Genetic variation in susceptibility to Trypanoplasma borreli infection in common carp (Cyprinus carpio L.)

A century of parasitology in fisheries and aquaculture

Fish parasitological research associated with fisheries and aquaculture has expanded remarkably over the past century. The application of parasites as biological tags has been one of the fields in which fish parasitology has generated new insight into fish migration and stock assessments worldwide. It is a well-established discipline whose methodological issues are regularly reviewed and updated. Therefore, no concepts or case-studies will be repeated here; instead, we summarize some of the main recent findings and achievements of this methodology. These include the extension of its use in hosts other than bony fishes; the improvements in the selection of parasite tags; the recognition of the host traits affecting the use of parasite tags; and the increasingly recognized need for integrative, multidisciplinary studies combining parasites with classical methods and modern techniques, such as otolith microchemistry and genetics. Archaeological evidence points to the existence of parasitic problems associated with aquaculture activities more than a thousand years ago. However, the main surge of research within aquaculture parasitology occurred with the impressive development of aquaculture over the past century. Protozoan and metazoan parasites, causing disease in domesticated fish in confined environments, have attracted the interest of parasitologists and, due to their economic importance, funding was made available for basic and applied research. This has resulted in a profusion of basic knowledge about parasite biology, physiology, parasite-host interactions, life cycles and biochemistry. Due to the need for effective control methods, various solutions targeting host-parasite interactions (immune responses and host finding), genetics and pharmacological aspects have been in focus.

Passive Immunization Delays Disease Outcome in Gilthead Sea Bream Infected With Enteromyxum leei (Myxozoa), Despite the Moderate Changes in IgM and IgT Repertoire

Passive immunization constitutes an emerging field of interest in aquaculture, particularly with the restrictions for antibiotic use. Enteromyxum leei is a myxozoan intestinal parasite that invades the paracellular space of the intestinal epithelium, producing a slow-progressing disease, leading to anorexia, cachexia and mortalities. We have previously demonstrated that gilthead sea bream (GSB, Sparus aurata) that survive E. leei infection become resistant upon re-exposure, and this resistance is directly related to the presence of high levels of specific IgM in serum. Thus, the current work was aimed to determine if passive immunization could help to prevent enteromyxosis in GSB and to study in detail the nature of these protective antibodies. Serum from a pool of resistant (SUR) or naïve (NAI) animals was intracoelomically injected 24 h prior to the E. leei-effluent challenge and at 9 days post-challenge (dpc). Effluent challenge lasted for 23 days, and then the injected groups were allocated in separate tanks with clean water. A non-lethal parasite diagnosis was performed at 56 dpc. At the final sampling (100 dpc), blood, serum and tissues were collected for histology, molecular diagnosis and the detection of circulating antibodies. In parallel, we performed an immunoglobulin repertoire analysis of the fish generating SUR and NAI sera. The results showed that, fish injected with parasite-specific antibodies (spAbs) became infected with the parasite, but showed lower disease signs and intensity of infection than the other groups, indicating a later establishment of the parasite. Repertoire analysis revealed that E. leei induced a polyclonal expansion of diverse IgM and IgT subsets that could be in part an evasion strategy of the parasite. Nonetheless, GSB was able to produce sufficient levels of parasite-spAbs to avoid re-infection of surviving animals and confer certain degree of protection upon passive transfer of antibodies. These results highlight the crucial role of spAb responses against E. leei and set the basis for the development of effective treatment or prophylactic methods for aquaculture.

Differential expression of innate and adaptive immune genes in the survivors of three gibel carp gynogenetic clones after herpesvirus challenge

BACKGROUND

Accompanied with rapid growth and high density aquaculture, gibel carp has been seriously threatened by Carassius auratus herpesvirus (CaHV) since 2012. In previous study, distinct CaHV resistances and immune responses were revealed in the diseased individuals of three gibel carp gynogenetic clones (A+, F and H). However, little is known about the gene expression changes in the survivors after CaHV challenge, particularly their differences of innate and adaptive immune system between susceptible clone and resistant clone.

