Foodborne Animal Parasites 693 the rediae through a birth pore A cercaria is a miniature fluke with a tail Cercariae leave the snail and swim through water in search of their next host—usually fish, clams, and the like They bore into the new host, shed the tail, and become surrounded by a cyst Within the cyst, further development leads to metacercariae, which develop further in the final host, usually a vertebrate, including humans Upon ingestion of metacercariae-containing fish, the cyst wall dissolves in the intestine, and the young flukes emerge They then migrate through the body to their final site, the bile ducts of the liver in the case of C sinensis where, among other problems, they may cause cirrhosis (see below) Liver flukes are common in China, Korea, Japan, and parts of Southeast Asia It is estimated that more than 20 million persons in Asia are infested with this parasite.45 In China, it is often associated with the consumption of a raw fish dish called ide Over 80 species of fish are known to be capable of harboring C sinensis.45 Symptoms, Diagnosis, Treatment, and Prevention Symptoms may not occur if the infection is mild, but in severe cases, damage to the liver may occur The liver damage may lead to cirrhosis and edema, and cancer of the liver is seen occasionally.79 Diagnosis is made by repeated microscopic examinations of feces and duodenal fluid for eggs An ELISA test is useful, but cross-reactions with other trematodes may occur Praziquantel is an effective chemotherapeutic agent The prevention of this syndrome is achieved by avoiding the deposition of human feces in fishing waters, but this seems unlikely in view of its wide distribution The avoidance of raw fish products and the proper cooking of fish are more realistic alternatives C sinensis can be inactivated in fish by the same procedures as for roundworms and flatworms According to Rodrick and Cheng,84 all captured fish must be considered to be potential carriers of parasites This applies to all flukes, flatworms and roundworms, and protozoa Diphyllobothriasis This infection is contracted from the consumption of raw or undercooked fish, and the causative organism, Diphyllobothrium latum, is often referred to as the broad fish tapeworm The definitive hosts for D latum are humans and other fish-eating mammals; intermediate hosts are various freshwater fish and salmon, where plerocercoid (or metacestode) larvae are formed When humans consume fish flesh that contains plerocercoid larvae, the larvae attach to the ileal mucosa by two adhesive grooves (bothria) on each scolex and develop in 3–4 weeks into mature forms As a worm matures, its strobila, made up of proglottids, increases in length to 10 m or nearly 20 m, and each worm may produce 3000–4000 proglottids that are wider than they are long (hence, broad fish tape) (see Figure 29–1) Over million eggs may be released each day into stools of victims Eggs are more often seen in stools than proglottids, and they are not infective for humans When human feces are deposited in waters, the eggs hatch and release six-hooked, free-swimming larvae or coracidia (also known as oncospheres) When these forms invade small crustaceans (copepods or microcrustaceans such as Cyclops or Diaphtomus), they metamorphose into a juvenile stage designated metacestode or procercoid larvae When a fish ingests the crustacean, the larvae migrate into its muscles and develop into plerocercoid larvae If this fish is eaten by a larger fish, the plerocercoid migrates, but it does not undergo further development Humans are infected when they eat fish containing these forms 694 Modern Food Microbiology Figure 29–1 Proglottid of Taenia saginata (top) and Diphyllobothrium latum (bottom), magnification: 360× Source: From S.H Abadie, J.H Miller, L.G Warren, J.C Swartzwelder, and M.R Feldman, Manual of Clinical Microbiology, 2nd ed.; copyright c 1974 by American Society for Microbiology, used with permission Foodborne Animal Parasites 695 Prevalence Although the first human case was reported in 1906, it was the scattering of cases during the early 1980s that brought new attention to this disease in the United States and Canada The cases in question resulted from the consumption of sushi, a raw fish product that has long been popular in parts of Asia but only relatively recently has become popular in the United States The incidence of diphyllobothriasis is high in Scandinavia and the Baltic regions of Europe It is estimated that million cases occur in Europe, million in Asia, and 100,000 in North America However, only of 275 asymptomatic natives of Labrador, Canada, examined in 1977 had a positive stool culture for this organism.100 Symptoms, Diagnosis, and Treatment Although most cases of diphyllobothriasis are asymptomatic, victims may complain