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The longer incubation period for parasitic infections (often 7-10 days) makes traceback efforts for epidemiological studies difficult.
Many control measures are available, but particular life cycle stages of some parasites might be very resistant to selected measures.
In 1996 over 1,400 people became sick due to a number of Cyclospora outbreaks along the East Coast of the U.S. and Canada. Epidemiologically, the infections were traced to fresh raspberries imported from Guatemala, although oocysts were not recovered from the fruit.
Cylospora is a protozoan, a one-celled organism. In one of its life stages, Cylospora forms an oocyst about 8 to 10 microns in diameter. This stage is excreted to the environment and ultimately infects another host. Evidence seems to indicate that this parasite has a very low infectious dose -- 100 oocysts, or might be even as few as 10. It is contracted by eating food or drinking water contaminated with oocysts from human feces.
Life Cycle of Cyclospora
Food with oocyst is swallowed.
Oocytst travels to intestine.
Oocyst excysts -- 2 sporocysts and then 2 sporozoites from the sporocysts.
Replicate enters more cells.
Oocysts pass out of intestine.
The oocysts are ingested and then excyst within the intestine.
The sporozoites penetrate intestinal cells.
The parasite undergoes multiplication and reproduction in the cells until new oocysts form and are excreted in the feces.
In the U.S., Cyclospora is usually transmitted on some type of produce -- raspberries, lettuce, basil -- that is served in a fresh, unfrozen dish. It can take days or even a week or more before the symptoms show up. For Cyclospora, it is usually 7-10 days. Then one might have severe diarrhea, nausea, and abdominal discomfort. Fortunately there is a treatment for the infection. Sulfa drugs, like Bactrim. If the infected person cannot take sulfa drugs then the infection must be allowed to run its usual 3-7 week course.
It is a common misconception that this illness can be prevented by washing produce. Washing with chlorinated water will not prevent infection by parasites. Many of the protozoan parasites, like Cyclospora, Cryptosporidium, and Giardia, are resistant to the levels of chlorine found in drinking water. In fact, they are all fairly resistant to the levels in swimming pools. Cyclospora is particularly resistant. The FDA laboratory uses straight bleach to clean oocysts before using them in experiments.
Available controls include cooking or freezing any produce suspected to be contaminated. There have been no confirmed reports of infections by Cyclospora from foods prepared by cooking or freezing. The oocysts are probably also susceptible to drying. If water is suspected to be contaminated, it can be boiled or filtered.
Unlike bacteria, parasitic protozoa do not usually multiply in or on a food. They need a host for that -- like viruses do. And they are not infectious when they first leave the human body, so they cannot be transmitted person to person. They have to sporulate first, and that can take 5-15 days under favorable conditions.
Cryptosporidium and Giardia
Cryptosporidium and Giardia are different in that they are infectious immediately upon leaving the human body and so can be transmitted person-to-person. This can happen when personal hygiene practices are questionable.
Cryptosporidium. There is no approved drug treatment for a Cryptosporidium infection. It takes anywhere from 2-14 days before the symptoms appear and then the infection, Cryptosporidiosis, normally lasts from 5-14 days.
The symptoms are the same as for Cyclospora unless the person is immunocompromised. Then the diarrhea can be very severe and contribute to the death of the host. There have also been some cases where the pulmonary system became involved.
In a serological survey, researchers estimated that 80% of the population has had Cryptosporidiosis. There have been several outbreaks at child day care centers, but most of the outbreaks have been associated with contaminated water. One outbreak in Milwaukee in 1994 involved over 40,000 infected people when one of the municipal water treatment plants failed.
Giardia. Giardia does not invade cells like Cyclospora and Cryptosporidium do. Giardia adheres to the intestinal wall and might interfere with fat absorption. The Giardia organism is bigger than Cryptosporidium, the trophozoites of Giardia -- that is the stage in the intestine -- is 8-16 microns long, and the cysts are 8-12. Cryptosporidium are usually 4-6 microns in diameter.
