Harrison's Practice: Pork Tapeworm Infections and Cysticercosis

Harrison's Practice

Pork Tapeworm Infections and Cysticercosis

Definition

The pork tapeworm Taenia solium can cause 2 distinct forms of infection.
- Taeniasis solium: adult tapeworm infection in the intestine
- Cysticercosis: larval (cysticercal) infection in tissues
Humans are the only definitive hosts for T. solium.
Pigs are the usual intermediate hosts.

Epidemiology

Incidence
- ~50 million people worldwide are infected by T. solium.
- ~50,000 people die annually of cysticercosis because of central nervous system (CNS) complications.
- Neurocysticercosis is the most common cause of seizures in the developing world.
- The prevalence of neurocysticercosis is increasing in the U.S., primarily in California.
Age
- All age groups are susceptible.
- The age of infection depends on the time of ingestion of poorly cooked pork (taeniasis solium) or of Taenia eggs (cysticercosis).
Sex
- Both sexes are equally affected.
Geographic distribution
- T. solium exists worldwide but is most prevalent in:
  - Latin America
  - Sub-Saharan Africa
  - China
  - Southern and Southeast Asia
  - Eastern Europe
- Cysticercosis occurs in industrialized nations largely as a result of the immigration of infected persons from endemic areas.

Risk Factors

Consumption of undercooked pork (risk factor for taeniasis solium)
Ingestion of T. solium eggs through close contact with a tapeworm carrier or an infected household member (risk factor for cysticercosis)
Current or prior residence in an endemic area

Etiology

Taenia solium (pork tapeworm)
- Helminth, cestode
- Normal life cycle
  - Eggs or gravid proglottids are passed with feces.
    - Eggs can survive for days to months in the environment.
  - Pigs become infected by ingesting vegetation contaminated with eggs or gravid proglottids.
  - Humans become infected by ingesting raw or undercooked infected pork.
  - Adult tapeworms produce proglottids that mature, become gravid, detach, and migrate to the anus or are passed in the stool.
Transmission
- Taeniasis solium
  - Intestinal tapeworms are ingested in undercooked pork containing cysticerci.
- Cysticercosis
  - T. solium eggs are ingested, usually as a result of close contact with a tapeworm carrier (as opposed to the ingestion of undercooked pork).
  - Autoinfection may occur if a person with an egg-producing tapeworm ingests eggs derived from his or her own feces.
Pathogenesis
- Cysticerci can be found anywhere in the body but are most commonly detected in the brain, skeletal muscle, subcutaneous tissue, or eye.
- The clinical presentation of cysticercosis depends on the number and location of cysticerci as well as the extent of associated inflammatory responses or scarring.
- Hydrocephalus results from obstruction of cerebrospinal fluid (CSF) flow by cysticerci and accompanying inflammation or from CSF outflow obstruction due to arachnoiditis.

Associated Conditions

Up to 15% of patients with cysticercosis have concomitant taeniasis solium.

Symptoms & Signs

T. solium intestinal infection (taeniasis solium)
- May be asymptomatic
- Proglottids are passed with feces.
- Infrequent signs/symptoms include:
  - Epigastric discomfort
  - Nausea
  - Polyphagia
  - Weight loss
  - Diarrhea
  - Vitamin B₁₂ deficiency
Cysticercosis
- Neurologic symptoms most common
- Seizures (generalized or focal)
- Hydrocephalus with signs of increased intracranial pressure
  - Headache
  - Nausea
  - Vomiting
  - Changes in vision
  - Dizziness
  - Ataxia
  - Confusion
  - Papilledema
  - Altered mental status
- Neurocysticercosis is often classified as:
  - Parenchymal, with lesions confined to the parenchymal CNS tissues
  - Extraparenchymal, including racemose (subarachnoid) and ventricular involvement

Differential Diagnosis

Intestinal T. solium infection (taeniasis solium)
- Taenia saginata infection
- Diphyllobothrium latum infection
Cysticercosis
- CNS lesions due to toxoplasmosis, echinococcosis, amebiasis
- Lymphoma
- Tumor
- Pyogenic CNS abscess

Diagnostic Approach

Intestinal T. solium infection (taeniasis solium)

Infection is diagnosed by the detection of eggs or proglottids in stool or the perianal area.
- Distinguishing T. saginata from T. solium requires examination of mature proglottids or the scolex.
- Serologic tests are not helpful.

