The Nemathelminths’ group of helminths are usually called nematodes, or more commonly roundworms because of the round shape of their body. The mature nematodes usually cause severe gastrointestinal symptoms, but roundworms can also infect the blood, the lymphatic system or the subcutaneous tissues. The larval form infects various types of body tissues. Numerous nematodes can affect humans. Below are some examples of the best-known roundworms infecting humans, but it is important to keep in mind that there are a lot more species that can be potentially involved.
Trichinella spiralis is the smallest species of nematode able to infect human and is responsible for the disease trichinosis (or trichinellosis) transmitted by the ingestion of undercooked pork or wild game infected by parasite’s larvae. It is present worldwide, but it is well controlled in developed countries where pork industry regulations are strict. It causes a wide range of symptoms, beginning with a severe gastrointestinal illness degenerating in systemic organs’ failure and nervous disorder leading ultimately to the death. This parasitic disease is usually diagnosed by muscle biopsy and treated with more or less success using anti-helmintics.
Dracunculus medinensis causes a disease called dracunculiasis, commonly known as the Guinea worm disease. This disease is transmitted by the ingestion of contaminated water. It is interesting to note that human is an obligate host for the reproduction of this parasite and that the complete elimination of infection in humans will lead to the extinction of this parasite. A vast eradication program begins in 1981 and had made a lot of progress since then. Now, in 2015, this parasite is almost completely eradicated with only few cases remaining in some sub-Saharan African countries.
Soil-transmitted helminths
The nematodes Ascaris lumbricoides, Trichuris trichiura, and the hookworms Necator americanus and Ancyclostoma duodenale belong to the larger group of soil-transmitted helminths. Approximately 2 billion people, representing a quarter of the global population, are infected by a least one of these so-called soil-transmitted helminths, mainly in tropical or subtropical regions. As they are transmitted by soil or by ingesting food or water contaminated with faeces from infected people, they are mainly present in developing countries where sanitation is deficient. Hookworms can also be acquired by walking barefoot on contaminated soil, as they are able to penetrate the skin. It is frequent for people living in these areas to be infected simultaneously with one or more roundworms and many other worms or protozoan parasites species. All these diseases have an important socio-economical impact in developing countries. Children are more at risk to be infected by these worms and such infections are linked to malnutrition and lead to physical and mental development retardation, such as learning capacity impairment.
Diagnosis is mainly done by the visualisation of the presence of live worms or eggs in the faeces. Anti-helminthics are usually used successfully to treat these infections, but surgical procedures might be necessary if a high number of worms cause an obstruction of the intestinal bowel.
The roundworm Ascaris lumbricoides is one of the most frequent human parasitic diseases. It is asymptomatic more than 85 % of the time, but it can also lead to gastrointestinal symptoms. It is thought that approximately 10 % of the people residing in developing countries harbour intestinal worms, which are mostly Ascaris. More than one billion people worldwide are infected with this parasite, primarily in Asia and Africa. This disease causes about 60 000 deaths per year worldwide, especially in children from developing countries.
The parasite Trichuris trichiura, also known as whipworm, infects a little less than 1 billion people, especially in Asia, but also in Africa and South America. This parasite is also present in rural Southeast United States where there could be up to 2.2 million infected people.
The parasites Necator americanus and Ancyclostoma duodenale, commonly known as hookworms, are responsible for infecting approximately 700 million people worldwide. Necator americanus is the most prevalent hookworm and is more frequent in Asia, sub-Saharan Africa and the Americas while Ancyclostoma duodenale is more frequent in Europe and the Mediterranean region.
Filarioidea type
Another important type of nematodes consists of the microscopic roundworms species from the Filarioidea type. Eight species of filarial nematodes are known to infect humans and can cause infections of the lymphatic system, the subcutaneous tissues or the abdominal cavity generally called filariasis. These parasites are transmitted by the bites of blood feeding infected black flies or mosquitos depending of the species involved. These parasites are mainly diagnosed by their visualisation in patient’s blood smear using a microscope. The most commonly known are Onchocerca volvulus, which cause «river blindness» disease, and Wuchereria bancrofti, which is the main cause of elephantiasis, a late-stage lymphatic filariasis disease characterised by the swelling of legs, arms or genitals.
Onchocerca volvulus is known to infect approximately 37 million people, the vast majority of the cases occurring in Africa with only a small proportion of the cases in Latin America and Yemen. Infection with Onchocerca volvulus is considered to be chronic. Actually, the progression of the disease is controlled by periodic administration of anti-helminthics that do not kill adult worms, but prevent their reproduction and kills the microfilariae form responsible for the symptoms. The World Health Organization and other NGOs are involved in the control and elimination programs of the «river blindness» disease by supplying drugs in endemic areas and by control black fly populations through the spraying of rivers with larvicide.
About 120 million people are affected by lymphatic filariasis worldwide, but they are mainly located in Asia, Africa, Latin America and the Pacific region. This affection is usually treatable with anti-helminthics, but long-term swelling caused by the infection is considered to be chronic.
Sources: Centers for Disease Control and Prevention; World Health Organization
