eMedicine Specialties > Infectious Diseases > Parasitic Infections

Nematode Infections

Author: Murat Hökelek, MD, PhD, Technical Consultant of Parasitology Laboratory, Associate Professor, Department of Clinical Microbiology, Ondokuz Mayis University Medical School, Turkey
Coauthor(s): 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; Arnold C Cua, MD, Physician, Department of Infectious Diseases, Renown Medical Center
Contributor Information and Disclosures

Updated: Oct 23, 2008

Introduction

Background

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

Pathophysiology

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.

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.

Frequency

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%.

International

• 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.
• 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 West and Central Africa.
• 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.

Mortality/Morbidity

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

Race

• Nematode infections have no known racial predilection.

Sex

• Nematode infections have no known sexual predilection.

Age

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

Clinical

History

Several clinical signs and symptoms can occur in patients with nematode infections.

• Lung invasion - Löeffler or Löefflerlike syndrome (ascariasis, hookworm infections, strongyloidiasis)
  • Fever
  • Cough
  • Blood-tinged sputum
  • Wheezing
  • Rales
  • Dyspnea
  • Substernal pain
  • Pulmonary consolidations
  • Eosinophilia
  • Urticaria
  • Asthma
  • Angioneurotic edema
• Intestinal invasion
  • May be asymptomatic (small number)
  • Abdominal pain (usually vague)
  • Abdominal cramps/colic
  • Diarrhea
  • Vomiting (rarely)
  • Constipation (occasionally)
• Muscle and other tissue invasion - Trichinosis
  • Myalgias
  • Fever
  • Edema and spasm
  • Periorbital and facial edema
  • Photophobia
  • Sweating
  • Conjunctivitis
  • Weakness or prostration
  • Pain upon swallowing
  • Subconjunctival, retinal, and ungual hemorrhages
  • Rashes and formication
  • Encephalitis, myocarditis, nephritis
  • Pneumonia, meningitis, neuropathy
• Lymphatic filariasis -W bancrofti, B malayi, B timori
  • Inflammatory signs (pain, tenderness, swelling, erythema)
  • Lymphadenitis/lymphangitis
  • Orchitis
  • Fever
  • Abscesses
  • Obstructive signs (lymphatic varices, hydrocele)
  • Lymphedema and elephantiasis
  • Chyluria
  • Hypereosinophilia
• Loiasis -Loa loa
  • Calabar swellings (recurrent subcutaneous inflammation/swelling)
  • Eye worm (adults or larvae migrating under conjunctiva)
  • Eosinophilia (may exceed 70%)
  • Fever, irritability, urticaria, and pruritus
• Onchocerciasis -O volvulus
  • Dermatitis
  • Nodules
  • Lymphadenitis
  • Ocular changes
    • Intraocular microfilariae
    • Punctate keratitis
    • Sclerosing keratitis
    • Anterior uveitis chorioretinitis
    • Optic neuritis
    • Optic atrophy
    • Glaucoma
    • Blindness (river blindness)
• Dracunculiasis -D medinensis, Guinea worm disease
  • Allergic manifestations
    • Erythema
    • Urticaria
    • Pruritus
    • Nausea
    • Vomiting
    • Giddiness
    • Syncope
    • Fever (occasionally)
  • Local lesions
    • Papule
    • Sterile blister
    • Ulceration
    • Abscesses
  • Worm protrusion from skin
• Toxocariasis -Toxocara canis/Toxocara cati, visceral or ocular larva migrans
  • Eosinophilia
  • Visceral larva migrans
  • Ocular larva migrans
• Cutaneous larva migrans -Ancylostoma braziliense, creeping eruption
  • Itching and red papules
  • Serpiginous track
  • Edema and acute inflammation
  • Scars
  • Secondary infection

Physical

• T trichiura infections: In heavily infected people, trichuriasis appears to manifest as mild anemia, eosinophilia, bloody diarrhea (classic trichuris dysentery syndrome), growth retardation (chronic trichuris colitis with growth retardation), abdominal pain, finger clubbing, and rectal prolapse (especially in children).
• E vermicularis infection (pinworm): The most common symptom of pinworm infection is nocturnal perianal pruritus. Occasionally, the migration of the parasite produces ectopic disease (eg, appendicitis, chronic salpingitis, ulcerative lesions in the small or large bowel). Enuresis has been blamed on the pinworm.
• A lumbricoides infection
  • A more serious complication of ascariasis is encountered when a mass of worms obstructs the lumen of the small bowel. This acute abdominal condition is commonly observed in children with heavy infections. The presentation is similar to that of acute intestinal obstruction, with vomiting, abdominal distention, and cramps.
  • Another obstructive syndrome is encountered when Ascaris worms invade the biliary duct and cause pancreatic-biliary ascariasis. The most common presenting feature is abdominal pain, observed in 98% of patients. Less common features include ascending cholangitis, acute pancreatitis, and, rarely, obstructive jaundice.
  • Ascariasis in pregnant women results in intrauterine growth retardation.
• Hookworms
  • The major manifestations of hookworm disease include iron deficiency anemia and chronic protein energy malnutrition. The development of these clinical features depends not only on the worm burden but also on the amount of absorbable dietary iron.
  • During the phase of worm attachment to small intestine mucosa, abdominal pain, diarrhea, and weight loss may be noted. In addition, malabsorption has been reported in children and, less commonly, in adults.
• S stercoralis infection
  • Burning or colicky abdominal pain, often epigastric, occurs and is associated with diarrhea and the passage of mucus.
  • Some patients with strongyloidiasis report nausea, vomiting, and weight loss, with evidence of malabsorption or of protein-losing enteropathy.
  • Massive larval invasion of the lungs and other tissues may occur with hyperinfection, usually in immunocompromised hosts. In this syndrome, the worm is able to complete its entire life cycle in the human, and the symptoms are related to the large burden of larvae migrating in the host.
  • Severe generalized abdominal pain, diffuse pulmonary infiltrates, ileus, shock, and meningitis or sepsis due to gram-negative bacilli may occur.
• T spiralis infection : Myositis with pain, swelling, and weakness is common; it usually first develops in the extraocular muscles and then involves the masseters, neck muscles, limb flexors, and lumbar muscles. Some patients report headache, cough, shortness of breath, hoarseness, and dysphagia.

Causes

• Ingestion of mature eggs in fecally contaminated food or drink
• Larval penetration of skin (hookworms and S stercoralis)
• Larvae introduced into human host by arthropod vector (mosquitoes for filariasis and flies for O volvulus)

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