RESULTS

We firstly confirmed the CaHV carrier state in the survivors of three gibel carp clones after CaHV challenge by evaluating the abundances of five CaHV genes. The assay of viral loads indicated the resistant clone H possessed not only stronger resistance but also higher tolerance to CaHV. Then, 2818, 4047 and 3323 differentially expressed unigenes (DEUs) were screened from the head-kidney transcriptome profiles of survivors compared with controls from clone A+, F and H. GO and KEGG analysis suggested that a persistent immune response might sustain in resistant clone H and F, while susceptible clone A+ had a long-term impact on the circulatory system which was consistent with the major symptoms of bleeding caused by CaHV. Among the top 30 enriched pathways of specifically up-regulated DEUs in respective clones, 26, 7 and 15 pathways in clone H, F and A+ were associated with infections, diseases, or immune-related pathways respectively. In addition, 20 pathways in clone F belonged to "metabolism" or "biogenesis", and 7 pathways involved in "circulatory system" were enriched in clone A+. Significantly, we revealed the differential expression changes of IFN system genes and immunoglobulin (Ig) genes among the survivors of three clones. Finally, myosins and Igs were identified as co-expression modules which were positively or negatively correlated to CaHV viral loads respectively.

CONCLUSIONS

Our results revealed the common and distinct gene expression changes in immune and circulatory system in the survivors of three gibel carp gynogenetic clones with different CaHV resistances. The current study represents a paradigm of differential innate and adaptive immune reactions in teleost, and will be beneficial to the disease-resistance breeding of gibel carp.

Immunity against selected piscine flagellates

This discussion is on immune response to Amyloodinium ocellatum, Cryptobia salmositica, Trypanoplasma borreli and Trypanosoma carassii. Piscidin and histone-like proteins enhance innate resistance to Amyloodinium. Fish that are naturally resistant to Cryptobia and Trypanoplasma can be bred. Cryptobia resistance in charr is controlled by a dominant Mendelian locus and protection is via the Alternative Pathway of Complement Activation. Studies on Cryptobia-tolerant charr may lead to production of transgenic Cryptobia-tolerant salmon. Innate response to T. borreli is associated with NO in macrophages. Transferrin regulates resistance and carp have been bred for transferrin genotypes. Recovered fish are protected from homologous challenge, and complement fixing antibodies are crucial in protection. Studies on antigens in T. carassii may lead to a vaccine. There are two vaccines against cryptobiosis; a single dose of the attenuated vaccine protects salmonids. On challenge fish inoculated with the metalloprotease-DNA vaccine do not have the disease and they recover faster.

Heterogeneity of macrophage activation in fish

In this review, we focus on four different activation states of fish macrophages. In vitro, stimulation with microbial ligands induces the development of innate activated macrophages whereas classically activated macrophages can be induced by stimulation with LPS in combination with (recombinant) IFNγ. Both types of macrophages show elevated phagocytic activity, expression of pro-inflammatory cytokine genes and radical production. Alternatively activated macrophages require the cytokines IL-4/IL-13 for induction of, among others, arginase activity. Until in vitro studies identify the effects of putative IL-4 and IL-13 homologues on fish macrophages, arginase enzyme activity remains the most reliable marker for the presence of alternatively activated macrophages in fish. The best evidence for the existence of regulatory macrophages, associated with the presence of IL-10, comes from in vivo studies, for example during parasitic infections of carp. Altogether, differentially activated macrophages in fish largely resemble the phenotypes of mammalian macrophages. However, the presence of fish-specific ligand recognition by TLRs and of duplicated genes coding for proteins with particular activities, poses additional challenges for the characterization of phenotype-specific gene signatures and cell surface markers.

Immune responses of resistant and sensitive common carp families following experimental challenge with Aeromonas hydrophila

Parameters of non-specific immune response and level of specific and natural antibodies were investigated after an experimental challenge with Aeromonas hydrophila in genetically different common carp (Cyprinus carpio) families. Ten resistant and ten sensitive families were used for the experiment, which had been selected out of 96 families, based on the results of a preliminary challenge test. Blood samples were collected 12 h, one week and 21 days following the challenge. Phagocytic and respiratory burst activities of phagocytic cells, lysozyme activity of the blood plasma were determined. Level of specific antibodies against A. hydrophila and level of natural antibodies were measured in the samples taken on the 28th day. Non-infected fish from resistant and sensitive families were used as controls. Significant differences of phagocytic and lysozyme activities were measured between the challenged resistant and sensitive families. The level of specific antibodies between the same families was also found to be significantly different. There were no significant differences of the studied parameters between the control groups. Based on our results, phagocytic activity of leukocytes, plasma lysozyme activity and specific antibody titre were found to be higher in the resistant families than in the sensitive ones following infection with A. hydrophila.

Living off a fish: a trade-off between parasites and the immune system

Research in fish immune system and parasite invasion mechanisms has advanced the knowledge of the mechanisms whereby parasites evade or cope with fish immune response. The main mechanisms of immune evasion employed by fish parasites are reviewed and considered under ten headings. 1) Parasite isolation: parasites develop in immuno-privileged host tissues, such as brain, gonads, or eyes, where host barriers prevent or limit the immune response. 2) Host isolation: the host cellular immune response isolates and encapsulates the parasites in a dormant stage without killing them. 3) Intracellular disguise: typical of intracellular microsporidians, coccidians and some myxosporeans. 4) Parasite migration, behavioural and environmental strategies: parasites migrate to host sites the immune response has not yet reached or where it is not strong enough to kill them, or they accommodate their life cycles to the season or the age in which the host immune system is down-regulated. 5) Antigen-based strategies such as mimicry or masking, variation and sharing of parasite antigens. 6) Anti-immune mechanisms: these allow parasites to resist innate humoral factors, to neutralize host antibodies or to scavenge reactive oxygen species within macrophages. 7) Immunodepression: parasites either suppress the fish immune systems by reducing the proliferative capacity of lymphocytes or the phagocytic activity of macrophages, or they induce apoptosis of host leucocytes. 8) Immunomodulation: parasites secrete or excrete substances which modulate the secretion of host immune factors, such as cytokines, to their own benefit. 9) Fast development: parasites proliferate faster than the ability of the host to mount a defence response. 10) Exploitation of the host immune reaction. Knowledge of the evasion strategies adopted by parasites will help us to understand host-parasite interactions and may therefore help in the discovery of novel immunotherapeutic agents or targeted vaccines, and permit the selection of host-resistant strains.

Trypanoplasma borreli cysteine proteinase activities support a conservation of function with respect to digestion of host proteins in common carp

Trypanoplasma borreli is an extracellular parasite that is transmitted by a leech vector and is naturally found in the blood of cyprinid fish. High parasitemia and associated severe anemia together with splenomegaly are typical of infection of common carp, Cyprinus carpio L. Papain-like cysteine proteinases expressed by trypanosome parasites contribute to the pathogenicity of trypanosomes, and are considered an important target for the development of new trypanocidal drugs. T. borreli is a member of the Parabodonida, sharing a common ancestor with the other Kinetoplastida. We demonstrate the presence of a cysteine proteinase expressed by T. borreli. Alignment of the sequence with other kinetoplastid cysteine proteinase sequences supports the phylogenetic hypotheses based on analyses of ribosomal RNA genes. We expressed the T. borreli cysteine proteinase in Escherichia coli, refolded the purified protein into a biologically active proteinase and showed it has cathepsin L-like activity. Addition of the (non)active proteinase to in vitro-derived carp head kidney-derived macrophages did not significantly modulate macrophage activity. Immunization of carp with the recombinant proteinase did induce a very high increase in proteinase-specific antibodies but only slightly lowered parasitemia. Digestion of host hemoglobin and immunoglobulin by the cysteine proteinase likely contribute to the pathogenicity of T. borreli. The possibility that digestion by the cysteine proteinase of host transferrin could contribute to an innate activation profile of macrophages in vivo is discussed. Our findings suggest a conservation of function with respect to cysteine proteinase activity in the Parabodonida in support of the hypotheses on the phylogeny of the Kinetoplastida.

Protective acquired immunity to Enteromyxum scophthalmi (Myxozoa) is related to specific antibodies in Psetta maxima (L.) (Teleostei)

The acquired protection of three groups of turbot that had survived enteromyxosis outbreaks was tested by challenging with E. scophthalmi in three different experiments. The relation of such a response with the kinetics and duration of antibody production (determined by an ELISA) was studied. The progression of the infection was evaluated by PCR. In experiments 1 and 2, in which turbot had cohabited with highly infected fish during outbreaks, parasite prevalence and mortality were very low or null, and there was a progressive and statistically significant increase in the mean antibody production up to 350 and 152 days post-exposure respectively. By contrast, in experiment 3, fish (coming from non-infected tanks during the initial outbreak), both infection prevalence and cumulative mortality reached 92.8%, and specific antibodies were detected only in two fish. The observed differences in mortality after challenge appear to be related to the production of specific antibodies and it is probably accompanied by a repertoire of mechanisms of innate immunity. The exploitation of the immune system through breeding selection programmes as a possible strategy to control the disease is discussed.

