Effects of migrating Gasterophilus intestinalis larvae (Diptera: Gasterophilidae) on the mouth of the horse.

Lesions were formed in the mouth of the horse by first- and second-stage Gasterophilus intestinalis larvae. The lesions resulted as larvae burrowed through the top millimeter of the tongue and then became embedded in the interdental gingiva. Appearance of the lesion was dependent on the degree of larval burrowing activity and healing that had taken place. Active tunneling production occurred at the posterior end of the lesion. Extensive change in configuration and color was apparent along the longer lesions with active larvae. Virtually all tissue in the path of the larvae was removed while forming a tunnel. Tissue destroyed included tops of the lamina propria mucosae evaginations and the deeper portions of the lamina epithelialis mucosae. Histopathologic findings showed that the tissue removed included the nerve and blood supply of the proprial evaginations. Hemorrhage and exocytosis into the tunnels included erythrocytes mixed with some macrophages, lymphocytes and scant numbers of eosinophils. Air shafts excavated in the tongue by larvae enabled bacteria to enter into the tunnels. Bacteria became adhered to larvae and initiated microabscesses. Microabscesses were composed of clotted erythrocytes, bacteria, disintegrating epithelial cells, and large numbers of neutrophils. Cells surrounding the tunnel exhibited pyknosis, epithelial hydropic degeneration and became separated from each other. Healing of tongue lesions occurred as epithelial cell growth below raised the tunnel upward and affected tissue desquamated. Interdental gingiva invaded by larvae were hyperemic and denuded of epithelia. Recession and ulceration of the gingiva produced periodontal pockets. Extensive invasion by larvae led to compound periodontal pockets. Larval mouth hooks were embedded in the submucosa of the pockets and tore at engorged capillaries. The cephalic portion of embedded larvae became surrounded by a cellular exudate containing erythrocytes and mononuclear cells. Attachment of second-stage larvae at the root of the tongue did not produce observable damage.