COVERT IRIDOVIRUS INFECTION OF BLACKFLY LARVAE

Iridovirus infections are typically isolated at extremely low frequencies from invertebrate species in moist or aquatic habitats. Crystalline arrangements of virus particles in heavily infected tissues cause an iridescent hue obvious to the naked eye; diagnosis of iridovirus infection has traditionally occurred when the host appeared bright lilac or blue. Evidence is presented here to show that there are two forms of iridovirus infection in larval blackfly (Diptera: Simuliidae) populations: (i) the classical patent form which causes larvae to develop blue iridescence before death; and (ii) a covert (inapparent) form which is not lethal. Patent infections are extremely rare in Simulium populations from the River Ystwyth, Wales (ca. 1 in 10(6)). However, a large proportion of the population harbours the disease without obvious symptoms. Covert iridovirus infection was shown by using two techniques. First, tissue homogenates from 8 out of 30 apparently healthy Simulium larvae produced patent iridovirus infections when injected into a permissive lepidopteran host species. The DNA restriction profiles from these covert isolates showed very close similarities in fragment pattern and variability to isolates causing patent infections in the same River Ystwyth populations. Second, the presence of iridovirus DNA in host tissues was confirmed by using the polymerase chain reaction (PCR) targeted at the iridovirus major structural protein gene. The frequency of PCR-positive larvae taken from the River Ystwyth in April 1992 varied between 17% and 37%, depending on site.