6 Chapter 6: Summary of the Digenea

General

Numerous species of digeneans exist, and, with proper attention, students can find infections in most vertebrate hosts although infection depends on habitats and general prevalence. There are about 150 families, and identification at the superfamily, family, subfamily, and genus levels of members can be best accomplished by using the 3 volume keys entitled Keys to the Trematoda (Gibson et al., 2002; Jones et al., 2005; Bray et al., 2008) or updated articles on specific genera or groups.  Digeneans are especially powerful for students to study because they come in all sizes and shapes; they exhibit a variety in feeding, reproducing, moving, and surviving; some can be harmful to their hosts, and, consequently, many have economical or medical importance. The taxon provides laboratory features that can utilize a variety of tools to investigate them.

A clear understanding of the biology of most digeneans remains poorly understood because of their lack of medical importance; little actual work has been done on the wildlife infecting species that are of little medical or veterinary importance. For example, most emphasis for intense study of trematodes has been focused on blood flukes, however, blood flukes are not the normal run-of-the-mill trematodes that comprise 95% of the species that exist. The blood flukes are truly exceptional because some groups even have separate sexes.

Phylogeny and Classification

Olson and colleagues (2003) combined their sequences from various digeneans to develop species-level phylograms based on Bayesian inference of combined data, ssrDNA + lsrDNA, and a revised classification based on the phylograms showing relationships among the different higher level taxa. Since that time, information gaps have been filled and much more is known about the relationships among families, genera, and species. A few general updates on methods and relationships followed (Nolan and Cribb, 2005; Olson and Tkach, 2005) and many have added to and sorted out the relationships.

Before the availability of marvelous molecular tools, various researchers used morphological and developmental means to show those relationships. Some were highly inaccurate, though a relationship tree developed by Cable (1974) was unexpectedly close. Cable’s accomplishment is truly amazing when one finds that worms that appear very similar morphologically are not phylogenetically related. Cable also showed convergent evolution of distantly related Microphallidae, Heterophyidae, and Fellodistomatiidae. On the other hand, worms that appear distinctly different may be shown to be closely related, as established by molecular means. Clearly, improvements in molecular tools, including entire genomes, will open new doors. They will also allow researchers to much better understand the biology and history of the digeneans.

Life cycles also have been used to assess the evolution of digeneans. A discussion of the importance of morphological features of adults and cercariae in understanding the phylogeny of digeneans occurs elsewhere in this chapter. Sinitsin (1931) and others, who considered that digeneans originated as gastropod parasites, were challenged by Heyneman (1960) who considered flatworms evolved from dalyelloid rhabdocoels. Cribb and colleagues (2001b, 2003) critically examined the nature and evolution of digeneans, looked at Diplostomida and Plagiorchiida separately, and still could not be definitive about how the complex cycle arose and how variation within the group evolved. Is a gastropod or vertebrate the primary original host? There is still lots of good reading such as that by Pearson (1972; 1988; 1992), who presumed that digeneans evolved from free-living rhabdocoels with a mollusc first origin, Cable (1965; 1974; 1982) and Gibson (1987) to compare with the molecular data that places the major helminth taxa (Trematoda, Monogenea, and Cestoda) and minor ones (Gyrocotylidea and Amphilinidea) together as the monophyletic Neodermata (Littlewood et al., 1999a; 1999b). That monophyly allowed the use of parsimony but has not definitively settled the origin of digeneans.

LaRue (1957) established a grouping based on embryological aspects of the excretory system of cercariae, which, with modifications, is similar to the accepted scheme used today. His suborders Anepitheliocystidia and Epitheliocystidia are no longer accepted because the cellular structure of the excretory vesicle as assumed by light microscopy for some members of Epitheliocystidia was shown to be a syncytium with transmission electron microscopical evaluation, the development of the excretory system in the tail was not clear cut, and most important, these features do not fit an acceptable phylogeny.

