Histochemical studies of enzyme activities and structural elements in Gyrodactylus derjavini Mikailov, 1975 parasitizing fins of Oncorhynchus mykiss Walbaum were conducted. Marked activities of non-specific esterase, acid phosphatase, alkaline phosphatase and amino-peptidase were found in the intestinal caeca of the parasite. A strong activity of acetylcholinesterase was seen in the nervous system. Extraintestinal non-specific and eserine-sulphate resistant esterase was localized in the distal part of the hamulus sheath. Activities of peroxidase and glucuronidase were not detected. In the embryo, developing hamuli were enclosed in a sheath rich in phospholipids. Deposits of neutral lipids were sparse. The fully developed ventral and dorsal hamulus bars stained strongly for calcium. Lectin binding assays showed a mannose rich region in the cephalic duct openings, strong reactions for galactose in the glycocalyx whereas reactions for lactose were weak. These findings arc discussed with respect to the parasite-host relationship.
In natural European waters, the congeneric monogeneans Gyrodactylus derjavini Mikailov, 1975 and G. salaris Malmberg, 1957 are primarily found on brown trout Salmo trutta L. and Atlantic salmon Salmo salar L., respectively. Interestingly, rainbow trout, Oncorhynchus mykiss (Walbaum), originating from North America, is as susceptible as brown trout to G. derjavini. However, the mechanisms involved in this host specificity are poorly understood but may include behavioural, mechanical and chemical factors affecting parasite attraction, attachment, feeding, reproduction and host responses. In the present laboratory work, this question has been studied. Detached parasites (either G. derjavini or G. salaris) were offered a choice in small aquaria between fry of rainbow trout, Atlantic salmon and carp Cyprinus carpio L. Within 48 hours more than 90% of G. derjavini colonised rainbow trout and left salmon almost uninfected. Some parasites were found on carp. During the same time span, more than 60% of G. salaris attached to salmon, the rest infected rainbow trout and none were found on carp. Following attachment, the parasites need appropriate stimuli to initiate feeding and reproduction but even such a successful specific colonisation can be followed by a host response. Both humoral and cellular elements have been suggested to participate in these reactions but in the present work it was demonstrated by immunoblotting and immunocytochemistry that no antibodies in host mucus and host plasma bound to any parasite structures or epitopes.
Host responses against skin inhabiting monogeneans are commonly obserÇetHtflfthe responsible immune mechanisms in the fish skin are insufficiently described. Based on recent knowledge of fish immunity and skin response mechanisms in mammals a model for the skin immunity in fish to monogcnean infections is proposed. Important cellular components of the model are the epithelial cells, the mucous cells and leucocytes. The release of cytokines, e.g. IL-1, following mechanical or chemical injury of the epithelial cells, initiates a series of events leading to decrease of the ectoparasite population. Cytokines (e.g. IL-1, TNF, INF) are suggested to affect secretions from mucous cell and attract neutrophils and macrophages. Leukotrienes are probably involved in the inflammatory reactions. The subsequent production of humoral substances (among others complement factors and peptides) could be responsible for the antiparasitic response in the later stages of infection. Although non-specific factors dominate the response, the involvement of specific antibodies and lymphocytes cannot be excluded.
Freshwater pulmonate snails from three locations in Lake Furesø north of Copenhagen were screened for infection with furcocercariae (by shedding in the laboratory) and recovered parasite larvae were diagnosed by molecular methods (by performing PCR of rDNA and sequencing the internal transcribed spacer [ITS] region). Overall prevalence of infection in snails was 2%. Recovered cercariae from Lymnaea stagnalis (Linnaeus) were diagnosed as Diplostomum pseudospathaceum Niewiadomska, 1984 (prevalence 4%) and cercariae from Radix balthica (Linnaeus) as D. mergi (Dubois, 1932) (prevalence 2%). Pathogen-free rainbow trout were then exposed to isolated cercariae and infection success and site location of metacercariae in these fish were determined. Infection experiments confirmed that both species could infect rainbow trout with the eye lens as infection site for the metacercarial stage although infection success differed. Combination of molecular and biological assays may contribute to improvement of our knowledge on diagnosis, distribution and biology of diplostomids in fish.
Response mechanisms of rainbow trout Oncorhynchus mykiss (Walbaum), experimentally infected with a Danish strain of Gyrodactylus salaris Malmberg, 1957 were investigated using molecular tools (qPCR) and immunohistochemistry. Expression of ten immune-relevant genes and reactivity with five different antibodies in the epidermis of skin and fin tissue were analysed in susceptible but responding rainbow trout. Rainbow trout were susceptible with regard to the parasite strain which initially colonised fins but relocated to the body region as infection progressed. The ten investigated genes encoding the cytokines IL-1β, TNF-α, IFN-γ, IL-10 and markers for adaptive immune activity, such as CD-4, CD-8, TCR-α, IgM, IgT and MHC II, were not found significantly regulated during the course of infection although IFN-γ showed a slight up-regulation. Immunohistochemical analyses showed positive reactivity with antibodies against CD3, B-lymphocytes, neutrophilic granulocytes and collectin but not with mAb against IgM. No staining differences between infected and non-infected skin and fin tissue were detected.