Experimental evaluation of pathogenicity and acquired immunity of species, and , infecting Japanese rock ptarmigans in a subspecies of the birds.

Japanese rock ptarmigans () are birds that inhabit only alpine regions of central Honshu Island, Japan, known as the Japanese Alps. The number of these birds has recently declined, and and national conservation programs for Japanese rock ptarmigans have been initiated. The infections of spp. as protozoan parasites of the phylum Apicomplexa, and , were frequently reported in the birds. However, the virulence of these parasites has not been determined. Here, we analyzed the pathogenicity of these parasites using experimental infections of a subspecies model of Japanese rock ptarmigans, Svalbard rock ptarmigans (), and evaluated acquired protective immunity against challenge in birds tolerant of low-dose inoculation with parasites. Following inoculation with two parasites derived from Japanese rock ptarmigans (dose range of 4 × 10 to 4 × 10 for and 1.7 × 10 to 4 × 10 for ), oocysts were detected at 6-8 days post-inoculation (PI), and the maximum number of oocysts per gram of feces was observed 7-10 days PI and then gradually decreased. The mortality rate and reduction in weight gain of chicks increased following high-dose inoculation of oocysts with abnormal feces (soft and diarrhea). Developmental zoites were detected histopathologically in epithelial tissues and sometimes the lamina propria from the duodenum to the colon. Chicks that survived low-dose inoculation did not show clear clinical symptoms after challenge inoculation. Our results suggest that the pathological characteristics of parasites infecting Japanese rock ptarmigans include abnormal feces and reduction in weight gain, resulting in mortality in cases of heavy infection due to high-dose inoculation. These findings provide helpful data for Japanese rock ptarmigan conservation efforts.