Parasite Population Density Research Articles

Transcriptomic analysis of the adaptation to prolonged starvation of the insect-dwelling Trypanosoma cruzi epimastigotes.

Trypanosoma cruzi is a digenetic unicellular parasite that alternates between a blood-sucking insect and a mammalian, host causing Chagas disease or American trypanosomiasis. In the insect gut, the parasite differentiates from the non-replicative trypomastigote forms that arrive upon blood ingestion to the non-infective replicative epimastigote forms. Epimastigotes develop into infective non-replicative metacyclic trypomastigotes in the rectum and are delivered via the feces. In addition to these parasite stages, transitional forms have been reported. The insect-feeding behavior, characterized by few meals of large blood amounts followed by long periods of starvation, impacts the parasite population density and differentiation, increasing the transitional forms while diminishing both epimastigotes and metacyclic trypomastigotes. To understand the molecular changes caused by nutritional restrictions in the insect host, mid-exponentially growing axenic epimastigotes were cultured for more than 30 days without nutrient supplementation (prolonged starvation). We found that the parasite population in the stationary phase maintains a long period characterized by a total RNA content three times smaller than that of exponentially growing epimastigotes and a distinctive transcriptomic profile. Among the transcriptomic changes induced by nutrient restriction, we found differentially expressed genes related to managing protein quality or content, the reported switch from glucose to amino acid consumption, redox challenge, and surface proteins. The contractile vacuole and reservosomes appeared as cellular components enriched when ontology term overrepresentation analysis was carried out, highlighting the roles of these organelles in starving conditions possibly related to their functions in regulating cell volume and osmoregulation as well as metabolic homeostasis. Consistent with the quiescent status derived from nutrient restriction, genes related to DNA metabolism are regulated during the stationary phase. In addition, we observed differentially expressed genes related to the unique parasite mitochondria. Finally, our study identifies gene expression changes that characterize transitional parasite forms enriched by nutrient restriction. The analysis of the here-disclosed regulated genes and metabolic pathways aims to contribute to the understanding of the molecular changes that this unicellular parasite undergoes in the insect vector.

Population Genetic Structure of Anisakis simplex Infecting the European Hake from North East Atlantic Fishing Grounds.

The European hake, one of the most commercially valuable species in ICES fishing areas, is considered an important neglected source of zoonotic risk by nematode parasites belonging to the genus Anisakis. Merluccius merluccius is, by far, the most important host of Anisakis spp. at the European fishing grounds, in terms of demographic infection values, and carries the highest parasite burden. These high parasite population densities within an individual fish host offer a chance to explore new sources of variations for the genetic structure of Anisakis spp. populations. A total of 873 Anisakis spp. third-stage larvae, originally sampled from viscera and muscular sections of hake collected at ten fishing grounds, were primarily identified using ITS rDNA region as molecular marker. After that, we used mtDNA cox2 gene to reveal the high haplotype diversity and the lack of genetic structure for A. simplex. Dominant haplotypes were shared among the different fishing areas and fish sections analyzed. Results indicate a clear connection of A. simplex from European hake along the Northern North Sea to the Portuguese coast, constituting a single genetic population but revealing a certain level of genetic sub-structuring on the Northwest coast of Scotland. This study also provides useful information to advance the understanding of parasite speciation to different fish host tissues or microenvironments.

ThePlasmodium falciparuminfection status of two major anopheline vectors in three hyper-, meso-, and hypoendemic districts in Odisha, India

AbstractIn-depth understanding of malaria transmission dynamics in a region can be assessed by identifying the vector populations of infected mosquitoes,Anophelesspp. (Diptera: Culicidae), and by quantifying the infectiousness extent. In this study of malaria transmission dynamics relating to vector incrimination in three districts of Odisha, India – hyperendemic Kalahandi, mesoendemic Bargarh, and hypoendemic Cuttack – we examined how quality and quantity of plasmodial infection rates vary among mosquito species and their organs and among districts. The minimum infection rate ofPlasmodium falciparumsporozoite forAnopheles culicifacieswas highest in Kalahandi and nil in Cuttack. However, forA. annularis, the rate was highest in Cuttack, followed by Kalahandi and Bargarh. In Kalahandi, the gland-positive rate was higher forA. culicifacies, but in Cuttack, it was higher forA. annularis. To quantify plasmodium infection in salivary glands and guts of anopheline vectors, quantitative polymerase chain reaction was performed. We observed higher parasite density in glands than in guts of vector mosquitoes in the three districts. The findings demonstrate that plasmodium-infected vectors preferentially bind sporozoites to salivary glands in the three study areas. The results improve understanding of infection status within malaria vectors and how parasite population density may affect transmissibility, which will, in turn, provide baseline evidence to develop intervention measures.