of epigastric pain, abdominal cramps, vomiting, loss of appetite, dizziness, and weight loss Intestinal obstruction is not unknown One of the consequences of this infestation is a vitamin B-12 deficiency, along with macrocytic anemia This disease is diagnosed by demonstrating eggs in stools Treatment is the same as for taeniasis The absence of overt symptoms does not mean the absence of the tapeworm in the intestines because the worms may persist for many years Prevention Diphyllobothriasis can be prevented in humans by avoiding the consumption of raw or undercooked fish Although the elimination of raw sewage from waters will undoubtedly help to reduce the incidence, it will not break the life-cycle chain of this organism, as humans are not the only definitive hosts Cooking fish products to an internal temperature of 60◦ C for minute or 65◦ C for 30 seconds will destroy the organism,6 as will freezing fish to −20◦ C for at least 60 hours.56,57 Cysticercosis/Taeniasis This syndrome in humans is caused by two species of flatworms: Taenia saginata (also Taeniarhynchus saginatus; beef tape) and Taenia solium (pork tape) They are unique among both flatworm and roundworm parasites in that humans are their definitive hosts; the adult and sexually mature stages develop in humans, whereas the larval or juvenile stage develops in herbivores These helminths have no vascular, respiratory, or digestive systems nor they possess a body cavity They depend on the digestive activities of their human hosts for all of their nourishment Their metabolism is primarily anaerobic The structure of a T saginata proglottid is illustrated in Figure 29–1 The adult worm consists of a scolex (head) that is about mm in size and lacks hooks but has four sucking discs Behind the scolex is the generative neck, which segments to form the strobila composed of proglottids The latter increase in length, with the oldest being the farthest away from the scolex Each proglottid has a complete set of reproductive organs, and an adult worm may contain up to 2000 proglottids These organisms may live up to 25 years and grow to a length of 4–6 m inside the intestinal tract T saginata sheds 8–9 proglottids daily, each containing 80,000 eggs The eggs are not infective for humans When proglottids reach soil, they release their eggs, which are 30–40 µm in diameter, contain fully developed embryos, and may survive for months When the eggs are ingested by herbivores, such as cattle, the embryos are released, penetrate the intestinal wall, and are carried to striated muscles 696 Modern Food Microbiology of the tongue, heart, diaphragm, jaw, and hindquarters, where they are transformed into larval forms designated cysticerci Cysticercosis is the term used to designate the existence of these parasites in the intermediate hosts The cysticerci usually take or months to develop after eggs are ingested by a herbivore When present in large numbers, the cysticerci impart a spotted appearance to the beef issue Humans become infected upon the ingestion of meat that contains cysticerci The infection caused by the pork tape (T solium) is highly similar to that described for the beef tape, but there are some significant differences Although humans are also the definitive hosts, the larval stages develop in both swine and humans In other words, humans can serve as intermediate (cysticercosis) and definitive (taeniasis) hosts, thus making autoinfections possible For this reason, T solium infections are potentially more dangerous than those of T saginata The infection caused by larval forms of T solium is sometimes designated Cysticercus cellulosae The T solium scolex has hooks rather than sucking discs, and the strobila may reach 2–4 m and contain only about 1000 proglottids Embryos of T solium are carried to all tissues of the body, including the eyes and brain in contrast to T saginata Although T saginata exists in both the United States and many other parts of the world, T solium has been eliminated in the United States However, it does exist in Latin America, Asia, Africa, and eastern Europe The incidence of T saginata in beef in the United States is below 1% as a result of federal and local meat inspections Symptoms, Diagnosis, and Treatment Most cases of taeniasis are asymptomatic regardless of the Taenia species involved, but symptoms differ when humans serve as intermediate host In these cysticercosis cases, the cysticerci develop in body tissues, including those of the central nervous system, and generally lead to eosinophilia Human taeniasis is diagnosed by demonstration of eggs or proglottids in stools and cysticercosis by tissue biopsies of calcified cysticerci or by immunological methods Complement fixation, indirect hemagglutination, and immunofluorescence tests are valuable diagnostic