Nearly two million years ago, an early hominid cooked the first steak, marking a turning point in our evolutionary history. Not only is cooked meat more easily digested and nutritious, but it is also sterilized of harmful pathogens. Nevertheless, there’s just something appealing about a rare steak, or fresh sushi. And when you’re shopping in climate-controlled grocery stores, it’s easy to forget that there are flesh-dwelling pathogens in the meat aisle. But sometimes, we are rudely reminded of their existence.
Recently, a 50-year old Calgarian man became the first Canadian known to be infected with anisakis, a parasitic nematode. An hour after eating homemade sushi, using raw, wild salmon from a large grocery chain, he went to the ER with severe abdominal pain and vomiting. An X-ray led doctors to send an endoscope to his stomach, whereupon they encountered numerous ulcers. At the center of each ulcer was a 1-2cm white, wriggling worm.
Once you’re finished being disgusted, maybe now you can be impressed. Within a single hour the worms were causing severe pain, presumably due to the formation of ulcers. While many other human parasites (like giardia, cryptosporidium, and the pork tapeworm for example) are eaten as cysts that must hatch before getting to work, anisakids are ready to go, already using their tooth to bore into the tissues of the gut.
To be clear, anisakid parasites want to end up in your stomach about as much as you want them to. Humans are a dead-end host. Here it is unlikely they will reach adulthood, instead remaining as pubescent larvae. Never mating, never laying eggs, and never passing along their genes.
Anisakids would much rather infect a marine mammal like a seal, dolphin, or whale. In the gut of these animals they develop into adults, mate, and lay eggs that are defecated into the water. When the eggs hatch into larvae, they make their way up the food chain. They are eaten by crustaceans, which in turn are eaten by squid or fish. Once the parasite migrates from the gut into the muscles, they develop into a stage that is infective to marine mammals-and potentially humans.
Prevention is, as always, the best medicine. While anisakids can live at refrigerator temperatures (4°C) for 2-3 weeks, they cannot survive when frozen at -20°C for 7 days, or -35°C for 15 hours. This is why reputable sushi bars will often freeze their fish before serving. If humans do become infected with anisakids, rather than prescribing anti-parasite drugs, doctors prefer to get rid of the parasite using an endoscope equipped with pincers at the end, much in the same way you use the claw crane to try and hook a stuffed toy at an arcade.
While North America has relatively few anisakid infections compared to the 2000 cases reported annually in Japan, experts predict infections will rise. Anisakids are well-travelled. They are found in all major oceans and seas, and their infective stage is found in many different commercially important species of fish—anchovies, salmon, and pollock, to highlight a few. Further, conservation efforts have led to rebounds in the populations of the marine mammals that serve as the final host for these parasites. Add in the increasing popularity of raw seafood delicacies like sushi, and it’s a perfect recipe for increased infection rates-even in the middle of the Canadian prairies.
Given the benefits of cooking, why are we drawn to rare and raw meat? From sushi to steak tartare, beef carpaccio to fermented herring, many countries boast raw meat delicacies, which are increasingly being exported beyond their borders. So if you still love sushi, but don’t want to host a parasite, perhaps best to learn about what might be lurking in your meat, and take a closer look at that piece of fish you’re about to eat.