Come with me if you want to live: sympatric parasites follow different transmission routes through aquatic host communities

Community composition, including the relative density of each host species, plays a vital role in the transmission of parasites or disease in freshwater ecosystems. Whereas some host species can effectively transmit parasites, others can act as dead ends (non-viable transmission routes), accumulating large numbers of parasites throughout their life, thus becoming important sinks for parasite populations. Although population sinks have been identified in certain host-parasite systems, robust field estimates of the proportions of parasites that are lost to these hosts are lacking. Here, we quantified the distribution of encysted larval hairworms (phylum Nematomorpha), common parasites in lotic ecosystems, in two subalpine stream communities of New Zealand. With parasite and host population densities calculated per m2, we identified which host species most likely contributed to the transmission of three sympatric hairworm morphotypes identified in both streams, and which species acted as population sinks. We also tested for seasonal patterns and peaks in the abundance of each morphotype in the two communities over the sampling season. Finally, we tested whether hosts emerging from the streams had comparable abundances of hairworm morphotypes throughout the sampling period. For each morphotype, different key sets of host species harboured more hairworms on average (abundance) than others, depending on the stream. For one morphotype in particular, two species of hosts were found to be important population sinks that inhibited over a third of these parasites from completing their life cycle. We also observed a clear peak in abundance for another hairworm morphotype during summer. Our data suggest that hosts emerging from the streams matched their aquatic counterparts with respect to hairworm abundance, indicating no infection-dependent reduction in emergence success. Our findings suggest that, depending on relative community composition, sympatric parasites follow different host transmission pathways, some of which lead to dead ends that potentially impact overall infection dynamics. In turn, this information can help us understand the spread or emergence of disease in both freshwater and terrestrial environments, since hairworms infect terrestrial arthropods to complete their life cycle.

How invasive oysters can affect parasite infection patterns in native mussels on a large spatial scale.

There are surprisingly few field studies on the role of invasive species on parasite infection patterns in native hosts. We investigated the role of invasive Pacific oysters (Magallana gigas) in determining parasite infection levels in native blue mussels (Mytilus edulis) in relation to other environmental and biotic factors. Using hierarchical field sampling covering three spatial scales along a large intertidal ecosystem (European Wadden Sea), we found strong spatial differences in infection levels of five parasite species associated with mussels and oysters. We applied mixed models to analyse the associations between parasite prevalence and abundance in mussels and oysters, and 12 biological and environmental factors. For each parasite-host relationship, an optimal model (either a null, one-factor or two-factor model) was selected based on AIC scores. We found that the density of invasive oysters contributed to three of the 12 models. Other biological factors such as host size (six models), and the density of target or alternative host species (five models) contributed more frequently to the best models. Furthermore, for parasite species infecting both mussels and oysters, parasite population densities were higher in native mussels, attributed to the higher densities of mussels. Our results indicate that invasive species can affect parasite infection patterns in native species in the field, but that their relative contribution may be further mediated by other biological and environmental parameters. These results stress the usefulness of large-scale field studies for detailed assessments of the mechanisms underlying the impacts of invasive species on native host communities.

Population structure of the monogenean, Paramazocraes thrissocles (Mazocraeidae) on the gills of clupeoid fish, Thryssa mystax of Visakhapatnam coast, Bay of Bengal.

An investigation was undertaken on the population biology of the mazocraeid monogenean Paramazocreas thrissocles occurring on the gills of the engraulid fish Thryssa mystax at Visakhapatnam coast , Bay of Bengal. The influence of season and host size on the parasite population density and the dispersion pattern of the parasite in the host population were analysed. A distinct seasonal pattern in the prevalence and intensity to infection is noted with higher parasite loads prevailing during warmer summer months and very low or no infection during winter months. The heavy infections are found to be promoted by high temperatures prevailing during summer months coupled with high host density and the larger size of fish examined during this period. A positive correlation between host size and prevalence and mean intensity to infection with the monogenean was noted, larger fish often carrying heavy infections. Further the monogenean showed an aggregated distribution on the host population as evidenced by the low value of negative binomial parameter 'K'.

