Quantitative data are presented on the spatial distribution of metacercariae of the digenean trematode Diploslomum phoxini (Faust, 1918) in the brains of minnows, Phoxinus phoxinus (Linnaeus, 1758), from two Scottish populations. Sequential examination of serial histological sections revealed metacercariae to be unevenly distributed throughout the brain, aggregating in specific regions including the cerebellum, the medulla oblongata and the optic lobes. In addition, a number of metacercariae were found in the anterior part of the spinal cord. The inferior lobe of the cerebellum, pituitary, olfactory lobes and olfactory bulbs were largely free of metacercariae. Reasons for the uneven distribution of metacercariae within the brains of infected minnows are discussed, including the possibility that the parasite may have evolved to enhance its transmission to subsequent hosts by aggregating in regions known to be important in the control of the host’s antipredator responses.
Two hundred and forty brown trout (Salmo trutta) and 49 rainbow trout (Oncorhynchus mykiss), obtained from 21 locations in Central Scotland between October 1990 and August 1993, were examined for endoparasitic helminth infections. Crepidostomum farionis (Digenea) was the most widely distributed helminth species, followed by Eubothrium crassum (Cestoda), Diphyllobothrium dendriticum and D. ditremum (Cestoda), Neoechinorhynchus rutili (Acanthocephala), Echinorhyn-chus truttae (Acanthocephala), Eustrongylides sp. (Nematoda), Capillaria salvelini (Nematoda), Cyalhocephalus truncatus (Cestoda), Raphidascaris acus (Nematoda) and Cystidicola farionis (Nematoda), in that order. The prevalences and intensities of each helminth infection were recorded. No evidence was found to indicate that even fish with the highest worm burdens (e.g. 339 plerocercoids of Diphyllobothrium spp.) were experiencing any obvious morbidity. An analysis of pairs of associations between species of helminths revealed a significantly positive association between N. rutili and C. farionis (P < 0.01). The results are discussed in terms of patterns in helminth communities in freshwater fish.