Round Worms from Past to Present: Evolution, Biodiversity, and Ecological Roles

1. Introduction

Nematoda, meaning “thread” in Greek, are separate sexual creatures whose bodies consist of a single piece, round, thread-like body structures and whose sizes can be between 1 mm and 1 m, depending on the species. They have a nervous, muscular, and reproductive system, but no circulatory and respiratory systems. Their digestive system starts with the mouth and ends with anus (Figure 1). Females lay eggs, and some species give birth to larvae. They complete their evolution through several larval stages. During larval stages, a new cuticle layer is synthesized. Parasitic nematodes have a dauer larval stage that allows them to infect their host. In this way, they can easily resist adverse conditions. Nematodes, of which more than 30,000 species have been described to the present day, have a very crowded biodiversity. It is estimated that there are more than 1 million species, including those from prehistoric times and those that have not yet been classified today. Their mouthparts vary according to species and can be in a simple tubular or cutting-piercing style.

The habitats of filamentous worms are quite diverse, ranging from humid nooks and crannies to arctic glaciers, desert dust, hot springs, and salty sea water. Parasitic nematodes are transmitted to humans and other vertebrates by the ingestion of eggs found in the external environment or by penetration of the skin by free-living larvae in the soil. Some species that live in blood and tissues are transmitted by vector arthropods. Commensalism is one of the most important associations between nematodes and insects and is often utilized in the transportation of larvae between hosts [2, 3, 4, 5, 6].

A substantial number of the members of the phylum Nematoda also include plant and animal parasites that cause damage to humans, animals, and cultivated crops. It is reported that half of the world’s population, mainly in tropical and sub-tropical regions, is infected with gastrointestinal nematodes (GIN). A significant proportion of these are children and women [2, 3, 5, 6]. All living things in nature are easily affected by nematode attacks. For example, more than 1 million people in the world suffer physical and mental incapacitation, often ending in death, due to nematode parasitising humans. Nematodes in animals that play a role in human nutrition cause huge economic losses. Likewise, soil nematodes, which have hundreds of species, cause global damage in agriculture worldwide. Nematodes can harm plant, animal, and human health in various ways. Among these, for example, plant nematodes cause significant damage such as disrupting the structure of root cells in plants, piercing the cell wall and feeding on the cell contents, and transmitting microbial agents such as bacteria, fungi, and viruses that they carry with them to their host. More than 300 nematode species are known to affect human and animal health in tropical and sub-tropical countries, especially in developing countries. A majority of these nematodes are the so-called “soil-transmitted nematodes” and still have high morbidity and mortality rates [2, 3, 4, 5, 6, 7, 8, 9, 10].

In this article, we will take a brief tour through the filamentous worms. Their histories, morphological structures, species diversity, factors affecting the diversity, evolutionary processes from past to present, effects of parasitic nematodes on living organisms, life cycles and reproductive habits, interactions with other organisms, positive and negative effects on the ecosystem, developments in diagnosis-treatment and prevention measures, current developments, and current research and future perspectives will be discussed.

2. Taxonomy of nematodes

The taxonomy of nematodes, a very large group in the world, is frequently updated with innovations in diagnostic methods and the discovery of new species. In the interest of space, here we present a summary classification of the entire Phylum Nematoda based on current molecular, developmental, and morphological evidence. The classification reflects the evolutionary relationships within the branch, as well as significant areas of uncertainty, particularly with regard to the early evolution of nematodes. Only numerical diversity will be discussed here due to the large area covered. Accordingly, the Phylum Nematoda includes 2 classes, 8 subclasses, 12 superorders, 32 orders, 53 suborders, 101 superfamilies, 276 families, 511 subfamilies, 3030 genera, and 28,537 species [11, 12, 13, 14, 15, 16, 17, 18, 19, 20].

How many Nematodes parasitize the Earth? The answer to this question is still not clear. Despite the developments in taxonomy, it is estimated to be about 100–350 thousand with current approaches and mathematical modeling. It is claimed that even half of the nematodes belonging to vertebrates that have been identified and collected in the world to date can be identified. Today, the power law formula is used to determine global helminth diversity. In modeling, studies have mostly focused on nematodes of vertebrates. It should be noted that there is still insufficient information on marine amphibians and reptiles. Calculations of species diversity in parasites were carried out according to certain rules. The possibility of completing this work in the twenty-first century seems difficult. For success, genome studies need to be supported by global projects [18, 19, 20, 21, 22, 23, 24].

