The infection rate and characteristics of metacercaria of
Paragonimus ohirai in the brackish water crab (
Sesarma dehaani) was investigated in Kikuyashima, a sandbank located near the mouth of the Maruyama River in northern Hyogo Prefecture, between 2001 and 2009. The infection rate was 76.5% in 2001 and 76.1% in 2002, but increased to 87.0% in 2003. It remained high at 84.6% in 2004 and 92.1% in 2005, dropped to 80.4% in 2006, then increased again to over 90%, specifically 93.4% in 2007, 95.6% in 2008, and 91.5% in 2009. The rate of metacercaria infection remained high for several years. The largest number of metacercariae found in a single crab was 293 (2003). Regarding seasonal variation in the infection rate, there were years in which the rate tended to be low during from May to July, and others in which the rate was above 80% from May to October. On the shore opposite Kikuyashima, the rate of metacercaria infection in
S. dehaani was about 40%, indicating spatial variation across short distances. On the shore area near Kinosaki-ohashi, where a high density of
S. dehaani and a high rate of metacercaria infection had been reported in the 1950s, few
S. dehaani were found, and no infecton by metacercaria was observed. The life cycle of
P. ohirai had been affected by environmental changes.
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