Anthropochore fish movements are a common route of parasite introduction. As such, Kennedy  estimated that approximately 68.7% of all introduced parasites were through the movement of ornamental fish species into the country. In the same year, Gibson  suggested that the introduction and establishment of M. wageneri into Britain may have been through the importation of infected ornamental varieties of tench, for sport angling and fisheries, although he also discusses the possibility of parasite introduction via infected oligochaete worms, the suggested intermediate host, which are also imported from the Continent for bait. There are no known published records regarding the pathogenicity of M. wageneri in tench from Italy and the host cellular reaction and the results from the current study are among the first.
Seven species exist in the genus Monobothrium Diesing, 1863, although M. wageneri and M. auriculatum Kulakovskaya, 1961, which is recorded as infecting Leuciscus danilewskyi (Kesseler) in the Ukraine , are the only European representatives of this genus . Monobothrium ingens Hunter, 1927 and M. hunteri Mackiewicz, 1963 recorded from the catostomidid cyprinids Ictiobus cyprinellus (Valenciennes) and Catostomus commersoni Lacépède from North America may exert pathogenic effects through the creation of lesions as a consequence of their attachment to the host's intestine [31, 37, 38]. The pathological changes induced by M. wageneri, however, differ markedly from those seen in other intestinal cyprinid-cestode systems (e.g. Bothriocephalus acheilognathi Yamaguti, 1934 and Khawia sinensis Hsü, 1935 infections in Cyprinus carpio L [39, 40]). These differences include the magnitude of the inflammatory response, the involvement of all layers of the intestine, and the complete loss of gut architecture, even in tench with light tapeworm burdens. Pronounced inflammatory nodules, leading to the partial occlusion of the intestinal tract were found in hosts infected with as few as 6 adult M. wageneri. Although the focal attachment of M. wageneri limits the area of intestine damaged by this cestode, the tight clustering of tapeworms accentuates the severity of the individual lesions. Intestinal occlusion and rupture are unusual and are, according to Williams & Jones , extreme consequences of tapeworm infection. These are among the most serious impacts caused by intestinal tapeworms, which have been associated with nutritional disturbance, debilitation and even the death of heavily infected fish .
Inflammation of the intestinal tract can be provoked by a variety of factors including, for example, feed , infectious agents and chronic stress . The level of infection and the tissue or organ affected can influence the range of histopathological responses initiated to an endoparasitic infection. These can range from benign encapsulation of the pathogen by host cells, to acute and chronic inflammation and necrosis . The relative importance of the body organ that is infected and whether its function is unduly compromised, therefore, dictates host survival. Most pathology associated with cestode infections, however, results from the deep penetration of the scolex into the intestinal wall . This is the case in M. wageneri, which induces marked pathological changes, penetrating the muscularis layer ( and current study), causing a significant inflammatory response in all layers of the intestine in both light and heavy infections.
Fish mucus is the intestine's first line of mucosal initiated defence . Parasitic infections can induce hyperplasia and hypertrophy of mucous cells and can increase the level of mucus secretion in the intestine [29, 47]. Recent investigations by Bosi et al.  and Dezfuli et al.  quantified the effects of enteric helminth infections on the density of mucous cells and on the composition of the mucus. Bosi et al.  determined the number of mucous cells in the intestines of uninfected brown trout, S. trutta, and in those infected with the acanthocephalan Pomphorhynchus laevis (Zoega in Müller, 1776) and found a significant difference in the number of cells between the two groups. Dezuli et al. , found a significant increase in the number of mucous cells staining positively for acid glycoconjugates in brown trout infected with both Cyathocephalus truncatus (Pallas, 1781) and Echinorhynchus truttae (Schrank, 1788). Data from the current study indicates that M. wageneri elicits a similar response with an increase in the number of two types of mucous cell - AB and AB/PAS positive cells. The study of Fairweather , likewise, documented different mucous cell secretions in response to the presence of parasites.
The attachment organs used by intestinal helminths during the process of attachment to their host's often induce inflammation in their host's alimentary tract [6–8]. RCs and two types of granulocytes, namely MCs [12, 16] and neutrophils [11, 14] have been repeatedly demonstrated to play a critical role in the immune system of fish as part of their defence function against pathogens. There is, therefore, a growing interest and accumulating body of evidence regarding the role of these inflammatory cells in the immune system of fish. Granulocytes are generally considered effector cells of the innate immune response . Innate defence provides a pre-existing and fast-acting system of protection which is non-specific and has several advantages over the slow-acting specific immune responses . The importance of each of these cells types i.e. RCs, MCs and neutrophils, will be discussed briefly in turn.