3. History and evolution of nematodes

Nematodes are believed to have evolved from their early ancestors in the ocean, and the first animal-parasitic nematodes are thought to have parasitized marine invertebrates. Paleoparasitological studies have shown that amber, stones, and coprolites containing fossils that have reached the present day contain various evidence related to the evolution of nematodes. The oldest of these is a plant nematode from the Devonian period. Nematodes parasitizing invertebrates first appeared in the Cretaceous period, while parasitic nematodes of vertebrates have been found in Triassic coprolites. Due to the size, structure, and soft bodies of most nematodes, fossil records remain limited. Although they can be large, their rapid decomposition after death has prevented fossilization. Among the remains shedding light on the history of nematodes, amber occupies an important place. Amber, a tree resin that can protect various microorganisms and vertebrates, has allowed for a journey into the history of nematodes. However, the nematodes found in amber are primarily arthropod nematodes related to their feeding habits. A significant portion of nematodes found in humans and other vertebrates is derived from coprolites (fossilized feces) and dates back up to 240 years. In later periods, it has been possible to understand more about these organisms from fossil sources found in wetlands [5, 12, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32].

Nematode fossils dating back approximately 2 million years are found in Holocene deposits. Additionally, a species of Strongyloides associated with vertebrates has been identified in a late-stage mummified horse fossil. Research suggests that animal nematodes likely evolved in marine environments before transitioning to land. The origins of plant nematodes are believed to trace back to the Devonian period. The data provide important insights into the evolution and history of nematodes [5, 12, 23, 27, 30].

The first remains of parasitic nematodes have been identified as a plant nematode from the Early Devonian period, dating back 396 million years. Their existence marks the initial symbiotic relationships between terrestrial plants and animal organisms. The oldest animal parasitic nematodes were recently discovered in Triassic synodont coprolites in Brazil. Among these are a 240-million-year-old ascarid, Ascarites rufferi, and a 240-million-year-old oxyurid, Paleoxyuris cockburni. The next oldest member of this group is a 130-million-year-old mermithid, Cretacimermis libani, found in Lebanese amber from the Early Cretaceous period. A wide variety of parasitic nematodes have been found in Baltic amber. The first nematode identified in amber belongs to a species of ant and dates back to the Pliocene epoch [5, 27, 28, 29, 30, 31, 32].

In Germany, freshwater accumulations in swampy areas have yielded nematode species dating back 2.5 million years, which are parasites of Tabanid flies. Various vertebrate-parasitic nematodes have been reported from deposits stretching back to the Pleistocene (up to 1.81 million years ago) and those from the Holocene era (from 11,500 BP to 3300 BC). The evolution of invertebrate nematode parasites likely first occurred with “aphasmid” nematodes in marine invertebrate hosts and later with Phasmid nematodes in terrestrial environments. Parasitic nematodes from the Cambrian period have been identified in nearshore areas.

The first evidence of nematode parasites in vertebrates has been found in freshwater habitats dating from the Cambrian period, located in nearshore regions. These findings suggest that the nematodes resided in primitive jawed organisms living in marine environments. It is proposed that modern oxyurid nematodes may represent the oldest parasites of terrestrial vertebrates. Evolutionary studies indicate that plant-parasitic nematodes have evolved at least three times, while animal-parasitic nematodes have gone through six evolutionary events [5, 19, 24, 33, 34].

Nematodes possess a broad range of vertebrate and invertebrate hosts, enabling them to transition easily between hosts and develop the ability to migrate within different organs of various hosts. Additionally, they have adapted their reproductive strategies according to their hosts. To further facilitate transitions between hosts, they have evolved attractive colors and morphological shapes. These adaptations have enhanced the evolutionary success of nematodes and allowed them to achieve widespread distribution in ecosystems [29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40].