Local diversity reduces infection risk across multiple freshwater host‐parasite associations

Summary In many host–parasite systems, infection risk can be reduced by high local biodiversity, though the mitigating effects of diversity are context dependent and not universal. In aquatic ecosystems, local fauna can reduce the transmission success of parasite free‐swimming infective stages by preying on them, acting as decoy hosts, or physically interfering with transmission. However, most prior research has focused on the effect of a single non‐host organism at a time and/or has been performed under simplified and artificial conditions. Here, using data on 11 trematode species sampled in different New Zealand lakes, we test whether local biodiversity affects infection risk in target second intermediate hosts, as well as total parasite population size (number of parasites per m2), under natural conditions. We considered four components of local biodiversity: total biomass of non‐host fish species, diversity (Simpson index) of non‐host benthic invertebrates, total density of zooplankton and macrophyte biomass. Our analyses also accounted for host density, a known determinant of parasite prevalence, intensity of infection and total parasite population density. The only influence of local biodiversity we detected was a negative effect of the diversity of non‐host benthic invertebrates on the prevalence achieved by trematodes in their second intermediate hosts: that is, the proportion of individual hosts that are infected. Interestingly, this effect was discernible in all 11 trematode species considered here, even if very weak within some species. Our findings suggest that higher non‐host benthic diversity may generally decrease infection risk for target hosts including snails, arthropods and fish. However, reduced infection success did not automatically mean smaller overall parasite population size, as other factors can maintain the parasite population in the face of high local diversity of non‐hosts.

Bottom–up regulation of parasite population densities in freshwater ecosystems

Theory predicts the bottom–up coupling of resource and consumer densities, and epidemiological models make the same prediction for host–parasite interactions. Empirical evidence that spatial variation in local host density drives parasite population density remains scarce, however. We test the coupling of consumer (parasite) and resource (host) populations using data from 310 populations of metazoan parasites infecting invertebrates and fish in New Zealand lakes, spanning a range of transmission modes. Both parasite density (no. parasites per m2) and intensity of infection (no. parasites per infected hosts) were quantified for each parasite population, and related to host density, spatial variability in host density and transmission mode (egg ingestion, contact transmission or trophic transmission). The results show that dense and temporally stable host populations are exploited by denser and more stable parasite populations. For parasites with multi‐host cycles, density of the ‘source’ host did not matter: only density of the current host affected parasite density at a given life stage. For contact‐transmitted parasites, intensity of infection decreased with increasing host density. Our results support the strong bottom–up coupling of consumer and resource densities, but also suggest that intraspecific competition among parasites may be weaker when hosts are abundant: high host density promotes greater parasite population density, but also reduces the number of conspecific parasites per individual host.

TcI/TcII co-infection can enhance Trypanosoma cruzi growth in Rhodnius prolixus

BackgroundRhodnius prolixus is an obligate haematophagous insect and one of the most important vectors of Trypanosoma cruzi, the causative agent of Chagas disease in the Americas. T. cruzi is a highly variable parasite which is not transmitted in the same efficiency by the different triatomine vectors. Because different T. cruzi genotypes are aetiopathologically divergent, further elucidation of the transmission abilities of different Chagas disease vectors is extremely important.FindingsIn the present study, the growth behaviour of two T. cruzi isolates, MDID/BR/1993/C45 (TcI) and TBRA/BR/1999/JCA3 (TcII), sharing the same microhabitat (intestinal tract) in single and mixed infections, was examined. The distribution patterns and parasite population densities were evaluated at 7, 14 and 21 days after feeding (daf) by quantification of parasites using Neubauer haemocytometric measurements and mini-exon PCR to identify TcI and TcII subpopulations. Parasitic colonization in the small intestine was more successful in the mixed infection model than the single infection models at 21 daf. In the rectal lumen and wall, the growth behaviour of the mixed infection was similar to that of the TcI group, although the total parasite number was lower. In the TcII group, no metacyclic trypomastigote forms were found. PCR analysis of the contents of each dissected region showed different genotype fractions in the mixed infection model, in which TcI seemed to be the predominant isolate.ConclusionThe different growth behaviour of the TcI and TcII isolates in single and mixed infection models demonstrated that possibly an intraspecific factor modulates parasitic development in the intestine of R. prolixus.

