Descrição morfológica do intestino posterior e comportamento diferencial do reto de Bombyx mori frente ao AlfaBV

Abstract

Bombyx mori nucleopolyhedrovirus (BmNPV) is an entomopathogenic and poliorganotrophic virus that infects Bombyx mori. BmNPV consists of a double strand DNA with two distinct phenotypes: the derived polyhedral virus (PDV), responsible for the primary infection in the insect s midgut; and the budded virus (BV), which disperses the infection in the hemocele, causing secondary or systemic infection. It is extremely virulent and when it infects silkworms, causes serious damages to the insect, usually leading to its death or harming the silk production, affecting all the productive chain, leading to economic losses. For a good silk production, a determinant factor is the functioning of the food duct, which is divided in foregut, midgut and hindgut, being the hindgut the place for water and mineral salts absorption and for the end of the digestive process. Several tissues have been established as virus targets, however, others have shown to be resilient to BmNPV. Given the importance of the hindgut, the present paper aimed to verify the susceptibility of its components, ileum, colon and rectum to an geographic isolated of BmNPV in Paraná, Brasil, the BmMNPV. In different post-inoculation days (dpi), the hindgut segment of 5th instar silkworms was dissected and processed for analysis in light microscopy, using conventional dyes and cytochemistry for viral detection and electronic scanning microscopy for the morphological details. The results revealed that the ileum, colon and rectum of the B. mori are constituted by simple epithelium, with alterations in cell morphology, covered by an intima in its luminal side, and that its organization is similar to that of other described insects. The cytological analysis of the ileum, colon and fore rectum revealed that its epithelial cells are not susceptible to BmMNPV in neither of the times analyzed. However, the posterior hindgut or anal duct showed itself to be susceptible to the virus after the 5º dpi, developing all the classic signs described for the infection with AlphaBV, as the presence of viroplasm, nuclear hypertrophy, polyhedra in formation and mature ones. At the end of the infectious cycle, occurs cell lysis with the liberation of viral polyhedra in the intestinal lumen and, consequently, to the external medium, coinciding with the death of the insect. Even with no infection in the other regions of the hindgut, the surrounding tissues have shown infected, affecting the normal functioning of this intestinal region, verified through changes in fecal pellet, which was less compact and changed format. These results will contribute in the establishment of the BmMNPV infectious cycle. Furthermore, the basic knowledge of viral behavior is important for the development of infection control, prevention and previous identification methods of this disease in the field, once it also makes possible the removal of infected silkworms, diminishing the horizontal transmission of the virus in the creation rooms, in a way to reduce the loss of cocoons to be used in the confection of silk yarn