Nematodes are microorganisms that occur on every continent and in deserts, swamps, oceans, and the tropics. They are some of the most essential decomposers, recycling minerals and nutrients into the soil.
Most nematodes live in the gut, but some infect other body parts, like the bug that causes trichinosis. Some cause infections that can be life-threatening.
Amebic Meningoencephalitis
The free-living nematode Naegleria fowleri causes an uncommon but devastating brain disease called primary amebic meningoencephalitis (PAM). It is found in warm freshwater, like lakes, rivers, and soil. The disease occurs when the ameba, a single-celled organism, enters the human body through the nose and travels to the brain, killing cells.
Most intestinal nematodes have adapted to their human host, and infected people tolerate them reasonably well. However, symptoms can become severe and even life-threatening when infection levels increase (as in developing countries).
Nematodes are also used to control insect pests in agriculture. Heterorhabditis bacteriophora and Steinernema carpocapsae are examples of such “entomopathogenic” nematodes. These nematodes seek out their insect hosts in the soil using chemical cues, temperature, touch, or vibration. Once they detect an insect, they actively hunt it or lie in ambush and attack it by entering through the insect’s natural openings (mouth, anus, or breathing holes). Once inside, they release symbiotic bacteria that produce insecticidal toxins, killing the host within a few days.
Hepatitis B
The multi-cellular nematode Caenorhabditis elegans is famous for being the first multi-cellular organism to have its genome sequenced. Another of its achievements was that it reproduces by androdioecy (infertile asexual reproduction), which is unusual among animals. It also demonstrated mitotic parthenogenesis, whereby most cells reproduce asexually, but some do so via sperm and eggs.
Infection with the human gastrointestinal nematodes, pinworms Ancylostoma duodenale and Necator americanus, hookworms Ancylostoma lumbricoides, and whipworms Strongyloides trichiura, and Enterobius vermicularis is a major global public health problem. These parasitic infections, referred to collectively as Neglected Tropical Diseases (NTD), extract an enormous toll in endemic areas with low incomes and poor standards of personal hygiene.
These diseases are endemic in warm, moist climates and occur most frequently, where poverty results in overcrowded living conditions with poor sanitation. These conditions allow the free-living larvae to survive, developing through two molts to the infective L3 stage. Eggs and larvae are transmitted from person to person by fecal contamination. Symptoms vary with the parasite. They range from itching where the worms penetrate the skin (“ground itch”) to a severe bowel infection and respiratory symptoms, including coughing and dyspnea with hemoptysis. Treatment is with antihelmintics.
Lymphatic Filariasis
The threadlike nematode Wuchereria bancrofti causes lymphatic filariasis (LF), which affects more than 100 million people worldwide. The condition is characterized by painful and disfiguring enlargement of the legs, arms, breasts, and genitals. In some cases, enlargement of the scrotum leads to severe, life-threatening swelling called hydrocele.
LF is caused by the parasite that causes elephantiasis, characterized by swollen legs, trunk, head, and abdomen. It is endemic in tropical and sub-tropical regions where poverty, overcrowded living conditions, and poor sanitation and hygiene promote transmission.
Symptoms of LF correlate with the parasite load: light loads are usually asymptomatic; heavy infections cause abdominal symptoms, diarrhea, and malnutrition. The infection is treated with a combination of safe medicines administered annually. The free-living larval stages of the intestinal nematode Ascaris lumbricoides and the skin penetration nematode Schistosoma stercoralis undergo repeated cycles in the soil to become infective. Infection is diagnosed by microscopic examination of feces or scrotal smears.
