharks. An award-winning public science educator, David has spoken to thousands of people around the world about marine biology and conservation and has bylines with the Washington Post, Scientific American, New Scientist, Gizmodo and more. Follow him on Twitter, Facebook and Instagram, where he’s always happy to answer any questions about sharks.

The views expressed in this article are those of David Shiffman, and not necessarily the views of Sport Diver or Scuba Diving magazines.