eMedicine Specialties > Infectious Diseases > CNS Infections

Cysticercosis

Author: Mossammat M Mansur, MD, Infectious Disease Fellow, Louisiana State University
Coauthor(s): Martin Montes, MD, Fellow, Department of Medicine, Section of Infectious Disease, Baylor College of Medicine; Research Associate, Instituto de Medicina Tropical ‘Alexander von Humboldt', Universidad Peruana Cayetano Heredia, Perú; Linda S Yancey, MD, Consulting Staff, West Houston Infectious Diseases
Contributor Information and Disclosures

Updated: Jul 25, 2008

Introduction

Background

Cysticercosis (ie, tapeworm infection) is an increasingly common medical problem in the United States, especially in the Southwest and other areas where large populations migrated from endemic areas and among populations that often travel to these areas.

Cysticercosis is caused by the metacestode, or larval, stage of Taenia solium, the pork tapeworm. The Clinical syndromes caused by T solium are categorized as either neurocysticercosis (NCC) or extraneural cysticercosis (intestinal tapeworm infection). 

Neurocysticercosis refers to CNS infection of the CNS with T solium. Neurocysticercosis, which is probably the most common parasitic infestation of the CNS, has gained increased recognition in the last two decades because of the development of MRI and CT scanning in the United States and in countries where neuro cysticercosis is endemic.

Neurocysticercosis is further divided into parenchymal and extraparenchymal disease. Parenchymal disease is characterized by infection with cysticerci within the brain parenchyma. Extraparenchymal disease develops when cysticerci migrate to the CSF of the ventricles, cisterns, and subarachnoid space or within the eyes or spinal cord.

Pathophysiology

When humans ingest undercooked pork that contains cysticerci of T solium, the scolex evaginates from the cyst and develops into an intestinal tapeworm. The tapeworm grows to a length of up to 10 meters and has hundreds of proglottids. Mature proglottids contain approximately 50,000 eggs each. Free eggs or whole proglottids are released periodically into the stool of the carrier and can survive in the environment for many months.

When pigs ingest the proglottids or eggs, the eggs hatch, penetrate the pigs' intestinal wall, and spread to skeletal muscle, especially the neck, tongue, and trunk. There, the larvae mature into encysted cysticerci over 2-3 months. The cysticerci suppress the host inflammatory response and survive in tissues for months to years. The life cycle is completed when humans ingest inadequately cooked pork that contains viable cysticerci or ingest eggs. Humans are end hosts of the larval stage and develop cysticercosis similar to that in pigs. Ingestion of encysted pork does not directly cause cysticercosis; rather, it produces an intestinal infection of the adult tapeworm and a carrier state for the T solium eggs that, when ingested by humans, produce the clinical syndrome of cysticercosis.

Humans can be infected with eggs through fecal-oral transmission or possibly through autoinfection. Fecal-oral contamination usually occurs via infected food handlers who do not appropriately wash their hands before working or via ingestion of fruit and vegetables fertilized with contaminated human waste. The eggs are sticky and can often be found under the fingers of tapeworm carriers. Thus, even populations who do not eat pork can develop cysticercosis. The egg-containing feces can contaminate water supplies in endemic areas. If the water is used to irrigate fruits and vegetables, eggs are ingested with the contaminated food. Thus, people who have never visited endemic countries can also develop infection. Autoinfection involves the retrograde transmission of proglottids from the intestines into the stomach with subsequent release of T solium eggs into the gut.

Human neurocysticercosis can result after ingestion of food contaminated with T solium eggs. The cystic larval stage that normally occurs in pigs develops in the human host and spreads to the skeletal muscle and brain. In this situation, the human becomes the end intermediate host.

Cysticerci are able to survive in the human brain by disarming host defenses. The cysticercus secretes prostaglandins and other compounds (paramyosin, taeniastatin, sulfated polysaccharides) that inhibit or divert complement activation and cytokine production, resulting in only minimal host inflammation around the viable cysticercus. In addition, humoral antibodies do not kill the mature metacestode. Taeniastatin and other poorly defined factors may also interfere with lymphocyte proliferation and macrophage function, inhibiting normal cellular immune defenses. The clinical manifestations commonly result when an inflammatory response develops around a degenerating cysticercus.

Over a period of years, the parasite may lose its ability to control the host defenses. Consequently, an inflammatory response leads to degeneration of the cysticercus. An inflammatory response that occurs in the CNS parenchyma causes seizures typical of parenchymal neurocysticercosis. As the degeneration continues, the parasite becomes encased in a granuloma, which either resolves or leads to scarring and calcification. In rare cases, patients with numerous parenchymal cysticerci develop a diffuse cerebral edema termed cysticercal encephalitis. Pathologically, cysticercal encephalitis may progress to meningoencephalitis, granulomatous meningitis, focal granulomas or abscess, hydrocephalus, ependymitis, or arteritis.

Approximately 10-20% of patients with neurocysticercosis present with extraparenchymal disease, often with concomitant parenchymal disease. Subarachnoid neurocysticercosis may form in the gyri of the cerebral convexities or in the fissures of the brain, especially the sylvian fissures. These forms of neurocysticercosis are associated with parenchymal inflammation and resemble parenchymal disease in manifestations and pathogenesis.