It is known that more than 4,000 plants in the world are infected with various nematodes. This situation will cause enormous agricultural damage and starvation for many people. Agricultural nematodes can also undergo continuous phylogenetic changes, facilitating their settlement in their hosts and developing genetic resistance to pesticides. Plant parasitic nematodes are divided into two groups as ectoparasites or endoparasites according to their feeding mechanisms and as resident or migratory according to their duration of persistence in their hosts. Endoparasitic nematodes, when they enter a plant, impede its normal development, disrupt the cellular structure, and create specialized feeding sites for themselves. Similarly, migratory plant-parasitic nematodes also damage the host cells through their feeding. They assist in the breakdown of plant cells by releasing peptidase inhibitors. Their active movements during migration within the host cause various degenerative changes in the host’s tissues. Additionally, to evade the host’s defense mechanisms, they secrete enzymes that resemble the molecular structure of the host, thereby protecting themselves from the host’s defense system. Plant-parasitic nematodes cause harm in different ways, such as disrupting the synthesis of cell walls and protein structures in plants through the transfer of parasitic genes [6, 8, 23, 26, 32, 35, 41].

4. Life cycle and parasite-host relationships in nematodes

Various studies on the life cycle of parasitic and free-living nematodes with phylogenetic similarities are ongoing. Although the basic mechanism of aging has not been fully analyzed, some experimental approaches are being used. For example, mutations seem to prolong the life span. Studies show that nematodes can be grouped according to their life span. Parasitic nematodes can live for up to 15 years unless they kill their host, as in the case of hookworms. The most important reason for this is that reproduction occurs continuously rather than seasonal variability as in free-living nematodes. For example, Enterobius vermicularis, which has a lifespan of 2–3 weeks in the human intestine, can sustain its life in its host for years thanks to its continuous egg-laying power and auto-infections. One of the most important evolved characteristics of parasitic nematodes is their high fecundity. Parasites are usually larger and have a longer life cycle than their free-living relatives [3, 12, 15, 20].

The most important factor here is their fecundity. For example, an Ascaris female living in the human intestine is 20–40 cm in size and lays about 200,000 eggs per day. Parasites are usually larger and have a longer life cycle than their free-living relatives. It probably survives in the intestines for a long time. In contrast, the free-living Caenorhabditis elegans is 1 mm long and can lay up to 300 eggs in a lifetime of about 3 weeks. However, according to some opposing views, it is difficult to prove whether the high morbidity of some nematodes, due to their aggressive lifestyle, such as Ascaris and hookworms, is due to the parasite’s own choice or to the death of the host. In addition, factors such as ambient temperature, foraging potential, oviposition potential, adaptation, and host immunity are other important factors affecting aging and longevity. However, current studies show that the aging process of nematodes varies depending on the state of free radicals. Therefore, it is very likely that they have a longer life cycle than free-living nematodes that depend on constant climatic changes [33, 34, 36, 37, 38].

Evolutionary studies have shown that plant parasitic nematodes have evolved at least three times and animal parasitic nematodes six times. Because they have a wide range of hosts, both vertebrate and invertebrate, they have been able to easily switch between them and migrate in different organs of different hosts. They have also been able to adapt their reproductive patterns to their hosts. Nematodes are colorless and transparent animals. Cyst nematodes develop brown to black color in their cuticle due to oxidation. In order to facilitate migration between hosts, they are able to transform into attractive colors and morphological shapes [8, 33, 34].

Many parasitic nematodes use chemical clues to find a host and establish themselves. While these are often assumed to be directly related to the host, occasionally, they are reported to originate from volatile compounds secreted by the host microbiome. These studies suggest that, instead of mutual adaptations that enable the host to better defend and the parasite to better attack, for each host and parasite, the microbiome mediating the host-parasite interaction seeks to prevent or overcome manipulation. The microbiome is an influential actor in maintaining animal and plant health and well-being and can be considered an important element in understanding host-parasite interactions, disease ecology and new disease emergence, and environmental impacts on biodiversity [26].

