Tracheliastes maculatus is a copepod crustacean parasite belonging to the family Lernaeopodidae. In the late spring months, around the time of spawning, large numbers of the imago stages of that parasite can be found attached to the scales of common bream in Lake Balaton every year. Due to their bright red colour, the small ulcers formed at the site of parasite attachment are highly conspicuous even to non-specialists, calling attention to the presence of infestation. In 2008, T. maculatus infestation affecting the bream stock of Lake Balaton proved to be more intensive than usual. The intensity of infestation of the 3- to 5-year-old bream specimens markedly exceeded the usual level of 7-12 parasites per bream, and infestation could be detected also on the 2-year-old bream specimens 12 to 14 cm in length. On the 2-year-old bream specimens, the inflammatory zone of bacterial origin, which was mostly limited to a single scale on breams of bigger body size, involved a larger area, resulting in scaleless areas and extensive superficial erosions developing in the place of the parasitic focus that had become detached together with the attacked scale. When samples from the haemorrhagic areas, from the serum accumulating in the scale pouches and from the kidney were inoculated onto blood agar, colonies of the facultative pathogenic bacterium Aeromonas hydrophila grew out. In addition to the sporadic mortality regularly occurring among older fish specimens in the late spring period simultaneously with parasite infestation, this year mortality could be observed also in the younger age groups of bream.