Cysticercosis

Diagnosis can be difficult.
Diagnostic certainty is possible only with definitive demonstration of the parasite (absolute criteria; see below).
In many cases, diagnostic certainty is not possible.
- Instead, a clinical diagnosis is made on the basis of clinical presentation, radiographic studies, serologic tests, and exposure history.
In patients from endemic areas, the following combination of clinical criteria is almost always indicative of neurocysticercosis.
- Single enhancing lesions presenting with seizures
- Normal physical examination
- No evidence of systemic disease (e.g., no fever, adenopathy, or chest radiographic abnormality)
- Constellation of rounded CT lesions 5–20 mm in diameter with no midline shift
- Evidence of calcifications on head CT (50% of patients)
A consensus conference has proposed criteria for diagnosis.
- Absolute criteria
  - Histologic observation of the parasite in excised tissue
  - Funduscopic visualization of the parasite in the eye (in the anterior chamber, vitreous, or subretinal spaces)
  - Neuroimaging studies demonstrating cystic lesions that contain a characteristic scolex
- Major diagnostic criteria
  - Neuroradiologic lesions suggestive of neurocysticercosis
  - Demonstration of antibodies to cysticerci in serum by enzyme-linked immunoelectrotransfer blot
  - Resolution of intracranial cystic lesions, either spontaneously or after therapy with antiparasitic agents
- Minor diagnostic criteria
  - Neuroimaging findings consistent with but less characteristic of cysticercosis
  - Clinical manifestations suggestive of neurocysticercosis (e.g., seizures, hydrocephalus, altered mental status)
  - Evidence of cysticercosis outside the CNS (e.g., cigar-shaped soft-tissue calcifications)
  - Detection of antibody in CSF by enzyme-linked immunosorbent assay
- Epidemiologic criteria
  - Exposure to a tapeworm carrier or to a household member infected with T. solium
  - Current or prior residence in an endemic area
  - Frequent travel to an endemic area
- The diagnosis is confirmed in patients with either 1 absolute criterion or a combination of 2 major criteria, 1 minor criterion, and 1 epidemiologic criterion.
- A probable diagnosis is supported by the fulfillment of:
  - 1 major criterion plus 2 minor criteria
  - 1 major criterion plus 1 minor criterion and 1 epidemiologic criterion
  - 3 minor criteria plus 1 epidemiologic criterion
Routine investigation before decisions are made about the need for medical therapy for suspected neurocysticercosis should include:
- Ocular examination for ocular cysts
- Careful physical examination of skin/soft tissues for nodules
- Careful examination of the spine
  - Spinal cysticerci are a potential indication for surgical intervention.

Laboratory Tests

Antibody detection of cysticerci
- An immunoblot assay using lentil lectin–purified glycoproteins is > 99% specific and highly sensitive.
- Patients with single intracranial lesions or with calcifications may be seronegative.
- With this assay, serum provides greater diagnostic sensitivity than CSF.
CSF analysis
- While the CSF is usually abnormal in neurocysticercosis, CSF abnormalities are not pathognomonic.
- Patients may have CSF pleocytosis with a predominance of lymphocytes, neutrophils, or eosinophils.
- Protein levels may be elevated.
- Glucose concentrations are usually normal but may be depressed.
- Detection of CSF antibody may be useful when only unfractionated antigens are used.
Antigen detection assays, especially those detecting antigen in CSF, may also facilitate diagnosis.
- However, these assays are not widely available, and published information on their use is limited.

Imaging

Neuroimaging (CT or MRI) (see Figure 1)
- Findings that constitute the primary major diagnostic criterion include:
  - Cystic lesions with or without enhancement (e.g., ring enhancement)
  - ≥1 nodular calcifications (which may also have associated enhancement)
  - Focal enhancing lesions
- Cysticerci in the brain parenchyma are usually 5–20 mm in diameter and rounded.
- Cystic lesions in the subarachnoid space or fissures may enlarge up to 6 cm in diameter and may be lobulated.
- For cysticerci within the subarachnoid space or ventricles, the walls may be very thin, and the cyst fluid is often isodense with CSF.
- Obstructive hydrocephalus or enhancement of the basilar meninges may be the only finding on CT in extraparenchymal neurocysticercosis.
- Cysticerci in the ventricles or subarachnoid space are usually visible to an experienced neuroradiologist on MRI or on CT with intraventricular contrast injection.
- CT is more sensitive than MRI in identifying calcified lesions, whereas MRI is better for identifying cystic lesions and enhancement.

Diagnostic Procedures

Brain biopsy of lesions in cases where the diagnosis is uncertain and other diagnostic criteria are not met

Classification

Neurocysticercosis is often classified as:
- Parenchymal, with lesions confined to the parenchymal CNS tissues
- Extraparenchymal, including racemose (subarachnoid) and ventricular involvement

Treatment Approach

Intestinal T. solium infection is treated with antiparasitic drugs.
The management of neurocysticercosis is multidimensional and includes:
- Symptom management with anticonvulsants if seizures occur
- Medical management with antiparasitic agents and anti-inflammatory drugs (glucocorticoids)
- Surgical management for extraparenchymal disease (e.g., ventricular, ocular, or spinal cysts)

Specific Treatments

Intestinal T. solium infection (taeniasis solium)

Single dose of praziquantel (10 mg/kg)
- Praziquantel can evoke an inflammatory response in the CNS if concomitant cryptic cysticercosis is present.
Up to 15% of patients with cysticercosis have concomitant taeniasis solium.

