Myxosporean and microsporidian infections in cultured Pacific bluefin tuna Thunnus orientalis in Japan

Abstract

During the parasitological survey of cultured juvenile Pacific bluefin tuna (PBT) Thunnus orientalis in 2007 and 2008, two myxosporeans and one microsporidian were found. Morphological and molecular analysis showed that the heart-infecting and brain-infecting myxosporeans are identified as Kudoa shiomitsui and K. yasunagai, respectively. This is a new host record for both species. High prevalence of infection (77-100%) with K. shiomitsui was observed in October to December (1-2 months posttransfer to sea cages), whereas only a few fish were infected with K. yasunagai. A microsporidian observed as white ''cysts'' in the trunk muscle of PBT had a resemblance to Microsporidium seriolae, the causative parasite of beko disease in yellowtail. Phylogenetic analysis showed that the microsporidian from PBT is closely related to but distinct from several other muscle-infecting species such as M. seriolae, Microsporidium sp. RSB, and Microsporidium sp. SH. Additionally, the spore dimension (2.7 9 1.5 lm on average) was remarkably smaller than the others. These results suggest that the present microsporidian is an undescribed species and designated Microsporidium sp. PBT. Prevalence and intensity of infection with Microsporidium sp. PBT reached a maximum of 100% and 20 cysts/fish, respectively. Although pathogenic effects of the two Kudoa species on fish health remain unknown, the microsporidian could be of concern to PBT aquaculture due to unsightly cysts in the musculature, reducing the market value of the fish.

Specificity of Infection Sites in Vertebrate Hosts

“…Plasmodia of some Kudoa spp., (e.g. K. neurophila and K. yasunagai) develop plasmodia in nerve cells (Grossel et al 2003, Zhang et al 2010a). Intracellular developmental stages of an unidentified species were also found in pericytes of the brain capillaries of the mole Talpa europaea (Friedrich et al 2000).…”

Transmission Biology of the Myxozoa

“…K. yasunagai forms numerous cysts in the brain, probably causing disorder of swimming performance of many fish species (Zhang et al, 2010a). Recently, enteromyxosis or myxosporean emaciation disease, caused by E. leei, has emerged as a new threat in various cultured marine fish, e.g.…”

“…The former two species are highly pathogenic to Chinese sea bass (Lateolabrax sp.) and red sea bream (Pagrus major), respectively (Yokoyama et al, , 2005a, whereas the pathogenic effects of K. shiomitsui are not clear (Zhang et al, 2010a). Many Kudoa infections in skeletal muscle may render the infected fish unmarketable by producing cysts (e.g., K. amamiensis and K. iwatai) or causing myoliquefaction (e.g., K. lateolabracis and K. neothunni).…”

Infection dynamics of Kudoa yasunagai (Myxozoa: Multivalvulida) infecting brain of cultured yellowtail Seriola quinqueradiata in Japan

“…Since the original finding from Japanese seabass Lateolabrax japonicus and striped knifejaw Oplegnathus fasciatus (Yasunaga et al 1981), the parasite has been reported from several important aquaculture fishes in Japan, including red sea bream Pagrus major, olive flounder Paralichthys olivaceus, tiger puffer Takifugu rubripes, yellowtail Seriola quinqueradiata and Pacific bluefin tuna Thunnus orientalis (Egusa 1986a,b, Whipps et al 2004, Zhang et al 2010. It has also been reported from several wild fish in other parts of the world, such as coral catfish Plotosus lineatus in the Philippines (Cheung & Nigrelli 1990), whitings Sillago spp., and monocle bream Scolopsis monogramma from Australia (Burger & Adlard 2010).…”