Parasitic nematodes, in contrast to other animals, have undergone a number of molecular and genomic adaptations during their adaptation to parasitism. The identification of these adaptations can be demonstrated experimentally by various methods. These include comparing the genomes of closely related parasitic and free-living species, comparing the gene expression of parasitic and free-living life cycle stages of parasitic nematode species, and analyzing the molecules secreted by parasitic nematodes. While many genomics-based studies on adaptation to parasitism have been conducted, there is still work to be performed given the species diversity of nematodes. Common findings in genomic-based studies are that some specialized gene families coding for proteases/peptidases, protease inhibitors, SCP/TAPS proteins, acetylcholine esterases, and so on are abundantly represented, and in most cases, these genes appear to be used and often secreted, especially during intra-host parasitic phases [8, 26, 40].

A high level of reproduction may have driven the parasite’s “virulence,” that is, parasite-induced host harm. Reproduction requires a large number of resources for parasites, which must come from the host. Virulence may also mean that it is a necessary part of parasitism as a consequence of the high reproductive capacity required by parasites. Nematodes are hermaphroditic and reproduce sexually, giving them an important evolutionary trait. Because of their sexual reproduction, adult nematodes do not multiply in numbers within the host. They therefore remain in the host in the same numbers as when they were acquired. Some of them are excreted by eggs and then undergo a maturation process, after which they are transmitted back to their host or pass from host to host via a vector. The lifespan of these species is directly proportional to their host. During their life in the host, they are in constant contact with their immune responses and have to develop a number of traits in order to survive. In order to protect themselves from the host defense system, they have developed features such as the ability to follow some molecules of the host, to change their own cellular structure, and to create an inflammatory response in order to protect themselves from the host defense system. These features are highly developed in nematodes living in the intestinal tract as well as in filarial nematodes living in blood and tissues. In some cases, they are also known to slow down their reproduction in order to be less affected by the host defense system. Likewise, nematodes living in the blood and lymphatic system have been shown to be sustained by secretions that attract various vertebrates as they migrate from one host to another. Parasitic nematodes have evolved in this way to maintain their species [37, 38, 39, 40].

5. Feeding habits of parasitic nematodes

Plant and animal nematodes have a wide variety of feeding habits. Accordingly, their damages cause significant losses both in human health and in agriculture and animal husbandry. The nutritional requirements and metabolic dependence of nematodes are directly proportional to the damage they cause. In nature, there are many vertebrate and invertebrate hosts as well as a wide variety of plants that can form the feeding group of nematodes. Phylogenetic studies have suggested that plant and animal parasitic nematodes have undergone multiple evolutionary processes, resulting in a diversity of feeding habits. They have a complex life cycle and biodiversity. The complexity of their diet has led to a number of changes in the evolutionary processes of both nematodes and their hosts. All parasitic nematodes are absolutely dependent on their hosts for nutrition and energy. Considering their diversity, all living organisms in the world have been attacked by nematodes at least once in their lifetime. Nematode attacks on human essential nutrients cause global losses of millions of dollars annually. In addition, the food supply of low-income countries becomes almost impossible. Parasitic nematodes carry out this process with the support of the mouthparts they have developed as well as the various enzymes they secrete. Depending on the variety of nematodes, their food requirements may also be different. Animal parasites feed on tissue fluid and blood, while plant parasites feed on sap secretions. Studies on the different nutritional needs of nematodes are continuing [2, 3, 8, 26, 34, 36, 39, 42, 43].

The life cycles of nematodes infecting humans are quite different. Some are transmitted to the host by their larvae penetrating the skin, some are transmitted through food and water, and some are transmitted through vectors. The feeding preferences of nematodes differ according to species. Those living in the blood and tissues feed on blood, tissue, and body fluids, while those living in the intestinal system feed on intestinal contents. Since a significant number of gastrointestinal nematodes are hematophage nematodes, such as hookworms, they cause blood loss and iron deficiency in their hosts. Likewise, nematodes living in tissues feed on the muscle tissue of the host, as well as leave their larvae in muscle ligaments through the circulation. Tissue nematodes survive in the muscles of many carnivorous hosts other than humans and cause a wide diversity of symptoms. Among the tissue nematodes, Filarial nematodes are another group of vector-borne nematodes that cause permanent damage to humans and are highly prevalent in certain endemic areas around the world. While some of them cause obstruction in the lymph flow and cause overgrowth in the organs (such as Wuchereria bancrofti, Brugia malayi, Brugia timori), others are also vector-borne and cause river blindness disease (such as Onchocerca volvulus, Loa loa). More than 200 million people in endemic areas are affected by lymphatic filariasis [2, 3, 42, 43].