Modulation of carp (Cyprinus carpio) neutrophil functions during an infection with the haemoparasite Trypanoplasma borreli

Trypanoplasma borreli is an extracellular blood parasite of common carp (Cyprinus carpio) transmitted by fish-biting leeches. The infestation with this parasite in juvenile carp may range between 75% and 100%, especially in fish recovering from the first hibernation period. T. borreli is perfectly adapted to its prolonged survival in a cyprinid host. Elevated numbers of activated neutrophils in peripheral blood and tissues are reported during T. borreli infection, but in context of the disease, the direct reason for elevated neutrophil numbers and their role during the infection remain unclear. In this study, a quantitative transmigration system, permitting the harvest of highly pure (> or = 97%) neutrophil populations was applied to investigate the modulation of carp neutrophil functions during T. borreli infection. We demonstrate time-dependent kinetics of a serum-induced down-regulation of neutrophil chemotaxis and an up-regulation of ROS production during the course of infection. With highly pure neutrophil populations, we could show that this divergent alteration of neutrophil functions was neither caused by T. borreli metabolites nor by the parasite itself. Moreover, when added to highly purified neutrophils, parasite metabolites did not alter the leukotriene B4-induced neutrophil chemotaxis nor the Staphylococcus aureus-induced ROS production. We conclude that the haemoparasite T. borreli does not interact with neutrophils directly, but indirectly modulates their functions via serum factors induced by parasite interaction with other components of the immune system.

Differential macrophage polarisation during parasitic infections in common carp (Cyprinus carpio L.)

In many parasitic infections both classically activated macrophages (caMF) and alternatively activated macrophages (aaMF) play a pivotal role. To investigate if both types of macrophages also play an important role during parasitic infections in fish, we infected carp with either Trypanoplasma borreli or Trypanosoma carassii and determined the activation state of the head kidney leukocytes (HKL). Nitrite production was used as read-out for caMF and arginase activity as read-out for aaMF. Basal nitrite production and arginase activity of HKL were moderately different between the two infections. Differences were observed, however, after ex vivo re-stimulation of HKL. Re-stimulation with LPS and T. borreli lysates increased nitrite production by HKL of T. borreli-infected fish. Re-stimulation with cAMP increased arginase activity in HKL of T. carassii-infected fish. Our results indicate that T. borreli-infected carp are more prone to increase nitrite production by caMF while T. carassii-infected fish are more prone to increase arginase activity by aaMF.

Humoral response and susceptibility of five full-sib families of Atlantic salmon, Salmo salar L., to the haemoflagellate, Cryptobia salmositica

Susceptibility and antibody production against pathogenic and vaccine strains of the haemoflagellate, Cryptobia salmositica were investigated in five full-sib families (A-E) of Atlantic salmon, Salmo salar. Humoral response and susceptibility of families were compared within three treatments: infection, vaccination and vaccination followed by challenge. Parasitaemias caused by the vaccine strain of C. salmositica were considerably lower than those caused by the pathogenic strain. All vaccinated families were protected when challenged with the pathogenic strain. Family B had significantly lower parasitaemias (with both strains) than the other families. When naïve fish were infected with the pathogenic strain, this family had a significantly lower and earlier peak parasitaemia (4.3 +/-1.3 x 10(6) parasites mL(-1) blood at 3 weeks post-infection; w.p.i.) than the other families. Family C had the highest peak (11.1 +/- 1.2 x 10(6) parasites mL(-1) blood), which occurred at 4 w.p.i. Antibodies against C. salmositica were detected earlier in Family B (3 w.p.i.) than in Family C (5 w.p.i.). This demonstrates an association of increased susceptibility with a delayed antibody response. Western immunoblot identified antibodies against 112, 181 and 200 kDa antigens earlier in more resistant fish (Family B). Antigenic stimulation leading to a stronger antibody response was shown with the vaccine strain and in the later stages of infection.

