Histopathology of Spiroplasma penaei Systemic Infection in Experimentally Infected Pacific White Shrimp, Penaeus vannamei

Penaeus vannamei shrimp were challenged with a suspension of a pathogenic isolate of Spiroplasma penaei prepared from a 72-h culture. The route of challenge was by intramuscular injection of the bacterial suspension into the third abdominal segment. Lesion development was evaluated in moribund shrimp collected and fixed in Davidson's fixative 96 h post challenge. The predominant host responses to infection by S. penaei observed by histological examination were the general systemic development of hemocytic nodules (often melanized) and poorly organized hemocytic infiltration. Such lesions were most prevalent in the lymphoid organ, gill filaments, heart, connective tissue, antennal gland, and skeletal muscle. The presence of S. penaei in the lesions was verified by in situ hybridization using a digoxigenin (DIG)-labeled probe specific to the spiralin gene of Spiroplasma spp. Transmission electron micrographs (TEM) showed S. penaei cells free in the cytoplasm of lymphoid organ cells. The cultures of S. penaei used for this study and infected abdominal tissue were verified by PCR using spiroplasma-specific primers that amplify a fragment from a variable region of the 16S rDNA gene sequence.