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Related FAQs; Roundworms, Worms, Parasitic WormsWorm Identification

Related  Articles: Worm Diversity, Feather Duster Worms

/A Diversity of Aquatic Life

If You Knew Sushi, Like I Knew Sushi, or,

Roundworms, Parasites and Raw Fish, Yichhh!

Bob Fenner  

Not a pretty story, but one that must be told, Dear Reader, for roundworms are indeed, important and literally everywhere. Most are benign, quite a few beneficial, but alas, there be a few noisome species that infect our favorite domestics and even (yipes!) ourselves. Read on should you have an inquiring mind, a strong stomach and would like to be in the know as to one of this planet's major-league phyla, though poorly known amongst aquarists, the roundworms!

Classification: Taxonomy, Relation With Other Groups

What, another esoteric group? Why can't that knucklehead just stick to fishes? Well will you understand the importance of this group as it relates to you and your fishy charges; patience! Let me tell/write you; roundworms are everywhere! Though there

are but some twelve thousand described species, the Phylum Nematoda includes some of the most widespread and numerous species of multicellular animals. They're in the sea, freshwater, soil, spread, tropics to arctics, hot springs to mountain heights to abyssal depths. In one study as many as 4,420,000 (yes, that's four million, four-hundred twenty thousand) roundworms were counted in one square meter of bottom mud off the Danish coast.

"A single decomposing apple on the ground of an orchard has yielded 90,000 roundworms belonging to a number of species." Yuhuck.

There are many parasitic species of nematodes, with all degrees of attacking virtually all groups of plants and animals, including our domestic crops and animals (including more than three hundred infecting fishes), and people too (!). Such ne'er do wells for ourselves are the ever-popular-at-luaus trichina worm Trichina spiralis, hookworms Necator, this-child-has-good parents, upbringing and pinworms Enterobius vermicularis, the giant intestinal nematode of humans and pigs Ascaris, whipworms Trichuris, the causative organism of elephantiasis Wuchereria bancrofti; the origin of the caduceus symbol, the guinea or fire worm Dracunculus medinensis; the African eyeworm (no thanks) Loa loa, and, and, and many more. Maybe one last mention that "worms", Toxocara of puppies and kittens and heartworms (Dirofilaria) of their adult equivalents, dogs and cats are also nematodes. Some fun group now!

Natural and Introduced Range

Check it out, they're everywhere! I heard a statement back in the old graduate college years that if every living thing but the nematodes was removed for Earth, you could make out their outlines from the nematodes inside and outside of everything. So if these darn things are so common and numerous, where are they and why don't we hear about them more often?

First of all, most are small, often microscopic, colorless, non-descript and otherwise un-exciting. As to the lack of chatting about the group, I guess it's due to the fact that not-a-whole-lot is known, other than descriptive biology; "Well, gee, lookey here, this fish is dead and lo and behold, another nematode, let's describe it", science. To cite a personal example: Some years ago (mid to late 70's) Chris Turk (of Ocean Nutrition fame) came to town (sunny southern cal.) to work for Sea World at their wanna be aquaculture facility. A friend and roomie, Mike Kent, who later got his M.Sc. and PhD. from work on a so-called free-living Turbellarian parasitic worm (those black spots on yellow tangs) written about in these pages, noticed some whitish worms protruding from some native clownfish, the garibaldi Hypsypops rubicunda' cloaca's. As this observation was positively correlated with mass mortality and the consequential closing of the facility and loss of their jobs, Mike and Chris were most anxious to determine the etiology (fancy term for study of causative mechanisms) and better still, treatment for aforementioned catastrophe. Well, yes, Dear Reader, the problem, perhaps not a priori, yet per accidens, turned out to be a nematode infection, and all was lost, with some sacrificed specimens found to be more than half full, dry weight, with worms.

Turns out such occurrences are not uncommon with wild caught and a few notable cultured species. Most marine and the majority of freshwater fishes do have endo-parasitic roundworms..., but are they "parasitic"? Why, yes, by definition "they live in or on another species where they derive nourishment and protection"; but to what degree are they detrimental to their hosts?

In most situations/circumstances apparently not much, as aquarium specimens tend to thrive and even reproduce, if not simply survive in captivity. Such semi-benign parasitic relations are ofttimes referred to as "successful", as the host apparently has little detrimental effect on their host.

But once again, to what degree does a zooparasitic fauna contribute to overall negative "stress" possibly resulting in diminished vitality? Hmmmmmm. And what's a mother to do, feed them Total? Well, actually yes, to some extent.

Allow me to march out my infamous pictograph illustrating the interactive relationship of three general sets of factors in the overall health of a system (fish). Or better still, see the articles in this series cited below under my name for 1987 and 89. Suffice it for here to write that optimizing the environment in the way of nutrition, water quality, & tankmates, selecting initially healthy livestock and reducing the introduction of other parasitic and infectious disease-causing organisms is the present state of the art. Chemical therapeutic control of nematodes has been tried with various substances. No trials get my Zalophus californianus sea-lion "seal" of approval.

