Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (510 page)

Guatelli JC, Siliciano RF, Kuritzkes DR, et al. Human immunodeficiency virus. In: Richman DD, Whitley RJ, Hayden FG.
Clinical Virolog
, 3rd ed. Washington, DC: ASM Press; 2009.

Howley PM, Lowy DR. Chapter 62, Papillomaviruses. In: Knipe DM, Howley PM, eds.
Fields Virology
. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2007.

Kimberlin DW. Chapter 55, Rubella virus. In: Richman DD, Whitley RJ, Hayden FG.
Clinical Virology
. 3rd ed. Washington DC: ASM Press; 2009.

Kimberlin DW, Rouse DJ. Genital herpes.
N Engl J Med.
2004;350:1970–1977.

Knuchel MC, Jullu B, Shah C, et al. Adaptation of the ultrasensitive HIV-1 p24 antigen assay to dried blood spot testing.
J Acquir Immune Defic Syndr.
2007;44:247–253.

Lambert JS, Harris DR, Stiehm ER, et al. Performance characteristics of HIV-1 culture and HIV-1 DNA and RNA amplification assays for early diagnosis of perinatal HIV-1 infection.
J Acquir Immune Defic Syndr.
2003;34:512–519.

Lane JM, Ruben FL, Neff JM, Millar JD. Complications of smallpox vaccination 1968; national survey in the United States.
N Engl J Med.
1969;281:1201–1208.

Markowitz LE, Preblud SR, Orenstein WA, et al. Patterns of transmission in measles outbreaks in the United States, 1985–1986.
N Engl J Med.
1989;320:75–81.

Poggio GP, Rodriguez C, Cisterna C, et al. Nested PCR for rapid detection of mumps virus in cerebrospinal fluid from patients with neurological diseases.
J Clin Microbiol.
2000;38: 274–278.

Young NS, Brown KE. Parvovirus B19.
N Engl J Med.
2004;350:586–597.

   
INFECTIOUS DISEASES CAUSED BY PARASITIC PATHOGENS

Parasites are eukaryotic pathogens; they may be single celled or multicelled. Parasites are responsible for an enormous disease burden worldwide. Infection and disease are especially common in developing nations, in which large segments of the population may be infected, and infection with multiple pathogens may be frequent. Improved sanitation and control of vector populations have reduced, but not eliminated, the burden of parasitic diseases in industrialized nations.

Parasites may have complicated life cycles, and there are varied modes of transmission to humans. Oral transmission is a common route for spread of infection; enteric parasites are responsible for the greatest burden of parasitic infection. Arthropod-transmitted parasites, such as
Plasmodium
spp., are also responsible for an enormous disease burden. Some parasites may be transmitted by direct invasion, as through skin, or other means of infection. Immunocompromised patients, like patients with AIDS, are at increased risk for severe disease.

Most parasitic disease is diagnosed by direct detection of organisms in infected specimens. Detection of specific antigens provides sensitive and specific diagnosis for several common parasitic pathogens, like
Giardia
and
Cryptosporidium
. Serologic assays may contribute to diagnosis and may be useful for epidemiologic studies. Isolation of parasites in culture is restricted to a few pathogens and is not widely available for routine diagnosis. Molecular diagnostic strategies are playing an increasingly important role in diagnosis and definitive speciation.

Common human parasites may be divided into different, genetically related groups:

   
Protozoa
: Protozoan species are single-celled parasites. There are four groups: amoeboid protozoa, ciliated protozoa, flagellated protozoa, and sporozoans.

   
Helminths
: Helminth species are parasitic worms. There are three major groups: cestodes (segmented tapeworms), nematodes (roundworms) and trematodes (flukes).

See: Macroscopic Examination, Parasites; Ova and Parasite Examination, Stool; Blood Parasite Examination in Chapter
17
, Infectious Disease Assays.

AMEBIASIS

   Definition

Invasive amebiasis is caused by the protozoan parasite
Entamoeba histolytica
.
Entamoeba histolytica
is primarily seen in Central and South America, Africa, and the Indian subcontinent.
Entamoeba histolytica
is transmitted by ingestion of fecally contaminated water or food. The trophozoites are able to invade into the intestinal mucosa, leading to the formation of flask-shaped ulcers. Trophozoites may gain access to the central circulation, providing access to distant organs, most commonly the liver, but also brain, lung, and others. During multiplication, some amebae revert to the cyst form, which is excreted in stool, leading to continuing transmission of infection.

   Who Should Be Suspected?

   Amebiasis is a symptomatic, but self-limited, disease in approximately 90% of infected patients; asymptomatic disease occurs in about 10% of patients. Most symptomatic patients present with GI disease manifested by low fever, abdominal pain, and diarrhea, which may be bloody. Organisms are able to penetrate into, and through, the intestinal mucosa, causing dysentery or extraintestinal disease. The liver abscess is the most common site of extraintestinal infection.

   Risk for symptomatic infection depends in part on immunity; travelers from nonendemic areas are at greatest risk when visiting endemic regions. In asymptomatic patients, it may be important to differentiate
E. histolytica
from
Entamoeba dispar
. The latter does not require eradication, but
E. histolytica
“carriage” poses a significant risk of progression to invasive disease, even after months of asymptomatic infection.

   Laboratory Findings