Mytilicola intestinalis (Red Worm Disease) of Oysters


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Mytilicola disease, Red worm disease.

Scientific name or taxonomic affiliation

Mytilicola intestinalis (Copepoda, family Mytilicolidae) [not a worm] (Steuer 1902, 1905).

Geographic distribution

Mytilicola intestinalis appears to be confined to European waters including coastal areas of the Adriatic Sea, Mediterranean Sea and North Sea (specifically, from Italy to Denmark including the British Isles and Ireland but not in the Baltic Sea). To date, it has been reported beyond the Mediterranean Sea and the west coast of Europe on only one occasion; in the unusual circumstance of being found in a plankton sample in the Indian Ocean-Malacca Strait area 67 nautical miles from the nearest shore and depth of 951 meters (Wickstead 1960). Although various reasons for finding intestinal parasites of European bivalves at this location in the water column were discussed (including the possibility of infected bivalves attached to the hull of the ship), Wickstead (1960) could not explain the phenomenon.

Host species

Ostrea edulis; and Crassostrea gigas, also reported from mussels, clams and cockles and in the laboratory, Crepidula fornicata. Hepper (1956) stated that M. intestinalis does not readily infest O. edulis in the presence of mussels and is unlikely to become epidemic in oysters. Thus, Cheng (1967) concluded that M. intestinalis prefers mussels but if they are not available, it will parasitize O. edulis. Crassostrea gigas was far less susceptible than O. edulis (Dare 1982).

Impact on the host

There is marked controversy with respect to the actual significance of this parasite to infected oysters and the oyster industry. Some scientists believe that it causes poor growth and condition, extensive tissue damage and metaplasia of the gut wall and sporadic mortalities. Others believe that the effect of the copepod is worst during sub-optimal growing conditions. However, in most infections there is no evidence of pathology caused by these parasites in oysters (Hepper 1956; Dare 1982; Aguirre-Macedo and Kennedy 1999a,b).

Diagnostic techniques

Gross Observations: Mytilicola intestinalis can be found in the dissected intestinal tract of its host examined under a compound (dissecting, binocular) microscope. The reddish colour and elongate morphology aids in the detection of this parasite. Because of the relatively elongate cylindrical morphology and small limbs of this parasitic copepod, it looks like a worm to the unaided eye, thus the common name of red worm. Hockley (1951) occasionally found active specimens with no colour. The chemical disruption of host tissues prior to examination should improve its detection (see below).

The body of adult M. intestinalis has thoracic segments with paired processes and the segmentation of the abdomen is incomplete. The male becomes sexually mature at about 2.8 mm in length and can grow to a maximum length of 4.5 mm. The female becomes sexually mature at about 4.6 mm in length and reaches a maximum length of about 9.0 mm. Paired egg sacs attached to the genital segment (located posterior to the thorax) of the female can extending beyond the posterior end of the abdomen. The head of M. intestinalis carries a median red eye spot, the first pair of antennae has four segments and the second has three. The second antennae are modified as a pair of stout hooks that are used as anchors for resisting expulsion from the host. There is an overall reduction in the length and complexity of the appendages in comparison to free living copepods. The loss of complexity is greatest in the mouth parts where the mandibles are entirely lacking and maxillulae, maxillae and maxilipeds are extremely simplified (Hockley, 1951). Juvenile stages of M. intestinalis (Copepodite II through V) and sexually immature preadult stages, all less than about 2.5 mm in length, also inhabit the intestinal tract of its host (Gee and Davey 1986).

The three species of Mytilicola can be differentiated by external morphological characteristics. Specifically, the caudal ramus of M. intestinalis is elongated (237 µm) and widely divergent, the caudal ramus of M. orientalis is also elongated (233 µm) but not widely divergent, and the caudal ramus of M. porrecta is short (96 µm) and not divergent. In adults of both sexes, the second antenna has three segments (podomeres) in M. intestinalis, two segments in M. orientalis and four segments in M. porrecta. The posterolateral thoracic protuberances are more prominent in M. orientalis, except for the first pair which is absent in male M. orientalis. The adult male of M. porrecta has reduced posterolateral thoracic protuberances that are almost indiscernible. The claw of the maxilliped of male M. porrecta is short, stout and strongly hooked in comparison to the elongated and not strongly hooked maxilliped claw of male M. intestinalis and M. orientalis. Also, female M. intestinalis (4.6 to 9.0 mm in length) tend to be shorter than female M. orientalis (10 to 12 mm in length) and longer than female M. porrecta (about 5 mm in length).

