''Contracaecum'' is a genus of parasitic

nematodes The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (he ...

from the family

Anisakidae The Anisakidae are a family (biology), family of intestinal nematodes (roundworms). The larvae of these worms can cause anisakiasis when ingested by humans, in raw or insufficiently cooked fish. Anisakidae worms can infect many species of fish, ...

. These nematodes are parasites of warm-blooded, fish eating animals, i.e. mammals and birds, as sexually mature adults. The eggs and the successive stages of their larvae use invertebrates and increasing size classes of fishes as intermediate hosts. It is the only genus in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

which can infect terrestrial, marine and freshwater animals.

Life cycle

The adults live as parasites in the stomachs of

piscivorous A piscivore () is a carnivorous animal that primarily eats fish. Fish were the diet of early tetrapod evolution (via water-bound amphibians during the Devonian period); insectivory came next; then in time, the more terrestrially adapted rept ...

birds and mammals. As third stage larvae they attach to the stomach of the species of fish which are preyed on by their

definitive host In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include ...

. When the intermediate host fish is eaten and reached the warm stomach of its predator the larvae of ''Contracaecum'' moult twice into adult males and females, producing eggs which are expelled into water in the

faeces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...

of the host. Where the water is shallow the eggs or larvae descend to the sea bed. Here they may be consumed by invertebrates while those that float in the water column are ingested by various

zooplankton Zooplankton are the heterotrophic component of the planktonic community (the " zoo-" prefix comes from ), having to consume other organisms to thrive. Plankton are aquatic organisms that are unable to swim effectively against currents. Consequent ...

. The small invertebrates and zooplankton are then eaten by larger organisms moving up the food chain until a fish suitable as a transport host consumes the larvae with the previous host. In this fish host, the larvae penetrate the wall of the intestinal tract into the organs and body cavity. The immune system of the fish reacts by producing a capsule of connective tissue around the larva, this capsule retains the larvae for the fish's life. Once an infected fish or the discarded guts of a cleaned fish are eaten by another fish, the capsule around the larvae are digested, freeing the larvae to restart this stage of its life cycle. In larger and older specimens of predatory fish there may be hundreds, possibly thousands of the larvae-containing capsules of connective tissue, these are all third-stage larvae and they are characterised by having and tooth-like structure on their head which is used to bore through the host's tissues. They have no reproductive organs at this stage. ''Contracaecum'' nematodes are common in wild salmonids in the marine environment where they may occur abundantly. When they reach their final host they develop into adults. Dependent on the species of ''Contracaecum'' the definitive host can be mammals, including humans, birds, and in some cases, fish. In experiments third stage larvae of '' Contracaecum osculatum'' were shown to be infective of

copepods Copepods (; meaning 'oar-feet') are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (living in the water column), some are benthic (living on the sediments), several species have ...

, for example the larvae measuring 300-320

micron The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...

s infected nauplius larvae of ''

Balanus ''Balanus'' is a genus of barnacles in the family Balanidae of the subphylum Crustacea. This genus is known in the fossil record from the Jurassic to the Quaternary periods (age range: from 189.6 to 0.0 million years ago.). Fossil shells withi ...

'' and small species of fish such as

stickleback The sticklebacks are a family of ray-finned fishes, the Gasterosteidae which have a Holarctic distribution in fresh, brackish and marine waters. They were thought to be related to the pipefish and seahorses but are now thought to be more close ...

and eelpout. Larger fish such as

flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish Order (biology), suborder Pleuronectoidei, also called the Heterosomata. In many species, both eyes lie on one side of the head, one or the other migrating through or around ...

and

gobies Gobiidae or gobies is a family of bony fish in the order Gobiiformes, one of the largest fish families comprising over 2,000 species in more than 200 genera. Most of gobiid fish are relatively small, typically less than in length, and the fam ...

were infected with the larvae by consuming the crustaceans while yet larger, predatory fish were infected by consuming these smaller fish. While the larvae were in the crustaceans they did not grow much, if at all. In the very small fish the larvae grew slowly and if a cod ate the small fish the smaller larvae died in the cod's liver and gut wall while the larger larvae travelled to the

parenchyma upright=1.6, Lung parenchyma showing damage due to large subpleural bullae. Parenchyma () is the bulk of functional substance in an animal organ such as the brain or lungs, or a structure such as a tumour. In zoology, it is the tissue that ...

of the liver. Here, some of the larvae attained lengths of up to 10 mm. Even so, some specimens of ''C. osculatum'' completed their life cycle using only a copepod host and a single intermediate host such as a stickleback.

Distribution

The nematodes in the genus ''Contracaecum'' are found in seas throughout the globe. It is the most speciose genus in the Anisakidae.

Effects on human health

''Contracaecum'' larvae can infect humans, the human disease caused by infection of Anisakid nematodes such as ''Contracaecum'' is called

anisakiasis ''Anisakis'' ( ) is a genus of parasitic nematodes that have life cycles involving fish and marine mammals. They are infective to humans and cause anisakiasis. People who produce immunoglobulin E in response to this parasite may subsequently ha ...

(or anisakidosis) which is a painful and severe condition with infection usually being caused by the consumption of raw or undercooked fish which are host to the third stage larvae. The

symptoms Signs and symptoms are diagnostic indications of an illness, injury, or condition. Signs are objective and externally observable; symptoms are a person's reported subjective experiences. A sign for example may be a higher or lower temperature ...

of anisakiasis include abdominal pain and distention, diarrhea and nausea, faeces with high proportions of blood and mucus and a mild fever. There can also be allergic reactions such as rash and itching, and occasionally there can also be

anaphylaxis Anaphylaxis (Greek: 'up' + 'guarding') is a serious, potentially fatal allergic reaction and medical emergency that is rapid in onset and requires immediate medical attention regardless of the use of emergency medication on site. It typicall ...

. Patients may require the removal of the parasite by

endoscopy An endoscopy is a procedure used in medicine to look inside the body. The endoscopy procedure uses an endoscope to examine the interior of a hollow organ or cavity of the body. Unlike many other medical imaging techniques, endoscopes are insert ...

surgery Surgery is a medical specialty that uses manual and instrumental techniques to diagnose or treat pathological conditions (e.g., trauma, disease, injury, malignancy), to alter bodily functions (e.g., malabsorption created by bariatric surgery s ...

to treat this condition.

Species

There are 142 species in the genus ''Contracaecum''.

References

{{Authority control Rhabditida Rhabditida genera