Encephalitozoon cuniculi Infection in Rabbits

intracellular parasite in the phylum Microspora, which is defined by its resistant spore stage. Microsporidia receive little attention in veterinary medical curricula because they are considered important only as parasites of rabbits and fish. Microsporidia are important parasites of humans as well, with some species causing severe disease and death in immunocompromised individuals. Microsporidia are among the smallest protozoa known and have the smallest genome (i.e., 2.9 million base pairs) of all eukaryotes. Microsporidia have many characteristics in common with prokaryotes. For example, they lack mitochondria, centrioles, and peroxisomes. In addition, the Golgi apparatus is atypical and primitive, and ribosomes of Microsporidia closely resemble those of prokaryotes. Sequencing of microsporidial genes has suggested a relationship with fungi. This theory is supported by the presence of chitin in the spore wall and by intranuclear division. LIFE CYCLE OF ENCEPHALITOZOON CUNICULI The life cycle of E. cuniculi takes 3 to 5 weeks to complete. Hosts are infected by ingestion or inhalation of spores or by transplacental transmission (Figure 1). The spore penetrates a host cell via the polar tube. The polaroplast swells, and the anchoring disk stabilizes the tube. The polar tube is inverted while extruded, and membranes from the polaroplast connect with the tube to extend its length. The extrusion occurs with great force and allows penetration of the host cell without disturbance of the host cell membrane. The posterior vacuole swells and forces the sporoplasm and nucleus through the polar tube (Figure 2). The tip of the polar tube inside the host cell envelopes the sporoplasm to become the parasite’s outer membrane. The sporoplasm divides, creating meronts, the major proliferative stage that divides by binary fission. This stage exists in a parasitophorous vacuole (PV) lined by the host cell membrane. Meronts may undergo several replications before converting into sporonts. In most species, the sporonts exist within a PV.