Plant parasitic nematodes, which are divided into two as ectoparasites or endoparasites according to their feeding mechanisms, are also divided into two as resident or migratory according to the duration of their stay in their hosts. After entering the plant, endoparasitic nematodes obstruct the normal development of the plant, destroy the cell structure, and create their own special feeding pools. Likewise, migratory plant-parasitic nematodes damage the host cell by feeding. They help plant cells to break down with the peptidase inhibitors they secrete. They cause degeneration with their active movements during migration. To protect themselves from host defense mechanisms, they secrete enzymes that resemble the molecular structure of the host. Plant parasitic nematodes damage plants in different ways, such as disrupting cell wall synthesis and protein structures by transferring parasitic genes [2, 8, 44, 45, 46].

6. Distribution of nematodes worldwide

Nematodes are the most widespread group of organisms in the world. They can therefore infect all kinds of multicellular animals and plants in all terrestrial and aquatic environments. Some authors consider parasitism by nematodes as a normal feature of life. Their biology and life cycle is an important factor in their distribution around the world. Soil-borne nematodes such as Ascaris lumbricoides, hookworms, Trichuris trichiura, and Strongyloides stercoralis, which are human parasites, are still among the most common parasitic infections in the tropics and subtropics worldwide [46, 47, 48, 49].

According to WHO reports, the morbidity and mortality of these nematodes, which affect millions of people, is quite high, especially in children. Although they have decreased with sanitation and hygiene practices in developed countries, they continue to be prevalent in developing countries and rural areas due to economic and social factors. Today, these infections are still on the WHO’s list of “Neglected Tropical Diseases” and are among the global health priorities. WHO has to frequently update its risk maps on the geographical distribution of nematodes according to geographical reports. Recent studies indicate a high prevalence of infection, especially in low- and middle-income countries. The situation is similar for nematodes living in blood and tissues. Despite all efforts, the prevalence of lymphatic filariasis remains high in the tropics and subtropics [46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58]. According to the latest WHO reports, 794 people in 44 countries have been treated against STHs in the last 10 years. This has led to significant reductions in the prevalence of the disease compared to previous years, but prevention and control strategies have suffered significant setbacks during the COVID-19 pandemic [54].

It is estimated that 1.5 billion people worldwide are infected with soil-transmitted helminths (STHs). Soil-transmitted helminths (STHs) are a group of intestinal parasites that are transmitted to humans through the ingestion of infective eggs or transcutaneous penetration of larvae excreted in human feces, contaminating soil and water resources. It mostly affects children in the world, especially in developing countries. It is an important source of morbidity and mortality. Distribution of nematode infections in the world’s most populous countries (Table 1) is indicated [54].

According to the WHO 2020 reports, STHs affect 820 million people in 102 countries worldwide. Among these, the most common species are Ascaris lumbricoides (820 million), Trichuris trichiura (460 million), and hookworm infections (Necator americanus and Ancylostoma duodenale) affecting 460 million people. In addition, in 2020, serological studies in some regions reported that 386 million people had Strongyloidiasis. This nematode, which can be diagnosed by serological tests rather than fecal examinations, is estimated to be more numerous than reported. The same can be said for Toxocariasis infections. Since the diagnosis of this zoonotic nematode can only be made by serologic methods, the number is estimated to be much higher than known. In a 2019 systematic meta-analysis study, the global prevalence of Toxocariasis was defined as 19%. These rates are estimated to be higher in Africa and Southeast Asia, where serologic testing is not available [54].

In endemic areas, these rates contribute to an estimated 3.4 million disability-adjusted life years (DALYs) and 6,000 deaths. It causes developmental delay and developmental and cognitive impairment, particularly in children, due to malnutrition, fat malabsorption, vitamin malabsorption, and iron deficiency anemia. In endemic areas, multiple nematode infections can often be observed. STHs not only cause intestinal obstruction and physical and mental retardation, but they can also migrate to different organs due to migration of their larvae in the case of corticosteroid or immunosuppression for any reason, carry various microbial agents with them, and cause hyperinfection syndrome such as bacteremia and meningitis. Among these, eosinophilia and hepatitis are the most common syndromes [46, 49, 54, 58, 59, 60, 61, 62, 63, 64].