Classification and phylogeny evolved into using morphological adult characters and characteristics, and characteris- tics of all life stages combined to produce cladograms from cladistic analysis (for example, Brooks et al., 1985; 1989; Brooks and McLennan, 1993). Classification developed further by utilizing phylogenetic relationships determined from genetic sequences of specific genetic fragments (Tkach et al., 2000; Olson et al., 2003). The 3 publications by Brooks and colleagues pointed out the ambiguous data for the unresolved Plagiorchiata and tried to better establish mem- bers of the clade. Tkach and colleagues (2000) considered those works valuable and an important basis for other in- vestigations. In fact, those molecular works supported some of the conclusions and straightened out other relationships.

Molecular analyses during the next 2 decades have clarified the higher level digenean taxa. Presumably, this classification will be perfected even more by using entire genomic sequences in the near future. Nevertheless, many articles on specific groups have added to or corrected the earlier phylogram of Olson and colleagues (2003). For example, Overstreet and Curran (2005a; 2005b) classified the haploporoids, all known to infect fishes only, based on morphological features. But once they collected and analyzed molecular sequences, they straightened out several aspects of the early classification (see the classification of Haploporoidea elsewhere in this chapter).

Classification

Littlewood and colleagues (2015) updated Olson’s work from the point of view of diversity, showing that at that time there were 24 major groups (superfamilies) of Digenea, with 150 families, 1,777 described genera, and 12,012 described species. Not all families include any sequenced individ- ual, and, in most groups, fewer than 5 or 10% have been se- quenced. Looking at the numbers from a different point of view, Bullard and Overstreet (2008) estimate that they amass the largest group of monozoic plathyhelminths, perhaps about 18,000 nominal species, with fishes hosting an astonishing number of digeneans. Considering there exist about 27,977 extant fish species, accounting for just over half of all living vertebrates, and considering the number of new digeneans named yearly, with approximately half the species being named and examined for digeneans, the number of digeneans infecting fishes will soon probably exceed the number of fish species. Moreover, most sequenced species from fishes represent fewer than 5% of the known members in their representative families (Littlewood et al., 2015).

Host-Digenean Relationships

Host-digenean relationships typically result in little pathological alteration, but when a cycle necessitates the intermediate host to be attractive to the definitive host, pathological alterations can be involved. In numerous cases, a definitive host may respond heavily to a parasite, but it usually relates to an abnormal/accidental host for the parasite or a condition that helps complete the cycle, like in aquaculture discussed later. When a large number of individuals harms the host, this relationship is usually referred to as a disease rather than an infection. For example, Lumsden (1979) reports a fibrotic response to an egg of Schistosoma mansoni in a mouse liver, a condition that can allow the ultimate passage to the intestinal lumen to be passed externally and does not cause disease unless heavily infected. He also shows an electron-micrograph of Paragonimus kellicotti in a cat lung. Responses occur around eggs and the worm; apparently if just 1 specimen obtained from a crayfish infection gets in a lung, it will migrate searching for a mate and create extensive host cellular response and, if the mate is found, the pair becomes protected from further response by fibrotic encapsulation (Lumsden and Sogandares-Bernal, 1970). Many cases of pathological responses are shown to occur in humans (for example, Beaver et al., 1984; Ash and Orihel, 2007) and wildlife (for example, Takashima and Hibiya, 1995; Randall and Reece, 1996; Jacobson, 2007).

In some cases, large numbers of cercariae penetrating into a host or of miracidia initially penetrating and then migrating within a host can cause pathological alterations or even kill the host (for example, Bullard and Overstreet, 2002; 2008). In the case of cercariae of Diplostomum sp., its odors did not have an effect on juvenile rainbow trout; however, when odors— alarm substances—from infected juvenile fish encircled free cercariae, the number of penetrations and length of time spent motionless by the cercariae increased (Poulin et al., 1999).

A different diplostome, Ornithodiplostomum ptychocheilus, acts differently in the brain of its fathead minnow host (Matisz et al., 2010). The cercaria can reach the brain within 3 hours by using different nerves, it then utilizes specific nerve tracts to reach the outermost tissue layer of the optic lobes where it grows for 4 weeks, and finally shifts its location to the adjacent meninges where it encysts. Associated with the shift in location occurs a massive inflammation, which lasts about 9 weeks and affected the health of the fish, with the amount apparently depending on the intensity of infection.

