Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (513 page)

CYSTICERCOSIS (PORK TAPEWORM,
TAENIA SOLIUM
)

   Definition

Pork tapeworm disease is caused by ingestion of viable metacestodes (cysticerci) or the eggs of
Taenia solium.
Ingestion results in small bowel infection by adult tapeworms.

   Who Should Be Suspected?

Most infections caused by adult pork tapeworms are asymptomatic, but intestinal, biliary, or pancreatic obstruction may occur in heavy infection. Neurocysticercosis is caused by hematologic spread of larvae to the brain. Cysticercosis is a significant cause of intracranial masses, with related symptoms, in endemic areas.

   Laboratory Findings

The diagnosis of cysticercosis relies on a combination of epidemiologic, imaging, histopathologic, and laboratory studies.

Direct detection
: Detection is usually achieved by identification of ova, proglottids, strobila, or scolices from feces. Tapeworm ova may be identified by O&P examination but cannot be distinguished from ova of
T. saginata
. Examination of portions of adult worms, like the uterine morphology of gravid proglottids, is required for speciation.

Serology
: The presence of detectable antibodies depends on the number and condition of cysticerci. Antibody detection using serum may be more sensitive than CSF for diagnosis of neurocysticercosis, especially in cases with degenerating cysts. ELISA detects antibody in serum or CSF in 75–80% with few or calcified cysts and 93% with severe CNS disease. Enzyme-linked immunoelectrotransfer blot (EITB) on serum or CSF has S/S of >94% with multiple CNS lesions and approximately 72% with single lesions. Change in titers is not reliable for judging cure. Solitary CNS lesions may not consistently induce antibody production.

Core laboratory
: Eosinophils may be slightly increased. Marked increase in ESR is unusual and suggests another diagnosis.

CSF findings
: May show increased eosinophils (in 10–77% of cases), increased mononuclear cells (≤300/μL), slightly increased protein, normal or mildly decreased glucose; parasites are not found.

GIARDIASIS

   Definition

Giardiasis is caused by infection with the flagellate protozoan
Giardia lamblia
. This pathogen has a worldwide distribution but is more prevalent in warmer climates. Infection is most commonly acquired by ingestion of cysts, with an incubation period of 2–3 weeks. After excystation and maturation, the trophozoites typically attach to the crypts of the duodenal mucosa by means of ventral disks. They do not penetrate the intestinal mucosa and typically cause minimal pathologic changes; villous atrophy may be seen in severe, chronic disease. Organisms are released and may encyst or pass in the feces as trophozoites.

   Who Should Be Suspected?

Children are most commonly infected. Although immunocompromised patients are at risk for severe disease, most infections occur in immunocompetent individuals. Acute infection may manifest with nausea, anorexia, and explosive, watery diarrhea. Systemic signs and symptoms are common with fever, malaise, and chills. The acute phase may be accompanied by a subacute or chronic phase, manifested by recurrent diarrhea. Chronic giardiasis may be complicated by weight loss, malabsorption, and electrolyte imbalance.

   Laboratory Findings

Direct detection
: Stool O&P testing should be performed on up to six specimens. Organisms may be excreted intermittently in chronic infection. Stool should be concentrated by centrifugation and permanent stains prepared. Examination of duodenal mucus, collected by duodenal aspirate or an enteric string capsule, may be used as an adjunct to stool O&P testing.

Serology
: Not useful for diagnosis because positive results cannot distinguish between acute and past infections.

Antigen detection
: Stool antigen detection or fluorescent staining provides rapid, sensitive, and specific detection of
Giardia
; sensitivity is greater than that of routine O&P examination. Antigen testing should not replace O&P testing. Multiple specimens should be examined by antigen testing to rule out giardiasis.

LARVA MIGRANS (CUTANEOUS AND VISCERAL)

   Definition

   Cutaneous larva migrans (CLM) is a skin eruption caused by migration of animal hookworms (usually
Ancylostoma caninum
or
Ancylostoma braziliense
) through the upper dermis. Filariform larvae of animal hookworms in the soil penetrate skin, usually of the feet or lower extremities, and then begin a sinuous migration through the upper dermis, causing inflammation with intense itching. These larvae cannot mature to adult stage hookworms, so they die out within several weeks.

   Visceral larva migrans (VLM) infections are caused by animal nematode larvae that are unable to mature into adult stage worms. Disease is caused by the migration of larvae through human organs. VLM occurs worldwide. Toxocariasis and classic VLM syndromes are caused by
Toxocara canis
and less commonly by
Toxocara cati
. Both of these ascarid nematodes have complex life cycles in dogs and cats, respectively, involving vertical transmission to pups and kittens, which excrete large numbers of embryonated eggs in their feces. When ingested by humans, the eggs hatch and the resulting larva is able to penetrate through tissues and migrate through different organs.