Occurrence and histological response of Raphidascaris acus (Nematoda: Ascaridoidea) in roach from four lakes differing in water quality.

Seasonality and size-related infection of Raphidascaris acus larvae from the inner organs and intestine of roach (Rutilus rutilus) were studied in 4 lakes of differing water quality and pollution level in Central Finland between August 1985 and November 1986. The influence of R. acus larvae on the liver and pancreatic tissues of roach was examined histologically in additional material from 1989 and 1990. The inner organs of roach were most heavily infected with R. acus in the eutrophic, polluted Lake Vatia (63% of fish infected with 4.0 nematodes/fish) and in the two eutrophic lakes, compared to the oligotrophic Lake Peurunka (23%, 0.8). The prevalence of free R. acus larvae in the intestine of roach was almost as high but the intensity only about half of that found in the inner organs. The prevalence of infection had significantly higher values in autumn in most cases, and larvae accumulated in the inner organs and intestine of older roach. In histological studies it was found that larvae occurred more often in the pancreatic tissue than in the liver, but in both organs the majority of the larvae were dead and partly destroyed. The most typical host response against R. acus was a chronic granulomatous inflammatory reaction. Granulomas and developing granulomas containing worms at different stages of degeneration are described; they were found in all of the lakes studied throughout the year and also in one and the same fish. On average only 37 and 21% of the worms in the liver and pancreas, respectively, were alive. No obvious difference in the histological response against R. acus was noted between the lakes.