Hyperparasitism

A hyperparasite is a parasite that occurs either in or on another parasite. Dollfus (1946) reviewed literature at that time on hyperparasites of helminths as well as added further information. Examples include an ectoparasite copepod attached to the hemiuroid Derogenes varicus in the buccal cavity of what is known today as the American plaice Hippoglossoides platessoides located in Northumberland, United Kingdom, and studied by Marie Lebour.

In the pharyngeal cavity of a puffer near Woods Hole, Massachusetts, United States, a trichodinid ciliate was noted by Edwin Linton infesting Lintonium vibex. Probably more likely not accidental are a multitude of internal ‘protozoans.’ He covered a variety of microsporidians from all stages of trematodes, including members of a few different families; a few haplosporideans, a flagellate, opalinids from amphistomes in frogs, and even a nematode. Canning (1975) provided more information on the same and additional microsporidians. She also described others, and Overstreet described yet more with Yuliya Sokolova (Sokolova and Overstreet, 2018; 2020).

Overstreet (1976b) found a flagellate species of Hexamita in the cecum of Crassicutis archosargi different than one from an acanthocolpid by Hunninen and Wichterman (1938) and others mentioned (Overstreet, 1976b). Overstreet has searched for ciliates and flagellates in digeneans from her- bivorous fish like mullets and rabbitfishes without success, but opportunities exist for future researchers.

The myxosporidian Fabespora vermicola infects C. ar- chosargi (see Overstreet, 1976a) and probably more digeneans will be infected by myxosporidians. A microsporidian has even been described from a myxosporidian in a rabbitfish (Diamant and Paperna, 1985). The haplosporidean Urosporidium crescens infects cercariae and metacercariae of microphallids in grass shrimp and the blue crab causing a condition called blackspot when the metacercariae become greatly hypertrophied (see for example, Overstreet, 1978; 1983).

Whether an accidental infection or not, Graham (1969) reported an alarid mesocercarium in Styphlodora magna. Overstreet has often witnessed these mesocercariae rapidly invade helminths in a stender dish containing saline, but has never seen an infected helminth when immediately transferred into saline.

Bacterial infections can occasionally be seen in digeneans. Overstreet has often seen the Brownian movement of a bacterium in the excretory vesicle of some haploporids from mullets. He tried unsuccessfully to obtain and culture specimens with a drawn out capillary tube and regular tryptic soy broth. Others are encouraged to use a similar technique with a micro-manipulator and a combination of different culture media and sequencing procedures.

As discussed elsewhere in this chapter (Curran and Over- street, 2004; Bullard and Overstreet, 2008), the diplostomatid Bolbophorus damnificus has caused millions of dollars of loss of cultured catfish annually. Infections can be asso- ciated with nephrotic pathological alterations in the catfish host. However, when as few as 4 metacercariae of B. dam- nificus are experimentally hyperparasitised by the bacterium Edwardsiella ictaluri, the commercial channel catfish (Ictalurus punctatus) died (Labrie et al., 2004). About 10% died by day 8 and cumulative mortality of 85% by day 21 com- pared with 45% mortality when exposed with the bacterium only (without the digenean) and 0% with controls and just the digenean group at day 21. Other studies reveal that different bacterial strains and different fluke genotypes influence host mortality, and interactions affect virulence and host health in surprising ways (Louhi et al., 2015).

A Few Notes on Ecological Methods in Parasitology

Although ecological studies take a long time to complete, they attract a lot of students and their mentors. With careful planning, a parasitologist can accompany an entomologist, ichthyologist, mammalogist, ornithologist, or herpetologist, and gather material—hopefully fresh—so it can be examined under a microscope and fixed properly. Of course, the parasitologist will probably spend the days collecting hosts and the nights collecting parasites. Studies can involve those parasites inhabiting specific hosts, those comparing infections in different hosts or the same host, or hosts in different localities, or under different conditions.