Ectoparasite load in European vespertilionid bats

AbstractThe parasite load of seven European bat species (Myotis daubentonii, Myotis emarginatus, Myotis myotis, Myotis mystacinus, Myotis nattereri, Nyctalus noctula, Pipistrellus pipistrellus) was compared and tested to see if there was a relationship between high parasite load and poor body condition in bats. Considerable variations of the parasite load between host species, ages and sexes were found, which can be attributed to roosting conditions and behavioural differences of the bats. Overall, male bats had the fewest parasites. Very different parasite population densities were found in different years, and also at the same time at different sites. A generally high parasite load during the nursing season was not observed. There was no strong trend of increasing parasite load in bats with decreasing condition. If weak bats had more parasites than other individuals, the high number of parasites seemed to be a symptom and not the direct cause of the bad condition.

Gene flow and the coevolution of parasite range.

The geographic range of many parasites is restricted relative to that of their hosts. We study possible evolutionary mechanisms for this observation using a simple model that couples coevolution and demography. The model assumes that the environment consists of two habitats connected by movement and that coevolution is governed by quantitative traits. Our results demonstrate that host gene flow is an important determinant of parasite geographic range. Fluctuations in the rate of host gene flow cause shifts in parasite population densities and associated range expansions or contractions. In extreme cases, changing the rate of host gene flow can lead to global extinction of the parasite. Through a process we term demographic compensation, these shifts in parasite density may occur with little or no change in parasite adaptation to the host. As a consequence, reciprocal adaptation between host and parasite can become uncoupled from the rate of host gene flow.

GENE FLOW AND THE COEVOLUTION OF PARASITE RANGE

The geographic range of many parasites is restricted relative to that of their hosts. We study possible evolutionary mechanisms for this observation using a simple model that couples coevolution and demography. The model assumes that the environment consists of two habitats connected by movement and that coevolution is governed by quantitative traits. Our results demonstrate that host gene flow is an important determinant of parasite geographic range. Fluctuations in the rate of host gene flow cause shifts in parasite population densities and associated range expansions or contractions. In extreme cases, changing the rate of host gene flow can lead to global extinction of the parasite. Through a process we term demographic compensation, these shifts in parasite density may occur with little or no change in parasite adaptation to the host. As a consequence, reciprocal adaptation between host and parasite can become uncoupled from the rate of host gene flow.

Sublethal infection and the population dynamics of host–microparasite interactions

Summary 1. A large group of parasites, including many of the larval pathogens of insects, cause an infection from which infectious hosts are unable to recover. In addition, a proportion of those individuals that do not develop the lethal disease on exposure to the parasite may still be harmed by it. 2. We examined the role of these sublethal effects on host‐population dynamics. Specifically we considered the case where there are three distinct classes of individuals: (i) susceptibles; (ii) infected and infectious individuals that will not reproduce and cannot recover; and (iii) sublethally infected individuals. 3. Parasites with sublethal effects are less likely to persist and control their host population. This is a consequence of the sublethally infected individuals not being infective. Less intuitively, the sublethal infection is highly destabilizing, increasing the risk of cyclic behaviour in host–parasite population densities. 4. Because sublethal infection acts as a destabilizing force in these host–parasite interactions, parasites with pronounced sublethal effects may be less effective as classical biological control agents.

Effects of the Protozoan Parasite Ophryocystis elektroscirrha on the Fitness of Monarch Butterflies (Danaus plexippus)

We evaluated the effects of the protozoan parasite Ophryocystis elektroscirrha on the survival and reproduction of monarch butterflies. Because larvae in natural populations are likely to experience a wide range of natural parasite population densities, we examined the effects of increasing spore density (0, 10, 100, or 1000 spores per larva) on host fitness. Parasites had little effect on monarch survival or reproduction, except at the highest dose. Monarchs inoculated with 1000 spores per larva had decreased survival to eclosion, and this effect was more severe when larvae were inoculated at an earlier stage (first versus third instar). Monarchs inoculated with higher spore densities also emerged with smaller wingspans and lower body mass than noninoculated adults. Infection with the highest dose of O. elektroscirrha led to decreased male lifespan and reproductive success, but females infected with O. elektroscirrha did not experience a significant decline in lifetime fecundity. However, heavily infected females in outdoor enclosures were less active than uninfected females and gained weight during their adult lifespan. Among samples of adult monarchs captured in natural populations, parasite loads were associated with butterfly condition and activity. Heavily infected adults captured breeding in western North America and southern Florida were smaller than uninfected monarchs. Among overwintering adults in Mexico and California, mating activity was positively associated with higher parasite loads. In addition, the proportion of adults with low and intermediate spore loads (as opposed to no spores) was higher among adults with greater wing tatter and scale loss. Our findings of minor effects of O. elektroscirrha on the survival and reproduction of monarch butterflies are consistent with the expectation that maternally transmitted parasites should have little or no effect on host fitness compared with horizontally transmitted parasites. However, because our laboratory studies demonstrated that monarchs exposed to the highest parasite density experienced decreased larval survival, smaller adult size, and shorter adult lifespans, additional transmission routes are likely to be important for parasite maintenance in natural populations.

