Pathological study of mouse mastitis model infected with four different Prototheca species

The genus Prototheca is a member of the family Chlorellaceae, and is widely distributed in moist environments such as the surfaces of plants, the digestive tracts of animals, soil, lakes, and sewage. Prototheca spp. is known to be a causative agent of bovine mastitis. Bovine mastitis affected by P. bovis, P. ciferrii and P. blaschkeae reveals swelling of the breast, induration, secretion of milk containing white flakes, and reduction in milk production. Since the 1990s, bovine mastitis caused by Prototheca spp. have been increasing, and is distributed in worldwide, including Japan. We experimentally infected immunocompromised mice and the mammary gland of healthy lactating mice with each Prototheca spp. to obtain knowledge on the pathogenicity of four different Prototheca spp. In this study, we used four different Prototheca spp., P. ciferrii, P. bovis, P. blaschkeae and P. vistulensis. Initially, the pathogenicity and organ tropism of Prototheca spp. in bloodstream infection were pathologically investigated by inoculating the SCID mice via the tail vein. Next, we inoculated the mammary glands of lactating BALB/c mice with Prototheca spp.by the teat orifice and examined pathologically for the establishment of the infection in the udder and oral transmission to the suckling mice. As a result of the experimental infection to SCID mice, only P. bovis showed infectivity in mice, and the pathological changes were observed in the brain and kidney. No blood-borne transfer of P. bovis to mammary tissue was observed. The mammary gland of lactating BALB/c mice, inoculated with P. bovis showed macroscopically discoloration, swelling, and hardening. Histopathology revealed loss of mammary gland epithelial cells and marked proliferation of organisms in the acinus. Mammary glands inoculated with P. ciferrii and P. blaschkeae showed macroscopically discoloration of mammary tissue and histologically focal inflammatory cell accumulations with mild organism proliferation. P. bovis, P. ciferrii and P. blaschkeae infected ascendingly in the lactating mammary glands of BALB/c mice. P. ciferrii and P. blaschkeae, whose virulency were unknown due to the few case reports of bovine mastitis, were found to infect the mammary gland of mice.