Virus-like particles in a monogenean (Platyhelminthes) parasitic in a marine fish.

In Microcotyle sp., a gill parasite in the marine fish Abudefduf analogus near Dakar, Senegal, various organs were studied by transmission electron microscopy. One of the six worms studied contained virus-like particles located only in the outer layer and in the cytons (deep cell bodies) of the tegument. The tegument ultrastructure is described for both healthy and infected monogeneans. The outer layer of the tegument, 2-5 microns thick, does not have microvilli. The virus-like particles are cytoplasmic, about 70 nm in diameter with a single-layered capsid-like structure 10-12 nm thick, and have an icosahedral symmetry. They originate from viroplasms and accumulate in paracrystalline arrays up to 1 micron in size. By their ultrastructural characteristics, these virus-like particles are related to the Reoviridae or, more probably, the Birnaviridae. This is the third report of viruses in monogeneans, and the first in a polyopisthocotylean monogenean. It is hypothesized that monogeneans could act as vectors of viral diseases among their host fishes.