A Proposed Density-Dependent Model of Long Slender to Short Stumpy Transformation in the African Trypanosomes

A simple arithmetic model is developed that is based upon the assumption: (1) that transformation of replicating long slender Trypanosoma brucei to nonreplicating short stumpy forms is parasite population density dependent; (2) that as the slender population increases there is a change in the external environment that triggers the slender to stumpy transformation; and (3) that stumpy forms of T. brucei do not induce the change in external environment that triggers slender to stumpy transformation or do so to a lesser extent than slender forms, thus preventing the proportion of stumpy forms in a population from reaching 100%. A simulation based on these assumptions shared many features with curves on numbers of long slender, intermediate, and short-stumpy forms of T. brucei during the first parasitemic wave of the 3 T. brucei subspecies in intact and immunosuppressed inbred mice.

The suprapopulation dynamics ofLernaeocera branchialisandL. lusciin the Oosterschelde: seasonal abundance on three definitive host species

Quantitative samples of the demersal fish fauna of the Oosterschelde were taken between May 1988 and July 1989. The instantaneous mortality rates of the 1988 year classes of whiting (Merlangius merlangus L.), sand goby (Pomatoschistus minutus Pallas) and bib (Trisopterus luscus L.) were estimated at 0.017 d −1 , 0.014 d −1 and 0.018 d −1 , respectively. These three species were examined for the presence of mesoparasitic pennellid copepods. Combination of host population densities and parasite infection intensities allowed for estimation of parasite population densities. Whiting were infected with Lernaeocera branchialis shortly after their arrival (April-May) in the Oosterschelde. By the end of the year approximately 80% of the first year class whiting were infected. Spawning sand gobies were infected with Lernaeocera lusci in May and June. Though the prevalence of L. lusci on juvenile sand gobies in autumn was low, the parasite population density in this period was high. First year class bib were mainly infected by L. lusci after their transition from a pelagic to a demersal behaviour in August. It is suggested that L. branchialis has one generation each year, whereas L. lusci has two. A biological model of the life cycle of both parasite species is presented.

Parasitic and Infectious Diseases. Epidemiology and Ecology

M.E. Scott, Populations are Dynamic. G. Smith, So You Want to Write a Model. G. Smith and M.E. Scott, Model Behavior and the Basic Reproduction Ratio. H. McCallum and M.E. Scott, Quantifying Population Processes: Experimental and Theoretical Approaches. G. Smith, Parasite Population Density Is Regulated. G. Smith, Ecological Epidemiology and Standard Measures of Disease Occurence. J.L. Aron and B.A. Silverman, Models and Public Health Applications. D. Wakelin, Host Populations: Genetics and Immunity. N.W. Solomons and M.E. Scott, Nutritional Status of Host Populations Influences Parasitic Infections. V. Apanius and G.A. Schad, Host Behavior and the Flow of Parasites through Host Populations. A.R. McLean, Control of Microparasites through Vaccination. G.F. Medley, Chemotherapy. C. Dye, Vector Control. D.W.T. Crompton, Ascaris lumbricoides. M.E.J. Woolhouse, Epidemiology of Human Schistosomes. B.T. Grenfell and B.M. Bolker, The Population Dynamics of Measles. M.J. Coyne and G. Smith, Trichostrongylid Parasites of Domestic Ruminants. M.G. Roberts and M.A. Gemmell, Echinococcosis. S. Vail, G. Smith, and C. Lord, The Population Biology of Ixodes scapularis, The Vector of Lyme Disease in the Eastern and North Central United States. M.E. Scott and G. Tanquay, Heligmosomoides polygyrus: A Laboratory Model for Direct Life Cycle Nematodes of Man and Livestock. A.P. Dobson and P.J. Hudson, The Population Biology of Trichostrongylus tenuis in Red Grouse, Lagopus lagopus scoticus. G.W. Esch, The Population Biology of the Diplostomatid Trematode, Uvulifer ambloplitis. F. Fenner, Myxomatosis. M.D.B. Burt, The Sealworm Situation. Index.

