I learned it from one of my outdoors mentors, Land Tawney of Backcountry Hunters & Anglers. Catch a trout in a beautiful Montana stream, bonk it, bleed it, fillet it, skin it, chunk it, then add a bunch of lime, cilantro, red onion, and jalapeño. Shake it up in a container, let it sit in the cooler for an hour, open a bag of chips and voilà—ceviche so good you might as well be on a beach in Mexico. But is eating raw freshwater fish safe?
Pretty much every time we make trout ceviche, people tell us we’re going to die.
Most of what you read online corroborates the widely-held belief that eating freshwater fish is dangerous. Lung flukes, tapeworms, anisakis roundworms: There are a number of parasites that can be passed to humans from raw fish. One article from VICE News loudly exclaims that all wild fish have worms. Many others detail and display the gruesome effects of rare, fishy parasites. In fact, some papers suggest that consumption of raw or undercooked fish is the primary vector for many of the human diseases related to these parasites. Some experts, however, say there really isn’t that much to worry about.
Thought to have been invented in Peru more than 2,000 years ago, ceviche has been embraced by most of the Spanish-speaking world and has spread around the globe. I’ve eaten fresh, never-frozen ceviche from Alaska to Mexico to Spain and was served Arctic char ceviche in Iceland just a few weeks ago. The dish is easy, delicious, and a bit adventurous.
Ceviche at its most basic level is raw fish marinated in lime, lemon, or other citrus juice. The citric acid denatures, or breaks down, the protein cells in the meat, giving it the texture and appearance of being cooked.
Contrary to some beliefs, a citrus bath does not actually “cook” meat in the same way heat does. Fish chunks in ceviche will turn opaque and firm, but the citric acid does not necessarily kill all the bacteria or parasites in the flesh. Most experts recommend freezing fish at -4°F for at least 48 hours before serving raw. This is common practice for sushi and sashimi, and required by the FDA and most states, but it should be mentioned that most home freezers do not get cold enough to kill all the worrisome parasites.
Tim Sly, PhD, epidemiologist and emeritus professor at Ryerson University’s School of Public Health in Toronto, specializes in food-borne illness and has written extensively about the parasites associated with fish. He says much of the concern over eating freshwater fish raw is overblown.
“Well you don’t have much to worry about actually,” Sly told MeatEater. “There’s a lot of strange things happening in marine fish, but freshwater fish you’ve really got hardly anything to worry about there at all.”
He says there are more prevalent parasites, dangerous bacteria, and toxins in saltwater fish, despite the higher prevalence of eating those species raw. Heavy metals may be of greater concern in freshwater, and cooking doesn’t affect that contamination.
According to Sly, the primary parasites of concern with eating raw fish from fresh or saltwater are a few species each of cestodes (tapeworms), nematodes (roundworms), and trematodes (flukes).
Of those, Sly says the parasites most commonly picked up from eating raw or undercooked freshwater fishes are cestodes of the Diphyllobothriidae family, especially D. latum, commonly known as fish tapeworm. They survive better in the human freshwater biome than do saltwater-specific parasites.
These worms are rendered dead and harmless through cooking or freezing and may not cause problems even if eaten alive in raw fish. However, it is possible for them to imbed in human stomach linings, causing Diphyllobothriasis. The symptoms of this disease can vary from completely unnoticeable to severe diarrhea, abdominal pain, and intestinal tract blockage.
Diphyllobothriasis has increased in prevalence in some regions the developed world, likely alongside the popularity of sushi, ceviche, and other raw preparations. It has become extremely common in Japan, where sushi is still often served without freezing. Yet, the disease also seems to have disappeared from other areas. Worldwide, estimates suggest that more than 20 million people are infected. These worms can persist for decades if left alone, but treatment with several different medicines is simple and effective. They pose little threat to a healthy person and most cases are mild. Some, however, can be exceptionally unpleasant.
Last year, the Washington Post reported on a California man who had a daily habit of eating salmon sashimi. That is, until he pulled a 5 ½-foot tapeworm out of his own, well, you know where. He brought it with him to the hospital, wrapped around a toilet paper roll. These revolting worms have been documented as long as 30 feet.
Likewise, many, if not most, saltwater fish can carry nematode roundworms, especially those of the genus Anisakis. They are regularly found in everything from cod to tuna. These parasites evolved with marine mammals like seals and whales as their definitive hosts, and do not survive the human stomach’s freshwater environment. When ingested, they burrow into the intestinal wall, get stuck and die, which triggers an immune and sometimes allergic response. Cysts form around the worms which can cause abdominal pain, vomiting, diarrhea, and malnutrition. But because humans are a dead-end host, Anisakiasis will eventually abate, though symptomatic treatment may be necessary.
Of note here, anadromous fishes like salmon can carry both fresh and saltwater parasites since they spend time in both environments.
More nasty parasites like lung and liver fluke of the clade Trematoda can be acquired from eating raw fish, crustaceans, and mollusks, but these are typically localized and less common in humans. A ceviche-like local delicacy called koi plaa from northeast Thailand that involves raw fish, lime juice, herbs, and live red ants has been linked to abnormally high rates of liver cancer, due to a high prevalence of liver flukes in the area’s fish. Liver and lung fluke are both rare in the United States, more often appearing in South and Central America, Asia, and Africa.
The distribution of tapeworms, roundworms, and flukes in North America is not well understood, though infection rates are known to be higher along the West Coast, alongside the prevalence of Diphyllobothrium latum in wild salmon. Certain waterways and fish populations carry these parasites, while others don’t. You can contact local biologists and health professionals to find out if there have been reported cases in your area.
According to Sly, you can be certain you’re not eating live parasites “only by heating/cooking the fish, by freezing, by drying, or by pickling.” A method called “candling,” holding translucent fish fillets against a light source to locate worms, is common in the commercial sushi industry, and may be a good starting point for knowing if a fish is dangerous to eat raw. Bacteria can be a concern with any raw meat, so always make sure fish is fresh, properly cleaned and carefully handled. Consider the environment where the fish came from and heed warnings about mercury, industrial pollutants, and sewage.
Scientific and popular publications are nearly unanimous: eating raw fish can have its consequences—even though many of those parasites induce mild, if any, symptoms. Few are ever lethal, according to Sly.
So, no, you’re not going to see any fresh carp ceviche recipes on themeateater.com/cook anytime soon. But are we going to tell you to never, ever get creative with a wild meal out in the woods? That isn’t really our style. Just be smart, and don’t say we didn’t warn you.