Psorospermium spp. (Protozoan Disease) of European Crayfish
Category 1 (Not Reported in Canada)
Common, generally accepted names of the organism or disease agent
Psorospermium haeckeli, Protozoan disease.
Scientific name or taxonomic affiliation
Psorospermium haeckeli and Psorospermium orconectis possibly conspecific to P. haeckeli. At least two different morphotypes (based on size, morphology and the method of embedding into the connective tissues of their host) have been reported in Europe but the significance of the difference to speciation has not been determined (Edgerton et al. 2002). Psorospermium spp. have been assigned to the DRIPs clade which is considered to constitute the most basal branch of the Animalia close to the animal-fungal dichotomy/divergence (Vogt and Rug 1999). This clade has been named Ichthyosprea, a class of the phylum Neomonada by Cavalier-Smith (Evans and Edgerton 2002)
Europe; in both wild populations and farmed crayfish.
All European crayfish (e.g., Astacus astacus, Astacus leptodactylus, Austropotamobius torrentium) and introduced American species (Pacifastacus leniusculus and Orconectes limnosus) cultured in Europe. Other species (or morphotypes) of Psorospermium occur in various species of crayfish in North America and Australia.
Impact on the host
Infection rates in Astacus astacus are not necessarily severe but other crayfish species may be more vulnerable to infection which may reach 100% prevalence and may be lethal especially in association with other invasive organisms. Thörnqvist and Söderhäll (1993) indicated that injection with P. haeckeli reduced the haemocyte count of P. leniusculus and biweekly injections resulted in latent benign infections of Aphanomyces astaci (cause of crayfish plague) becoming lethal in the normally resistant P. leniusculus. The life cycles of Psorospermium spp. are unknown but studies suggest that the life cycle may be diphasic (histozoic and free-living phases) and/or direct from crayfish to crayfish by cannibalism (Vogt and Rug 1999, Edgerton et al. 2002). Astacus astacus fed infected P. leniusculus acquired P. haeckeli (Gydemo 1996). However, Henttonen et al. (1997) was unsuccessful in transmitting the infection to juvenile laboratory-reared A. astacus by feeding them Psorospermium-infected crayfish tissues. The apparent increase in infection intensity and prevalence with crayfish age suggests that infection tends to persist and increases over time by repeated re-infections (Vogt and Rug 1999).
In addition to amoeboid forms reported in A. astacus, six different morphological forms were described by Henttonen et al. (1997). The round naked forms (25-30 µm in diameter) contained one or two nuclei. The ellipsoid forms had a gelatinous-looking cell wall. The elongated forms were longer than the ellipsoid form, contained uniform globules and had a thin cell wall wit a reticular pattern. The mature forms were the typical most common form of Psorospermium spp. and are described below. In addition, large naked forms and abnormal forms have been reported (Henttonen et al. 1997). Some abnormal forms consist of two or three lobes and other abnormal forms, which increase rapidly when a crayfish dies, have an innermost membranous layer that loses its transparency.
Gross Observations: Infected animals may show characteristic orange spots on the carapace. Some haemocytic encapsulation observed; melanization rare.
Squash Preparations: Large (60 x 100 µm) ovoid sporocysts with multi-layered refractive cell walls (6-8 µm thick) and highly refringent globules of varying size in the cytoplasm. The elongate form (50 x 150 µm) contains small uniform lipid globules. In mature forms, the cell wall consists of three layers: an outer layer composed of shell plates, a homogenous medial layer and an inner layer consisting of two closely associated membrane-like structures. In addition, many sporocysts are enveloped by a layer of host connective tissue. Early stages of development consisting of thin walled spheroid forms containing abundant globules and melanized sporocysts may also be present. Although the infection is systemic, highest concentrations occur in connective tissue including the collagenous layer of the thoracic arteries and subepidermal tissues under the carapace of astacid crayfish (Vogt and Rug 1995) and mainly in the abdominal muscles of cambarid crayfish (Edgerton 2002). Sporocysts persist in dead crayfish for up to three days and may withstand desiccation. Incubation of the tissues in pepsin-hydrochloride ('digestion') prior to microscopic examination appears to be detrimental to observing P. haeckeli in P. leniusculus (Gydemo 1996). Prolonged (about 2 to 3 weeks) incubation of connective tissue from infected crayfish in fresh water may induce the emergence of a spore "receptical" from each sporocyst. The spore "receptical" eventually ruptures (within hours to days) to release free-living amoeboid forms (7 to 10 µm in diameter) with filose pseudopodia and a single nucleus (Vogt and Rug 1999).
Histology: Early developmental stages often appear irregular (amoeboid) in shape. In mature sporocysts, the large alcohol-soluble lipid globules are not evident but the sporocysts contain intensely staining small globules with crystalline and vacuolar inclusions, and two closely associated nuclei (Vogt and Rug 1995, 1999). The outermost layer of the multilayered wall is composed of several irregular thick proteinaceous plates separated by sutures that are strongly eosinophilic when stained with haematoxylin and eosin stain.
Electron Microscopy: The free-living amoeboid form contains a large nucleus (about 4 µm in diameter) with a prominent nucleolus, spherical platycristate mitochondria, numerous ribosomes and vacuoles surrounded by ribosome-like particles (Vogt and Rug 1999).
Methods of control
Methods of transmission are not understood. No known methods of control.
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Vogt, G., M. Keller and D. Brandis. 1996. Occurrence of Psorospermium haeckeli in the stone crayfish Austropotamobius torrentium from a population naturally mixed with the noble crayfish Astacus astacus. Diseases of Aquatic Organisms 25: 233-238.
Bower, S.M. (2006): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Psorospermium spp. (Protozoan Disease) of European Crayfish.
Date last revised: June 2006
Comments to Susan Bower
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