Parasitic Infections

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99 Parasitic Infections

Parasitic disease causes extensive morbidity and mortality worldwide in children, particularly Plasmodium falciparum, which causes 1 to 2.7 million deaths annually. The morbidity of parasitic disease disproportionately affects the developing world with an estimated 39 million disability-adjusted life years attributable to infections with parasitic worms. Children in the United States are still at risk for parasitic infection. More common infections include giardiasis, pinworm infections, and head lice. Contaminated water and pets can serve as exposures to particular parasites including Giardia lamblia, Cryptosporidium parvum, Toxoplasma gondii, and Toxocara canis. Although covered in Chapter 89, it should be kept in mind that infection with Trichomonas spp. is the most common parasitic infection in the United States with 7.4 million infections each year. A comprehensive review of all parasitic infections in children would exceed the scope of the chapter, so an overview discussion of the most common intestinal parasites, systemic parasites, and specific coverage of malaria is presented.

Intestinal Parasites

Etiology and Pathogenesis

Intestinal parasites infect millions worldwide, particularly in developing countries with poor access to potable water and in patients with comorbidities. Intestinal parasites that are the most common in children in the United States include: G. lamblia, Entamoeba histolytica, and C. parvum. Table 99-1 reviews additional causes of intestinal parasitic infections (Figures 99-1 and 99-2).

G. lamblia is a flagellated protozoon and the most common cause of parasitic enteritis in the United States. Outbreaks can occur from contaminated water supplies, including pools, because it is resistant to chlorination as well as person to person in daycare centers. The mode of transmission is often fecal–oral or direct person-to-person contact (Figure 99-3).

E. histolytica is a large intestinal amoeba occurring in 1% to 5% of people around the world and causes amebiasis. Cysts are ingested from contaminated water from which they enter and inhabit the colon lumen and may form shallow ulcers. Amebomas can form on the intestinal wall and present with obstruction. With penetration into intestinal wall, invasion of the bloodstream occasionally occurs, leading to amebic dysentery. By this mechanism, the parasite can pass to the biliary system in the liver and form amebic abscesses and can be transmitted to other tissues as well. Interestingly, when E. histolytica is disseminated to the liver or other tissues, peripheral eosinophilia is not seen on complete blood count.

Cryptosporidium spp. are a worldwide intracellular protozoon seen mostly in immunocompromised patients. The clinical presentation is commonly with diarrhea. Outbreaks occur in healthy individuals as well, especially because Cryptosporidium spp. are chlorine resistant and may be spread through infected water and swimming pools. Fecal–oral contamination can transmit the parasite to epithelial cells in the stomach and intestine.

Enterobius vermicularis (pinworm) is the most common helminth of industrialized nations, and causes pruritus ani. Cysts are often ingested by the host via hand contamination. The eggs hatch in the duodenum, and then adult females lay eggs on the perineum.