Mytilicola intestinalis (Red Worm Disease) of Mussels


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). Distribution along the west coast of Europe and around Italy is patchy with a high prevalence in some locations (up to 100%) and not detectable in others (Lauckner 1983, Canestri Trotti et al. 1998). 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 an 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

Mytilus edulis and Mytilus galloprovincialis are believed to be the primary host for Mytilicola intestinalis. However, other bivalves are known to be infested including oysters, clams and cockles and in the laboratory, Crepidula fornicata.

Impact on the host

Usually there is a positive correlation between host size and the intensity of infestation with larger mussels harbouring more parasites, and juvenile mussels less than 10 mm in length are rarely infested (Williams, 1967). This positive correlation is unlikely to represent an accumulation of parasites with increased host age because of the short life span of M. intestinalis (Davey and Gee 1976). Larger hosts accumulate more parasites as a result of a higher filtration rate in larger bivalves (Davey and Gee 1976; Paul 1983). Prevalence of 100% is common and more than 30 copepods may be dissected from a single mussel (Lauckner, 1983). In a population of mussels near Cornwall, England, some mussels contained over 90 M. intestinalis but in such cases, about two thirds of the parasites were copepodites stages I to III (which are less than 1 mm long) or immature later stages (Davey et al. 1978). In this population, female M. intestinalis produced two broods and two generations of parasites coexist for most of the year, with recruitment taking place in summer and autumn (Davey et al., 1978, Davey, 1989). However, more northerly populations are limited to a single generation per year and in southerly populations, such as in Galicia, NW Spain and the Mediterranean Sea, there is less seasonal variation in M. intestinalis numbers and generations overlap continuously throughout the year (Andreu, 1963; Rayyan et al., 2004). Temperature dependant development rates throughout the life cycle of the parasite seem to account for differences in population dynamics over the geographical range M. intestinalis (Gee and Davey, 1986a; Davey and Gee, 1988). Mytilicola intestanilis may be enzootic in low proportions of mussels in various areas and become epizootic only under adverse environmental conditions (Lauckner, 1983).

The effect of infection on the host is the subject of intense debate. Prior to the late 1960s, M. intestinalis was frequently accused of causing disease and mass mortalities in various populations of mussels (Mytilus galloprovincialis and Mytilus edulis). For example, M. intestinals was thought to have caused large scale mortalities among mussels bringing the related industry to a standstill first in the Netherlands in 1949 and then in Germany in 1950. However, in none of the cases were the mussels assessed for the presence of microbial or other pathogens or adverse environmental factors that could have caused the diseases (Dollfus 1951, Lauckner 1983). Also, many of the studies lacked the experimental protocols and statistical rigor that would be demanded today casting doubt upon their credulity (Davey and Gee 1988). Most findings could not be duplicated by modern workers and mass mortalities of European mussels have virtually subsided in spite of persistent high prevalences of M. intestinalis in some areas (Davey et al. 1978, Davey and Gee 1988, Figueras et al. 1991, Stock 1993). An exception to this last statement was the heavy losses among mussels cultured in Mont Saint-Michel’s Bay, northern Brittany, France between 1982 and 1984 that were associated with thin mussels with heavy infestations of M. intestinalis (Blateau 1989). However, the problem was resolved after the mussel densities were reduced even though M. intestinalis persisted but at a lower level of infestation (Blateau et al. 1992, Jean-Pierre Joly, personal commmunication, and Daniel Gerla, unpublished report (RIDRV-90.25 –CSRU), IFREMER, France).

