Introduction

The parasites of humans are classified into five major divisions: Protozoa (amebae, flagellates, ciliates, sporozoans, coccidia, microsporidia); Platyhelminths (cestodes, trematodes), Acantho-cephela (thorny headed worms); Nematodes (roundworms) and Arthropods (insects, spiders, mites, ticks). Geohelminths are a sub-group of soil-transmitted intestinal nematodes with similar epidemiological characteristics. They include Strongyloides, hookworm, Ascaris and Trichuris.1 In this chapter, we focus on the common protozoa, nematodes and trematodes which affect the intestine (Table 11.1). Out of the large numbers of children infected by these parasites in the developing world, most are asymptomatic and do not present to the health services for treatment.

The World Health Organization (WHO) estimates that some 3.5 billion people worldwide are infected by intestinal parasitic and protozoan infections, including 350 million with morbidity from ascariasis, 220 million from trichuris and 150 million from hookworm.2 A nationwide survey of intestinal parasites in China during 1988-92 on 1.5 million people reported a prevalence of 62.6%, of whom 43.3% had multiple parasites. The five common parasites were Ascaris lumbricoides (47.0%), Enterobius vermicularis (26.4%), Trichuris trichiura (18.8%), Giardia lamblia (2.5%) and Entamoeba histolytica (0.9%).3 In sub-Saharan African schoolchildren, on the other hand, estimates of the prevalence of helminth infections were: hookworms 32.1%, Ascaris 30.2%, Trichuris 29.5% and Schistosoma mansoni 14.0%.4 These prevalence rates have been fairly stable over the past 50 years with only a modest decrease in ascariasis. However, higher rates have been reported, such as surveys reporting that only 14% of Tanzanian and 37% of Ghanaian children were free from worm infections.5 Intestinal parasites also occur in industrialized countries, with a third of 5792 fecal specimens positive for parasites in the USA in 2000, peaking between July and October.6

The peak age for intestinal parasites tends to be school-age children but overt sickness from them is the exception. A large Tanzanian morbidity survey of schoolchildren found that no sign or symptom was significantly associated with any helminth infection, only blood in stool and hepatomegaly with Schistosoma mansoni.7 The major determinant of morbidity from parasitic diseases is the intensity of infection, but this is rarely reported in children. A Zairean study found that the highest mean intensities (measured as eggs per gram of feces (epg)) of Ascaris and Trichuris were among 2-4-year-olds, whereas school-age children excreted the most hookworm ova.8 Using disability-adjusted life years lost (DALY), WHO estimated that intestinal helminthiasis caused 39 million DALYs, with 57% attributed to hookworm, 27% to ascariasis and 16% to trichurasis.2

The most common symptoms of intestinal parasites are diarrhea and/or failure to thrive from anorexia and malabsorption. The protozoa Giardia lamblia and Cryptosporidium parvum are common causes of diarrhea in children, but other protozoa such as Cyclospora cayetanensis, Entamoeba histolytica, and Microsporidia species may also cause diarrhea, and these include Isospora belli in immunodeficient (e.g. HIV-seropositive) patients. Only five helminthic parasites are associated with diarrheal disease: Strongyloides stercoralis, Trichuris trichiura, Schistosoma mansoni, Trichinella spiralis and Capillaria philippinensis.9 Strictly intraluminal worms (e.g. Ascaris lumbri-coides) only rarely affect intestinal function

Table 11.1 Classification of intestinal parasites

Infection

Other name

Symptoms

Transmission

Alternative treatments

Treatment of choice

PROTOZOA

Intestinal amebae

Entamoeba histolytica

amebiasis

dysentery

fecal-oral

metronidazole

Entamoeba dispar

asymptomatic

fecal-oral

nil

Flagellates

Giardia lamblia (duodenale

giardiasis

chronic diarrhea,

fecal-oral

metronidazole

tinidazole

or intestinalis)

malabsorption

Ci Mates

Balantidium coli

asymtomatic

fecal-oral

nil

Coccidia

Cryptosporidium parvum

cryptosporidiosis

persistent diarrhea

fecal-oral

nitazoxanide*

Cyclospora cayetanensis

cyclosporiasis

diarrhea

fecal-oral

co-trimoxazole

Isospora belli

isosporiasis

diarrhea with AIDS

fecal-oral

co-trimoxazole

NEMATODES

Ascaris lumbricoides

roundworm

intestinal obstruction

fecal-oral

levamisole, pyrantel

albendazole1

Enterobius vermicularis

pinworm, threadworm

nocturnal anal pruritis

fecal-oral

levamisole, pyrantel

albendazole1

Ancylostoma duodenale

hookworm

iron deficiency anemia

percutaneous

levamisole, pyrantel

albendazole1

Necator americanus

hookworm

iron deficiency anemia

percutaneous

levamisole, pyrantel

albendazole1

Strongyloides stercoralis

strongyloidiasis

diarrhea

percutaneous

albendazole

ivermectin

Trichuris trichiura

whipworm, trichuriasis

dysentry, rectal

fecal-oral

levamisole, pyrantel

albendazole1

prolapse (rare)

CESTO DES

Hymenolepis nana

dwarf tapeworm

asymptomatic

fecal-oral

praziquantel

nitazoxanide*

TREMATODES

Schistosoma mansoni

bilharzia

melena, portal

percutaneous

oxamniquine

praziquantel

hypertension

Fasciolopsis buski

giant intestinal fluke

percutaneous

praziquantel

*Nitazoxanide is the drug of choi

ce (if available) where treatment is indicated; +or mebendazole

adversely when the intensity of infection is high. Infections such as giardiasis, cryptosporidiosis and severe trichuriasis may contribute to growth failure via malabsorption, so their resolution or treatment should lead to catch-up growth. Malnutrition appears to predispose children to an acute diarrheal syndrome in strongyloidiasis, which may then exacerbate the nutritional status through anorexia and enteropathy.10 Owing to the prolonged carriage of many parasites, only Strongyloides and Cryptosporidium have a consistently significant association with diarrheal disease when compared to non-diarrheal controls (Table 11.2).

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