A. lumbricoides is the largest and most prevalent human nematode, with an estimated one billion cases worldwide; most cases of death are because of intestinal obstruction. Although it is most commonly found in tropical and subtropical climates, it is present throughout the United States. Ascariasis is most common in preschool and early-school age children. From an egg measuring 65 μm by 45 μm, this nematode can grow to a length of 30 cm (Fig. 65-1). After being deposited in the stool, the egg matures over 3 weeks. Upon ingestion, the egg hatches in the small intestine. The larvae burrow through the gut mucosa, enter the bloodstream, and migrate to the lungs. They cause shortness of breath, hemoptysis, eosinophilia, fever, and Loffler pneumonia as they break through the alveoli, migrate up the bronchial tree, and are swallowed. Maturing to the adult form, A. lumbricoides can live freely in the small intestine for up to a year, shedding eggs in the stool. At this stage, it usually remains asymptomatic but can cause gastrointestinal symptoms, including pain, protein malabsorption, biliary duct or bowel obstruction, and appendicitis.
Ascaris lumbricoides. (Used with permission from S. Margaret Paik, MD.)
Although stool testing for ova is diagnostic, serologic hemagglutination and flocculation tests are available. Albendazole (400 mg orally as a single dose), mebendazole (100 mg orally twice daily for 3 days or 500 mg orally once) or ivermectin (150–200 μ/kg orally as a single dose) is curative. Dosing is the same in adults and children, though ivermectin is approved only for children weighing 15 kg or more. Piperazine salts (50–75 mg/kg for 2 days) are recommended for ascariasis complicated by intestinal or biliary obstruction, as they cause relatively rapid expulsion of the worms.1–3 If multiple infestations are present, Ascaris should be treated first, as treatment of other parasites may stimulate a large worm burden to migrate simultaneously, causing obstruction.4 Preventive therapy in endemic regions may be considered.
E. vermicularis (pinworm) is present in all parts of the United States and affects individuals of all ages and socioeconomic levels. The most common presentation is that of a toddler or small child with anal itch. The egg is oval, approximately 50 μm by 25 μm in size. It is inhaled or ingested and hatches between the ileum and ascending colon, growing to an adult length of 3 mm to 10 mm. The adult may live and reproduce in the colon for 1 to 2 months. The gravid female migrates to the anus, where it deposits embryonated eggs, usually during early morning hours (Fig. 65-2A). When the host stirs, the adult will migrate back into the body, causing symptoms of pruritus ani, dysuria, enuresis, and vaginitis. Scratching and hand–mouth behavior reinoculates the host, and the cycle repeats. Granulomas of the pelvic peritoneum and female genital tract may occur.
A. Pinworms. Multiple tiny pearly white worms are seen at the anus. B. This photomicrograph depicts the eggs of the nematode, or round worm, Enterobius vermicularis, mounted on cellulose tape. (Reproduced with permission from Centers for Disease Control Public Health Image Library.)
Scotch tape, placed sticky side to perianal skin when the child first awakens and then viewed under low power (Fig. 65-2B), is usually diagnostic; but repeated examination may be necessary to find the eggs. Treatment is with albendazole, 400 mg orally, pyrantel pamoate (11 mg/kg), or mebendazole (100 mg).1,5 Each drug is given as a single dose, with a repeat given 2 weeks later to remove secondary hatchings.
T. trichiura (whipworm) is found in southern Appalachia, southwest Louisiana, and other warm rural areas. The life cycle mimics that of E. vermicularis. The eggs are of similar size and configuration, with the addition of a rounded cap at each pole (Fig. 65-3A). The adult resembles E. vermicularis, with a long whip-like projection at one end (Fig. 65-3B). It lives predominantly in the cecum and can cause malabsorptive symptoms, pain, bloody diarrhea, and fever but is usually asymptomatic. A heavy worm burden may cause a colitis-like picture and rectal prolapse and can be associated with anemia and developmental and cognitive deficits.6 Treatment is with albendazole (400 mg daily for 3 days) or mebendazole (100 mg bid for 3 days). Community control should be considered in heavily endemic areas.1,3
A. This micrograph depicts an egg from the “human whipworm,” Trichuris trichiura, the causal agent of “Trichuriasis.” B. This micrograph of an adult Trichuris female human whipworm reveals that its size in centimeters is approximately 4 cm (A and B.) (Reproduced with permission from Centers for Disease Control Public Health Image Library.)