Although health education, provision of clean water, and antiparasitic treatment programs have been successful in preventing morbidity in STD control strategy programs implemented by WHO in endemic areas, they have not been fully successful due to other outbreaks, economic inadequacies, and labor deficits from time to time. Increased global migration to developed countries due to climate change has led to the emergence of Neglected Tropical Diseases (NTDs) in non-endemic countries. With increased global migration, people infected with NTDs may move to countries where these diseases are not present. This can lead to difficulties and delays in diagnosis and treatment of infection [58, 59, 60, 61, 62, 63, 64].

7. Diagnostic methods in nematodes

The diagnosis of nematodes in the intestinal tract is made by macroscopic and microscopic examination of feces. Since these nematodes, most of which are of soil origin, have a larval migration period shortly after infection, larvae can be searched for in sputum and vomit samples during this period. To increase the chance of finding nematode eggs, feces should be prepared by sedimentation. Here, methods such as Kato-Katz, Wisconsin flotation, and Haroda-Mori increase the chance of detection. These methods have their own positive and negative advantages and disadvantages [65, 66].

Molecular methods allow for better species identification of many nematodes. However, their use is limited by the need for a variety of equipment. Serologic tests for nematode coproantigens are not suitable for field work due to low sensitivity and specificity and cannot distinguish whether infection is present or past. ELISA and Latex tests are among the tests used in field studies, but their specificity is low due to the common antigenic structure of nematodes. In large-scale epidemiological studies, samples are mostly collected and used for PCR applications in specific laboratories [67, 68].

In the diagnosis of soil-transmitted nematodes, high eosinophilia in blood parameters and mild or severe anemia are also common signs of infections. Helminths should be considered in the presence of Charcot-Leyden crystals in stool and sputum and the presence of fecal occult blood. Colonoscopy is important in the diagnosis as whipworms are located in the rectum [69, 70].

Similar practices are also practiced in veterinary parasitology. In addition to microscopic examination, immunodiagnostic methods are also used for rapid diagnosis. Recently, in addition to rapid and reliable methods, quantifiable molecular and immunodiagnostic methods such as artificial intelligence have been used to perform the necessary examination in a large number of farm animals. However, due to the variability of reproductive cycles of nematodes with different evolutionary cycles, the results are not always successful. In addition, microscopic diagnosis requires experience [71, 72].

Investigating the presence of plant nematodes is very difficult as large areas need to be investigated. Statistical methods are mostly used to describe their quantity or presence. The results obtained by examining samples of soil or plant samples scanned by random sampling methods in large areas of cultivated land are calculated by statistical methods. Research on agricultural nematodes is important in terms of determining damage and identifying species diversity, population trends, soil types, and climate changes. For this purpose, 250–300 ml of soil samples around the cultivated roots in agricultural fields are placed in polyethylene bags, and the necessary data are noted. The collected samples should be extracted and analyzed in a short time. Extraction processes are applied according to various techniques. After the samples are prepared according to the enrichment method, they are examined by microscopy, serological, or molecular methods using various methods. Biological, biochemical, and cytogenetic methods are used to identify some plant nematodes. In this way, their taxonomy is also made [72, 73, 74].

8. Treatment, prevention, and control strategies for nematodes

Unfortunately, the number of drugs used in the treatment of nematodes has been limited to a small number of drugs from the past to the present. Today, the development of resistance to anthelmintics is quite common. Due to the development of economic and anthelmintic resistance, there are great losses due to parasitic nematodes in various animal groups and agricultural products in different geographies of the world [75, 76, 77].

Today STHs infections are still the most prevalent of neglected tropical diseases, primarily affecting child populations in low- and middle-income countries. In these areas, more than 1 billion people are infected with STHs. WHO has announced that significant success has been achieved with chemotherapy in risk groups consisting of pre-school, school age, and women of childbearing age by 2020 with the control program launched in 2012. The organization has decided to continue prevention and control practices until 2030 to ensure the success of the work.