Neurocysticercosis

Seizures can usually be controlled with antiepileptic agents (see Approach to Seizures).
- If parenchymal lesions resolve without development of calcifications and patients remain free of seizures, antiepileptic therapy can usually be discontinued after 2 years.
Use of antiparasitic drugs
- Complex decision that depends on the number, viability, location, and extent of lesions
- Careful assessment of parenchymal versus extraparenchymal involvement required (see Symptoms & Signs)
  - Determines whether additional methods are necessary (e.g., surgical intervention for extraparenchymal involvement)
- Conflicting data on utility of antiparasitic treatment
  - Some studies have shown a trend toward faster resolution of neuroradiologic abnormalities with treatment.
  - A recent trial demonstrated greater resolution of cysts and fewer generalized seizures in patients treated with albendazole.
  - Each patient must be assessed individually, given the complexity of the decision and the potential for neurologic deterioration due to the inflammatory response during antiparasitic treatment.
- Preferred antiparasitic agents
  - Albendazole (15 mg/kg per day for 8–28 days)
    - Drug of choice; better CNS penetration than praziquantel
    - Must be taken with food for adequate absorption
  - Praziquantel (50–60 mg/kg daily in 3 divided doses for 15 days or 100 mg/kg in 3 doses given over a single day) is an alternative.
  - Both agents may increase the inflammatory response around the dying parasite, exacerbating seizures or hydrocephalus.
    - Thus, concomitant use of glucocorticoids is recommended.
- For patients with only calcified lesions or cysticercotic encephalitis, antiparasitic treatment is not recommended.
Hydrocephalus
- Emergent reduction of intracranial pressure is the mainstay of therapy.
- In the case of obstructive hydrocephalus or ventricular cysts, the preferred approach is surgical removal of the cysticercus.
- An alternative approach is initially to perform a diverting procedure, such as ventriculoperitoneal shunting.
- Historically, many shunts have had high failure rates, but lower failure rates have recently been attained with:
  - Treatment with antiparasitic drugs or
  - Long-term administration of immunomodulators (e.g., glucocorticoids) or
  - Use of flow-sensitive shunts
- Open craniotomy to remove cysticerci is now required only infrequently.
- For patients with subarachnoid cysts or giant cysticerci, glucocorticoids are needed to reduce arachnoiditis and accompanying vasculitis.
- Most authorities recommend prolonged courses of antiparasitic drugs and shunting when hydrocephalus is present.
In cases with cerebral edema and elevated intracranial pressure due to large numbers of inflamed lesions, glucocorticoids are the mainstay of therapy, and antiparasitic drugs should be avoided.
For ocular and spinal medullary lesions, drug-induced inflammation may cause irreversible damage.
- Most patients should be managed surgically and should be screened for cysts in these locations before antiparasitic therapy is initiated.

Monitoring

Patients should be monitored serially during treatment, given the risk for neurologic deterioration in the setting of inflammatory reactions to dying parasites.
Patients should be followed for ongoing seizures after treatment and should undergo serial CNS imaging.

Complications

Chronic meningitis
Arachnoiditis
Hydrocephalus
Stroke
Seizures

Prognosis

Most patients with parenchymal cysticercosis either remain asymptomatic or develop a self-limited seizure disorder.
The presence of calcifications in the brain (inactive cysts) has been associated in some series with a risk of ongoing seizures.
Extraparenchymal neurocysticercosis (subarachnoid, ventricular, or spinal) carries a relatively poor prognosis.
Subarachnoid disease
- Previously associated with a case-fatality rate of ~50%
- Prognosis is much improved with a combination of:
  - Shunting procedures for hydrocephalus
  - Antiparasitic agents
  - Surgical extirpation of cysts, when required

Prevention

Measures for prevention of intestinal T. solium infection entail the killing of cysticerci by:
- Adequate cooking of infected pork
- Exposure to temperatures as low as 56 °C for 5 minutes
- Refrigeration or salting for long periods
- Freezing at –10 °C for 9 days
Cysticercosis is prevented by minimizing opportunities for ingestion of fecally derived eggs through:
- Good personal hygiene
- Effective fecal disposal
- Treatment and prevention of human intestinal infections

ICD-9-CM

123.0 Taenia solium infection, intestinal form (includes pork tapeworm)
123.1 Cysticercosis