Seafood Safety
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 140°F 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 -35°C 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 -10°F 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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