It proposes an integrated approach that includes access to proper sanitation, hygiene education, and preventive chemotherapy (large-scale and periodic distribution of anthelminthic drugs). To this end, WHO has been providing Benzimidazole treatment in endemic areas with donors since 2010. Approximately 4 billion Benzimidazole treatments have been given in schools, and significant reductions in morbidity have been reported. Treatment and monitoring of lymphatic filariasis is also being implemented in the same areas. Donors are needed to ensure the success of preventive treatment programs in endemic areas [75, 76, 77, 78, 79].

In recent years, some countries have been searching for alternative prevention methods against the development of resistance and side effects of drugs. Biological methods used as alternative methods will prevent human-animal and environmental damage. Nematophages, the natural competitors of nematodes, are potential biological control agents like microbes. These natural enemies include both bacteria and fungi and use a variety of methods to infect and kill nematodes. For example, nematode-capturing fungi can sense host signals and produce specialized capture devices to capture nematodes, while endo-parasitic fungi can kill nematodes through spore adhesion and invasive growth to break the nematode cuticle [79, 80, 81, 82].

It should also be able to prevent transmission and prevent various complications with simple measures to be taken. Personal hygiene, clean water, sewage and treatment systems, cooking and consumption of food, public education, and improvement of living conditions are extremely important criteria for elimination. It is important that individuals in risk groups (children, pet owners) are periodically checked by a doctor [54, 77, 80].

The cost of praziquantel and other anthelmintics has decreased to such an extent that the cost should no longer prevent epidemic countries from easily delivering them to those in need. Regular treatment can be provided in an affordable and sustainable way with available medicines. This strategy, aimed at reaching those at the highest risk of morbidity from schistosomiasis and soil-transmitted helminthiasis, was endorsed by the World Health Assembly in May 2001 [83, 84].

From the past to the present, according to WHO criteria, people diagnosed with geo helminths must receive antihelminthic treatment. The prevalence has decreased considerably in many countries due to antihelminthic treatment since the 1970s. Nowadays, oral rehydration is an important step in the treatment in order to make the treatment more successful. Treatment reduces parasite load and complications and prevents transmission. In endemic areas, treatment is recommended to be repeated every 6 months or once a year to prevent new infections.

Prevention and control strategies for nematodes have not yet achieved the desired success. It is necessary to facilitate community access to anthelmintics and to make them available to the widest masses. In addition, there is a need for the discovery of new drugs with fewer side effects and that are more economical. However, treating soil-borne nematodes with drugs alone may not be a permanent solution because vaccination seems to be a better way for elimination due to the high rates of re-infection [78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88].

9. Challenges, solution, and future expectations

Nematoda is a large group of plant and animal parasites that cause significant damage to humans and agricultural crops. The most extensive studies of members of this group have focused on human, animal, and agricultural pests. It is estimated that about half of the world’s human population is infected with nematodes. Parasitic nematodes have to constantly modify their individual life cycles in order to live long lives with their hosts. Their life history suggests that the process of future evolution is still ongoing. The appearance of nematode infections in different geographical regions is an important indicator of this. Factors such as various climatic changes and migration of human populations from one place to another for various reasons and tourism also contribute to this diversity and evolution. In addition, one of the most important evolutions is the resistance they have developed against various antiparasitic drugs used for therapeutic purposes. In the twenty-first century, nematodes are still an important source of morbidity and mortality in children in certain areas of the world. According to WHO’s target, solutions, such as finding donors and establishing training guidelines and rapid and effective diagnosis and treatment methods have been planned to control SHTs by 2030. However, increasing economic problems make it difficult to find donors, and control is becoming increasingly difficult due to other epidemics, wars, and migrations.

A type of roundworms, nematodes are widespread throughout the world, and a significant number of them are free-living in nature. Some live in freshwater, some in the oceans, and some in the soil. They decompose various harmful microorganisms that they use to feed on and transform them into useful ones. They decompose organic matter and transform it into something useful for soil and agricultural plants. They contribute to biological control by consuming many harmful parasites. They are an important model in genetic research. Nematodes are indispensable elements of nature’s complex and balanced ecosystems. Their evolutionary processes, diversity, and ecological roles have had a profound impact on both natural and human life. Understanding them is of great importance both in terms of biodiversity and in the fields of sustainable agriculture and health. Getting rid of their harmful effects requires the collective fight against them.