RCs occur in a wide range of tissues of teleosts and have been, most commonly, associated with epithelia . The results of several recent investigations on both wild and farmed fish suggests that RCs represent an immune cell type closely linked to other piscine inflammatory cells [8, 12, 26]. In the present survey, however, the number of RCs in infected and uninfected tench were not determined because M. wageneri destroys the epithelia at the site of attachment. RCs were found to co-occur with MCs and neutrophils within the submucosa of cestode induced nodules. The findings of intestinal RCs at this site is unusual and requires further investigation.
Most teleosts possess MCs and these have been likened to mammalian mast cells in that they possesses similar structural and functional properties [16, 18]. In fish infected with helminths, it has been observed that MCs tend to migrate and accumulate in large numbers at the site of infection [16, 20]. In some fish-acanthocephalan systems, numerous MCs have been found at the sites of infection [8, 51] while they have been notably lower in uninfected fish; a similar situation was seen in the current study in the tench infected with M. wageneri. Descriptive data exists detailing how MCs degranulate in response to their exposure to a variety of known degranulating agents and pathogens [18, 19, 52, 53]. In parasitised tench, an intense MC degranulation was observed at the site of M. wageneri infection, notably in the immediate zone surrounding the scolex. It is possible that the secretions produced by the mast cells may have a role in attracting other cell types (i.e. neutrophils) involved in the inflammatory process, particularly during the period of initial pathogen challenge [17, 22]. Murray et al.  suggested that MCs may be involved in the direct destruction of pathogens, adding to their multifunctional role in teleosts.
In the current study, a high number of neutrophils were found to co-occur with MCs within the submucosa at the attachment sites of M. wageneri. A similar finding was reported by Dezfuli et al.  investigating the livers of minnows, Phoxinus phoxinus (L.), infected with Raphidascaris acus (Bloch, 1779) larvae. Neutrophils are one of the first cell types to arrive at the sites of inflammation and are a critical component of the teleost innate immune defence system [23, 54]. Neutrophils are present in high numbers in the blood and in hematopoietic organ pools as a reserve, and, under normal conditions, are rare in the tissues and body cavities . Generally, macrophages co-occur with neutrophils to engulf extracellular pathogens into intracellular phagosomes and, through a series of events that lead to the maturation of the vacuole, destroy the invading agent in the newly "armed" phagolysosome . Macrophages from the head kidney of rainbow trout, Oncorhynchus mykiss (Walbaum), were reported by Whyte et al.  to kill the larvae of the eye fluke Diplostomum spathaceum (Rudolphi, 1819), although this was dependent on the ratio of cells to parasites. Interestingly, no macrophages were found at the sites of M. wageneri attachment in the current study and yet the data gathered does not permit a definitive explanation for this lack of macrophages.
Neutrophils and macrophages might co-occur when the infecting agent is small and can be readily engulfed  or is of a small size (e.g., diplostomules of D. spathaceum) that it can be killed by host macrophages (see ). Monobothrium wageneri, however, is a tapeworm measuring several centimetres in length and so it is improbable that the scolex or the entire worm can be engulfed by host macrophages. The findings from the current study suggest that, M. wageneri may preferentially induce the recruitment of neutrophils and MCs.
Hayunga  draws attention to the important detail that despite the poorly developed mechanical apparatus of caryophyllideans, they do possess numerous frontal glands. Histochemical and experimental examinations suggest that secretions from the tegumentary glands of caryophyllidean tapeworms consist of neutral glycoproteins that may assist in protecting the worm from the host's immune response . In the absence of definitive studies, the function of these glandular elements remain speculative . From the TEM observations that were made within the current study, several glandular cytons within the syncytial tegument along the anterior and lateral parts of the M. wageneri scolex were observed (not shown). No discharge from these glands or the presence of an adhesive layer between the host and the cestode was evident. The occurrence of abundant immune cells at the site of attachment and in close contact with the scolex of M. wageneri perhaps dismisses the possibility that the secretions produced by the scolex tegumentary glands are responsible for distancing the cellular responses of infected T. tinca.