For a chapter on patterns and processes in parasite communities, Esch and colleagues (1990a) introduced the his- torical aspects by saying that perhaps most ecological parasitologists agree that the earliest body of ecological studies was conducted by the Russian academician V. A. Dogiel and colleagues (for example, 1966), that H. D. Crofton (1971a; 1971b) introduced quantitative approaches to population dynamics, and that J. C. Holmes (for example, 1979) initiated a quantitative approach to helminth community dynamics. That chapter (Esch et al., 1990b) and other books (for exam- ple, Combes, 2001; Bush et al., 1997; 2001) can be used separately or in conjunction to understand terms and approaches. There exist a variety of books and publications that treat different aspects of ecology. For example, Poulin and Morand (2004) wrote a good general book on parasite diversity and models. Chapters should encourage readers to ask themselves many questions regarding their research and course topics. Diversity of trematodes in freshwater fishes is poorly understood and requires more research (Choudhury et al., 2016).

General Digenean Ecology

Marcogliese (2004) presented an opening address to a group of fish researchers entitled “Parasites: Small players with crucial roles in the ecological theater.” He told how parasites could have pronounced or subtle effects on the behavior, growth, fecundity, and mortality of the host as well as regulate host population dynamics and influence community structure.

Digeneans seldom kill their definitive host. They occasionally harm their intermediate hosts but seldom kill them unless the hosts are being reared, such as in aquaculture. The majority of the commercial channel catfish Ictalurus punctatus grown in the United States comes from ponds in Mississippi. Eggs from Bolbophorus damnificus are deposited with the feces of its host, the American white pelican, into the ponds where the pelican feeds on the catfish along its flyway. Snail intermediate hosts in the shallow water of the ponds along their borders obtain heavy infections and produce very large numbers of cercariae. Consequently, since fingerling catfish occupy the shallow water, they become heavily infected, and losses of over US $10 million in catfish have occurred annually. As discussed elsewhere in this chapter, hyperinfection with the bacterium Edwardsiella ictaluri can kill the catfish when only 4 metacercariae occur per fish. Normally, the hyperparasitized metacercariae kill the fish. The surviving fingerlings usually have about 40 to 50 metacercariae per fish, suggesting that more—and there can be hundreds—kill the fish intermediate host (Overstreet and Curran, 2004; Bullard and Overstreet, 2008). Infected fish often have necrotic kidneys.

In addition to the adult of Bolbophorus damnificus in the American white pelican occurs a cryptic species, Bolbopho- rus sp., often just a few centimeters away in the same individual bird’s intestine. That digenean uses sunfishes and Gambusia spp. as a second intermediate host, and it readily kills them in the same ponds (Overstreet et al., 2002).

As shown elsewhere, infected hosts serve as indicators of many biological activities as well as historical biogeography and phylogenetics (Brooks and Hoberg, 2000). Parasites can indicate trophic interactions over weeks or months as opposed to 24 hours or less when analyzing gut contents. When mullet fry enter the estuary from offshore plankton, the parasites reflect a copepod diet, but when the same sized fry is sampled from the nearby bottom, it adds haploporid trematodes acquired by feeding on the bottom (Paperna and Overstreet, 1981).

On the basis of 1 short collecting trip, Bush and colleagues (1993) collected metacercariae from 2 crab species from a small key in the Florida Keys, found that 1 crab species had 5 different microphallid species, and a few individuals of the other crab harbored 1 microphallid clumped in masses of a few thousand. They suggested that a single definitive host bird briefly feeding on the first crab species may be colonized by 6 different species and that the infrapopulation can increase rapidly by feeding on the other crab species. Consequently, understanding colonization processes in definitive hosts may be a critical underpinning to many community level studies. Consequently, community-level studies on in- vertebrate hosts (intermediate hosts as source communities) may be easier and more informative than conducting such studies on definitive hosts.

Long term studies on 1 or more parasites are important in understanding many aspects of ecological relationships. Esch and colleagues (1986) and Marcogliese and colleagues (1990) investigated Crepidostomum cooperi in the burrowing mayfly for 16–20 years and determined the dynamics were driven by eutrophication.

