More Food-Borne Parasites

by Food and Drug Administration (FDA)

(Page 3 of 4)

"There are more than 80 food-borne parasites," says FDA's George Jackson, "but, fortunately, not all are of great significance in this country at this time. However, the food market is becoming international — we're getting not only preserved foods, but fresh foods flown in from all parts of the world."

Jackson says parasites are important not only because of the direct infections they cause, but because of their secretions and excretions. This is particularly true of the helminths. Even if the worms are pulled out of the food, their waste products — biologically active materials — are left behind in the flesh. Studies are getting started to discover what long-term effects they may have on humans.

"Our own habits are also a big factor," Jackson says. "We like to eat raw vegetables in salads and we're eating more raw fish. While most parasites are easily killed by proper cooking, right now we're not doing that well enough." Probably the best-known — and most serious — food-borne parasitic disease in this country is trichinosis. Larvae of the Trichinella roundworm infect pigs and some game animals whose meat ends up on our dinner tables. The incidence of trichinosis has declined, however, with an average of only 44 cases per year from 1984 through 1988 reported to CDC. This is due partly to legislation requiring that garbage fed to pigs be cooked, killing any larvae. The sporadic outbreaks that occur are primarily among new immigrants from Asia. "There is very little Trichinella in pork in Asia," Jackson explains. "Therefore, Asian immigrants do not cook pork as thoroughly as we do."

Trichinosis symptoms vary with individual immunity and the intensity of the infection. The adult worms develop and reproduce in the human digestive tract, where they may cause mild diarrhea. They then die and leave the body in feces. The new generation of larvae may then invade cells of the diaphragm, skeletal muscles or heart, causing serious damage, if present in large numbers.

"If you get a very few worms, you probably will not know it, but if you get a large dose, you will," Jackson says. "The problem is that if it's a subclinical infection, you probably don't need to treat it, and if it's severe, you're in trouble. Once symptoms develop, it's already in the muscles where it causes so much damage."

Treatment is aimed at helping the patient survive the acute infection. Effectiveness of the anthelmintic drug Mintezol (thiabendazole) varies among patients. Those who develop heart or central nervous system problems or who have allergic reactions such as hives and swelling are also given corticosteroids.

New Recipes, New Risks

While trichinosis is on the decline, fish-borne parasitic illnesses are on the rise, corresponding with the growing popularity in this country of raw fish dishes. Japanese sushi and sashimi, Latin American ceviche, Scandinavian gravlax, lomi-lomi salmon, and other raw fish recipes may tempt our taste buds, but if they're not carefully prepared, our stomachs may revolt.

Seals, dolphins, porpoises, and other large sea mammals are host to a group of parasitic worms called anisakids. The parasites' eggs pass out of the mammal's body in feces. In the water, they hatch into larvae, which are then eaten by fish, such as cod, salmon or herring. When these infected fish reach our mouths raw or undercooked, trouble may ensue. "Fortunately, the most common symptoms of anisakiasis are more annoying than life-threatening," says Jackson. He explains that the larvae burrow into the mucosa of the stomach or intestine, producing sometimes painful 'attachment ulcers,' and sometimes nausea and vomiting. "Usually the worms don't last long — we're not their usual hosts — and they die or try to get out of us. They may be coughed or vomited up," he says. "Many people feel a tickling at the back of the throat. They reach back there and pull out this spaghetti-like worm."

In the normal course, the disease usually subsides spontaneously. Sometimes gastroscopy (inserting a tube through the mouth to the stomach) is used to remove the larvae. If chronic illness develops, surgery may be required to remove lesions that have developed.

Jackson says that on rare occasions the larvae penetrate the intestinal wall and go wandering in the body or settling in and affecting other organ systems. After the larvae begin to die, the body responds to their presence with a cellular reaction, which may be misdiagnosed as cancer. FDA is working on a new policy to minimize the public's exposure to fish-borne parasites.

"We've decided to try to implement good manufacturing practice levels of allowable parasites on a species group basis," says Jeffrey Bier, Ph.D., research microbiologist in FDA's Office of Seafood. "The policy will be based on species groups because parasites are more visible and more easily detected in the flesh of certain species than in others." Bier explains that the cods, flounders and sea basses, for example, are similar in the incidence of parasites and in the ease with which the parasites can be detected visually. The process to detect them is candling, in which light is used to look through the flesh of the fish.

Bier says the agency is collecting data on which to base proposed rules for good manufacturing practice levels of parasites for the cod and flounder families.

About the Author

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FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.