Covert infections as a mechanism for long‐term persistence of baculoviruses

Abstract

The prevalence of pathogens in wild populations has often been estimated by the appearance of overt symptoms in the host, and this is typically used as the sole gauge of the impact of the pathogen on host dynamics. However, the development of molecular methods has increased the sensitivity with which we can detect asymptomatic infections. Baculoviruses are insect pathogens that, like many microparasites, are usually only found when their hosts reach outbreak densities, when a disease epizootic occurs. Conventional wisdom is that the long-term persistence of baculoviruses relies on their survival in the external environment in the form of occlusion bodies. These are proteinaceous matrices in which the virus particles are embedded, and which provide a degree of protection from UV irradiation. However, Mamestra brassicae has also been shown to harbour a persistent, non-lethal baculovirus infection (M. brassicae nucleopolyhedrovirus) in laboratory culture, which may represent another putative persistence mechanism. Here, we present evidence that such covert infections are also present and frequent in natural populations of the moth. The persistent infections were triggered into the lethal overt state by exposure to another baculovirus, and two closely related but different baculoviruses were subsequently identified as persistent infections within the populations sampled. These results have broad-ranging implications for our understanding of host pathogen interactions in the field, in the use of pathogens as biocontrol agents, and in the evolution of virulence.

Prevalence of latent virus in Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) and its activation by a heterologous virus

“…Our findings are similar to those of Eastwell et al (1999), who detected latent virus in laboratory cultures, and in 23% of larvae of codling moth populations from British Columbia, using the PCR reaction. Application of PCR permitted Burden et al (2003) to demonstrate a higher rate (50 to 100%) of latent infection in natural cabbage moth populations in England, and, similar to our results, the virus was present in insects from geographically remote regions. Moreover, they demonstrated that the viral infection rate of these insects when reared in the laboratory (under conditions without external viral contamination) usually did not decrease over the five generations during which they were observed.…”

Horizontal transmission of deformed wing virus: pathological consequences in adult bees (Apis mellifera) depend on the transmission route

“…Such vertically transmitted infections are mainly if not exclusively covert, which might present a mechanism for long-term persistence in the honeybee population as has also been described for other insect viruses (Bonsall et al, 2005;Burden et al, 2003;de Miranda & Genersch, 2010;Hails et al, 2008). Only in association with the ectoparasitic mite V. destructor have overt outbreaks of DWV infection, characterized by deformed wings as the most prominent symptom, been recorded (Ball, 1989;Ball & Allen, 1988;Bowen-Walker et al, 1999;Martin, 2001).…”

Intra- and Intergenerational Persistence of an Insect Nucleopolyhedrovirus: Adverse Effects of Sublethal Disease on Host Development, Reproduction, and Susceptibility to Superinfection

“…Molecular techniques have been based on the detection of viral DNA or mRNA transcripts (3,(11)(12)(13)32) or, more recently, proteomics-based techniques (1). The mechanisms by which the virus persists in the cells of sublethally infected insects without triggering apoptosis remains unclear, although advances in the field of microRNA research may soon shed light on this issue (33).…”

“…The mechanisms by which the virus persists in the cells of sublethally infected insects without triggering apoptosis remains unclear, although advances in the field of microRNA research may soon shed light on this issue (33). Covert infections are believed to persist in one of two forms: as a latent infection in which the virus is not replicating and transcriptional activity is minimal, or as a sublethal infection in which the virus is transcriptionally active and replication occurs at a low level (2,3,9). Compared to healthy conspecifics, sublethally infected insects can experience slower development rates, lower pupal and adult body weights, reduced reproductive capacity, or altered preoviposition period (26,30,38).…”