Nematodes are among the most species-rich genera of worms. Some subtypes can infect humans and cause disease.
What are nematodes?
Nematodes are also known as eelworms or nematodes. They are considered to be exceedingly diverse, bringing the total to more than 20,000 different species as well as 2000 different genera. Since some species can infest both humans and animals, they are classified as parasites. Known nematode species that are considered harmful to humans are, for example, the pinworm, the whipworm and the roundworm. However, since the 1950s, the number of nematode cases in humans has been declining.
Occurrence, distribution and characteristics
Nematodes are found almost everywhere. Thus, they live in terrestrial biotopes as well as in salt and fresh water. Among the nematodes, numerous parasite species are also common. Particularly well known is the roundworm (Ascaris lumbricoides), whose habitat extends from East Asia through Africa to South America. Other species include the pinworm (Enterobius vermicularis), the dwarf pinworm (Strongyloides stercoralis) and the wandering filarial (Loa loa). Because the nematodes reach only microscopic size, they are classified as meiofauna. However, a few species, such as the horsepulworm, reach a length of several meters. Nematodes are triploblastic primordial mouths. Their typical shape is that of a worm. Thus, they become long and have a round cross-section. Nematodes do not have segmentation. A narrow pseudocoel serves as the body cavity. The head of a nematode is equipped with small directional organs that serve as eyes, so to speak. In addition, the worm has a large mouth opening including the pharynx. The mouth, located on the front, often contains projections. These function for the groping as well as for the admission of food. The anus is located at the pointed rear end. The largest nematode is Placentonema gigantissimum, which is found in the placenta of sperm whales. Thus, females can reach a length of about 8.40 meters. Their diameter is 2.5 centimeters. In contrast, the males reach a length of only 4 meters. The skin cell layer of the nematode is considered interesting. Unlike other animals, it is not composed of single cells. Instead, it consists of a mass of cellular material. This is not divided into individual cells by membranes. In addition, there are several cell nuclei. A multilayered thick cuticle is secreted from the epidermis, which has the property of protecting the nematode from adverse environmental conditions and desiccation. In parasites, this cuticle can even protect the worm from the digestive juices of the host body. Similar to roundworms, nematodes are also equipped with longitudinal muscles that are used for locomotion. They extend from the head to the tail. The nervous system of nematodes is considered to be very simple. Thus, it is composed by a circumoesophageal or cirumpharyngeal ring. From there, a ventral as well as a dorsal main cord run in the posterior direction. The ring is able to perceive and process different stimuli. In contrast to other animals, the muscle cells of the worm can extend independently to the nerve tracts. Depending on the species of nematode, the diet varies. For example, free-living species feed on algae, fungi, bacteria, feces and carrion. Sometimes animals are also preyed upon. With the help of the projections on the mouth, the food can be picked up and crushed with the strong muscles. Processing and digestion of the food then takes place in the intestinal tract. Reproduction of nematodes is sexual, and there are usually two separate sexes. Occasionally, however, hermaphrodites occur that self-fertilize. This includes, for example, the species Caenorhabditis elegans. Since nematodes have the property of molting, they are counted among the molting animals. Threadworms are transmitted primarily through the consumption of raw meat. This already contains the larvae of the worms. However, the ingestion of worm eggs contained in feces can also lead to a worm infestation, which is often the case with dogs. Food contaminated with worm eggs is also considered dangerous.
Diseases and ailments
If humans are infested with nematodes, medical science speaks of filariasis.Worm infestation causes various diseases, depending on the species. These include onchocerciasis (river blindness), lymphatic filariasis and loiasis. The worm species Wuchereria bancrofti is considered particularly contagious, affecting an estimated 108 million people around the world. A further 12 million carry Brugia species. Around 17 million people are suspected of being infected with Onchocerca volvulus. The disease occurs almost entirely in Africa. In Germany, infections with threadworms are very rare. Almost always the affected persons stay in tropical countries before. An infestation with threadworms is usually noticeable by considerable itching in the anal region. This is particularly noticeable at night and is intensified by heat. If girls and women are infected by a threadworm, the inflammation sometimes spreads to the vagina. The discovery of a threadworm infestation is usually made by the fact that the long worms can be seen in the feces or on the anus of the patient. To combat the parasites, anthelmintics (vermifuges) are administered to kill the worms. In addition, consistent hygienic measures in the anal area are needed.