Plasma Chloride and Gill Epithelial Response of Channel Catfish to Infection withIchthyophthirius multifiliis

Abstract Because infection with Ichthyophthirius multifiliis has been reported to result in osmotic stress, genesis of the plasma chloride and chloride cell response of channel catfish Ictalurus punctatus to the infection was studied from day 0 to day 7 postexposure (PE). Parasite population densities ranged from 0.08 to 0.22 trophonts per gill filament during days 1–5 PE and increased to 2.1 at day 7 PE. Population increases were the result of infection by a new generation of infective theronts and reproduction by the parasite on the host. Mean plasma chloride concentrations of fish were not significantly different among days 0–5 PE (range of means, 93.01–97.87 meq/L), but by day 7 PE the mean had declined significantly, to 83.73 meq/L. Maintenance of relatively stable plasma chloride concentrations for 5 d was associated with chloride cell proliferation. The number of interlamellar filament chloride cells had significantly increased by day 1 PE, and the number of chloride cells in the lamellae had signi...

The population biology of parasite-induced changes in host behavior.

The ability of parasites to change the behavior of infected hosts has been documented and reviewed by a number of different authors (Holmes and Bethel, 1972; Moore, 1984a). This review attempts to quantify the population dynamic consequences of this behavior by developing simple mathematical models for the most frequently recorded of such parasite life cycles. Although changes in the behavior of infected hosts do occur for pathogens with direct life cycles, they are most commonly recorded in the intermediate hosts of parasites with complex life cycles. All the changes in host behavior serve to increase rates of transmission of the parasites between hosts. In the simplest case the changes in behavior increase rates of contact between infected and susceptible conspecific hosts, whereas in the more complex cases fairly sophisticated manipulations of the host's behavioral repertory are achieved. Three topics are dealt with in some detail: (1) the behavior of the insect vectors of such diseases as malaria and trypanosomiasis; (2) the intermediate hosts of helminths whose behavior is affected in such a way as to make them more susceptible to predation by the definitive host in the life cycle; and (3) the behavior and fecundity of molluscs infected with asexually reproducing parasitic flatworms. In each case an expression is derived for R0, the basic reproductive rate of the parasite when first introduced into the population. This is used to determine the threshold numbers of definitive and intermediate hosts needed to maintain a population of the pathogen. In all cases, parasite-induced changes in host behavior tend to increase R0 and reduce the threshold number of hosts required to sustain the infection. The population dynamics of the interaction between parasites and their hosts are then explored using phase plane analyses. This suggests that both the parasite and intermediate host populations may show oscillatory patterns of abundance. When the density of the latter is low, parasite-induced changes in host behavior increase this tendency to oscillate. When intermediate host population densities are high, parasite population density is determined principally by interactions between the parasites and their definitive hosts, and changes in the behavior of intermediate hosts are less important in determining parasite density. Analysis of these models also suggests that both asexual reproduction of the parasite within a host and parasite-induced reduction in host fecundity may be stabilizing mechanisms when they occur in the intermediate hosts of parasite species with indirect life cycles.(ABSTRACT TRUNCATED AT 400 WORDS)

The occurrence and effects of Mytilicola intestinalis in Mytilus edulis

Mytilicola intestinalis was observed in the mussel Mytilus edulis in increasing numbers for the first time at Brighton (England), in October 1966; the populations here and at Whitstable were examined. Mussels exposed high in the littoral zone were less heavily infected than those lower down, the degree of infection being directly related to the duration of exposure in each tidal cycle. Silt in the intestine of the mussel is considered to act as a controlling factor in numbers of parasites present at Whitstable. Egg-bearing copepods were present in samples throughout the year, suggesting that breeding is not interrupted by the winter. Evidence indicates that juvenile stages of the parasite cause most damage to the host, due in part to their presence in the ramifications of the hepatopancreas. Recovery of the mussel from the effects of parasitation is rapid, following a reduction in parasite population density and number of juveniles. In the laboratory, M. edulis is more rapidly affected by lack of food at 10 °C than M. intestinalis. No dead parasites were seen during 4 months of laboratory storage. Juvenile parasites continned to mature, indicating that this period of time may be required for Mytilicola intestinalis to reach maturity at 10°C.