Reports this week that a US man's sushi habit ended with a hospital trip with an almost two-metre tapeworm, has sparked much revulsion and discussion. The Guardian assesses how worried should raw fish lovers should be.
A California man’s daily sushi habit ended in a trip to hospital with a stomach-churning item to show doctors: a 5ft tapeworm that “wiggled” out of his body. The Guardian quotes Fresno emergency department doctor Kenny Banh as saying he was sceptical when the man walked into his hospital, asking for treatment for a worm. But when the patient opened a plastic bag, the “giant” parasite was inside, wrapped around a toilet roll. “Apparently it was still wriggling when he put it in the bag but it had died in transit,” Banh said.
The report says the patient, whose identity has not been revealed, told the doctor that during a bout of bloody diarrhoea he looked down and thought a piece of his intestine was hanging from his behind. When the man pulled on it and it kept coming out, he realized it was moving and must be a worm.
The incident happened last August but came to light because Banh spoke about it on a medical podcast called This Won’t Hurt a Bit. He unravelled the worm, he said, and laid it on paper towel on the floor of the trauma room at the Community Regional Medical Centre in Fresno. It was 5ft 6in long. The patient was given a deworming pill which, Banh explained, was no different from the kind generally given to pets.
The man said he ate raw fish, especially salmon sashimi, almost every day.
The report says last January, the US Centres for Disease Control and Prevention (CDC) put out a warning of an elevated risk of parasitic larvae that can grow into tapeworms being found in Pacific Ocean salmon, including Alaskan wild salmon that is popular in the US and elsewhere.
The report says cooking kills the tiny larvae and sushi is supposed to be flash-frozen to kill such parasites too. But larvae may survive in poorly prepared raw salmon, then take up residence in a human digestive tract.
Most people, Banh said, think tapeworms survive on food in intestines. In fact, a worm attaches its head to the wall of the small bowel, just below the stomach, allowing it to suck its host’s blood “like a leech”. “It’s not eating your pizza, it’s eating you,” Banh said.
The report says many human hosts experience no symptoms. Worms often die and are passed at the end of their lifecycle. They can also slip out alive, as in the case in Fresno.
The good news, said Bahn, was that the patient who had turned up at the emergency department was not dying. But, says a report in The Guardian, that is about the only happy element of the story. The story has attracted attention all over the world, as these things tend to do, says Peter Olson, a tapeworm expert and a researcher at the Natural History Museum’s life sciences department, “because they’re gross”. The worm, he says, was “almost certainly something called the broad fish tapeworm … salmon is one of the main ways you would pick it up, if you don’t cook the meat.”
The life of the broad fish tapeworm involves more than one host. “A typical life cycle might include a bear that feeds on salmon, then defecates back into the river. The larvae would be passed into the environment and, in the case of an aquatic life cycle like this, it would be eaten by something like a copepod, a little crustacean. When that copepod is eaten by a fish, it would transform into a larval tapeworm and that’s what is being transmitted to a human in this case. That would go to the intestine and grow into this giant worm.”
The report says the tapeworm is a monstrous and impressive creation. It has a segmented body, with male and female reproductive organs in each segment, so it is capable of self-fertilisation. It does not have a head as such – its “head” is only useful for holding on to its host’s gut, rather than for “eating” (it absorbs nutrients through its skin). In many cases, you would not know you were infected. You might spot bits of tapeworm segment in your stool – small, pale, rice-like bits – or experience stomach pain or vomiting.
Of the more than 10,000 known species of tapeworm, only a small number can infect humans. The report says the type of tapeworm humans might pick up from sushi or undercooked fish is deeply unpleasant, but relatively harmless – although it may cause gastric symptoms and allergic reactions. In rare cases, it can obstruct the intestine (a worm can reach a length of 15 metres or even 25 metres, according to one scientific paper). It is fairly easily treated with the type of worming tablets given to pets.
In those cases, Olson says chillingly, the human is the “final host”. But it is when we are the “intermediate host” that tapeworms can cause significant damage. “The problem is that the larvae don’t grow in the intestine, they move to other parts of the body and in particular to the central nervous system,” says Olson. They can cause cysts in the body, such as in the brain, causing seizures and headaches. In some cases, they can be fatal.
The transmission is far more likely to come from faecal contamination, rather than eating under-cooked meat, he says. “It’s a huge problem in places such as Central America, where there is a large pork industry and amount of pig farming; the worse the sanitation conditions, the more likely there is to be transmission,” says Olsen. Tapeworm infection is thought to be a cause of up to one-third of cases of epilepsy in the region.
Doctors have warned that contracting diseases through eating raw fish or sushi is a risk of which we should become more aware. A case report in the British Medical Journal last March told of a 32-year-old man in Portugal who was admitted to hospital with severe abdominal pain; an endoscopy revealed a parasitic worm was attached to his gut wall and it was removed. It is thought he ingested the parasite while eating sushi. “Owing to changes in food habits, anisakiasis (disease caused by worms from fish, causing symptoms such as pain, vomiting and diarrhoea) is a growing disease in western countries, which should be suspected in patients with a history of ingestion of raw or uncooked fish,” wrote the researchers.
In the UK, EU food hygiene legislation requires fish that is to be eaten raw or cooked lightly be frozen first (although some farmed fish is exempt), since cooking fully or freezing kills any parasites. Tapeworms from fish probably cause more economic damage, says Olson, than human damage. “If you have contaminated meat, you can’t sell it. If you’re farming salmon and lots end up with the parasite, you’ve got a lot of meat you can’t sell. In terms of humans getting this parasite, it’s not a killer. It’s just a gruesome thought.”
[link url="https://www.theguardian.com/world/2018/jan/22/how-the-sushi-boom-is-fuelling-tapeworm-infections"]The Guardian report[/link]
[link url="http://casereports.bmj.com/content/2017/bcr-2016-218857.full"]BMJ case report[/link]