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Nematode Infections

  • Author: Murat Hökelek, MD, PhD; Chief Editor: Mark R Wallace, MD, FACP, FIDSA  more...
Updated: Jan 22, 2015


Nematode infections in humans include ascariasis, trichuriasis, hookworm, enterobiasis, strongyloidiasis, filariasis, and trichinosis, among others. The phylum Nematoda, also known as the roundworms, is the second largest phylum in the animal kingdom, encompassing up to 500,000 species. Members of Nematoda are elongated, with bilaterally symmetric bodies that contain an intestinal system and a large body cavity.

Many roundworm species are free living in nature. Recent data have demonstrated that approximately 60 species of roundworms parasitize humans. Intestinal roundworm infections constitute the largest group of helminthic diseases in humans. According to a 2005 report by the World Health Organization (WHO), approximately 0.807-1.221 billion humans have ascariasis, 604-795 million have trichuriasis, and 576-740 million have hookworm infections worldwide.[1]



The life cycle of parasitic nematodes is clinically important. Some nematode infections can be transmitted directly from infected to uninfected people; in others, the nematode eggs must undergo a process of maturation outside the host. In a third category, the parasites may spend a part of their life cycle in the soil before becoming infective to humans.

As with other parasitic infections, definitive diagnosis of nematode infections depends on demonstration of the stage of the life cycle in the host. Nematodes, as with most other worms infectious to humans, almost never complete their entire life cycle in the human host.

The life cycles of nematodes are complex and highly varied. Some species, including Enterobius vermicularis, can be transmitted directly from person to person, while others, such as Ascaris lumbricoides, Necator americanus, and Ancylostoma duodenale, require a soil phase for development. Because most helminthic parasites do not self-replicate, the acquisition of a heavy burden of adult worms requires repeated exposure to the parasite in its infectious stage, whether larva or egg. Hence, clinical disease, as opposed to asymptomatic infection, generally develops only with prolonged residence in an endemic region.

Unlike with protozoan infections, a casual or a low degree of exposure to infective stages of parasitic nematodes usually does not result in patent infection or pathologic findings. Repeated or intense exposure to a multitude of infective stage larvae is required for infection to be established and disease to arise. Anisakis species cause erosive and/or hemorrhagic lesions in or near the main lesion, forming a tunnel through the gastric mucosa to the submucosa.

Eosinophilia and elevated serum immunoglobulin E (IgE) levels are features of many nematode infections; when unexplained, these symptoms should always prompt a search for occult roundworm infection. Humans do not appear to develop significant protective immunity to intestinal nematodes, although the mechanisms of parasite immune evasion and host immune responses to these infections have not been elucidated in detail.




United States

Trichuriasis: Recent estimates indicate that 2.2 million people in the United States are infected with Trichuris trichiura (whipworm), mainly in the rural Southeast.

Enterobiasis: This is the most common of all helminthic infections, with an estimated 42 million cases.

Ascariasis: An estimated 4 million people, mainly in the Southeast, are infected with Ascaris species.

Hookworm: This infection still has a low degree of prevalence in the Southeast.

Strongyloidiasis: A prevalence rate of 0.4-4% has been estimated in southern states.

Trichinosis: The prevalence rate is 4-20%.

Anisakiasis: Annually, fewer than 10 cases occur in the United States.


Trichuriasis: Infection with T trichiura is one of the most prevalent nematode infections worldwide; approximately 800 million persons have trichuriasis worldwide, most abundantly in warm moist regions. Infection rates of up to 75% were found in young schoolchildren in Puerto Rico.

Enterobiasis: Pinworm is also highly prevalent throughout the world, particularly in countries of the temperate zone. Children are most commonly infected. Estimated prevalence rates among children in various world regions are 4–28%.

Ascariasis: Ascaris, or roundworm, infection is the common helminthic infection in humans, with an estimated worldwide prevalence of 1 billion. The causative organism, A lumbricoides, is cosmopolitan in distribution, being most abundant in tropical countries.

Hookworm: Human infection with the 2 species of hookworm, A duodenale and N americanus, is estimated to affect approximately 550-750 million people.[2]

Strongyloidiasis: The infection is more common in tropical countries with poor sanitation, especially in countries of Southeast Asia and parts of Africa. Strongyloides stercoralis is also endemic in Jamaica and presumably elsewhere in the Caribbean. An estimated 30-100 million persons worldwide have strongyloidiasis.

Trichinosis: Trichinella species are distributed throughout the world and are spread widely in nature among a large number of carnivorous animals, with humans acting as an incidental host. Trichinosis has been a major public health problem and has been reported in many Asian countries, including China, Japan, Korea, and Thailand.

