The Occurrence of a Virus of the Pox Group in a Field Population of Chironomus salinarius Kieffer (Diptera: Chironomidae) in Italy

Abstract

A larval population study on Chironomus salinarius Kieffer in two saltwater lakes surrounding the city of Orbetello, central Italy, was recently reported by Ali and Majori (1984). In this study, a part (18%) of the C. salinarius larval population in a sample taken from one station in June 1982 was patently infected with an entomopoxvirus. Further sampling of that station in June-July 1982 showed the overall disease incidence to be 6% (A. Ali, unpublished). A detailed description of the two lakes (East Lake and West Lake) is available in Ali and Majori (1984). The diseased larvae occurred at station 8 of sampling row G in the West Lake where the lake received domestic and industrial discharges. Patently infected larvae were recognized in the laboratory by the appearance of whitish areas beneath the integument of the entire body caused by massive accumulations of virus polyhedral bodies (PB). Infected 4th instars were randomly separated and placed in a rearing chamber (Biever 1965) to study their survival. Wet mount preparations (in saline) of larval smears of diseased larvae were observed under a phase-contrast microscope and the PB from cells of different host specimens were randomly measured. For electron microscopy, infected 4th instar C. salinarius were fixed in a 2.5% glutaraldehyde in Millonig buffer (pH 7.4) for 1 hour. After rinsing in the buffer, the tissues were postfixed for an hour in 1% OSO4, dehydrated in acetone series and embedded in Spurr's resin. Thin sections were examined with a Siemen's Elmiskop 102 and Zeiss EM 10 C electron microscope. Suspensions of purified virions (Huger et al. 1970) were negatively contrasted with 2% phosphotungstic acid at pH 7.2 and placed on carbon grids. Microscopic examination of smears of infected larvae revealed oval shaped PB with a mean size of 7.30 x 5.52 p.m but ranging in length from 3.4-13.23 pLm and width of 2.47-9.26 ,um. Phase-contrast microscopy showed that a variable number of PB may