Lymphoidal parvovirus-like particles in Australian penaeid prawns

Abstract

Prawns in northern Queensland exhib~ted mult~nucleated g ~ a n t cell formation in hypertrophied lymphoid organs. Mostly farmed Penaeus monodon, P, merguiensls and P esculentus were involved The transformation involved mild nuclear hypertrophy, marglnated chromatin and basophilic da rken~ng of increased cytoplasm In a multinucleate giant cell. Areas of transformation were d~scre te , often bounded by elongated f~brocytes and sometimes fully encapsulated Basophilic inclusions occurred con~monly and eos~nophlhc lntranuclear inclus~ons rarely. These inclus~ons talned for DNA with acndlne orange fluorescence. Associated \nth lympho~dal changes was the presence of rounded F l~ke cells with basophilic, pyknolc nucleoli. Under the electron microscope, transformed lympho~dal cells d~splayed virus-hke parlcles of 18 to 20 nm diameter associated with inclusion b o d ~ e s in the nucleus These particles were similar to a parvovlrus and named lymphoidal parvovirus-like (LP\/). Their similarities and relationship to hepatopancreatic parvovirus and infectious hypodermal and haematopoletic necrosis virus are discussed. INTRODUCTION MATERIALS AND METHODS The lymphoid organ (or Oka organ) is a relatively newly discovered organ in penaeid prawns (Oka 1969) and is becoming increasingly important in disease studies. Lightner et al. (1987) described Oka organ hyperplastic spheroids and metastatic foci in ectopic localities in both Penaeus monodon Fabricus and P. penicillatus from Taiwan. Prlmary cell cultures from the lymphoid organ of P. n~onodon were capable of supporting in vitro replication of Monodon baculovirus when inoculated into the cultures (Chen & Kou 1989). Lymphoid t~ssue necrosis and haenlocytic infiltration lead to encapsulation, nodule formation and melanization and are often the earliest changes associated with bacterial infections (Nash 1990). In Australia, the lymphoidal spheroids in Penaeus nlerguiensis de Man have been associated with moribund wild spawners (Owens & Hall-Mendelin 1990). Suggestions were made that there were similarities between tissues showing lymphoid metastasis with the lymphoidal spheroids and tissues attacked by infectious haematopoietic and hypodermal necrosis virus (IHHNV). This paper reports on further investigations of lymphoidal changes in prawns and the probable causahve agent. Banana prawns Penaeus merguiensis were collected by a cast net in the Burdekin Rver , northern Queensland. Tiger prawns (P. monodon and P. esculentus Haswell) were collected from prawn farms in the local area. Prawns were split longitudinally, and fixed in Davidson's fixative. Histological sections were cut and stained using standard techniques (Culling et al. 1985). For electron microscopy, the diced lymphoid organ was fixed in 2 .5 % glutaldehyde/2 O/O paraforn~aldehyde in cacodylate buffer and postfixed in 1 O/O osmium tetroxide. The tissue was dehydrated and mounted in Spurr (Tm) resin. Sections were cut on a LKB Ultratome at 50 nm stained with uranyl acetate/ 70 O/O methanol and lead citrate and viewed at 80 kV on a Joel 2000FX transmission electron microscope