# Fecal Transmission of Nucleopolyhedroviruses: A Neglected Route to Disease?

## TL;DR

This paper explores how nucleopolyhedroviruses can spread through insect feces, a transmission route that has been largely overlooked despite its potential importance.

## Contribution

The paper highlights fecal transmission as a neglected but potentially significant route for nucleopolyhedrovirus spread among lepidopteran larvae.

## Key findings

- Infected larvae release virus-containing occlusion bodies in their feces, which can infect other larvae.
- Fecal transmission may be as effective as other known routes like cannibalism or predator interactions.
- The quantity of virus in feces increases as the infection progresses in the host.

## Abstract

Lepidopteran nucleopolyhedroviruses are virulent pathogens of the larval stages of butterflies and moths, and they are widely used as the basis for biological insecticides. The virions are occluded within a matrix of protein to form highly resistant polyhedral occlusion bodies (OBs) that protect the virus outside of the insect. Larvae become infected after consuming foliage contaminated with OBs that dissolve in the intestine, release virions and infect gut cells, from where they spread to the cells of other tissues and cause a lethal disease. The virus-killed insect releases millions of progeny OBs into the environment for the following cycles of transmission. Here, I review the evidence that infected intestinal cells can produce biologically significant quantities of OBs that are released in the feces. These can be transmitted to other susceptible larvae and represent a little-recognized route for the transmission of these viruses. I compare fecal transmission with other alternative routes of transmission and make a series of suggestions for future lines of research to establish the importance of virus contamination of feces in the transmission and dispersal of these pathogens.

Nucleopolyhedroviruses of lepidopteran larvae (Alphabaculovirus, Baculoviridae) form the basis for effective and highly selective biological insecticides for the control of caterpillar pests of greenhouse and field crops and forests. Horizontal transmission is usually achieved following the release of large quantities of viral occlusion bodies (OBs) from virus-killed insects. In the present review, I examine the evidence for productive midgut infection in different host species and the resulting transmission through the release of OBs in the feces (frass) of the host. This has been a neglected aspect of virus transmission since it was initially studied over six decades ago. The different host–virus pathosystems vary markedly in the quantity of OBs released in feces and in their ability to contaminate the host’s food plant. The release of fecal OBs tends to increase over time as the infection progresses. Although based on a small number of studies, the prevalence of transmission of fecal inoculum is comparable with that of recognized alternative routes for transmission and dissemination, such as cannibalism and interactions with predators and parasitoids. Finally, I outline a series of predictions that would affect the importance of OBs in feces as a source of inoculum in the environment and which could form the basis for future lines of research.