Abstract
Many pathogens and parasites are present in host individuals and populations without any obvious signs of disease. This is particularly true for baculoviruses infecting lepidopteran hosts, where studies have shown that covert persistent viral infections are almost ubiquitous in many species. To date, the infection intensity of covert viruses has rarely been quantified. In this study, we investigated the dynamics of a covert baculovirus infection within the lepidopteran crop pest Spodoptera exempta. A real-time quantitative polymerase chain reaction (qPCR) procedure using a 5' nuclease hydrolysis (TaqMan) probe was developed for specific detection and quantification of Spodoptera exempta nucleopolyhedrovirus (SpexNPV). The qPCR assay indicated that covert baculovirus dynamics varied considerably over the course of the host life-cycle, with infection load peaking in early larval instars and being lowest in adults and final-instar larvae. Adult dissections indicated that, contrary to expectation, viral load aggregation was highest in the head, wings and legs, and lowest in the thorax and abdomen. The data presented here have broad implications relating to our understanding of transmission patterns of baculoviruses and the role of covert infections in host-pathogen dynamics.
Highlights
Diseases can be an important factor in the population dynamics of insect pests of agricultural importance as well as being useful model systems for exploring host pathogen interactions
Studies have shown that when larvae are subjected to periods of stress, covert infections may be triggered into lethal overt infections that produce occlusion bodies (OB) for subsequent horizontal transmission (e.g., [4,15])
All insects used in this study were asymptomatic for Spodoptera exempta nucleopolyhedrovirus (SpexNPV) infection, with no overt viral mortality observed during the experiment
Summary
Diseases can be an important factor in the population dynamics of insect pests of agricultural importance as well as being useful model systems for exploring host pathogen interactions. The viral insect diseases caused by Baculoviruses, double-stranded DNA virus [1] are among the most studied and have been developed for both classical biological control and as biological pesticides. These viruses are characterised by having the infectious virions embedded in proteinaceous occlusion bodies (OB) adapted for persistence in the environment and classical horizontal transmission between susceptible hosts [2]. The various protocols for triggering covert viruses have proved inherently unreliable and unrepeatable indicating that our fundamental knowledge of this phenomenon is deficient and that we have much to learn about the dynamics and behaviour of non-persistent infections and their role in the ecology of lepidopteran populations, especially those of economic and agricultural importance
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