Histological evidence indicates that M. intestinalis causes local metaplastic changes in the gut epithelium involving the replacement of normal ciliated columnar cells with non-ciliated cuboidal cells. Accumulation of haemocytes, the usually response by bivalves to disease, was not evident (Moore et al. 1978, Robledo et al. 1994b). But, limited haemocytic infiltration in the intestinal epithelium and surrounding connective tissue near the copepod was noted by Figueras et al. (1991) and Villalba et al. (1997). Histochemical examination of the gut contents of M. intestinalis observed in situ revealed that the diet of the copepod is primarily herbivorous, presumably the copepod feeds on the gut contents of the host and not on host tissues (Moore et al., 1978). Campbell (1970) determined that juvenile stages of the parasite cause most damage to the host, due in part to their presence in the ramifications of the hepatopancreas. However, Moore et al. (1978) found no evidence of this pathology and indicated that the few copepodites that penetrated the tubule epithelium and lodged in the connective tissue were encapsulated and killed. Given the motility of the copepod, repair of the damaged area was rapid with no apparent significant effect on the basic cellular function of the host (Campbell 1970; Moore et al. 1978; Robledo et al. 1994a,b). Davey et al. (1978) detected no evidence to support the contention that heavily infested mussels are killed, and deduced that the reduction in parasite infestations during the spring was attributed to parasite deaths not host deaths. Although Gee and Davey (1986b) speculated that at high concentrations of M. intestinalis resulting in mechanical blockage of the mussel intestine could cause mussel mortality, they suggested that density-dependent mortality of parasites operated during M. intestinalis development and would prevent the occurrence of this situation.

Some investigations since 1969 indicated that mussels infested with M. intestinalis had a significantly lower condition index than non-infested mussels (Theisen 1987, Pérez Camacho et al. 1997, Rayyan et al. 2004). Apparently, this effect can be masked in samples of mussels that have a high variation in condition thereby requiring large sample sizes to demonstrate this effect (Theisen 1987). However, Pérez Camacho et al. (1997) noted that M. intestinalis did not have a measurable effect on absorption efficiency and ingestion rate but the sample size of infested mussels was small (n=9) and Davey (1989) claimed that Theisen (1987) did not use any of the standard formulae for assessing condition conventionally applied by other authors. Andreu (1963) noted an inverse relationship between the average weight of mussel flesh and the number of parasites in the gut of their hosts but did not observe any external signs that could demonstrate the influence of the parasites in the most heavily infested mussels. Bayne et al. (1978) determined that more than 10 M. intestinalis per mussel at a time of high metabolic demand for the host (e.g., during gametogenesis or when environmental temperatures exceeded 22° or 23 °C and food availability was low) would result in a decline in the condition of the host. Other studies found no or infrequent negative correlations between condition index (meat content) and infestations of M. intestinalis (Dethlefsen 1975, Paul 1983, Pascual et al. 1987).

Additional information pertaining to the impact by M. intestinalis on its mussel host was published by several other authors. Williams (1969a) determined that differences in lipid, protein, carbohydrate and ash between parasitized and non-parasitized mussels were rarely significant, but differences were observed during the times of the year correlated to the breeding cycle of the mussel when the greatest changes in biochemical composition were occurring. Durfort et al. (1982) observed a series of ultrastructural alterations within the ovocytes of mussels infested with M. intestinalis similar to those in mussels parasitized by trematode sporocysts and mussels from an oil spill site. However, Williams (1969a) reported that breeding did not cease in infested mussels but may be somewhat retarded by the presence of the parasite. Robledo et al. (1995) reported significant reduction in total carbohydrate concentrations in the haemolymph of M. galloprovincialis from Galicia (NW Spain) when M. intestinalis was present with other parasites but not as the only parasite detected. And, Carballal et al. (1998) found no significant differences in the haemograms (number of haemocytes, granulocytes and hyalinocytes) in M. galloprovincialis from the same location whether M. intestinalis was present or absent.

Overall, the effects of M. intestinalis on the condition index and biochemical constituents of mussels seems to fall within the general seasonal variations in these parameters in mussels with exception to the very highest infestations during periods of extreme environmental conditions (Lauckner 1983, Davey and Gee 1988). Gee and Davey (1986b) and Davey and Gee (1988) contended that M. intestinalis and the mussel form more of a commensal association in which the copepod uses the bivalve to concentrate algal food and then feeds on that fraction not used by the mussel. However, these authors indicate that additional research is needed to fully understand the feeding mechanisms of the copepod especially that of the juvenile stages that inhabit the stomach and digestive tubules of the host. Also, further work is required in the area of pathogenicity and biological and anthropogenic synergisms, particularly in relation to other pathogens and to pollutants, before the pest status of M. intestinalis can be fully decided (Davey and Gee 1988).

