Experimental Horizontal Transmission of Enterospora nucleophila (Microsporea: Enterocytozoonidae) in Gilthead Sea Bream (Sparus aurata)

Simple Summary Enterospora nucleophila is a microsporidian parasite infecting gilthead sea bream (Sparus aurata) that has been associated with increased mortality, growth arrestment and emaciation in aquaculture settings. The investigations on this parasite depend on material from spontaneous outbreaks in fish farms, which hampers advancements in infection research and control. In the present study, different ways of horizontal transmission were performed in order to establish an in vivo transmission model. Parasite transmission was achieved with all the assayed routes, but with milder clinical signs than in natural infections. Enterospora nucleophila is a microsporidian enteroparasite that infects mainly the intestine of gilthead sea bream (Sparus aurata), leading to an emaciative syndrome. Thus far, the only available information about this infection comes from natural outbreaks in farmed fish. The aim of the present study was to determine whether E. nucleophila could be transmitted horizontally using naturally infected fish as donors, and to establish an experimental in vivo procedure to study this host-parasite model without depending on natural infections. Naive fish were exposed to the infection by cohabitation, effluent, or intubated either orally or anally with intestinal scrapings of donor fish in four different trials. We succeeded in detecting parasite in naive fish in all the challenges, but the infection level and the disease signs were always milder than in donor fish. The parasite was found in peripheral blood of naive fish at 4 weeks post-challenge (wpc) in oral and effluent routes, and up to 12 wpc in the anal transmission trial. Molecular diagnosis detected E. nucleophila in other organs besides intestine, such as gills, liver, stomach or heart, although the intensity was not as high as in the target tissue. The infection tended to disappear through time in all the challenge routes assayed, except in the anal infection route.

Automated summary

Enterospora nucleophila is a microsporidian parasite that infects the intestine of gilthead sea bream, leading to emaciation. This study successfully established an experimental in vivo transmission model through different horizontal transmission routes, with milder clinical signs observed compared to natural infections. The parasite was detected in naive fish in all challenge routes, with infection levels decreasing over time except in the anal transmission route.