Dracunculiasis: Estimates of the number of people infected with Dracunculus medinensis in Africa, the Middle East, India, and other tropical areas range from 50-150 million. An aggressive eradication campaign has been underway to eliminate D medinensis, which is called the Guinea worm.

Filariasis: An estimated 120 million people are infected with Wuchereria bancrofti, Brugia malayi, and Brugia timori.

Loiasis: Loa loa is irregularly distributed in Africa. It is estimated that between 3 and 13 million people in West and Central Africa are infected.

Onchocerciasis: Onchocerca volvulus infects 20 million people in West, Central, and East Africa and another 1 million people in scattered foci in Central America and South America. The disease caused by this filarial worm is called river blindness.

Anisakiasis: Approximately 20,000 cases of anisakiasis are reported annually worldwide; over 90% are from Japan and most others from Spain, the Netherlands, and Germany, depending on the habits of fish consumption.[3]


Nematode infections are usually asymptomatic or subclinical.

Strongyloidosis may be fatal in immunocompromised patients and in newborns.

Intestinal nematodes can cause some GI problems (eg, abdominal pain, diarrhea, anorexia, weight loss, malaise).

Hookworms can cause serious anemia.

Onchocerciasis can result in blindness (river blindness).

Trichinosis can cause life-threatening manifestations, including myocarditis, CNS involvement, and pneumonitis.

Larvae of Anisakis species may become embedded in the gastric mucosa; endoscopy may reveal erythema, edema, severe erosive gastritis, a tumorlike nodule, or ulcerations.

Some of the more serious nematode infections result in symptoms from inflammatory responses in vital organs and nutritional deficiencies.


Nematode infections have no known racial predilection.


Nematode infections have no known sexual predilection.


E vermicularis infection (pinworm) is more common in children than in adults.

Contributor Information and Disclosures

Murat Hökelek, MD, PhD Professor, Department of Clinical Microbiology, Istanbul University Cerrahpasa Medical Faculty, Turkey

Murat Hökelek, MD, PhD is a member of the following medical societies: American Society for Microbiology, Turkish Society for Parasitology

Disclosure: Nothing to disclose.


Larry I Lutwick, MD Professor of Medicine, State University of New York Downstate Medical School; Director, Infectious Diseases, Veterans Affairs New York Harbor Health Care System, Brooklyn Campus

Larry I Lutwick, MD is a member of the following medical societies: American College of Physicians, Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Arnold C Cua, MD Physician, Department of Infectious Diseases, Renown Medical Center

Arnold C Cua, MD is a member of the following medical societies: American College of Physicians-American Society of Internal Medicine, Infectious Diseases Society of America

Disclosure: Nothing to disclose.

Specialty Editor Board

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Received salary from Medscape for employment. for: Medscape.

Chief Editor

Mark R Wallace, MD, FACP, FIDSA Clinical Professor of Medicine, Florida State University College of Medicine; Clinical Professor of Medicine, University of Central Florida College of Medicine

Mark R Wallace, MD, FACP, FIDSA is a member of the following medical societies: American College of Physicians, American Medical Association, American Society for Microbiology, Infectious Diseases Society of America, International AIDS Society, Florida Infectious Diseases Society

Disclosure: Nothing to disclose.

Additional Contributors

Joseph R Masci, MD, FACP, FCCP Professor of Medicine, Professor of Preventive Medicine, Icahn School of Medicine at Mount Sinai; Director of Medicine, Elmhurst Hospital Center

Joseph R Masci, MD, FACP, FCCP is a member of the following medical societies: American Association for the Advancement of Science, American College of Chest Physicians, American College of Physicians, American Medical Association, American Society for Microbiology, American Society of Tropical Medicine and Hygiene, Infectious Diseases Society of America, International AIDS Society, International Society for Infectious Diseases, New York Academy of Medicine, New York Academy of Sciences, Physicians for Social Responsibility, Royal Society of Medicine, Association of Program Directors in Internal Medicine, Physicians for Human Rights, Association of Professors of Medicine, HIV Medicine Association, American Academy of HIV Medicine, Association of Specialty Professors, International Association of Providers of AIDS Care, Federation of American Scientists, American Society of Tropical Medicine and Hygiene

Disclosure: Nothing to disclose.

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Strongyloides eggs in native examination from feces of a newborn.
Ascaris lumbricoides egg in feces (formalin-ethyl acetate sedimentation method).
A typical Trichuris trichiura egg in feces.
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