Seafood Safety

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Life Cycle

The life cycle of Anisakis was described by Oshima (1972 as cited in Pinkus et al., 1975; Oshima, 1987). Adult Anisakis worms live in the stomach of various marine mammals including: whales, dolphins and porpoises. The eggs are passed in the feces of the final hosts into seawater where they molt and develop to second-stage larvae. The second-stage larvae are consumed by euphausiids (krill), where they molt to the third stage. The infected euphausiids are consumed by either whales, where they develop to the adult stage; or by fish and squid, where they migrate to the viscera and musculature and remain in the third larval stage. Fish and squid, infected with the third stage larvae, are consumed by piscivorous marine mammals. In the mammals, the larvae attach to stomach and develop to fourth-stage larvae and finally to the adult stage. Humans are infected by consuming raw or undercooked fish and squid infected with third-stage larvae (Deardorff et al., 1986).
                            ADULT
    ------> Marine Mammals ------> Eggs Released
    ^      (Whales, dolphins, porpoises)   | in feces
    |                                      v
    |                               2nd STAGE LARVAE
    |                                   Seawater
    |                                      v
 4th STAGE LARVAE                        3rd STAGE LARVAE
  Marine Mammals  <----------------  Euphausiids
    ^                                      |
    |             3rd STAGE LARVAE         v
    <------ Fish, Squid <-------
                               |
                               v
                            Humans
                       Accidental Hosts

Symptoms & Treatment

Symptoms of anisakiasis have been detailed by a number of authors (Deardorff et al., 1986; Pinkus et al., 1975; Olson, 1986). When ingested by humans the larvae often die or are passed out of the body without affecting the individual. The incidence of anisakiasis appears to be low in relation to the number of larvae consumed. Larvae which persist in the body can invade the mucosa, causing severe abdominal pain (may be spasmodic) sometimes accompanied by nausea, vomiting, and occasionally fever. The onset of symptoms varies depending on the location of larval penetration. Penetration in the stomach usually causes symptoms within 1 - 12 hours of eating infected fish. The onset of intestinal anisakiasis usually occurs within 1 - 5 days of eating infected fish.
If the larva penetrates the mucosa, surgical removal is necessary. No detrimental effects to the patient have been documented after the larva has been removed. Anthelminthic drugs have not been useful in treating cases of anisakiasis (Schantz, 1989). Since the nematodes do not develop to full maturity and produce eggs in humans, stool examination is not useful for diagnosis (Deardorff et al., 1986; Schantz, 1989).

Statistics

Anisakiasis is associated with the consumption of raw or undercooked fish. Most documented cases of anisakiasis have occurred in areas where raw fish is commonly eaten, such as, Japan, the Netherlands and Western U.S. (Hawaii, Alaska, and California).
Since the enactment of the Marine Mammal Protection Act of 1972, the population of marine mammals has increased (Bonnell et al., 1983 as cited in McKerrow et al. 1988). The number of human Anisakis infections has also increased. Between 1980 and 1988 there was a 70% increase in the number of reported cases (McKerrow et al., 1988). However, it is unclear whether the recent increase in human infections is due to the increase in the marine mammal population, or if it is due to better diagnosis and reporting of the disease.
Documentation and reporting of anisakiasis is ambiguous. Anisakis simplex looks very similar to Pseudoterranova decipiens, or cod worm, and the symptoms of both infections are similar. As a result, cases are sometimes misdiagnosed or incorrectly reported. Also, anisakiasis is not reportable to the CDC, so there is no definitive record of the number of cases.
The majority of Anisakis infections are acquired from dishes prepared at home, restaurants have been incriminated only rarely (Schantz, 1989). The first case of anisakiasis in the U.S. was reported in 1958 (McKerrow et al., 1988). Since then there have been 30 to 50 documented cases in this country (Schantz, 1989).

Detection & Prevention

Processing plants often use candling techniques to detect larvae in fillets. This technique is not completely effective in detecting Anisakis since the larvae are small and whitish in color, making it difficult to differentiate between the larvae and the flesh of light fish. One study indicated that candling efficiency could be improved 16% by bleeding fish. Another study suggested that detection efficiency could be improved by scoring or stripping the fillets, a method which would result in loss of product quality. Research has been done on acoustic detection of larvae, but this technique is still not commercially viable (Margolis, 1977).
Since there are no infallible methods of detecting and removing larvae, the best prevention is to ensure that the larvae are killed before eating the fish. Cooking fish to a temperature of 140F will kill Anisakis larvae (USFDA, 1987). If the fish will be eaten raw it is advisable to freeze it first. Commercial blast-freezing to at least -35C for 15 hours kills 99% of Anisakis larvae and those that survive are presumed damaged beyond the ability to cause human illness (Deardorff and Throm, 1988). For home freezing fresh fish, the temperature and time required to kill Anisakis larvae can depend on the species of fish, the depth of penetration, and the physiological condition of the larvae. The USFDA recommends freezing fish at -10F for at least 168 hours (7 days) (USFDA, 1987).
Storing ungutted fish on ice after capture may result in post-mortem migration of Anisakis larvae from the viscera to the flesh of some species of fish (Myers, 1979). Post mortem migration of larvae is evident in herring and mackerel, but not in pollock, whiting or blue whiting, suggesting that migration occurs in "fatty" fish species, but not in "non-fatty" species (Smith, 1984). For this reason it appears that the length of time between the catch of the fish and actual gutting may be an important factor in governing the number of larvae in some fish flesh. The incidence of Anisakis in fillets could possibly be reduced if "fatty" fish are processed more quickly or gutted at sea.


Pseudoterranova Decipiens or Cod Worm

Description

Pseudoterranova decipiens (formally Phocanema, Porrocaecum and Terranova) (Olson, 1986), commonly called "codworm" or "sealworm", is a parasitic nematode. The final hosts of Pseudoterranova are grey seals, sealions and walruses. In the larval stage, the stage that infects some fish, codworms are is 5-58 mm in length, 0.3-1.2 mm in width and yellowish, brownish or reddish in color (Hafsteinsson and Rizvi, 1987). Pseudoterranova larvae are mostly found in Atlantic Ocean fish since the Atlantic has a large population of grey seals (Myers, 1979).
Humans are accidental hosts of sealworms and infections are associated with eating raw or undercooked fish (sushi, sashimi, ceviche, sunomono, green herring, marinated and cold smoked fish).

Contaminated Species

The incidence of sealworm infection in fish generally increases with length, weight and age of the fish host (Scott and Martin, 1857; Templeman et al., 1957; Young, 1972; as cited in Hafsteinsson and Rizvi, 1987). Codworms are mostly found in temperate and polar fish and are more common in bottom dwelling fish than pelagic fish (Margolis, 1977). The infection rate in fish can be highly variable, but often especially high in cod. In one study of the Gulf of St. Lawrence the infection rate of cod was estimated to be 70-90% (Margolis, 1977). The species of fish most often infected with sealworm include: cod (Margolis, 1977; Oshima, 1987); pollock (Oshima, 1987); halibut (Margolis, 1977; Oshima, 1987); greenling (Oshima, 1987); squid (Oshima, 1987); and flatfish (Oshima, 1987).



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