Common Parasitic Infections

Harold H. Osborn


Pathophysiology Clinical.Features Diagnosis Helminths

Nematodes, „(Roundworms) TrematodesiFlukes) Ce.stodes jfiatworms) Protozoa Amebas

Protozoan ,immunocompromised...H.o.sts Respiratory Tract

Gastrointestina l . Tract

Central .NervoMs. .System

Treatment Disposition Chapter References

Despite significant advances in medical knowledge and technology over the last half century, parasitic disease remains prevalent world-wide. It is estimated by the World Health Organization that 200 million people, living mostly in the rural tropics, suffer from schistosomiasis and that hookworm infects approximately a quarter of the world's population.1 Such parasites as Ascaris and Enterobius each infect 1 billion people. Ascaris is said to infect 3 million people in North America; Enterobius infection rates among children in the United States vary from 10 to 45 percent.

Worldwide, malaria is the most prevalent infectious disease among humans, affecting over 500 million and killing over 2 million people each year. 2 In the United States, parasitic disease is becoming increasingly recognized. In addition to the persistence of endemic parasites in this country, three factors account for this trend: (1) immigration to the United States of infected individuals from the countries of Asia, Africa, and Latin America; (2) increased travel by Americans, particularly to the underdeveloped parts of the world; and (3) the rise of parasitic infections among immunosuppressed patients, especially those afflicted with HIV. 3

Serologic surveys in the United States have demonstrated that 20 to 70 percent of the population have antibodies to Toxoplasma and over 90 percent have antibodies to Pneumocystis carinii. These two parasites can cause significant morbidity and mortality when they reactivate in immunosuppressed individuals and create opportunistic infections.

Finally, Cryptosporidium and Giardia are now recognized as causes of traveler's diarrhea and have become major causes of diarrhea in immunocompetent individuals in the United States. Person-to-person spread of these parasites has led to diarrheal illness in day care centers and other institutions, and contamination of municipal water supplies has led to major outbreaks. In 1993, 400,000 people in Milwaukee became ill after Cryptosporidium contaminated the drinking water, and 54 people died as a result.4 It is estimated that 10 to 15 percent of the chronic diarrhea and wasting in AIDS patients is due to infection with Cryptosporidium parvum. Recently, surveys of water throughout the United States indicate that 50 percent of rivers and lakes may be contaminated with Cryptosporidium5 This parasite cannot be contained by chlorination, iodination, or ozonation of the water; it must be eliminated by filtration.

Outbreaks of diarrhea in the summer of 1996 due to the consumption of strawberries contaminated with Cyclospora, a coccidian, highlights the challenge of emerging parasitic diseases. Another problem in the 1990s was the increasing emergence of drug-resistant strains of various parasites. Malaria provides a notable example of drug resistance and two species (Plasmodium falciparum and Plasmodium vivax) now demonstrate resistance.2 In addition, strains of leishmania are resistant to antimonial drugs in many areas, and reports of metronidazole-resistant trichomoniasis and giardiasis are increasingly common. 6

The agents that cause parasitic diseases belong to three major groups: helminths (worms), protozoa, and arthropods. The multicellular helminths include nematodes (round worms), cestodes (flat worms), and trematodes (flukes). The protozoa are single-celled organisms that cause a variety of diseases ranging from malaria to amebiasis. Arthropods are classified as ectoparasites and are medically important as obligatory intermediate hosts and as mechanical vectors in many diseases. This chapter reviews diseases caused by helminths and protozoa. Malaria is discussed in Chap 142, "Malaria."

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