Digeneans as Indicators Feeding Behavior of Hosts

The same approach can provide information about feeding habits and other biological parameters of the hosts. For example, studies on parasites of 21 species of grebes worldwide by Storer (2000), and those by Overstreet and Curran (2005c) investigating the American white pelican and brown pelican, relate digeneans and other parasites to specific feeding habits. Both studies also show how the digeneans, digenean hosts, and other parasites show evolution of the hosts, evolution of the parasites, health of bird hosts, health of intermediate hosts, public health risks, migratory patterns, and other aspects.

Variations in results from sampling hosts for digeneans obviously differ when the presence of necessary hosts differ. However, when compared ecosystems have variation in temperature and other environmental factors, the prevalence of infection (percentage of hosts infected divided by those examined in a sample) and mean intensity of infection (the number of a specific parasite divided by the number of hosts infected by the specific parasite) may also exhibit variation. Note that high prevalence and mean intensity of the digeneans indicate a healthy host and environment.

Collections made during different seasons from the same locations will usually reflect differences in infections of some of the parasites, depending on the longevity of the infection and other factors. There are also unusual conditions such as collections from near a nuclear power plant discharging hot water. Cercariae shed a month earlier in that water than those not in the heated location (Höglund and Thulin,1988).

Overstreet (1993) discusses a variety of natural and anthropogenic cases involving temperature as well as other environmental factors on host-parasite relationships. Another example reveals dynamics of infections of Metadena cf. spectanda in the Atlantic croaker Micropogonias undulatus during subsequent similar seasons. This worm may be the same as Metadena spectanda in Brazil (Overstreet, 1971a); however, sequencing a few Brazilian specimens and comparing them with the larger specimens from Mississippi will probably show that the specimens from the northern Gulf of Mexico represent a new species. Both the prevalence and mean intensity reached high values in the early 1970s in Mississippi, United States. The fish fed on a wide variety of prey, but crustaceans, annelids, molluscans, and small fishes served as the principal diet, at least in inshore water (Overstreet and Heard, 1978). Prevalence of infection with M. cf. spectanda became increasingly higher in fish over 60 mm-long (standard length) demonstrating when the croaker fed more on fishes. These trematode infections probably differ seasonally and annually because when the temperature and salinity are high, anchovies are abundant, and they are a favorite prey for the croaker but not a host of the trematode. In contrast, when the salinity and temper- ature are low, anchovies are rare or absent, and the croaker is more energy efficient when searching out gobies as their fish prey. A few different gobies serve as the second intermediate host for M. cf. spectanda, and during these periods, the croaker served as a super host for that parasite (Overstreet, 1973; 1982; personal observations).

Feeding studies provide a good background for studies dealing with indicators, zoogeography, diversity, and other fields. In a presentation at a symposium, Marcogliese (2003) asked whether parasites were the missing link to food webs and biodiversity. He also pointed out the need for integrating several disciplines (as was done in classical parasitology) and how these fields are no longer highly regarded. This is a shame considering the importance of using dige- neans as indicators (see, for example, Gibson, 1972; MacK- enzie et al., 1995; MacKenzie, 1999; 2002; Marcogliese and Jacobson, 2015).

Digeneans, especially when in combination with nematodes, represent an ecological link between mesozooplank- ton and relatively large pelagic animals (Noble, 1973; Campbell, 1983; 1990; Marcogliese, 1995; Klimpel et al., 2010; Andres et al., 2016a).

Health of Ecosystems (Including Toxicology)

Using digeneans as monitors of environmental health requires selecting the appropriate animal host to study. Considerable work has been conducted with fish model systems (for example, Overstreet, 1997). Criteria for a good fish model include having a restricted home range, serving as host for a relatively large number of digenean species, and being common and easily sampled. Depending on how good a model fish is will determine whether it will answer questions and solve problems. Additional features are usually needed to support and refine a study such as parasites other than digeneans, histological findings, or genetic markers.

