Mechanism of Viral Infection

Most insect viruses gain access to the insect host by ingestion of contaminated food, as none of them are able to penetrate the insect tissues through the insect cuticle (exoskeleton). Still, there are some other alternative ways of infection, such as the internal or external contamination of the host eggs, and by the contaminated ovipositor of parasites.83 Particularly for baculoviruses, insect hosts ingest the OBs present on the food (leaves, for instance), and once in the insect midgut the OBs are dissolved due to the highly alkaline environment of the gut (pH 9 or higher) releasing the occluded virions.84 Once the virions are released, their envelopes are fused to the membrane of the midgut epithelial cells and the bare nucleocapsids are able to penetrate the cell and reach the nucleus where the virus replicates forming a generation of non-occluded virions. The bare virions bud out of the host cell, surrounded in part by the host cell membrane and disseminate throughout the whole insect body cavity (haemocoel) by the insect blood (hemolymph), infecting susceptible tissues and causing a systemic infection. The budded virions enter the cells and replicate in the nuclei, producing a second generation of virions, most of them occluded in polyhedra which are formed de novo, but others bud out of the cell and infect other cells. Once the infection spreads out in the whole body, the insect is killed and remains attached to the plant by their prolegs, head-down, and millions of OBs are released to the environment as the insect cuticle (exoskeleton) is degraded, constituting new inocula for other susceptible insects (see Figure 8.5).84

Infections caused by entomopoxviruses are very similar, as they start by the ingestion of sphaeroids by the susceptible insects. Sphaeroids are also degraded in the insect midgut, and virions are released and infect the susceptible fat body cells, where the virus replicates.85 Infected insects become weak and eventually die in 12 to 15 days post-infection, although some may survive up to 25 to 30 days. Cypovirus infections, on the other hand, are restricted to the midgut epithelial cells, causing mostly chronic infections on the gut of lepidopteran and dipteran (mosquitoes) insects.86 Cypovirus OBs are also degraded in the insect midgut, where the icosahedral virions are released and fused to the microvilli membrane of the midgut cells before they penetrate and replicate in the cytoplasm. Infected larvae show a whitish coloration in the midgut and later tend to

Figure 8.5 Trichoplusia ni infected with baculovirus TnSNPV. In baculoviruses, infected insects become whitish in color because of the massive infection of the fat body. The infected larva crawls up and hangs head down from its crochets in an inverted ''V'' position. A greyish to creamy liquid is released, where billions of OBs are suspended, which facilitates the spread of inocula in the field.

Figure 8.5 Trichoplusia ni infected with baculovirus TnSNPV. In baculoviruses, infected insects become whitish in color because of the massive infection of the fat body. The infected larva crawls up and hangs head down from its crochets in an inverted ''V'' position. A greyish to creamy liquid is released, where billions of OBs are suspended, which facilitates the spread of inocula in the field.

dry up. As for the iridoviruses, they replicate both in the cytoplasm and in the nuclei of the cells infected by pinocytosis;87 however, the assembling (para-crystalline arrangement) of the virions particles occurs only in the cytoplasm. Virions replicate mostly in the mosquito fat body, although other tissues are eventually susceptible. Infected larvae show iridescence in several parts of the body, with orange, blue-green or turquoise tonalities. Frequently, iridoviruses are not lethal to the infected larvae, causing chronic infections.

Aromatherapy Aura

Aromatherapy Aura

This powerful tool will provide you with everything you need to know to be a success and achieve your goal of breaking into the mighty wellness arena. All the same the issue with getting hold of all that content is the huge expense. If you don't have time to compose all that content yourself, you're going to have to pay somebody to do it for you. And not only that, but if you've done outsourcing before, then you'll know that quality may often be 'questionable'.

Get My Free Ebook


Post a comment