Thelohania |
Classification: Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000. Allen Press) Protista (unicellular eukaryotes) Thelohania spp. [these species cause cotton-tail disease in aquatic crustaceans]Parasite morphology:The parasites form monomorphic ovoid spores (5 x 4ยตm) 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. 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. |