Histology: Examine body cross sections for large copepods within the lumen of the gut. Copepods may attach by hooked appendages to the intestine wall. Focal tissue metaplasia may be present in the intestinal epithelium.

Digestion: Chemical disruption of tissues will expose copepods for easy quantification. Specifically, pepsin digestion of the flesh that was removed from the shells of bivalves followed by filtration of the disintegrated tissues through sieves (348 µm and 124 µm pore size) and examination of the residues for Mytilicola under a compound microscope is a technique used for the detection of all parasitic stages including egg sacs and early infective stages (0.45 µm long) intact (Dare, 1982). This process is recommended for large scale surveys rather than for diagnostic identity of the parasite.

Methods of control

No known methods of prevention or control. Results of field exposure experiments by Dare (1982) confirmed that the oyster juveniles (seed) of Ostrea edulis and Crassostrea gigas could be infested by M. intestinalis but C. gigas was far less susceptible than O. edulis. From these experiments, Dare (1982) concluded that the risk of moving M. intestinalis to new areas through the transportation of C. gigas juveniles less than 25 mm in shell length was negligible and O. edulis should be regarded as a potential vector for this parasite although the risk is likely to be slight with juveniles less than 15 mm in shell length. Bivalves from areas known to be affected (currently or historically) should not be introduced to Canada.


Aguirre-Macedo, M.L. and C.R. Kennedy. 1999a. Patterns in metazoan parasite communities of some oyster species. Journal of Helminthology 73: 283-288.

Aguirre-Macedo, M.L. and C.R. Kennedy. 1999b. Diversity of metazoan parasites of the introduced oyster species Crassostrea gigas in the Exe estuary. Journal of the Marine Biological Association of the United Kingdom 79: 57-63.

Baird, R.H., G.C. Bolster and H.A. Cole. 1951. Mytilicola intestinalis, Steuer, in the European Flat Oyster (Ostrea edulis). Nature 168: 560.

Cheng, T.C. 1967. Marine molluscs as hosts for symbioses with a review of known parasites of commercially important species. In F.S. Russell [ed.]. Advances in Marine Biology. Volume 5. Academic Press Inc., London, p. 286-296.

Dare, P.J. 1982. The susceptibility of seed oysters of Ostrea edulis L. and Crassostrea gigas Thunberg to natural infestation by the copepod Mytilicola intestinalis Steuer. Aquaculture 26: 201-211.

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Gee, J.M. and J.T. Davey. 1986. Stages in the life cycle of Mytilicola intestinalis Steuer, a copepod parasite of Mytilus edulis (L.), and the effect of temperature on their rates of development. Journal du Conseil International pour l'Exploration de la Mer 42: 254-264.

Hepper, B.T. 1953. Artificial infection of various molluscs with Mytilicola intestinalis, Steuer. Nature 172(4371): 250.

Hepper, B.T. 1956. The European flat oyster, Ostrea edulis L. as a host for Mytilicola intestinalis Steuer. The Journal of Animal Ecology 25: 144-147.

Hockley, A.R. 1951. On the biology of Mytilicola intestinalis (Steuer). Journal of the Marine Biological Association of the United Kingdom 30: 223-232.

Lauckner, G. 1983. Diseases of Mollusca: Bivalvia. In: O. Kinne (ed.) Diseases of Marine Animals. Vol II: Introduction, Bivalvia to Scaphoda. Biologische Anstalt Helgoland, Hamburg, p. 817-828.

Steuer, A. 1902. Mytilicola intestinalis n. gen. n. sp. aus dem Darme von Mytilus galloprovincialis Lam. Zoologischer Anzeiger 25: 635-637.

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Citation Information

Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Mytilicola intestinalis (Red Worm Disease) of Oysters.

Date last revised: December 2009
Comments to Susan Bower

Date modified: