Haplosporidian parasite of Abalone

Category

Category 3 (Host Not in Canada)

Common, generally accepted names of the organism or disease agent

Haplosporidian parasite of paua.

Scientific name or taxonomic affiliation

Parasite not yet specifically identified. Transmission electron microscopy and molecular studies indicate that the parasite is a novel species in perhaps a new genus at the base of the phylum Haplosporidia and ancestral to other genera in this group (i.e., Urosporidium, Haplosporidium, Bonamia, and Minchinia) (Reece et al. 2004).

Geographic distribution

A commercial abalone culture facility in New Zealand.

Host species

Haliotis iris.

Impact on the host

Moribund H. iris from a population of juveniles experiencing mortalities (peak of 24% per week with a total mortality approaching 90% within 6 months) at a commercial abalone culture facility had heavy systemic infections of a uni- to multi-nucleate protist. The parasite was confirmed by transmission electron microscopy and molecular studies to be a haplosporidian. Mortalities associated with the infection were most severe during the summer and early autumn when water temperatures peaked at 21 °C. Results of experimental exposure in the laboratory indicated that the parasite was not transmitted between H. iris by cohabitation (for 3 months) nor injection (Diggles et al. 2002). A subsequent survey of 1094 H. iris from 5 spat production farms and 3 grow-out farms in New Zealand did not detect this parasite (Diggles and Oliver 2005).

Diagnostic techniques

Gross Observations: Haliotis iris from raceways with increased mortalities that exhibit behavioural abnormalities including lethargy, loss of righting reflex and were easily detached from the surface. Apart for these behavioural abnormalities that are not specific to infection with this parasite and oedema and pale lesions in the foot and mantle, no reliable gross signs of disease were noted.

Figure 1. Gross appearance of a juvenile Haliotis iris from a commercial culture facility heavily infected with a haplosporidian parasite. Note the blotchy appearance of the epipodium (arrows). Image provided by Ben Diggles PhD, DigsFish Services, www.digsfish.com, ben@digsfish.com.

Wet Mounts: Multinucleate plasmodia up to 25 µm in diameter with up to 17 nuclei were detected in wet preparations of haemolymph.

Figure 2. Unstained hemolymph preparation from infected Haliotis iris showing spherical haplosporidian plasmodium with prominent lipid droplets (arrow) and 6 nuclei above a typical irregularly shaped haemocyte (He) adhered to the glass slide. Image provided by Ben Diggles PhD, DigsFish Services, www.digsfish.com, ben@digsfish.com.

Smears: Up to 17 nuclei in one plasmodium were evident in air-dried haemolymph smears stained with a Giemsa type stain.

Histology: Systemic infection with uni- to multi-nucleate haplosporidian-like plasmodia (ranging is size from 3.5-8.9 x 3.5-16.7 µm with an average of about 5.5 x 6.9 µm) in cultured juvenile H. iris. Up to 9 spherical nuclei, each with emarginated chromatin and about 2.3 µm in diameter, were observed within cross sections of a single plasmodia but on average only about 2 nuclei were observed in each plasmodium. Each individual plasmodium was usually surrounded by a clear area, which facilitated detection. These distinctive halos may be evidence of extra cellular toxic products produced by the plasmodia or differential shrinkage of the plasmodia during fixation. Early stages of infection were characterised by a small number of plasmodia in the connective tissues surrounding the gut, amongst glial cells adjacent to nerves in the mantle and foot and within gill lamellae.

Although haemocytes were sometimes associated with the plasmodia in light infections, relatively few or no host haemocytes were visible in heavily infected H. iris (even in comparison to apparently healthy uninfected paua). In heavy infections, large numbers of average sized plasmodia were present in the haemolymph sinuses of all organs and within the tissues of the gills, heart, kidneys, mantle, epipodium of the foot, and connective tissue of the digestive gland. Spore stages of the parasite have not been described.

Figure 3. Large numbers of haplosporidian plasmodia (arrows) in the haemal sinuses of the gills of a heavily infected Haliotis iris.

Figure 4. Haplosporidian plasmodia in the connective tissues (arrows) and occasionally in the tubule epithelium (arrowheads) of the right kidney of a heavily infected Haliotis iris.

Figure 5. Haplosporidian plasmodia (arrows) in the connective tissue adjacent to the gut and associated with multifocal haemocytosis (*) of a moderately infected Haliotis iris.Images in Figs. 3 to 5 provided by Ben Diggles PhD, DigsFish Services, www.digsfish.com, ben@digsfish.com.

