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.
Clinical Presentation
Intestinal infection with parasites is often initially asymptomatic in a carrier state. Children with intestinal parasites can present with failure to thrive, diarrhea, abdominal pain, a protuberant abdomen, anemia, blood in the stool, and delayed development. A careful history with particular attention to travel and food and water intake is valuable. Additional details about sources and geography can be found in Table 99-1
Differential Diagnosis
The differential diagnosis for intestinal parasite infections is broad and includes noninfectious malabsorption and infectious diarrhea caused by viruses or bacteria. Anemia and failure to thrive seen in chronic infection with some parasites can also be misdiagnosed as other etiologies of iron-deficiency anemia and other systemic reasons for failure to thrive. Therefore, chronic infection should be considered in the differential of failure to thrive.
Evaluation and Management
Laboratory evaluation for parasitic infection can start with sending stool for ova and parasites (O+P). Several parasites are not found on standard testing such as Cryptosporidium, Cyclospora, and Microspora spp.; if these pathogens are suspected, the microbiology laboratory should be notified. Rapid immunoassays for Giardia spp., Cryptosporidium spp., and amebiasis can be sent and are more sensitive than O+P. Pinworms can be diagnosed by tape test to the anus. Mucosal biopsy can aid in diagnosis of Giardia spp. and Entamoeba histolyticum. Serology can be used for helminthic infections.
In addition to rehydration and supportive care, antimicrobial recommendations for each intestinal parasite covered in this chapter can be found in Table 99-1.
Extraintestinal Parasites
Extraintestinal parasites include bloodborne parasites and parasites that affect other sites of the body. Asymptomatic infection with these agents may underestimate their prevalence in the United States; for example, infection with Toxoplasma spp. may be as high as 22% of the United States population. The following sections discuss particular pathogens.
Etiology and Pathogenesis
The pathogenesis of individual parasites differs by the type of parasite and the mode of transmission. Protozoa are free-living, single-celled eukaryotic organisms. Systemic protozoan infections in humans include T. gondii, Plasmodium spp., Babesia spp., Naegleria fowleri, and Acanthamoeba spp. A systemic parasite particularly important in pediatrics because of congenital infection is T. gondii. T. gondii is the third leading cause of death from foodborne illness in the United States. It is estimated that about 22% of the U.S. population has been infected with Toxoplasma spp. from undercooked meat or contaminated food or utensils in contact with the parasite, zoonotic transmission often from Toxoplasma oocytes shed in cat litter or soil, or newly infected mothers transmitting the organisms to unborn children. Toxocara is also a roundworm infection transferred to humans from domestic animals, with estimates of almost 14% of the United States population estimated to be infected.
Leishmania and Trypanosoma are bloodborne flagellates that are transmitted by the bite of blood-sucking insects. Leishmania spp. are spread by the sandfly. African trypanosomes, the cause of African sleeping sickness, are spread by the tsetse fly, and American trypanosomes, the cause Chagas’ disease, are spread by reduviid bugs.
Systemic helminth infections, including Strongyloides stercoralis, Trichinella spiralis, Ascaris lumbricoides, Toxocara canis, and Trichuris trichiura, are spread via fecal contamination with worms penetrating into the toes of barefoot humans or through ingestion of cysts (Figure 99-4). Discussion of individual parasites is included in Table 99-2.
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