Thelohania

Classification: Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000. Allen Press)

Protista (unicellular eukaryotes)
Microspora (form unicellular spores containing coiled polar tubes)
Microsporea (oval-tubular spores, well-developed polar tube opening terminally)
Pansporoblastina (spore formed in sporophorous vesicles bounded by pansporoblast membrane)

Family: Thelohaniidae
These microsporans (microsporidia) proliferate in host tissues by merogony (asexual division) followed by sporoblastogenesis and sporogony (spore formation). The meronts are diplokaryotic (containing 2 identical nuclei) while mature spores are unikaryotic. Eight spores are formed within each sporophorous vesicle. Mature spores contain a coiled polar tube which everts forcibly to inject the infective sporoplasm into pierced host cells. Most thelohaniid species are parasitic in insects and crustaceans.

Thelohania spp. [these species cause cotton-tail disease in aquatic crustaceans]

Parasite morphology:The parasites form monomorphic ovoid spores (5 x 4µm) bound by a dense membranous exospore wall overlaying a thick lucent endospore wall. Mature spores are unikaryotic and contain an isofilar polar tube arranged in 17-19 coils in two layers. The spores divide in rosette formation from a sporogonial plasmodium to produce 8 sporoblasts bound by a sporophorous vesicle.

Host range: Infections have been detected in most freshwater crayfish species, both wild and cultured animals (marron, yabbies, redclaw). The prevalence of infections can be so high in some streams and ponds that no commercial return for wild or cultured crayfish is possible. Microsporidia are common histozoic parasites of fish and arthropods, although clinical infections are now being detected in humans, especially immunocompromised individuals. Some infections in humans are similar to species found in animals (suggesting their zoonotic origin), but others are unique and only found in humans.

Site of infection: Parasites are usually found at all stages of development within the muscles, although other tissues and organs can be infected.

Pathogenesis: The parasites undergo obligate intracellular development resulting in cell lysis (subacute presentation) and tissue cyst (xenoma) formation (chronic presentation). Mature spores are very refractile and heavily infected muscles become porcelain white in appearance (hence the term cotton-tail). The muscles are unpalatable and are rejected from human consumption. Mildly infected individuals may be stunted in growth and exhibit weak tail-flick responses, while heavy infections may be fatal. Other microsporidia in fish have been associated with respiratory distress (Loma), anaemia (Nucleospora), myeloencephalitis (Microsporidium) and lesions/xenomas (Glugea), while some species in arthropods have been associated with bee dysentery (Nosema) and silkworm disease (Loma?). Infections in humans involving pansporoblastic (Pleistophora, Trachipleistophora, Brachiola, Thelohania and Vavraia) and apansporoblastic genera (Nosema, Enterocytozoon, Septata, Encephalitozoon and Vittaforma) have variously been associated with neurologic (convulsions, vomiting, headaches, fever, coma), ocular (keratoconjunctivitis, chronic sinusitis), muscular (atrophy, muscle fibre degeneration), enteric (diarrhoea, fever, malaise, weight loss) and pulmonary (respiratory) signs.

Mode of transmission: While the developmental cycle within the host has been well studied, the route of spore transmission between hosts remains speculative. Transmission has been assumed to be direct via water-borne carriage of infective spores or their ingestion by carnivorism, but neither has been substantiated by experimental studies. It is known that mature spores contain a coiled polar tube which is forcibly everted to penetrate adjacent cells and inject the infective sporoplasm which subsequently divides and ultimately forms new spores.

Differential diagnosis: Heavy infections can be detected macroscopically by visual examination of crayfish tails which are opaque and chalky in appearance, rather than translucent and clear. Most infections, however, are diagnosed by the microscopic detection of cysts and spores in squash preparations or histological sections of musculature. Mature spores are best visualized using phase-contrast or interference-contrast microscopy, as they have a phase-bright refractile appearance due to the chitinous nature of the spore wall.

Treatment and control: No drug treatments have proven totally effective, but some successes have been reported when treating human infections with albendazole (ocular, intestinal and disseminated infections), metronidazole (intestinal infections) and trimethoprim-sulfamethoxazole (disseminated infections). Recent studies have shown that the coccidiostat toltrazuril may be effective against microsporidial infections in fish, insects and decapod crustacea. Various forms of control have been attempted in aquaculture systems, the most successful being to drain culture ponds and lime them or bake them over summer before restocking.