Anyway, how are you going to know when your fish are infected so you can treat them? Most aquarists would like to do this before their specimens start kicking (finning?) off. Good luck; these things are generally packing the gut cavity or encysted in the fishes muscles. Microscopic fecal examination, sacrificing or necropsying (yep, cutting up a dead specimen) are possibilities, as is general observation of an individual that eats and eats, but perhaps for "wormy" reasons doesn't gain weight. See Anderson below for more meaty treatment modes and materials. It is my decided opinion that vermifuges (de-wormers) should only be applied orally to pet-fish, through their feeding.

Okay! No more tangential, anecdotal, getting-off-the-trail, digression/asides. Back to the issue at hand: Nematodes in general, their importance to you and I as sushi-bar guests and their implications/significance.

What Are They? Selection: General Characteristics

Nematodes are:

1) Round bodied, elongate, cylindrical and unsegmented.

2) Their bodies are covered with a thick, chemical resistant cuticle, capable of being shedded.

3) They possess longitudinal musculature, no circular.

4) Their body cavity is a pseudocoel, not a true coelom. Useful character for separating the group from other "advanced" worm groups like the earthworm and feather-duster phylum Annelida.

5) Complete alimentary canal; mouth, (triradiate) esophagus, mesenteron, anus. Good trait for differentiating from less-advanced worm types like the fluke, tapeworm and flatworm phylum Platyhelminthes.

6) They are of separate sexes; little boys, little girls.

Departing Friendships:

Do you, Dear Reader, sport (on your car, silly) one of those bumper stickers reading "Be Kind to Animals, Don't Eat Them"? If so, it may be time for us to part ways; adieu. For I must admit my accomplice with the Japanese infidel, yes I eat fish (!!!) And further, in an ongoing attempt at un-bolstering our pathetic economy...at the sushi bars! Oh no, but oh yes, I've tested barbecued Pleco (yum, dee-lish), pickled gouramis (I could take 'em or leave 'em) and much more, but my most expensive past-time (worse than the mortgages) is sushi-bar-going. Thus, and even more, my interest, and our key example, in the nematode parasites responsible for anisakiasis or herring worm disease, the family, what else (?), Anisakiidae! Save yourself and don't get hooked on sushi! Actually, as sources of health problems go, you're more likely to have aircraft parts fall on you. But there is the issue of high cost. Oh well, as there are other "uncooked" fish meal derived dishes in our cultures as well as our pets, I think you'll find this discussion most interesting. 

Anisakis Descriptive Life Cycle: Descriptive Life Cycle:

To reduce noise and confusion as the genus Anisakis species have a complex life cycle involving at least two intermediate hosts and four molts with changes in morphology (structure and form), I'll try to describe just the most important variety A. marina appearance and life cycle together. (Hopefully Dick Kidd at FAMA will be able to render something reasonable from my scratchings).

(A) Adult worm in the final or definitive host, (@ to Young, 1972) found in at least twenty five species of cetaceans (whales, dolphins) and eight species of pinnipeds (seals, sea-lions, walruses) in stomachs, upper intestines. Males 3.5 to 7 cm., females 4.5 to 15 cm. (six inches to you).

(B) Adults lay morula stage eggs, passed out in feces, 48-54 microns. Skipping ahead here>

(F) Larva 7-14 days fully formed within shell breaks out. 200 microns in length, molted within egg are in second stage. Infectious second stage larva, surrounded by first larval cuticle is eaten by an invertebrate intermediate host. These L2 larvae are ingested by many species of invert.s. Echinoderms, worms, bivalves, most any filter feeder. Most reports list as the principal carriers or first intermediate hosts, a copepod or euphausiid. (Smith, J.W. 1975)

(H) Penetrates to haemocoel and remains till either it is eaten by>

(I) another invertebrate intermediate host where it will re-encyst>

(J) as a L2 larva or

(K) possibly be eaten by a fish where it migrates to tissue and encapsulates and encapsulates (Young, 1972).

(L) The fishes: at least three hundred species (Yamaguti); herring, cod, haddock, mackerel, salmon, pike, halibut, tuna... Encysts in the mesenteries, peritoneum, abdomen wall, liver of fish. Usually found coiled in a spiral, enclosed in a thin walled cyst in fish hosts. Completely colorless except for opague esophagus. Smooth cuticle. Average length 21-33 mm, maximum width 0.49-0.55 mm. Mouth surrounded by a lip mass, mouth opening a triangular apex, ventral with a prominent boring tooth. Lip mass covered with large number of minute teeth or "hair". Excretory pore just outside border of fine teeth on ventral side. At extreme posterior end of body is a small papilla. Esophagus in two parts, anterior muscular, 2.4-2.6 mm, posterior glandular 0.7-0.8 mm, esophagus and intestine straight (Grainger, 1959). Yes, this is what scientist's do all day.

(M) This fish intermediate host may be eaten by another fish intermediary where the L3 re-encysts as a third stage larvae or by a

(N) definitive marine mammal host. L3 nematode molts to L4 or pre-adult, reaches maturity in stomach and/or intestine of final host. & we're back to the beginning except for... Homo 

Implications and Significance:

Members of the family Anisakidae commonly are found as larvae in various organs, especially mesenteries and peritoneum of marine fishes; almost all adults in stomachs and intestines of marine mammals. Family includes genera Anisakis, Contracaecum, Multicaecum, Phoconema (the latter responsible for lung-worm disease, hauling-out possibly leading to pneumonia of pinnipeds).

High incidence of anisakine nematodes in some fishing waters of North Pacific. So high in one rockfish (sometimes called snappers in the west) Sebastes paucispinus that it was removed from fisheries. High concentration possibly due to large whale populations (Jackson et al. 1978).

The history of this parasitic relationship spans almost five hundred years. Recognized first in fish and marine mammals in the thirteenth century. (Myers, 1976).

Until 1955 anisakine infected fishes were a concern chiefly for aesthetic reasons; i.e. no one wants to buy or eat a "wormy" fish. Since their life cycle includes three types of hosts attempts were made to break the cycle; selective fishing sites (offshore), reduction of seal populations...none with much success.

Recognized first as a human problem in 1955 in Holland. Then anisakiasis known as "herring-worm disease". In 1955 there was a change in how green herring were process. Previously fresh caught herring were headed and gutted before being brine stowed. Smith & Wooten (1975) describe large scale migration of anisakids into the flesh of ungutted herring, especially brined or cold smoked. Smoking at 60 degrees C. does not kill them (Hauck 1977). Fish to be consumed raw should be frozen at minus 20 degrees C for sixty hours or cooked completely.

Human ingestion results in "severe gastrointestinal syndrome". For you microscopists, intestinal eosinophilic granuloma; symptoms resemble peptic ulcer or tumor, including nausea, vomiting, bloody diarrhea and fever, resulting from the worms trying to bore through the digestive lumen.

Between 1964-1976 over one thousand cases reported worldwide. By 1976 only eight cases reported for North America, only one in California (!). With the American appeal for raw and exotic fish delicacies on the rise (ceviche, matzoh, gravlax, sashimi)... But I refuse to give up my sushi!

By the way, with careful preparation, including "candling" (holding up fillets against a bright light source), the chance of even ingesting an anisakid worm is very small. Don't worry.


Okay, so there's another group of bad-guys you should/have to/want to be familiar, but not too familiar with. You can't do much to treat your livestock if they're already on the down spin and/or too heavily parasitized. You can select for good specimens, optimize their environment and otherwise prevent the introduction of more nematodes. Ah, once again the warning against using "live foods" from an infected (same water conditions) source. Thus the strong pro-argument for irradiated, frozen otherwise disease-neutralized prepared foods. 

Bibliography/Further Reading:

Anderson, James A. 1989. Worm Infestations in Fish (shouldn't that be infection, infestation is external, and fishes as in more than one species? picky picky). Seascope, Vol. 6, 1989.

Barnes, Robert D. 1987. Invertebrate Zoology. Saunders, Orlando.

Davey, J.T. 1971. A revision of the genus Anisakis Dujardin, 1845

(Nematoda: Ascaridata). J. Helminthology 45:51-72

Fenner, Bob & Dave Huie. 1987. A Livestock Treatment System. FAMA 1/87.

Fenner, Who Else?. 1989.Parasitic Diseases of Cultured Fishes: Methods of Their Prevention and Treatment: FAMA 10/89.

Grainger, J.N.R. 1959. The identification of the larval nematodes found in the body muscles of the cod (Cadus callarias L.) Parasitology 49:121-131.

Hauck, A.K. 1977. Occurrence and survival of the larval nematode Anisakis sp. in the flesh of fresh, frozen, brined and smoked pacific herring, Clupea harengus Pallas. J. Parasis. 63:515-

Jackson, G.J. et al. 1978. Nematodes in fresh marine fish. Washington, D.C. J. Food Protection 41(8):613-620.

Myers, B.J. 1976. Research then and now on the Anisakidae nematodes. Trans. Amer. Micros. Soc. 95(2):137-142.

Noble, E.R. & Noble, G.A. 1976. Parasitology- The Biology of Animal Parasites. Lea & Febiger, Philadelphia. pp. 309-310.

Pippy, J.H. & van Banning, P. 1975. Identification of Anisakis larva. J. Fish Res. Bd. Can. 32:29-32.

Smith, J.W. & R. Wootten. 1975. Experimental studies on the migration of Anisakis sp. larvae (Nematoda: Ascaridata) into the flesh of herring, Clupea harengus L. Intl. J. Parasit. 5:133-136.van Banning, P. & Becker, H.B. 1978. Long term survey data (1965-1972) on the occurrence of Anisakis larvae in herring from the North Sea. J. Fish. Biol. 12(1):25-34.

Young, P.C. 1972. The relationship between the presence of larval anisakine nematodes in the cod and marine mammals in British home waters. J. Appl. Ecol. 9:459-485.


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