Trichinella spiralis is found throughout the United States, with increasing prevalence in the Northeast and Mid-Atlantic states. Although less than 100 cases of clinical disease are reported annually, cysts are found at autopsy in the diaphragms of 4% of patients. Current control efforts include laws governing the feeding of swine destined for sale to the public (e.g., treatment of garbage used as feed and recommendations for the preparation of meat in the home).7
Digestive enzymes liberate the encysted larvae that lodge in the duodenum and jejunum, grow, and within 2 days, mature and copulate. The females give birth to living larvae that bore through the mucosa, become blood-borne, and migrate to striated muscle, heart, lung, and brain. Host defenses produce inflammation at each site. Although a classic triad of fever, myalgia, and periorbital edema has been described, symptoms of gastroenteritis, pneumonia, myocarditis, meningitis, and seizures can occur.
Most cases are mild and self-limited. The history and physical examination, along with elevation of muscle enzymes and eosinophilia, may suggest the need for further investigation. Serologic tests are available from the Centers for Disease Control and Prevention. Muscle biopsy is confirmative. Treatment with aspirin and steroids, is initially aimed at reducing the inflammatory symptoms. Mebendazole (200–400 mg tid for 3 days and then 400–500 mg tid for 10 days) or albendazole (400 mg bid for 8–14 days) is indicated for severe disease but may not be effective after encystment.1,7
The hookworms N. americanus and A. duodenale are found between 36-degree north and 30-degree south latitude and are one of the most prevalent infectious diseases of humans, with an estimated one billion individuals affected.7 The eggs hatch in the soil, releasing rhabditiform larvae 275-μm long that feed on bacteria and organic debris (Fig. 65-4). They double in length, molt, and may survive as filariform larvae for several weeks. Upon contact, they burrow through the skin, causing pruritus (ground itch), enter the blood, travel to the lung, and are ingested, like A. lumbricoides. Although a broad spectrum of symptoms is possible, the hallmark of hookworm infestation is the microcytic, hypochromic anemia of iron deficiency. Each adult hookworm may ingest up to 0.05 mL of blood a day. Children with chronic hookworm disease may develop a characteristic yellow–green pallor called chlorosis. Although more commonly seen with the dog and cat hookworms (Ancylostoma braziliense), these hookworms can also cause the serpentine track of cutaneous larva migrans (Fig. 65-5). Finding the ova in stool is diagnostic.8 Albendazole (400 mg qid for 2–3 days), mebendazole (100 mg bid for 3 days), or pyrantel pamoate (11 mg/kg maximum 1 g qid for 3 days) is recommended. Cutaneous larva migrans is usually self-limited, but topical application of 10% thiabendazole, ivermectin (150–200 μg orally), or albendazole (400 mg qid for 3 days) may hasten resolution.1,3,8
This micrograph depicts a hookworm rhabditiform larva, which represents its early, noninfectious immature stage. (Reproduced with permission from Centers for Disease Control Public Health Image Library.)
Pruritic cutaneous plaque characteristic of cutaneous larva migrans. (Used with permission from Peter Lio, MD.)
Strongyloides stercoralis (threadworm) is found in southern Appalachia, Kentucky, and Tennessee. Like the hookworm, it penetrates the skin, producing pruritus and cutaneous larva migrans. Pulmonary and gastrointestinal symptoms occur as the larvae migrate. The human is a definitive host. Ongoing autoinfection is slowed by the host's immune response, but immunocompromised patients and the elderly may suffer fatal infestation. The rise in the acquired immunodeficiency syndrome (AIDS) has been mirrored by a rise in reported cases of Strongyloides infestation, and infestation is increasingly common in patients who are immunocompromised for other reasons such as organ transplantation.9 A definitive diagnosis is made by recovering Strongyloides in stool, sputum, or duodenal aspirate. Ivermectin (200 μg qid for 2 days) is recommended. Albendazole (400 mg bid for 7 days) or thiabendazole (50 mg/kg/d divided bid, maximum 3 g/d for 2 days) may also be used. In disseminated strongyloidiasis, treatment may need to continue for up to 2 weeks1,9,10 (see Chapter 59, Evaluation and Management of the Immunocompromised Patient).