Overstreet (1997) used the western mosquitofish Gambusia affinis in Mississippi as an indicator of parasitism because it was host for many different metacercarial and other larval species that showed that the environment contained many specific teleost fishes, birds, mammals, turtles, snakes, the alligator, as well as many specific gastropods and bivalves. It shows this because specific harsh conditions can eliminate 1 of those specific hosts (break a link in the parasite life cycle) and consequently the associated digenean. He determined that metacercaria of many different species remain in the fish for periods of over a year. Consequently, the relative number of animals in the environment can be determined by sampling the model fish just once or maybe twice a year, whereas sampling the biota requires numerous collections and a variety of biologists to identify the different animals.

If all the parasites in the model in addition to the digeneans are sampled, the number of non-parasitic invertebrate and vertebrate hosts in the ecosystem can be detected, making assessing parasitic data much more economically valu- able than sampling animals monthly or bimonthly because many of those animals may remain in the environment for just a short time. Of course, reference stations are necessary for comparisons. When trying to evaluate specific areas, a variety of similar locations containing the model fish with and without the suspected conditions have to be sampled as those reference locations.

Anthropogenic contaminants

Anthropogenic contaminants can act in a distinct manner relative to host, parasites, and each other as well as being influenced by natural environmental conditions. When a sample of a specific fish host from a specific area exhibits a lower number or mean intensity of 1 or more digenean species than in samples from nearby localities, that finding suggests contamination. Further assessment of the samples for bacterial contamination, histopathological alterations, and other parasites can often pinpoint the source of contamination. Multiple samples of the western mosquitofish from one Back Bay, Mississippi, United States location designated as a superfund toxic clean-up site revealed a low prevalence and mean intensity of digeneans compared with samples from reference sites. In another nearby site contaminated with specific chemicals used to treat timbers, a low number of only 1 of the local digeneans occurred, and a myxosporidian with associ- ated histopathological alterations was also unique to that location. When a live sample from that location was transferred to a laboratory and reared, about 50% died from the myxosporidian infection. No fish from 2 of the reference sites died or exhibited the infections when reared concurrently (Overstreet, 1997).

In another example from a Texas river in the United States using the same fish model, the same group of researchers determined that contamination occurred upstream from an integrated pulp and paper mill effluent canal, primarily on the basis of the mean intensity of a digenean metacercaria, which was most prevalent in the effluent canal, and invasion of a usually free-living ciliate and macrophage aggregates in the spleen, both of which occurred at the upstream location. The effluent canal, which had been accused of being a toxic site because of the coffee-like appearance, gave the impression of being the healthiest of the 5 sampled locations (Over- street et al., 1996).

In another study, Sun and colleagues (1998; 2009) were charged with assessing a large number of sampling locations along 2 contaminated rivers in southern Taiwan. As it turned out, because of the pollution, only fish species and hybrids of tilapia could tolerate the rivers and no intermediate hosts of expected parasites could tolerate the conditions. Results had to be obtained from the amount of morphological and histopathological abnormalities in the fishes.

Bioaccumulation

In addition to parasites indicating the presence of toxicants in the ecosystem, parasites can also concentrate toxins from host tissues. Sures (2001) reviewed this problem in fishes where helminths, primarily acanthocephalans and secondarily cestodes and nematodes, can concentrate numerous heavy metals to concentrations several orders of magnitude higher than those in host tissues or the environment. Most digeneans do not concentrate as much as other helminths, but Fasciola hepatica inhabiting the bile ducts of cattle has been shown to accumulate lead concentrations 172 and 115 times higher than values in muscle and liver, respectively (Sures et al., 1998). Perhaps this occurs because lead binds to the erythrocytes, is transported to the liver where the majority of lead is stripped from the blood, and is excreted into the intestine by means of bile. Apparently the site of F. hepatica with high concentrations of lead allows the worm good access to it. As a point of interest, this ability of many helminths protects hosts from acquiring too high of concentrations of many heavy metals shows that parasites/digeneans can be good guys!

