Cytopathology of Bombyx mori (Lepidoptera: Bombycidae) larva integument infected by Bombyx mori nucleopolyhedrovirus
Bombyx mori nucleopolyhedrovirus (BmNPV) is an entomopathogenic virus of the family Baculoviridae that causes disease in the silkworm, Bombyx mori, leading to a series of metabolic disorders that severely affect silk production. The main external sign of BmNPV infection can be observed in the integument of infected caterpillars, which is an important diagnostic indicator that can help direct control measures to prevent viral dispersion in rearing facilities. We analyzed the integument of B. mori caterpillars infected with BmNPV in a cytopathological study of epidermal cells and cuticle, to help identify signs of infection. Fifth instar larvae were inoculated with viral suspension and integument segments were processed for light microscopy and transmission electron microscopy. Signs of infection were monitored daily until cocooning. Cytopathology of epidermal cells showed signs of baculovirus infection, leading to cytolysis and release of viral polyhedra into the hemocoel, similar to what occurs in other BmNPV target cells. Considerable modifications were observed in the apical region of the epidermal cells, with disarrangement and involution of the microvilli, and loss of contact of the plasma membrane plaques with the endocuticle. Plasma membrane plaques are involved in the transport of precursors for chitin synthesis and assembly of chitin myofibrils. The disorganization of the integument of silkworms infected with BmNPV shows the fragility of this organ, which easily ruptures due to infection, leading to the release of BmNPV polyhedra to the external environment, perpetuating the infection.