Parasitic diseases are ubiquitous. Despite worldwide advances in sanitation, new medications, and the heightened awareness of health care providers, between one-fourth and one-half of the world’s population has a parasitic infestation at any given time. Children’s normal developmental oral exploratory behavior places them at particular risk for acquiring parasites. Travel, immigration, the importation of vectors via international trade, and the increased number of immunocompromised hosts have all led to an increase in reported disease (see Chapter 60, Evaluation and Management of the Immunocompromised Patient and Chapter 67, Travel-Related Infections). Parasitic diseases endemic to the United States are primarily described here.

Three major groups of parasites cause human disease: helminths, protozoa, and arthropods. Three important subgroups of helminths cause human disease: nematodes (roundworms), cestodes (flatworms), and trematodes (flukes).

Important factors to be elicited in the history are included in Table 66-1. Parasitic infection may affect virtually all organ systems with symptoms depending on the system(s) involved. The varied and often nonspecific symptoms produced (Table 66-2) place parasitic infestation on the expanded differential diagnosis of most patients presenting to the emergency department (ED). Symptoms produced depend on the stage of the parasitic life cycle.

ASCARIASIS

A. lumbricoides is the largest and most prevalent human nematode worldwide, with an estimated one billion cases. 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. 66-1). Eggs are deposited in the soil, and upon ingestion the eggs hatch in the small intestine. An intermediate larval stage burrows through the gut mucosa, enters the bloodstream, and migrates to the lungs. Symptoms associated with this phase include shortness of breath, hemoptysis, eosinophilia, fever, and Loffler pneumonia. The larvae then break through the alveoli, migrate up the bronchial tree, and are swallowed. In the adult worm form, A. lumbricoides can live freely in the small intestine for up to a year, shedding eggs in the stool. At this stage, the disease usually remains asymptomatic but can cause gastrointestinal symptoms, including pain, protein malabsorption, biliary duct or bowel obstruction, and appendicitis. Rarely, death can result from complications related to intestinal obstruction, especially in developing countries.

Stool testing for ova or direct visualization of the worm is the gold standard for diagnosis. Serologic hemagglutination and flocculation tests are available but not typically used in the clinical setting. 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 μg/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. Although commonly used off-label, albendazole is not approved by the United States Food and Drug Administration (FDA) for the treatment of ascariasis._1,2 In the United States, mebendazole is only available through compounding pharmacies. Limited data exist on the safety of albendazole and mebendazole in young children; however, albendazole can be used for children as young as 1 year and mebendazole in children greater than 2 years.^2,3 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 Mass treatment campaigns in endemic regions may be considered.

ENTEROBIASIS

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, and is ingested or, less commonly, inhaled. Emerging larvae mature and migrate to the cecum. The adult worm, measuring 3 mm to 10 mm in length, is typically found in the colon, where it may live and reproduce for 1 to 2 months. The gravid female migrates to the anus, where it deposits embryonated eggs, usually during early morning hours (Fig. 66-2A). When the host stirs, the motile larvae migrate back into the body, causing symptoms of pruritus ani, dysuria, enuresis, and vaginitis. Scratching and hand–mouth behavior leads to reinoculation of the host and allows the cycle to repeat while also contributing to spread of infection. Granulomas of the pelvic peritoneum and female genital tract may occur.