The Histopathology of Experimentally Induced Infections of Spiroplasma taiwanense (Class: Mollicutes) in Anopheles stephensi Mosquitoes

Abstract

Histopathology visible in premortem female Anopheles stephensi mosquitoes was examined using light and transmission electron microscopy at 5 days postintrathoracic inoculation with Spiroplasma taiwanense strain SP2/sl/mut/P9. Light microscopy showed thoracic flight muscle to have undergone extensive degradation and polysaccharide depletion. Transmission electron microscopy revealed spiroplasmas both extra- and intra-cellularly and occurring in hemolymph, thoracic flight muscle, glial cells, neural lamella, hemocytes, an unknown cell type found in juxtaposition with axon bundles, connective tissue surrounding diverticulum and midgut, trophocytes, and tracheocytes. Replication of spiroplasmas within cells caused cellular lysis and the liberation of spiroplasmas into the hemolymph. Necrosis of infected muscle cells involved, successively, condensation, swelling, and lysis of mitochondria, Z-disc loss, and splitting of myofibrils. Nerve cord axons surrounded by infected glial cells were distended by swollen mitochondria. Spiroplasmas were able to survive and replicate in hemolymph and within phagocytic vacuoles of hemocytes, independent of any host immune response. Pathologies observed within thoracic flight muscle resulted from intracellular replication of spiroplasmas and probably produced the loss of flight capacity observed for these mosquitoes. Neurological involvement and/or the presence of toxic metabolites may represent additional pathogenic determinants. The existence of life-history stages described previously in vitro were confirmed in vivo.