Migration, site selection, and development of Ornithodiplostomum sp. metacercariae (Digenea: Strigeoidea) in fathead minnows (Pimephales promelas)

A congeneric and non-randomly associated pair of larval trematodes dominates the assemblage of co-infecting parasites in fathead minnows (Pimephales promelas)

Individual hosts are often co-infected with multiple parasite species. Evidence from theoretical and empirical studies supports the idea that co-occurring parasites can impact each other and their hosts via synergistic or antagonistic interactions. The fundamental aim of understanding the consequences of co-infection to hosts and parasites requires an understanding of patterns of species co-occurrence within samples of hosts. We censused parasite assemblages in 755 adult, male fathead minnows collected from 7 lakes/ponds in southern Alberta, Canada between 2018 and 2020. Fifteen species of endoparasites infected fathead minnows, 98% of which were co-infected with between 2 and 9 parasite species (mean species richness: 4.4 ± 1.4). Non-random pairwise associations were detected within the overall parasite community. There were particularly strong, positive associations in the occurrences and intensities of the 2 congeneric larval trematodes Ornithodiplostomum sp. and Ornithodiplostomum ptychocheilus that comprised >96% of the 100 000+ parasites counted in the total sample of minnows. Furthermore, the occurrence of Ornithodiplostomum sp. was a strong predictor of the occurrence of O. ptychocheilus, and vice versa. Positive covariation in the intensities of these 2 dominants likely arises from their shared use of physid snails as first intermediate hosts in these waterbodies. These 2 species represent a predictable and non-random component within the complex assemblage of parasites of fathead minnows in this region.

Mapping a brain parasite: Occurrence and spatial distribution in fish encephalon

Parasites, especially brain-encysting trematodes, can have an impact on host behaviour, facilitating the transmission to next host and completion of the life cycle, but insufficient research has been done on whether specific brain regions are targeted. Using Cardiocephaloides longicollis as a laboratory model, the precise distribution of metacercariae in experimentally-infected, wild and farmed fish was mapped. The brain regions targeted by this parasite were explored, also from a histologic perspective, and potential pathogenic effects were evaluated. Experimental infections allowed to reproduce the natural infection intensity of C. longicollis, with four times higher infection intensity at the higher dose (150 vs 50 cercariae). The observed metacercarial distribution, similar among all fish groups, may reflect a trematode species-specific pattern: metacercariae occur with highest density in the optic lobe area (primarily infecting the periventricular gray zone of optic tectum) and the medulla oblongata, whereas other areas such as the olfactory lobes and cerebellar lobes may be occupied when the more frequently invaded parts of the brain were crowded. Mono- and multicysts (i.e. formed either with a single metacercaria, or with 2-25 metacercariae encapsulated together) may be formed depending on the aggregation and timing of metacercariae arrival, with minor host inflammatory response. Larvae of C. longicollis colonizing specific brain areas may have an effect on the functions associated with these areas, which are generally related to sensory and motor functions, but are also related to other host fitness traits such as school maintenance or recognition of predators. The detailed information on the extent and distribution of C. longicollis in fish encephalon sets the ground to understand the effects of brain parasites on fish, but further investigation to establish if C. longicollis, through purely mechanical damage (e.g., occupation, pressure and displacement), has an actual impact on host behaviour remains to be tested under controlled experimental conditions.

The versatility of simplicity: Structures of Cardiocephaloides longicollis used for different purposes during cercarial transmission