The parasitic ciliate Ichthyophthirius multifiliis induces expression of immune relevant genes in rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract During an infection with the parasitic ciliate Ichthyophthirius multifiliis, expression of genes encoding complement factor C3, inducible nitric oxide synthase (iNOS), immunoglobulin (IgM) and major histocompatibility complex (MHC-II) was examined in the skin, head kidney and spleen of rainbow trout using semi-quantitative reverse transcriptase-polymerase chain reaction. Induction of C3 transcription levels was evident in the skin and spleen showing extra-hepatic production of C3. MHC-II and IgM levels were increased in both head kidney and skin suggesting a production of antibodies at the site of infection, as well as in the lymphoid organs. iNOS expression was only increased briefly in the skin during the infection. These data suggest that complement is involved in immune reactions against I. multifiliis and that mucosal antibodies might be produced at the site of infection.

Head kidney neutrophils of carp (Cyprinus carpio L.) are functionally modulated by the haemoflagellateTrypanoplasma borreli

In the present work responses of carp (Cyprinus carpio) head kidney-derived neutrophils to the blood parasite T. borreli, and the consequences of these responses for parasite survival and other host response mechanisms, were studied. In co-cultures of head kidney leucocytes (HKL) with viable and lysed T. borreli a prominent shape change of neutrophilic granulocytes towards increased size and complexity was observed. In addition, the longevity of neutrophils in vitro was prolonged in the presence of T. borreli antigens. In these cultures, neutrophils also exhibited an increased phagocytosis activity. An up regulation of the production of nitric oxide (NO) and reactive oxygen species (ROS) was observed in T. borreli- and mitogen-stimulated HKL cultures. However, addition of live, fluorescence-labelledT. borreli to previously stimulated HKL cultures, revealed neither killing nor phagocytosis of the parasite by activated neutrophils. Moreover, viable T. borreli, when added to HKL cultures of infected carp, reduced their phagocytosis activity and NO production. Supernatants of co-cultures between T. borreli and HKL also contained mediators, which suppressed a mitogen-induced proliferative response of peripheral blood leucocytes (PBL) in vitro. Thus, while T. borreli itself appeared not to be sensitive to responses of activated neutrophils, the flagellates interferes with the production of immunomodulatory signals of these cells, probably resulting in a partial immunosuppression, which may favour the parasite development in vivo.

The haemoflagellate Trypanoplasma borreli induces the production of nitric oxide, which is associated with modulation of carp (Cyprinus carpio L.) leucocyte functions

In an attempt to characterise the role of nitric oxide (NO) in immune responses of carp, carp leucocytes obtained during an acute T. borreli infection were examined, for their capacity to generate NO. In a second set of experiments the impact NO on viability of the parasite and on the modulation of functional carp leucocyte responses were tested in vitro. Both in carp head-kidneys and in the peripheral blood, the fractions of lymphoblasts among separated leucocytes were increased. However, the relative proportions of granulocytes among head-kidney leucocytes (HKL) significantly decreased during infection, whereas granulocytes appeared among peripheral blood leucocytes (PBL). The cellular dynamics of HKL and PBL of infected carp were paralleled by an enhanced spontaneous NO release in vitro. NO production was further increased after addition of viable parasites to these cultures. The hypothesis that NO had a possible role in granulocyte activation and lymphocyte proliferation in carp was supported by the reduction of mitogen-induced proliferative responses of PBL from healthy carp in the presence of NO donor substances. The negative effects of NO on lymphocyte proliferation were contrasted by enhancing effects on granulocyte functions: the inhibition of NO generation in T. borreli-stimulated HKL cultures by the l-arginine analogue L-NMMA reduced the viability of granulocytes and their phagocytic activity. Even massive amounts of nitric oxide produced by donor substances (up to 600 micromol l(-1) NO(-)(2)) caused no reduction in the numbers of viable T. borreli flagellates in vitro. Thus, in carp, T. borreli seems to induce high amounts of NO in vivo which are apparently not harmful for the parasite but which may interfere with co-ordinated interactions of activated cells aiming at the defence of the parasite.

Molecular and functional characterization of carp TNF: a link between TNF polymorphism and trypanotolerance?

