Histopathological, ultrastructural and immunohistochemical investigations were conducted on 26 specimens of powan Coregonus lavaretus (L.) from Loch Lomond (Scotland). The hearts of all 26 powan (15 females and 11 males) investigated harboured metacercariae of the digenean trematode Ichthyocotylurus erraticus (Rudolphi, 1809). The vast majority of metacercariae were located either singly or as an aggregation of white
cysts on the surface of the bulbus arteriosus. The intensity of
infection ranged from 2 to 200 larvae heart(-1), although the number of metacercariae found on male powan did not exceed 13. Histochemically, the parasite
cyst wall gave a strong positive reaction with
periodic acid schiff (PAS) and a faint positive signal with Azan-Mallory
stain. All the metacercariae
cysts were embedded in a granulomatous proliferation of heart epicardium tissue, forming a reactive fibroconnective
capsule around the parasite. The
capsule enclosing the parasite (produced by the host's reaction to the parasite) measured 13.57 to 90.20 microm (37.43 +/- 3.56) in thickness. Within the
capsule wall, eosinophilic granular cells (EGCs), granulocytes, melanocytes and, in some instances, partially degenerated or vacuolated epithelioid cells were observed in close proximity to the
cyst wall. Pigment-bearing macrophages were scattered throughout the granulomatous host-tissue reaction and as macrophage aggregates (MAs) within the capsules surrounding parasites. Immunohistochemical tests were applied to infected heart sections using 12 different
antisera. Nerve fibres immunoreactive to
bombesin,
substance P (SP), and
atrial natriuretic peptide (
ANP)
antisera were observed in close proximity to the parasite larvae. The presence of a
serotonin-like substance was also observed within host immune-cells surrounding trematode
cysts. Large cells of the epicardium were found to be immunoreactive to
met-enkephalin and
vasoactive intestinal peptide (VIP)
antisera but not immunoreactive to anti-
protein gene-product 9.5 (PGP9.5) sera.