During the acute stage, which occurs 7-10 days after eating the cysts, the person can expel numerous cysts, which makes it easy to find in a stool sample. In older infections, it is harder to find the cysts and it might take as many as three different samples before the doctor finds it. The treatment is a drug called Flagyl. However the doctor will not prescribe it to one unless they confirm the organism. Untreated the infection can last 4-6 weeks. But some infections have been known to last months or years. Giardiasis is the most common non-bacterial diarrhea in North America. Scientists estimate that there are 200 million infections annually.
It is very hard to trace the source of a single infection of giardiasis. Campers and backpackers get it from drinking mountain water, kids get it at day care centers, and tourists pick it up when they travel to other countries. Howeve,r outbreaks are often traced to contamination of potable water or poor sanitation by food handlers.
Animals might also be a source. Giardia has been found in dogs, rabbits, muskrats, and raccoons. However, Cryptosporidium would probably more likely found in animals. It has been reported from various vertebrate hosts, including calves, sheep, and deer.
Nematodes are one of groups of parasitic worms. The larval forms are commonly found in the flesh and viscera of marine fishes. The most common ones are Anisakis and Pseudoterranova decipiens. Pseudoterranova is also known as herring worm or codworm. One might have seen them before in wild caught salmon. Aquacultured salmon do not have these worms if they only eat pelleted food. Many other fish have these worms, including arrowtooth flounder, Pacific red snapper, and sole.
The adult worms live in marine mammals and the eggs pass out in the feces into the water. The eggs hatch and crustaceans eat the larvae. Then fish eat the crustaceans and get the worms. The worms penetrate through the intestine and encyst in the viscera or -- the worms penetrate into the flesh and encyst there. If another fish eats the infected fish -- the worms are not digested -- they just penetrate into the new fish. So -- the bigger the fish and the older the fish -- the more parasites it generally has. If a seal or whale eats the fish, it becomes infected with the worms.
What happens when humans eat the infected fish. If the fish has been completely cooked or frozen properly - nothing. If the worms are still alive, which they will be if one eata raw fish, the worm is usually just passed out of the body. But sometimes, Anisakis larva is able to penetrate the stomach or intestinal mucosa and hang on. Fortunately, because humans are the wrong host, Anisakis will not develop into an adult. Anywhere from 12 hours to several days after consumption, one will feel a sharp stabbing pain, usually on the right side, they will rush to the emergency room, and, if they are lucky, the doctor will put an endoscope down their throat and pull out the worm. In most cases one will be misdiagnosed. They will either have their appendix removed, at which time the doctor will probably find the worm too. Or, the doctor will tell them that they have an ulcer, or Crohn's disease, or even stomach cancer. Anisakis infection is not normally a fatal condition. No drug treatment has been approved for anisakis infection.
Pseudoterranova is not as good at penetrating the mucosa. So it does not result in as serious an illness as that from Anisakis. From 12 hours to several days after consumption, one gets what is called "tickling throat syndrome" where they feel a tickling at the back of their throat and they cough up the worm.
There are two methods to control Anisakis and Pseudoterranova. The fish can be cooked until the thickest part reaches a minimum of 145oF for 15 seconds. With microwaving, because there are "cold spots", one should heat to at least 170oF. Or, the fish can be frozen. The fish needs to be held at -31oF for 15 hours or -4oF for 7 days -- the colder the temperature the shorter the time.
If the fish is hot smoked, then the cooking temperatures noted above apply, but cold smoking will not kill the parasitic worms. Most cold smoked salmon is frozen either before processing or afterwards following the above temperature guidelines.
Pickled and marinated fish like pickled herring or lomi lomi must be treated to destroy the parasitic worms. A 6% salt solution with 4% acid -- similar to most pickling solutions, would take 70 days to kill the nematodes. Also, as the salt content decreases, the amount of time it takes to kill the worms increases. The worms normally live in the stomachs of marine mammals where the acid content is very high.
Irradiation can be used to destroy these parasitic worms if one uses a high level -- 6 to 10 Kilogray. But at those levels, the texture and taste of the fish is changed.
