Kudoa neurophila in striped trumpeter : identification, diagnostic development and histopathology

Striped trumpeter, Latris lineata (Forster), are being experimentally cultured by the Tasmanian Aquaculture and Fisheries Institute (TAFI) at Taroona, Hobart, Tasmania, Australia. Fish that survive beyond 30 days develop nervous aberrations associated with a severe granulomatous meningoencephalomyelitis. The myxozoan parasite Pentacapsula neurophila was described as the parasite causing the disease in the striped trumpeter juveniles. Molecular Bayesian phylogenetic analysis using small subunit ribosomal DNA (ssu rDNA) gene sequence and the covariotide evolutionary model, has shown P. neurophila to reside firmly within the clade comprised of Kudoa species, histozoic parasites of fish from the order Multivalvulida with 4 or more shell valves containing polar capsules. This has provided molecular evidence resulting in the proposed new combination of the Kudoidae to include this Pentacapsula species. A polymerase chain reaction (PCR) diagnostic assay was developed from the ssu rDNA gene sequence to detect Kudoa neurophila (formerly known as Pentacapsula neurophila). The assay is sufficiently species specific and sensitive enough to detect a small fragment of the parasite ssu rDNA gene (0.1 spore or 60 fg DNA or 4 spores g-1 /25 microlitres PCR reaction). Specifically, the test is capable of detecting early stages of the life cycle within the fish host and consequently diagnosing an infection not normally detected using histology. The PCR test can also be used to screen water supplies and prey cultures throughout the hatchery system to determine bio-security efficacy, assist in epizootiology studies, identify infected alternative or other primary hosts indicating the location of the disease reservoir, and enable a targeted approach to disease prevention in an aquaculture situation. Histology and in situ hybridisation techniques were incorporated into the study of the histopathology of the disease caused by this parasite to elucidate its entry point on the fish host and migratory pathway to terminal stage sporulation. Kudoa neurophila enters the fish via epithelial cells as early as 25 days post hatch (dph). Parasite cells then appear within plasmodia in skeletal muscle tissue between 50-80 dph. This appears to be the first key proliferation stage which is followed by another plasmodial stage in the peripheral nerve pathways near the spinal cord (70-115 dph). It is during this stage that clinical signs of the disease, such as whirling, become apparent. The presporogonic cells then enter the spinal cord where terminal stage sporulation occurs throughout the brain and spinal cord (105-130 dph) causing acute pathology, eventually resulting in the death of the animal. The information gained from the research conducted in this thesis, along with incorporated epizootiology studies, form the foundation of understanding that has assisted the hatchery management team to make informed health management decisions with an outlook to produce healthy juvenile striped trumpeter for on-going research into the development of this species for marine sea cage aquaculture.