Patterns of covert infection by invertebrate pathogens: iridescent viruses of blackflies

Abstract

RecentIy, it has been recognized that blackfly populations mar host two forms of infection by iridescent viruses (IVs)¡ a covert (inapparent, nonlethal) form which was common in springtime populations in the River Ystwyth, Wales, and a patent (obvious, lethal) form which was rafe. This study aimed to investigate the changes in frequency of the two types of infection in blackfly populations ayer the reproductive period of the flies, April-September 1992. Blackfly larvae sampled from three different sites along the river were bioassayed for the presence of covert IV infection. Of 870 larvae assayed, 17 were found to be infected. AII the infected larvae appeared to be Simulium variegatum, the dominant species during the sampling periodo IV infections were common in the spring (17-37% depending on site) but appeared absent in the S. variegatum population for most of the summer months, reappearing again in the autumn (0-20% infected). These fluctuations were concurrent with biotic and abiotic factors: elevated levels of covert infection occurred at low population densities, high water flow rates, low temperatures (and presumably slower growth rates), although it is not clear if any cause-and-effect relationship exists. Patent infections occurred immediately alter the peak of covert infection in the spring, and again in the autumn. Virus characterization of isolates from covertly infected larvae showed that three distinct groups of isolates were present in the blackfly population. Isolates from the springtime populations were mostIy variants of an isolate found in patentIy infected blackfly larvae in the 1970s (Aberystwyth IV). Isolates from the autumn populations were mostIy variants of an isolate from a patentIy infected larva found in September the previous real. A third group comprised a single novel isolate which wás detected in a covertly infected larva. The mechanisms by which IVs persist in blackfly populations remain unknown, although the role of altemative hosts is a possibility which needs to be studied.

Natural invertebrate hosts of iridoviruses (Iridoviridae)

“…However, in the case of natural populations of blackfl ies and mayfl ies and laboratory populations of Aedes aegypti (L.), the prevalence of covert infection is between 10 and many thousands of times that of patent infections (Williams 1993(Williams , 1995Marina 1999Marina , 2003Tonka & Weiser 2000). In contrast, covert infections are rarely observed in terrestrial isopods or Spodoptera frugiperda (J. E. Smith).…”

Parasitoid-mediated transmission of an iridescent virus

“…Unlike the IIV isolates infecting Simulium variegatum Meigen populations in Great Britain, and the mayfly Ecdyonurus torrentis Kimmins in the Czech Republic (Tonka and Weiser, 2000;Williams, 1995), the IIV isolated from S. frugiperda larvae in Mexico did not induce abundant covert infections. This isolate is currently being characterized in more detail but initial results suggest that it is a possible member of the oligoiridovirus complex described by Williams and Cory (1994) within the genus Iridovirus.…”

“…Patent IIV infections are usually rare although epizootics have occasionally been reported (Fowler, 1989;Hern a andez et al, 2000;Ricou, 1975). In contrast, inapparent IIV infections have been detected by PCR, insect bioassay or electron microscopy and are reported to be prevalent in certain insect populations during certain periods (Tonka and Weiser, 2000;Williams, 1993Williams, , 1995. Such infections are not fatal but may cause a number of sublethal effects on the reproduction and longevity of covertly infected hosts (Marina et al, 1999).…”

Transmission dynamics of an iridescent virus in an experimental mosquito population: the role of host density

“…Volumes of 8.4 ml of the supernatant were injected into groups of 15 third-instar G. mellonella using a manual microinjector (Burkard Ltd, Rickmansworth, U.K.). Injected G. mellonella were placed in plastic cups containing a semisynthetic diet and reared for 12-14 days, after which they were checked for signs of patent IIV infection, indicative of the presence of a covert infection of the original mosquito adult (Williams, 1995). Identical procedures were performed with control mosquitoes at all time points.…”

“…IIVs can cause two types of infection (Williams, 1995;Tonka & Weiser, 2000). Patent infection results in an obvious blue coloration of immature stages followed later by death, whereas covert infection is not obvious and infected insects can develop to the adult stage and may reproduce.…”