Catastrophes

By using similar methods for determining biological richness, Overstreet (2007) sampled a variety of locations and known hosts continually for digeneans after a hurricane to assess habitat recovery. Hurricane Katrina in August 2005 reached gusts of 433 km/hour and surges penetrating 20 km inland along bays, rivers, and bayous of coastal Mississippi, Louisiana, and Alabama in the United States. Resulting devastation covered a landmass of about the same size as that of the island of Great Britain, United Kingdom. They investigated a variety of situations involving hurricanes, but regarding digeneans, they noted how long it took various digeneans to become reestablished following Hurricane Katrina. Loss of biota resulted from perturbations of sediments and surge of high salinity water into estuarine and freshwater habitats. Clay and sandy sediments were lost from some areas and added to others, with the storm’s energy being most influential offshore and at a depth of 25–30 m, where 1 m of sediment was scoured from the bottom and re-suspended, with the corresponding loss of the infauna. The surge of over 9 m in some locations with water of 32 ppt replacing water of 15–0 ppt saline, flushing out and killing nearly all of the biota.

The reader must keep in mind that it may take 1 or more years for the invertebrates serving as intermediate hosts to become reestablished and additional years for those invertebrates to become infected by their digenean parasites. Of course, migrating fishes that acquire infections in Texas or Florida in the United States do not show a loss of infections nor do local fishes that migrated to avoid the effects of the storm. In the latter case, the authors considered reestablishment as infections in juvenile fish that had not been born until after the storm.

By the time of the first scientific presentation on reestablishment (Overstreet, 2007), only a few fish species became infected, and with a low mean intensity of digeneans. Sampling continued, and updated results on specific digeneans and other parasites were presented at various scientific meetings, and finally the compiled data were published (Overstreet and Hawkins, 2017), showing that reestablishment can take a short period for some species and many years for others.

Climate Change

Parasites, and digeneans in particular, allow researchers to investigate large scale events. Since change takes place over evolutionary and ecological time scales resulting from natural and anthropogenic causes, Marcogliese (2001) considered temperature and parasites of boreal regions of North America as a good focal point for investigations of climate change. Because different hosts in a cycle follow range constrictions, the presence of a parasite will also become modified in unpredictable ways since the host-parasite systems are intricately interwoven with the environment, and changes in physical processes at different temporal and space scales will affect parasite populations differently.

Introduced Species

Occasionally when a megafaunal organism becomes introduced outside its typical location, other organisms are in- cluded or the range of the organism spreads. Also, a parasite can be included in the transfer or spread. As an example, tropical fishes are reared in outside facilities and are shared with other growers. This has happened probably on numer- ous occasions and has involved vegetation and the invasive snail Melanoides tuberculatus (common name, red rim melenia). The snail was introduced by the 1970s into southern Florida, United States (Roessler et al., 1977). Unfortunately, the heterophyid Centrocestus cf. formosanus infects the snail and follows it around the southeast United States, and probably elsewhere. This parasite has an unusual characteristic of promoting proliferation of cartilage surrounding the metacercarial cyst, usually in the gills of the host. This abnormal proliferation occurs extensively in a few of the many fishes the cercariae can infect. Some of the fishes are rare, such as the federally listed endangered fountain darter Etheostoma fonticola, which is highly susceptible to and easily killed by the infection. Mitchell and colleagues (2005) reported on the history of the introduction and the life cycle of the worm.

Digenean species that had once been considered to be introduced are occasionally determined by molecular comparisons to be sister species. For example, what had thought to be Bolbophorus confusus introduced from Europe appears to be B. damnificus or other Bolbophorus species (Overstreet et al., 2002).

Migration of a Model Host

Using parasites of pelicans and grebes as a variety of indicators, including migration, was mentioned elsewhere in the chapter. Most species of these are useful to examine because they host many digeneans. However, the use of digeneans and of other parasites has also been very useful for determining migration of fish hosts and stock separation. For example, Blaylock and colleagues (1998) examined Pacific halibut from 15 localities from northern California to the northern Bering Sea for all parasites, including many digeneans. The fish clustered into 3 groups on the basis of parasites, and these depended on temperature and geography, features that have a large effect on digeneans. These and associated data suggest 3 separate stocks of this commercially important fish.

Host Stocks

Not all fishes make good models for using parasites to separate or distinguish fish stocks, and often digeneans do not provide the best parasite indicator. The sablefish Anoplopoma fimbria off Canada’s west coast is an example of a good model (Kabata et al., 1988). These fish contained 7 digeneans and their prevalence, mean intensity, relationship with host age, and locations (13) differed enough to show the seamount and slope host populations constituted separate stocks. That development of the localized fisheries provided a significant yield to Canadian fishermen.

There are other cases where salmonids infected with a single freshwater digenean species allow researchers to know from which specific freshwater sources the infected individuals arose. This may be because the digenean has a lengthy longevity in both freshwater and marine phases, such as the metacercaria of Nanophyetes salmincola and adult of Plagioporus shawi in juvenile trout from the United States Pacific Northwest, for example.

Dalton (1991) reported tagged steelhead trout 5,000– 5,500 km from their area of origin in the central North Pacific Ocean. Monitoring of chinook salmon smolts from the Trinity River, California, United States detected annual differences, possibly because of differences in temperature and the resulting shed of cercariae (Foott et al., 1997). In this study, fish and snails were placed in a shallow trough, and 10 fish were examined and sectioned. In a wet mount of the most infected tissue, the mid-kidney, the most infected individual contained 10,220 cysts/gram, and the mean number of metacercariae in sections of the posterior kidney was 28.0 ± 14.7. The Puget Sound Steelhead Marine Survival Work- group (Berejikian et al., 2018) reported abstracts on various projects on Nanophyetes salmincola, including cumulative mortality of fish at 46 days in seawater (mortality leveled at 7% after day 12 for infected individuals versus 0% for unin- fected ones), susceptibility of waterborne cercariae to chemo- therapeutics (100 ppm hydrogen peroxide, Perox-Aid^®, and various doses of formalin), plus others.

Detective Work/Forensics

Many of the findings resulting from using parasites, primarily digeneans and other helminths with complicated life cy- cles, as indicators can be considered detective work. However, some cases clearly can be defined as detective work in the literal sense. An example concerns a truckload of red drum Sciaenops ocellatus that had been stopped and examined by dif- ferent authorities, including United States Customs officials. The fishermen operating the truck said the fish, which they planned to sell, came from the Carolinas, from where the catch would have been legal. Professionals had Overstreet examine a sample of the fish, and he found a bucephalid endemic to the northern Gulf of Mexico where limits and seasons were stricter than along the Atlantic coast. Neither Overstreet nor other researchers who had examined the red drum from the Carolinas found any infection with that worm. That evidence was used to find the fishermen guilty of illegally catching and trying to sell Gulf fish (Overstreet et al., 2009).

Ichthyologists considered the Pascagoula River in the late 1960s to be free of striped bass. Consequently, a few hatch- ery-reared individuals fed commercial feed were released in the area, and 1 year later Overstreet (1971b) discovered several specimens of a new digenean species, Neochasmus sogandaresi, in a specimen of the fish. Then and later, a great deal of effort was unsuccessfully spent trying to see if the parasite also occurred in another host. None was discovered, suggesting that a small wild stock of striped bass had occurred in the area and represented at least enough striped bass to maintain a population of the digenean.

Digeneans as a Human Food Source

Numerous books and articles treat public health. Overstreet (2003) took the other point of view. He wrote about people eating parasites on purpose, with the assumption that there was no public health risk. For example, different people eat, or have eaten in earlier times, the giant liver fluke of various species of deer, Fascioloides magna: Hunters eating what they call little livers, Cajuns eating double-fried puffed flukes, Native Americans of the southeast United States eating what they call little flapjacks, and some members of the Sioux Nation in North America eating them and other liver flukes as a portion of their game or domesticated mammal with the intention to transfer the life force. Some Indigenous people in Africa eat the paramphistomes from the stomach lining of hippopotamus calling them the juicy part of the hippo, and members of the tribes of Meghalaya, India relish paramphistomes from the rumen of cattle and buffaloes. Lots of parasites other than flukes are eaten fresh or cooked with smiles on the face of the consumers.

Acknowledgements and Disclosure

The author thanks Jean Jovonovich and Janet Wright for their tireless help with the references and reading over portions of the text. Some of the investigations described in this section were supported in part by a grant from BP Exploration and Production, Inc.

 

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