aids A single-dose oral treatment with niclosamide, which acts directly on the parasites, is effective in ridding the body of adult worms This drug apparently inhibits a phosphorylation reaction in the worm’s mitochondria Another effective chemotherapeutic agent is praziquantel With cysticercosis, surgery may be indicated Prevention The general approach in the prevention and elimination of diseases that require multiple hosts is to cut the cycle of transmission from one host to another Because the eggs are shed in human feces, taeniasis can be eliminated by the proper disposal of sewage and human wastes, although T solium infections in humans present a more complex problem Cysticerci can be destroyed in beef and pork by cooking to a temperature of at least 60◦ C.52 The freezing of meats to at least −10◦ C for 10–15 days or immersion in concentrated salt solutions for up to weeks will inactivate these parasites Freezing times and temperatures necessary to ensure the death of all cysticerci from infected calves were found by one group to be as follows: 360 hours at −5◦ C, 216 hours at −10◦ C, and 144 hours at −15, −20, −25, or −30◦ C.46 ROUNDWORMS The disease-causing roundworms of primary importance in foods belong to two orders of the phylum Nematoda The order Rhabditida includes Turbatrix aceti (the vinegar eel), which is not a human pathogen and is not discussed further Foodborne Animal Parasites 697 Phylum Nematoda Class Adenophorea (= Aphasmidia) Order Trichinellida Genus Trichinella Class Secernentea (= Phasmidia) Order Rhabditida Genus Turbatrix Order Ascaridida Genus Ascaris Subfamily Anisakinae Genus Anisakis Genus Pseudoterranova (Phocanema) Genus Toxocara Trichinosis Trichinella spiralis is the etiological agent of trichinosis (trichinellosis), the roundworm disease that has been of greatest concern from the standpoint of food transmission, especially in the United States The organism was first described in 1835 by J Paget in London, and the first human case of trichinosis was seen in Germany in 1859.61 Although most flatworm and roundworm diseases of humans are caused by parasites that require at least two different host animals, the trichinae are transmitted from host to host; no free-living stages exist In other words, both larval and adult stages of T spiralis are passed in the same host It is contracted often from raw or improperly cooked pork or bear meats The adult forms of T spiralis live in the duodenal and jejunal mucosas of mammals such as swine, canines, bears, marine mammals, and humans that have consumed trichinae-infested flesh The adult females are 3–4 mm long, and adult males are about half this size Although they may remain in the intestines for about a month, no symptoms are produced The eggs hatch within female worms, and each female can produce around 1500 These larvae, each about 0.1 mm in length, burrow though the gut wall and pass throughout the body, ultimately lodging in certain muscles Only those that enter skeletal muscles live and grow; the others are destroyed The specific muscles affected include those of the eye, tongue, and diaphragm When assaying for trichinae larvae in pork, the U.S Department of Agriculture (USDA) employs diaphragm muscle or tongue tissues In a recent study, the Crus muscle of the diaphragm was found to yield more larvae per gram than several others.64 As the larvae burrow into muscles several weeks later, severe pain, fever, and other symptoms occur, which sometimes lead to death from heart failure (see below) The larvae grow to about mm in muscles and then encyst by curling up and becoming enclosed in a calcified wall some 6–18 months later (Figure 29–2) The larvae develop no further until consumed by another animal (including humans), but they may remain viable for up to 10 years in a living host When the encysted flesh is ingested by a second host, the encysted larvae are freed by the enzymatic activities in the stomach, and they mature in the lumen of the intestines Prevalence About 75 species of animals can be infected by T spiralis, but avians appear to be resistant.69 During the 1930s and 1940s, about 16% of Americans were infected.69 For the period 1966–1970, 4.7% of ... Clinical Microbiology, 2nd ed. ; copyright c 1974 by American Society for Microbiology, used with permission Foodborne Animal Parasites 695 Prevalence Although the first human case was reported in... muscles 696 Modern Food Microbiology of the tongue, heart, diaphragm, jaw, and hindquarters, where they are transformed into larval forms designated cysticerci Cysticercosis is the term used to designate... developed embryos, and may survive for months When the eggs are ingested by herbivores, such as cattle, the embryos are released, penetrate the intestinal wall, and are carried to striated muscles