Transmission and infection strategies are critical for completing the life cycles of trematode parasites, which are characterized by complex life cycles involving multiple hosts and stages. Transmission between the first and second intermediate hosts typically relies on cercariae, a free-swimming larval stage that displays a series of behaviors to efficiently disperse, locate, attach to, and infect the next host. The aim of this study is to provide detailed information on behaviors used by furcocercariae (bifurcated tail) during its transmission from the snail to the fish host, using the laboratory-established model of Cardiocephaloides longicollis (Strigeidae). These cercariae are released from snails into seawater, where they swim, locate, penetrate the skin of fish, and encyst as metacercariae in their brain. In a series of in vivo assays, freshly-emerged cercariae were used to visually study their behavior and locomotion. Histopathology of experimentally infected gilthead seabreams with C. longicollis, taken at sequential post-infections times, were analysed to localize the migrating cercariae to the fish brain. Our results show that simplicity and versatility are the key features for the success of cercariae transmission by using their organs for different purposes. While 80% of the behavior was spent in a resting position, the most common swimming behavior was with tail-first, which is commonly described in furcocercariae to reach the host microhabitat. However, C. longicollis relies more on the furcae of the tail by using them as a propeller providing thrust and guidance when they swim, instead of using the tail stem. After attaching to the fish skin, cercariae rapidly creep on it using the oral- and ventral-suckers simulating a leech-like movement until they find a suitable penetration site. To penetrate, cercariae press the cephalic structures against the skin, while the ventral sucker anchors the cercariae to it. After this, they switch their locomotion to a slow peristaltic movement, opening the path through tissues with the help of their cephalic structures and anchoring their body with their surface spines. This is consistent with the post-penetration histological analyses, which suggested that C. longicollis cercariae move between the cells of the connective tissue and muscle fibers when migrating towards the fish's brain, without provoking relevant tissue damage or host responses. Understanding the versatility of cercarial structures to adapt to external conditions enriches our knowledge on parasites and their transmission ecology, opening the door to the design of avoidance methods in fish farms struggling with harmful parasites.

Architecture of the nervous system in metacercariae of Diplostomum pseudospathaceum Niewiadomska, 1984 (Digenea)

The development of metacercariae of Diplostomum pseudospathaceum Niewiadomska, 1984 is accompanied by profound morphological transformations often characterized as metamorphosis, which makes these metacercariae an interesting case for studying the morphogenesis of the digenean nervous system. Although the nervous system of D. pseudospathaceum is one of the most extensively studied among digeneans, there are still gaps in our knowledge regarding the distribution patterns of some neuroactive substances, most notably neuropeptides. The present study addresses these gaps by studying pre-infective metacercariae of D. pseudospathaceum using immunochemical staining and confocal microscopy to characterize the distribution patterns of serotonin (5-HT) and two major groups of flatworm neuropeptides, FMRFamide-related (FaRPs) and substance P-related (SP) peptides. The general morphology of the nervous system was examined with antibodies to alpha-tubulin. The nervous system of the metacercariae was shown to conform to the most common morphology of the nervous system in the hermaphroditic generation, with three pairs of posterior nerve cords and four pairs of anterior nerves. The patterns of FaRP- and 5-HT immunoreactivity (IR) were similar to those revealed in earlier studies by cholinesterase activity, which is in accordance with the known role of these neurotransmitters in controlling muscle activity in flatworms. The SP-IR nervous system was significantly different and consisted of mostly bipolar cells presumably acting as mechanoreceptors. The architecture of the nervous system in D. pseudospathaceum metacercariae is discussed in comparison to that in cercariae of D. pseudospathaceum and metacercariae of related digenean species.

Possible mechanism of host manipulation resulting from a diel behaviour pattern of eye-dwelling parasites?

Parasitic infection often results in alterations to the host's phenotype, and may modify selection pressures for host populations. Elucidating the mechanisms underlying these changes is essential to understand the evolution of host–parasite interactions. A variety of mechanisms may result in changes in the host's behavioural phenotype, ranging from simple by-products of infection to chemicals directly released by the parasite to alter behaviour. Another possibility may involve parasites freely moving to certain sites within tissues, at specific times of the day to induce behavioural changes in the host. We tested the hypothesis that parasites shift to certain sites within the host by quantifying the location and activity of the trematode Tylodelphys sp., whose mobile metacercarial stages remain unencysted in the eyes of the second intermediate fish host, the common bully (Gobiomorphus cotidianus). This parasite's definitive host is a piscivorous bird feeding exclusively during daytime. Ocular obstruction and metacercarial activity were assessed within the sedated host's eye at three time points 24 h−1 period, using video captured via an ophthalmoscope. Although observed metacercarial activity did not change between time periods, ocular obstruction was significantly reduced at night. Increased visual obstruction specifically during the foraging time of the parasite's definitive host strongly suggests that the parasite's activity pattern is adaptive.