Electron Microscopy: Ultrastructural features consistent with other haplosporidians that were identified in this parasite include: multinucleate plasmodia, mitochondria with tubular cristae, anastomosing endoplasmic reticulum, multivesicular bodies and haplosporogenesis (haplosporosome-like bodies produced from nuclear membrane-bound Golgi that matured into haplosporosomes). This abalone parasite differs from previously described haplosporidians in the apparent absence of a persistent mitotic spindle and the presence of intracytoplasmic coccoid to rod-shaped bacteria resembling Rickettsiales-like prokaryotes (Hine et al. 2002).

Figure 6. Electron micrograph of a haplosporidian plasmodium containing a section through the edge of one nucleus (N), mitochondria (M), haplosporosomes (arrows) and Rickettsiales-like organisms (arrowheads). Image provided by Ben Diggles PhD, DigsFish Services, www.digsfish.com, ben@digsfish.com.

DNA Probes: The in situ hybridisation assay designed to detect rDNA of Bonamia ostreae (Cochennec et al. 2000) gave a moderately strong reaction with the paua haplosporidian and demonstrated the presence of low numbers of plasmodia in the connective tissue adjacent to the gut and amongst glial cells around the periphery of the nerves in lightly infected H. iris (Diggles et al. 2002). A fragment of the small-subunit rDNA gene has been sequenced (1967 bp in length after removal of the primer sequences) and the sequence deposited into GenBank (accession number AF492442). This sequence was used to determine the taxonomic affiliations of the parasite (Reece and Stokes 2003).

Figure 7. Serial section to that illustrated in Figure 5 but stained by in situ hybridisation with the DNA probe described by Cochennec et al. (2000) indicating a staining reaction with the haplosporidian plasmodia (arrows). Image provided by Ben Diggles PhD, DigsFish Services, www.digsfish.com, ben@digsfish.com.

Methods of control

No known control methods. Research on another species of haplosporidian (Haplosporidium nelsoni, Ford et al. 2001) suggests that passing incoming water through a 1 µm filter followed by UV irradiation may exclude infective stages from the water supply of a commercial tank facility. Surveys of wild stocks of juvenile and adult H. iris could also be used to establish the distribution of the parasite in wild stocks to hopefully avoid the inadvertent transfer of the disease.

References

Cochennec, N., F. LeRoux, F. Berthe and A. Gerard. 2000. Detection of Bonamia ostreae based on small subunit ribosomal probe. Journal of Invertebrate Pathology 76: 26-32.

Diggles, B.K. and M. Oliver. 2005. Diseases of cultured paua (Haliotis iris) in New Zealand. In: Walker, P.J., R.G. Lester, M.G. Bondad-Reantaso (eds.) Diseases in Asian Aquaculture V. Proceedings of the 5th Symposium on Diseases in Asian Aquaculture. Fish Health Section, Asian Fisheries Society, Manila. pp. 275-287.

Diggles, B.K., J. Nichol, P.M. Hine, S. Wakefield, N. Cochennec-Laureau, R.D. Roberts and C.S. Friedman. 2002. Pathology of cultured paua Haliotis iris infected with a novel haplosporidian parasite, with some observations on the course of disease. Diseases of Aquatic Organisms 50: 219-231.

Ford, S.E., Z. Xu and G. Debrosse. 2001. Use of particle filtration and UV irradiation to prevent infection by Haplosporidium nelsoni (MSX) and Perkinsus marinus (Dermo) in hatchery-reared larval and juvenile oysters. Aquaculture 94: 37-49.

Hine, P.M., S. Wakefield, B.K. Diggles, V.L. Webb and E.W. Maas. 2002. Ultrastructure of a haplosporidian containing Rickettsiae, associated with mortalities among cultured paua Haliotis iris. Diseases of Aquatic Organisms 49: 207-219.

Reece, K.S. and N.A. Stokes. 2003. Molecular analysis of a haplosporidian parasite from cultured New Zealand abalone Haliotis iris. Diseases of Aquatic Organisms 53: 61-66.

Reece, K.S., M.E. Siddall, N.A. Stokes and E.M. Burreson. 2004. Molecular phylogeny of the haplosporidia based on two independent gene sequences. The Journal of Parasitology 90: 1111-1122.

Citation Information

Bower, S.M. (2006): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Haplosporidian parasite of abalone.

Date last revised: December 2006
Comments to Susan Bower

Date modified: