The class Trematoda is the largest group of Platyhelminths and includes two subclasses: Aspidogastrea and Digenea. Trematodes, and particularly Digeneans, is a large group of organisms with significant medical and veterinary interest. Over 100 species of digenetic trematodes have been reported infecting humans. Although the significant mortality and morbidity that some of these infections cause, they are among the most neglected tropical diseases. Apart from their impact in public and animal health, the Digenea constitutes an intriguing group of organisms that has a vast interest in experimental biology. Systematics and taxonomy of this group constitute a challenge for biologists in relation to the difficulty entailed in the establishment of phylogenetic relationships between trematodes and the determination of valid diagnostic features. Moreover, their complex life cycles, using at least two hosts and alternating free-living and parasitic stages or sexual and asexual multiplication, constitute a paradigm of how organisms can evolve to become adapted to different biotic and abiotic environments to enhance survival. In this review, we briefly summarize the major features of trematodes in relation to both biological and medical areas.
The class Trematoda constitutes a large group of Platyhelminthes parasites that is divided into two subclasses: the Aspidogastrea and the Digenea. The Aspidogastrea, also called aspidobothrea, is a small group comprising 4 families, 12 genera and ∼80 species. In general, they are parasites of freshwater and marine mollusks, and cartilaginous or bony fishes and turtles may serve as facultative or obligatory hosts. Commonly, Aspidogastrea are external parasites of these hosts and have a single-generation life cycles lacking asexual reproduction. Adults are conspicuous for having a large ventral disc often divided into several compartments. There are no species of Aspidogastrea with medical or economic importance. However, further information can be found in the works by Prof. Klaus Rhode [1,2].
In contrast, the subclass Digenea is a markedly larger group and has a significant medical, veterinary and economic importance. Digeneans, also called digenetic trematodes or flukes, comprises more than 18 000 species that are obligatory parasitic in invertebrate intermediate and vertebrate definitive host . Owing to their greater importance, this review will be focused on the subclass Digenea. Most digenean species inhabit the intestine as adults, but several species colonize other habitats within the vertebrate host. Digenetic trematode infections occur worldwide and over 100 species have been recorded infecting humans causing serious problems in them. For example, it is estimated that almost 300 million people are currently infected with Schistosoma spp. causing ∼6,000 deaths annually . Moreover, a large list of digenetic trematodes actually parasitize animals both of veterinary interest and wildlife.
Apart from their medical and veterinary importance, digenetic trematodes have a vast interest in experimental biology due to many characteristics of these parasitic organisms: (i) the paradigmatic systematics of this group of parasites in relation to the morphological similarity between members of different taxa and the inadequate or poor specific diagnosis (or both) of several newly established taxa. Furthermore, the absence or loss of type material for several taxa further complicates the situation. As a result, this group has been characterized by a long history of synonymies and inadequate diagnosis. (ii) The complexity of the life cycles that has led to the development of an important number of adaptive strategies to enhance parasite survival and transmission converts these organisms to an excellent model to study the evolutionary adaptation of an organism to an environment.
Taxonomy and functional morphology
The taxonomy of the Digenea has long been a challenge due to the difficulties in establishing relationships and diagnostic features, even for the higher taxa . In fact, more than 2500 genera have been historically accepted within this group. Relatively recently, an important effort was done to clarify the systematics of this group by Dr Gibson and colleagues. This work was reflected in a series of three volumes (Keys to the Trematodes) that exhaustively review the taxonomy of these parasites [5–7]. This relevant piece of work provides historical background and novel approaches for the systematics and taxonomy at the higher taxa and a re-appraisal of the generic diagnosis. In fact, only 148 families with 1577 genera were considered valid. In spite of this detailed work, there are still several gaps in our knowledge on this topic and further effort is required. In this context, molecular analysis may be key to achieve an improved picture of the digenetic taxonomy.
Figures 1 and 2 show details on the morphology of adult Digenean Trematodes. Digenetic trematodes have a dorsoventrally flattened body, bilateral symmetry and a definite anterior end. They are leaf-shaped and may range in size from a few millimeters to ∼8 cm (i.e. Fasciolopsis buski). The form and function of major organ systems of the digenetic trematodes has been recently reviewed by Peoples and Fried .
Organography of an adult Trematode.
Microphotographs of Trematodes with details on several organs.
The body surface of flukes comprises a syncytial tegument. The epidermis comprises a distal anucleate cell layer. Cell bodies containing the nuclei lie beneath a superficial layer of muscles, connected to the cytoplasm by several channels . Moreover, tegument can be covered by spines and also molecules released by the worm can be found . Adult flukes possess an oral sucker around the mouth and a ventral sucker or acetabulum that can be used for the attachment to the host tissues (Figures 1 and 2). Apart from adult stages, life cycle of trematodes may include up to six larval stages known as egg, miracidium, sporocyst, redia, cercariae and metacercaria. In some of these larval stages, some particularities can be found. The miracidium is covered by ciliated epithelial cells that are shed during transformation to either sporocyst or redia. Early embryos of cercariae are covered with a primary epidermis, and the definitive epithelium is formed underneath of this primary epidermis. The wall of the metacercarial cyst is formed by a material released from the cystogenic cells found within the parenchyma of the cercariae. Recently, it has been shown that several trematodes are able to produce secretomes that are actively secreted containing molecules of interest in communication with the host . The nervous system consists of longitudinal cords connected by transverse ring commissures . Moreover, flukes possess a variety of sensory adaptations including photosensory, chemosensory and mechanosensory [11,12]. Commonly, muscles are circular lying beneath the basal lamina of the tegument in lateral areas.
Internal organs are embedded in connective tissue parenchyma. Flukes have an alimentary canal including pharynx and esophagus. The intestine is commonly a branched tube. The main branches may end blindly or open into an excretory vesicle. The excretory vesicle also accepts the two main lateral collecting ducts of the excretory system which is of a protonephridial type with ciliated flame cells. These cilia facilitate excretion of fluids through an excretory pore, commonly located at the posterior end .
Most trematodes (excluding members of the family Schistosomatidae) are hermaphroditic having each individual with both male and female reproductive system. Male organs include two testes with accessory glands and ducts leading to a cirrus that extends into a common genital atrium (Figure 2c). The female system consists of an ovary including seminal receptacle and vitellaria that connect with the oviduct and it expands into an ootype. The uterus opens into the genital pore into the genital atrium. Both self- and cross-fertilization may occur depending on the species. In the case of schistosomatids, the structure of the reproductive systems is similar, but these flukes are dioecius and there are separate male and female individuals.
Biology of trematodes
One of the most relevant features of flukes is the complexity of their life cycle which are characterized by: (i) alternation of several generations known as the adult, egg, miracidium, sporocyst, redia, cercariae and metacercaria; (ii) alternation of sexual and asexual multiplication; (iii) an intensive asexual multiplication enhancing the transmission of the parasite and (iv) inclusion of two or three hosts, one invertebrate intermediate host (usually a mollusk) in which asexual multiplication occurs and a vertebrate definitive host in which adult worms reproduce sexually. All these characteristics enable the trematodes to be an excellent model to study the parasite–host interactions in both invertebrate and vertebrate organisms [13,14].
Although each species has a unique life history, a general cycle is as follows. Adult worms produce eggs in the vertebrate definitive host. Eggs are passed in the feces, urine or sputum, depending on the location of adult worms, to reach the external environment. The eggs are either eaten by a mollusk and the miracidia hatch, and invade this first intermediate host, or hatch in the external environment allowing the miracidium to locate and penetrate the appropriate molluskan host. Within the mollusk first intermediate host, asexual multiplication occurs, which is critical to enhance the transmission of the parasite. The miracidia produce sporocysts that contain germ cells which will give rise to a large number of the subsequent generations of larval stages. Sporocysts are asexual sacs containing a large number of germ cells each of which produces either a subsequent generation of sporocysts or rediae. Rediae are similar to sporocysts, but have a mouth and pharynx to feed directly on the host tissues. Rediae share the ability of sporocysts to produce a large number of the subsequent generations and, ultimately, to cercariae that is the last stage produced in the mollusk and responsible for the transmission to the next host in the life cycle. Although this ability of asexual multiplication may imply serious damage for the molluskan host, it enhances the transmission of the parasite in spite of the difficulty entailed in the encounter between miracidium and first intermediate host and the hostile environment by ensuring the generation of a large number of the next infective, larval stage infective, from a single miracidium. For example, it has been shown that an individual miracidium  of Echinostoma caproni infecting Biomphalaria glabrata resulted in the production of almost 1500 cercariae during the first month of cercarial emission . Cercaria, the ultimate stage of asexual multiplication, is a tailed free-living stage that is released from the mollusk. In some species (i.e. blood flukes), cercariae enter the vertebrate definitive host where they mature to adult worms. In other species, cercariae penetrate, or are eaten by, the second intermediate host and encyst as metacercariae, and the definitive host became infected after ingestion of the second intermediate host, harboring the metacercaria. Finally, in other species (i.e. Fasciola hepatica), cercariae swim in an aquatic environment to reach the aquatic plants where they attach as metacercariae. The vertebrate host becomes infected by eating the metacercariae.
Trematodes of medical interest
As mentioned above, more than 100 species of digenetic trematodes have been reported infecting humans. Although most of them are irrelevant since they do not cause symptoms or are mild and transient, there are many flukes that constitute an important public health problem. Species with medical interest are commonly classified on the basis of their location in the human host: blood flukes (Schistosoma spp.), liver flukes (F. hepatica, F. gigantica, Clonorchis sinensis and Opisthorchis spp.), lung flukes (Paragonimus spp.) and intestinal flukes. According to their mode of infection and the subsequent measures of control, these parasitic diseases also are classified into blood flukes, in which cercariae actively penetrate into the human skin in aquatic environments, and food-borne trematode infections, including liver, lung and intestinal flukes, in which humans become infected by ingestion of foods harboring the metacercariae. Although the considerable public health impact and the emerging nature of food-borne trematodiases [4,16,17], they are among the most neglected of the so-called neglected tropical diseases and it was recognized by WHO . It should be noted that the neglected tropical diseases are found predominantly in the world's poorest populations in low-income countries, and where these diseases are common, they exacerbate poverty. Major features of these parasitic infections have been recently compiled in the book edited by Toledo and Fried .
Blood flukes cause schistosomiasis or bilharziasis that is one of the most important parasitic diseases of humans causing significant morbidity and mortality on several continents. Schistosomiasis is transmitted in over 70 countries, throughout a wide belt of the tropics and subtropics [20,21]. The three major species causing schistosomiasis are Schistosoma haematobium, S. mansoni and S. japonicum. It is difficult to determine the number of schistosomiasis cases, but it has been estimated that ∼300 million people were infected in 2013 worldwide and ∼280,000 annual deaths occur [4,22]. Although the number of deaths caused by schistosomiasis decreased from 1990 to 2013, the number of active infections increased 31% in the same period which is evidence for the need to implement campaigns of control . In addition to active infections, it is estimated that almost 800 million people worldwide are at risk of infection .
It is estimated that ∼80 million people worldwide were infected with any species of food-borne trematodes in 2013, showing a growing trend from 1990 (∼51% greater number of cases in 2013) . However, these data are controversial, since other authors  reported an estimation of ∼56 million people using a systematic review and meta-analysis.
Fasciola spp. causes an important liver disease called fascioliasis that is an important medical and veterinary problem of worldwide distribution. The estimates on the incidence of human fasciolasis are confusing and range from 2.4 to 17 million people . Opisthorchiasis, caused by Opisthorchis spp., and clonorchiasis, caused by C. sinensis, are the major fish-borne flukes . It is estimated that ∼24 million people are currently infected . Although symptoms of these diseases are commonly mild, cholangiocarcinoma is the most serious complication. Opisthorchis viverrini is classified by the International Agency for Research on Cancer (IARC) as definitely carcinogenic (class 1) and C. sinensis as a probable carcinogen (class 2A) . Cholangiocarcinoma is a malignant tumor starting in the epithelium of the intrahepatic biliary tree and might invade the sinusoids of the liver parenchyma . The prognosis of this tumor is extremely poor . Lung infections or paragonimiasis are caused by Paragonimus spp. There are ∼15 species of Paragonimus known to infect humans. P. westermani is the most common worldwide, while P. heterotremus is the etiological agent of human paragonimiasis in PR China, Laos PDR, Vietnam and Thailand . About 23 million people are infected with lung flukes and almost 300 million people are estimated to be at risk of infection [24,28]. The intestinal food-borne trematodes category constitutes a large assemblage of species that induce parasitic zoonoses. Collectively, these parasites affect the health and economy in a big way in developing countries of the tropics and subtropics in Asia, Africa, Europe and the U.S.A. A total of 70 species (14 families and 36 genera) of intestinal flukes have been isolated from humans [29–31]. About 7 million people were estimated to be infected in 2012 .
Another point of interest of human trematode infections is related to the treatment of these diseases. Praziquantel is the drug of choice, though other drugs such as triclabendazole or tribendimidine also can be used. In general, all of them provide satisfactory results, but there are several concerns in relation to factors that include: (i) the limited number of available drugs; (ii) the side effects of these drugs; (iii) the commercial availability of these drugs is limited and is authorized only for human use in particular countries and (iv) there is increasing evidence in relation to the development of resistance against these drugs . Thus, further efforts for the developments of new active compounds are required. In this context, several promising synthetic and natural products are being studied .
Trematodes, and particularly digenetic trematodes, constitute an amazing group of parasites for both their medical and veterinary importance and their interest in more basic biological sciences. Trematode infections have been considered for a long time as minor diseases confined to low-income countries and their importance has been underestimated. However, their frequency is greater than previously considered and some of them have relevant mortality and morbidity. Moreover, these diseases are re-emerging worldwide, including in developed countries in relation to the ‘globalized world’. Although considerable efforts are being made to control these diseases, the emergent nature of them requires new epidemiological and pharmacological approaches. Additionally, trematodes constitute an intriguing group of parasites in relation to the taxonomy and other factors such as their complex life cycles and the evolutionary adaptations of these organisms to guarantee survival in different biological environments.
The class Trematoda includes a large number of species of medical and veterinary interest.
Further studies on the systematics and taxonomy of these organisms are required.
This constitutes an intriguing group of organisms in relation to their complex life cycles and the biological adaptations to enhance their survival.
This work was supported by the Projects PROMETEO2014-083 Fase II from Generalitat Valenciana, BFU2016-75639-P, from the Ministerio de Economía y Competitividad, and No. RD12/0018/0013, Red de Investigación Cooperativa en Enfermedades Tropicales — RICET, IV National Programe of I+D+I 2008-2011, ISCIII — Subdirección General de Redes y Centros de Investigación Cooperativa and FEDER from the Ministerio de Sanidad y Consumo.
The Authors declare that there are no competing interests associated with the manuscript.