Diagnostic techniques

Gross Observations: Mytilicola intestinalis can be found in the dissected intestinal tract of its host examined under a compound (dissecting) microscope. The reddish colour and elongate morphology aids in the detection of this parasite. Because of the relatively elongate 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. Female M. intestinalis in young mussels (between 10 mm and 35 mm in length) were significantly shorter in length than those from adult mussels possibly because of the smaller dimensions of the intestine in small mussels (Williams, 1967). 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 (Davey 1989, Gee and Davey 1986a).

Mytilicola intestinalis can be differentiated from the other two species in the genus (Mytilicola orientalis and Mytilicola porrecta) by external morphological details (Humes, 1954; Cheng, 1967). 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 gut lumen. Copepods may attach by hooked appendages to the gut wall. Focal tissue metaplasia may be present in the gut epithelium.

Digestion: Chemical disruption of the tissues exposes 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 binocular 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). Alternately, mussels can be digested in the shell using 10 to 15 g papain powder per 100 ml of raw mussel flesh in 500 ml water at 30 °C for 48 hr after the mussels were deep frozen and thawed to induce the mussels to gape. The resulting digested material was sieved through 2.5 mm mesh to remove the shell, 1.05 mm mesh to remove large Mytilicola, byssus strands and finer debris and 0.35 mesh mm to retain small Mytilicola. Material retained by the two finer sieves was placed in water and scanned with a low power binocular microscope (Dare, 1977). Dare (1977) determined that Mytilicola cuticle was resistant to the action of papain at concentrations of 10 g to 15 g in 500 ml of water for at least 72 hr. at 30 °C. This process is recommended for large scale surveys rather than for diagnostic identity of the parasite.

Methods of control

The infestation of mussels was shown to be essentially a passive process dependant first on the chance encounter between M. intestinalis Copepodite I and the field of filtration of the host and secondly on the strength of the inhalant current of the host. Also, once established within a host, M. intestinalis cannot transfer from one host to another (Gee and Davey 1986b). Factors that may determine infestation rates and spread of M. intestinalis include the age and availability of larvae, host population size and density, water temperature, the amount of flushing in the water column, wave action, current speed, turbulence, salinity, depth of water, and location within an estuary (Davey and Gee 1976, Robledo et al. 1994b). Hosts from lower shore levels and from sheltered areas are invariably more heavily invaded than individuals from higher-shore levels and exposed localities (Lauckner 1983). Fuentes et al. (1995) found that the prevalence of M. intestinalis in mussels (M. galloprovincialis) cultured in the Ría de Arousa (Galicia, NW Spain) was not affected by any of the 3 factors investigated; location in the ría, mussel stock deployed and situation within the raft. However, infestation increased with the depth from the surface on cultivation ropes in the Rías of Galicia, north west Spain and the Thermaikos Gulf, Greece (Paul 1983, Fuentes et al. 1998, Rayyan et al. 2004). Infestations of M. intestinalis can apparently be reduced by growing mussels on stakes, fences or ropes in fast moving water and brackish water. Epidemics attributed to M. intestinalis in mussels cultivated in Mont Saint-Michel’s Bay, northern Brittany, France during the early 1980s were alleviated by restricting the density of mussels farmed in the area (Blateau et al. 1992). In addition to restricting the density of farmed mussels, Andreu (1963) recommended obtaining mussel seed less than 20 mm in length from less infested areas and not to grow the mussels on rafts placed close to shore where tidal currents are reduced. Brienne (1964) listed these as well as other strategies that may reduce the intensity of infestations.

Development in Italy of mussel culture in the open sea over the last 10 years may explain current lower prevalences of M. intestinalis in comparison to past reports (Canestri Trotti et al. 1998). In addition to farm management strategies, Blateau (1989) and Blateau et al. (1992) indicated that out of seven drugs tested, an organophosphorate Dichlorvos (emersion in 30 mg/l concentration for 2 hr) appeared to be the most efficient at freeing mussels of Mytilicola without any mortalities of the mussels. The most significant control on M. intestinalis populations may be the survival of the planktonic larvae, the ability of the infective stage to find a suitable host and density-dependant factors within the host (Davey et al. 1978).

Bivalves from areas known to be affected (currently or historically) should not be introduced to Canada.


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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 Mussels.

Date last revised: December 2009
Comments to Susan Bower

Date modified: