Cysticercosis (Pork Tapeworm Infection) Workup

The diagnosis of cysticercosis is often based on clinical presentation, abnormal findings on neuroimaging, and serology. Occasionally, more invasive procedures (eg, brain biopsy) are required. ^[15]

Del Brutto et al have previously defined and modified the diagnostic criteria for definitive and probable neurocysticercosis in 2001 and 2012, respectively. These criteria were revised in 2017. ^[16]

Absolute criteria

Absolute criteria include the following:

• Histological demonstration of the parasite on biopsy from the brain or spinal cord lesion
• Visualization of subretinal cysticercus
• Conclusive demonstration of a scolex within a cystic lesion on neuroimaging studies

Neuroimaging criteria

Major neuroimaging criteria include the following:

• Cystic lesions without a discernable scolex
• Enhancing lesions

Confirmative neuroimaging criteria include the following:

• Resolution of cystic lesions after cysticidal drug therapy
• Spontaneous resolution of single small enhancing lesions
• Migration of ventricular cysts documented on sequential neuroimaging studies

Minor neuroimaging criteria: Obstructive hydrocephalus (symmetric or asymmetric) or abnormal enhancement of basal leptomeninges

Clinical/exposure criteria

Major clinical/exposure criteria include the following:

• Detection of specific anticysticercal antibodies or cysticercal antigens via well-standardized immunodiagnostic tests
• Cysticercosis outside the CNS
• Evidence of household contact with T solium infection

Minor clinical/exposure criteria include the following:

• Clinical manifestations suggestive of neurocysticercosis
• The patient came from or lives in an area where cysticercosis is endemic

Degrees of diagnostic certainty

Definitive diagnosis requires one of the following:

• One absolute criterion
• Two major neuroimaging criteria plus any clinical/exposure criterion
• One major and one confirmative neuroimaging criteria plus any clinical/exposure criterion
• One major neuroimaging criterion plus two clinical/exposure criteria (including at least one major clinical/exposure criterion), together with the exclusion of other pathologies that produce similar findings

Probable diagnosis is supported by one of the following:

• One major neuroimaging criteria plus any two clinical/ exposure criteria.
• One minor neuroimaging criteria plus at least one major clinical/ exposure criteria.

Findings from laboratory studies such as routine CBC counts and liver function tests are not specific. The WBC count is usually within the reference range, and most patients do not have eosinophilia unless a cyst is leaking, in which case the eosinophilia may be pronounced.

Serologic studies can be helpful in the diagnosis of cysticercosis but are limited in their usefulness in a community setting by general lack of availability, as follows:

• An enzyme-linked immunoblot transfer blot (EITB) assay can demonstrate serum or CSF anticysticercal antibodies. The findings in the serum are more sensitive than those in the CSF. The assay is highly specific for exposure to T solium. The sensitivity is high (94%) in patients with multiple lesions or extraparenchymal infection but may be as low as 28% in patients with a single parenchymal lesion. EITB assay findings may revert to negative after the cysticercus dies and are often negative in patients with only calcified lesions.
• ELISAs that use unfractionated antigen are fraught with problems regarding sensitivity and specificity, and they are reliably diagnostic only when performed on CSF.
• The observed cross-reactivity of infected sera with antigens of other parasites has limited the accuracy of serologic techniques such as ELISA. The western blot kit (QualiCode) offers qualitative detection of immunoglobulin G (IgG) antibodies to T solium with a sensitivity of 95% and a specificity of 100%. It yields rapid results (< 90 min).
• The sensitivity of ELISA using CSF is approximately 80%. False positive results may occur in patients with hydatid cysts, filariasis, tuberculous meningitis, or viral encephalitis. An active inflammatory response is likely to cause high titers; intraventricular cysts cause a low titer.
• An assay using monoclonal antibody HP10 to detect parasite secretory/excretory antigens performed well in CSF samples, with results similar to those from the EITB assay. ^[17]
• A 2017 meta-analysis found that ELISA for antibodies and EITB yield a similar diagnostic value. ^[18]
• No polymerase chain reaction (PCR) tests are available.

Carriage of tapeworms in humans can be diagnosed via detection of proglottids or eggs in feces, Taenia antigens in stool, or specific antibodies in serum. ^[19]

Stool examination for ova and parasites can occasionally be used to diagnose intestinal infection with T solium. However, most people diagnosed with cysticercosis do not have viable T solium tapeworm in their intestine, so eggs are not typically found.

• Because eggs are shed intermittently, most cases of taeniasis cannot be detected with a single stool test.
• Furthermore, egg morphology is the same for T solium and Taenia saginata.
• Antigen detection tests for stool and serologic tests for tapeworms are being studied; none is available for use in clinical practice.
• Identifying tapeworm carriers does not usually help in diagnosing neurocysticercosis, but it may be useful in detecting the source of infection in cases among US residents who have not traveled.

Neuroimaging with contrast-enhanced CT scanning or MRI is the mainstay of diagnosis.

MRI is better for detecting intraventricular types and extraparenchymal disease and visualizing the scolex within the cysticercus (as high intensity inside a cyst). Fluid attenuation inversion recovery (FLAIR) sequences are particularly helpful for identifying associated edema and the scolex. ^[20]

CT scanning is better for detecting intracerebral calcifications.

Both modalities can reveal hydrocephalus and active intraparenchymal lesions.

The ventricles may be narrowed with extensive low attenuated areas in the parenchyma, sparing the cortex.

A ring enhancing active lesion with surrounding edema is the second-most-common finding.

A homogenously enhancing lesion represents a dying larva.

Calcified lesions are also common on CT scans.

Nonenhanced CT scan of the brain demonstrates the multiple calcified lesions of inactive parenchymal neurocysticercosis.

Neurocysticercosis is classified as parenchymal or extraparenchymal based on the location of the parasite and surrounding host tissue on neuroimaging. ^[20]

Parenchymal form

The viable parenchymal form is characterized by vesicular lesions, often with evidence of associated contrast enhancement and/or surrounding edema. The scolex is often visible on high-definition imaging. This represents parasites with an intact cyst wall, vesicular fluid, and scolex, with variable degrees of inflammation surrounding the parasite, sometimes invading the cyst wall.

A single small enhancing cystic or nodular enhancing lesion smaller than 2 cm represents parasites in the process of degeneration with surrounding inflammation and variable cyst fluid.

Nodular calcifications smaller than 20 mm in diameter with or without surrounding edema and/or contrast enhancement are seen in nonviable form, representing calcified granuloma with or without surrounding inflammation and/or gliosis.

Extraparenchymal  form

Intraventricular cysticerci can be seen within the ventricles and may cause obstructive hydrocephalus or loculated hydrocephalus with disproportionate dilation of the ventricles on CT/MRI.

Subarachnoid cysticerci are found in the sylvian fissure, basilar cisterns, or interhemispheric spaces. These can present as stroke or meningitis without discrete cysts. The cysticerci often appear in clusters with proliferating membranes (racemose) and may lack a scolex.

Spinal cysticerci are seen within the spinal subarachnoid space with or without evidence of inflammation/diffuse spinal arachnoiditis. Intramedullary cysticerci can also be within the spinal cord.

EEGs are frequently obtained in patients who have experienced seizures. The EEG is abnormal in up to 50% of cases, demonstrating various findings (diffuse slowing, focal paroxysmal activity, generalized spike waves) depending on lesion number, size, and location.

No pattern is diagnostic for neurocysticercosis.

Focal abnormalities may be present in persons with active disease.

Seizures may also be caused by inactive disease (calcified nodules of the residual phase), but, in these cases, the EEG does not usually reveal focal abnormalities.

In patients who will likely require prolonged corticosteroid therapy, screening for latent tuberculosis and Strongyloides stercoralis infection is suggested before therapy is initiated. ^[20]

Lumbar puncture: Lumber puncture for a CSF study is usually unnecessary in the diagnosis of neurocysticercosis. This procedure is also contraindicated upon suspicion of increased ICP. If a lumber puncture is performed, examination of CSF shows a normal glucose concentration and protein levels and WBC counts that are usually only mildly elevated. CSF studies in individuals who have a leaking cyst that communicates with the CSF may reveal prominent CSF eosinophilia.

Biopsy: Biopsy may be required in patients with suspected neurocysticercosis who have a single brain lesion with no characteristic scolex and negative serology findings. Biopsy specimens may be taken from subcutaneous nodules or a muscle lesion. Biopsy of CNS lesions is rarely necessary.

Occasionally, CNS lesions are mistakenly identified as tumors and are diagnosed only at surgery. Upon gross examination, the cysticerci appear as 5- to 10-mm semiopaque cysts with a 1- to 2-mm mural nodule containing the scolex.

Histopathologic examination reveals a superficial tegument layer covered with microtriches, a cellular layer below that containing the cell nuclei and musculature, and a loose reticular layer characterized by canaliculi. When the parasites are viable, little surrounding inflammation is observed. Degenerating parasites, on the other hand, are invaded with an inflammatory infiltrate including lymphocytes, macrophages, plasma cells, neutrophils, and eosinophils. Cavellani et al used autopsy protocols to study the influence of age and gender on cardiac and encephalic inflammation caused by cysticercosis. They concluded that inflammation decreases with age and depends on the stage of the disease; women have a more intense response during senescence. ^[21]

Histologic studies have shown that viable cysticerci in humans and pigs have little or no surrounding inflammation. ^[22]

Cysticerci can persist in the human host for long periods, often years, without eliciting a surrounding inflammatory reaction.

In contrast, the immune-mediated inflammation around one or more degenerating cysts may precipitate symptomatic disease.

When the parasite begins to involute, either naturally or after treatment with anticysticercal drugs, granulomatous inflammation develops around the cysticerci. The predominant components of this inflammatory response include plasma cells, lymphocytes, eosinophils, and macrophages. The latter engulf parasite remnants, eventually leaving a gliotic scar with calcifications.

Early granulomas in cysticercosis are predominantly associated with a Th1 response, whereas later granulomas, in which parasite destruction is complete, have a mixture of Th1 and interleukin-4 (IL-4). The Th1 response appears to play an important role both in the pathogenesis of disease and in the clearing of the parasites, with IL-4 involved in downregulation of the initial response.