Two carp tumor necrosis factor alpha (TNFalpha) genes have been cloned and sequenced. Both TNF1 and TNF2 sequences have several polymorphisms in the 3' UTR and TNF2 has a polymorphism in the coding sequence. Lipopolysaccharide and the protozoan blood flagellate Trypanoplasma borreli induced expression of TNFalpha in carp head kidney phagocytes when added in vitro. Differential expression was observed, with TNF2 being higher expressed than TNF1. We used the TNFalpha-specific inhibitor pentoxifylline to demonstrate the involvement of carp TNFalpha in the induction of nitric oxide and in the stimulation of cell proliferation. In addition, two carp lines differing in their resistance to T. borreli were typed for the TNF2 polymorphism and association between one isoform and resistance was found.

The occurrence and mechanisms of innate immunity against parasites in fish

Parasitic infections in teleost fish are limited by constitutive innate defence mechanisms that render the host refractory or reduce the severity of infection. Controlled challenge trials using naive animals provide indirect evidence of innate immunity as well as identifying the host range or specificity of a parasite, often when specific details of defence mechanism(s) are lacking. Examples of parasites for which innate immunity may be inferred from cross-infectivity studies include Gyrodactylus spp., Lepeophtheirus salmonis, Cryptobia spp., Trypanosoma spp., Ceratomyxa shasta, Myxobolus cerebralis and Kudoa thyrsites. Recent studies however, have begun to clarify the relative roles of innate and acquired immunity against parasitic infection in teleosts by recognizing the presence and significance of specific innate effector mechanisms. The physico-chemical characeristics of skin mucus, the presence of bioactive substances including lysozyme, complement, C-reactive protein, haemolysins and lectins and the epidermal migration of inflammatory cells and their secretions may affect the establishment and proliferation of ectoparasitic copepods, ciliates or monogenea. Similarly in refractory species, haematozoic parasites are lysed via the alternative complement pathway and in susceptible and refractory hosts, protease inhibitors associated with the plasma neutralize proteolytic virulence factors. Detailed knowledge of innate resistance mechanisms against histiozoic parasites are lacking although non-specific cytotoxic lymphoid cells and macrophages probably play a role. The demonstration in certain disease models that innate resistance traits are under genetic control and may be inherited in a simple Mendelian fashion suggests opportunities for selective breeding for resistance against parasitic disease. Beyond a small number of well-described models however, research programs focussing on innate immunity against parasites in fish are lacking. Given the relative importance of innate immunity in fish, particularly as disease losses continue to have an economic impact in aquaculture, this area deserves considerable attention.

Immunogenetics of disease resistance in fish: a comparative approach

The study of the genetic regulation of infectious disease resistance depends on the availability of inbred lines or selection lines of the species under investigation. The small numbers of such lines of fish has limited the strategy in teleosts to studies of associations between disease and immune/health traits. Attempts to correlate genetic differences in immune responsiveness with survival after experimental challenge with pathogenic bacteria have failed to define immune parameters that can substantially aid selection for genetic resistance to infectious diseases. Advantages and disadvantages of selection strategies as illustrated by mouse and chicken models are discussed. In this study we summarize the present situation in fish as well as our attempts to develop gynogenetic lines of carp for immunogenetic research.

Investigations into the ubiquitous nature of high or low immune responsiveness after divergent selection for antibody production in common carp (Cyprinus carpio L.)

This paper reports on the selection of individual carp with a high or low antibody response, in combination with reproduction by gynogenesis, in order to develop well-characterised inbred carp lines consisting of practically unlimited numbers of carp with the same genotype. Two homozygous progenies, previously characterised as having a high or low immune response to dinitrophenyl keyhole limpet haemocyanin (DNP-KLH), were immunised with either a T-dependent (DNP-human serum albumin (DNP-HSA)) or T-independent (trinitrophenyl lipopolysaccharide (TNP-LPS)) hapten-carrier complex. In comparison with the antibody response after DNP-KLH immunisation, the response to DNP-HSA was observed to be highly variable and did not differ between the divergently selected progenies. This suggests that the divergent selection for antibody production to DNP-KLH has been carrier-specific. Immunisation with T-independent TNP-LPS induced a very rapid response which differed between the high and low responders, and likely measured changes in the DNP-specific precursor pool of B cells caused by the selection. A number of selected individuals with a high immune response to DNP-KLH were infected with Trypanoplasma borreli, a haemoflagellate parasite of carp, to examine a possible relationship between the increase in immune responsiveness and disease resistance, but no change could be detected. However, individual homozygous carp were able to escape inbreeding depression and survive the infection. Such carp would be likely candidates for gynogenetic reproduction to obtain viable inbred carp lines.