Putting the fish on a light table can also be used to remove some of the parasitic worms. This is called candling. It is done for some fish, but it does not remove all of the worms. The thickness, the oil content, pigmentation, even the experience of the person doing it all affect the effectiveness of candling. One can also cut away the belly flaps from the fillets -- the flesh that covers the viscera. Cutting that off will remove quite a few of the worms, but still -- it will not remove all. The best way to control are to freeze or cook the fish. These nematodes are the most common parasitic worms, but they are not the only ones.
Trematodes are a second group of parasitic worms. Nanophyetus salmincola is a parasitic flatworm that can be found in its metacercaria (larval stage) in the kidneys from a salmon. They appear as little white spots or fat deposits in the kidney. The kidney is not the only place they are found in the fish, howeve, that is the easiest place to find them. These trematodes penetrate the skin of the fish and encyst anywhere in the fish -- under the skin, in the brain, in the flesh, in the kidney. They are carried by raccoons, dogs, and people. The eggs are passed in the feces of these animals. If the eggs get into freshwater, they hatch and the miracidium infects a snail. The worm develops in the snail and eventually leave the snail and penetrate any freshwater fish, frog or salamander that it encounters. It encysts and waits for the fish to be eaten by the mammal, where it will develop into an adult.
These trematodes are found in freshwater and anadromous fishes -- like salmon. Salmon get infected before they leave for the ocean and when they return to spawn . These worms are only found in rivers and streams where one species of snail lives. If the snail is not there, the life cycle cannot be completed. One can find this worm in freshwater in western Washington; Oregon, along the Columbia River; and in Northern California.
After contracting this parasite, in about one week, one might get diarrhea, abdominal discomfort, nausea, weight loss, or fatigue. One will have what is known as nanophyetiasis -- or more commonly -- the fish flu. Except it lasts a lot longer than the flu. There are several different drugs that are effective for treatment of nanophyetiasis. Praziquantel or niclosamide appear to work the best. The fish flu (nanophyetiasis) is not transmitted person to person. Humans pass eggs and eggs are not infective to other people -- only to the host snails mentioned earlier.
Tapeworms are a third group and the last of the parasitic worms that will be discussed.
Tapeworms of the genus Diphyllobothrium can be transmitted by marine or freshwater fishes, depending on the species. The infectious stage for people is the (plerocercoid) larval form. The larvae do not look like tapeworms. They are small, unsegmented little white worms that often resemble fat deposits. If they are not alive and moving, most people would not even know they were worms.
The eggs pass out of the definitive host -- the host with the adult parasite, which can be mammals or birds, including gulls, bears, dogs, and humans. The eggs pass out in the feces. In order to infect the fish, the eggs have to be deposited in water. As the eggs hatch a coracidium leaves the egg and infects a crustacean -- usually a copepod. There it develops into a procercoid. If one accidentally swallows an infected crustacean -- nothing will happen. However, if a fish swallows the crustacean, it will develop into the next larval stage -- the plerocercoid. Sometimes, these larvae of Diphyllobothrium behave just like the anisakid nematodes. If another fish eats the infected fish, the tapeworm larva just infects the new fish host. But, if the infected fish is eaten by a bird or mammal, the adult tapeworm develops.
Most people do not know they have diphyllobothriasis until much later -- three weeks or more -- when they pass a section of the tapeworm in their stools. However, some people will have vague gastrointestinal symptoms about a week after becoming infected. Some people are genetically susceptible to developing a severe form of anemia as a result of their tapeworm infection. This anemia is caused by the high affinity the tapeworm has for vitamin B12. The infection is easy to treat. The tapeworm is killed with Praziquantel or niclosamide and the patient is given vitamin B12.
The same measures used to kill or remove the anisakid nematodes from fish will work for both Nanophyetus and for Diphyllobothrium. The only exception is that net pen salmon can have the trematode, Nanophyetu,s if the young fish were raised in hatcheries that also had the host snail necessary for the trematode's life cycle.
Angela M. Fraser, Ph.D., Associate Professor/Food Safety Education Specialist, NC State University. All content was adapted from the FDA course "Food Microbiological Control" prepared in 1998.