In severe cases, cysticerci in the sylvian fissures may enlarge to several centimeters in diameter and cause mass effects. Cysticerci can form in the ventricles of the brain, where they can cause hydrocephalus by blocking the outflow of CSF. Obstructive hydrocephalus may also be caused by associated ependymitis. If cysticerci form in the basal cisterns, they can cause basilar arachnoiditis. Arachnoiditis may result in communicating hydrocephalus or vasculitis. Involvement of the arteries may lead to lacunar infarctions or, occasionally, large-vessel strokes.

Cysticerci may be located in the spinal subarachnoid space and the spinal cord medulla. Medullary cysticerci may cause cord compression or other symptoms related to their location. Ocular cysticercosis is generally intravitreal or subretinal. Skeletal muscle cysticerci are common but usually cause only minor local symptoms unless they are present in overwhelming numbers. Subcutaneous cysticerci manifest as painless, palpable, cystic lesions. CNS parenchymal cysticerci may be present in patients with suspected extraparenchymal or extra-CNS disease.

Frequency

United States

Approximately 1,000 new cases of cysticercosis are reported annually in the United States. Most occur among Latin American immigrants in locations such as California (particularly Los Angeles), Phoenix, and Albuquerque. Less frequently, cysticercosis is observed in immigrants from other areas, including Asia and Africa. A small number of cases of cysticercosis develop in people born in the United States who have traveled to areas in which the infection is endemic. These travelers are often the children of immigrants. Locally acquired infection is rare and is associated with contact with a tapeworm carrier. All tapeworm carriers acquire infection from areas of endemic disease.

In a mortality study using data from the NationalCenter for Health Statistics from 1990 to 2002; 62% of patients with cysticercosis had emigrated from Mexico.¹

International

An estimated 50-100 million people are infected with cysticercosis worldwide. This is probably an underestimate since many infections go undiagnosed. Neurocysticercosis is one of the leading causes of adult-onset seizures worldwide. CT scanning and MRI of the brain have greatly improved the diagnosis of neurocysticercosis.

Areas of endemic disease include Central and South America, India, China, Southeast Asia, and sub-Saharan Africa. Studies in Latin America and India have noted adult-onset seizures in approximately 2% of the population, with as many as half due to neurocysticercosis. In Latin America, the seroprevalence rate ranges from 4.9-24%. In India, the estimated prevalence is similar. Rural China and Korea have lower infection rates. The seroprevalence in certain rural South American communities is as high as 10-25%.²

Mortality/Morbidity

• Neurocysticercosis is one of the leading causes of adult-onset seizures and is estimated to cause as many as 50% of adult-onset seizure cases in developing countries where T solium is endemic. Neurocysticercosis was found to be responsible for 10% of newly onset seizures in one Los Angeles, California, emergency department.³ Overall, among patients who presented to emergency departments with newly onset seizure, neurocysticercosis was found to be responsible for 2.1-5.7% of cases.⁴
• A total of 221 deaths were attributed to cysticercosis in the United States from 1990-2002.¹
• Although some patients die of status epilepticus in areas with poor access to medical care, mortality due to parenchymal disease is rare. With modern medical and surgical care, mortality due to extraparenchymal disease is also unusual. However, without aggressive surgical management, hydrocephalus is potentially life-threatening. Even with shunting procedures, subarachnoid cysticercosis is associated with a high 10-year fatality rate.

Race

Immigrants from countries where T solium is endemic are more likely to be infected. While most of these immigrants are Hispanic and some are Asian, prevalence rates appear to be related more to exposure than to genetic predisposition.

Sex

• Cysticercal encephalitis, a severe form of cysticercosis, is more common in children and young females. The cause is unknown.
• No other sex predisposition has been noted.

Age

Patients with cysticercosis are typically aged 10-40 years. However, cases have been described in every age group.

Clinical

History

Postmortem studies in endemic areas suggest that 80% of neurocysticercal infections are asymptomatic.⁵ Consequently, many cases are never diagnosed or are found incidentally during imaging procedures.

The peak severity of neurocysticercosis has been estimated to occur 3-5 years after initial infection, but it can be delayed for more than 30 years. After a variable period of degeneration, cysts can become calcified and may then become inactive. Once they are calcified, they may cease to cause symptoms or may serve as a focus for epileptic activity. The symptoms of neurocysticercosis depend on the stage, site, and number of cysticerci. Cysts frequently develop in multiple locations, and a combination of active and inactive cysts in the same patient is not uncommon.

Symptoms of cysticercosis may include seizures, elevated intracranial pressure (ICP), meningoencephalitis, psychiatric disorder, stroke, and/or radiculopathy or myelopathy, if the spinal cord is involved.

The symptoms are mainly due to mass effect, an inflammatory response, or obstruction of the foramina and ventricular system of the brain. The most common symptoms include seizures, focal neurologic signs, and intracranial hypertension.

Generally, the patient’s history includes exposure to an area where the parasite is endemic and an adolescent- or adult-onset seizure disorder. Symptoms of hydrocephalus should raise concerns about extraparenchymal disease.

• Parenchymal CNS disease
  • Seizures may be focal, focal with secondary generalization, or generalized.
  • Headaches are common and may be migrainelike or tension-type.
  • Neurocognitive deficits may include learning disabilities, depression, or even psychosis.
• Extraparenchymal disease
  • Most patients present with headaches or symptoms of hydrocephalus.
  • Symptoms of increased ICP may include headache, nausea or vomiting, altered mental status, dizziness, and decreased visual acuity due to papilledema.
  • Patients with numerous cysticerci in the basilar cisterns may present with communicating hydrocephalus, meningismus (without fever), symptoms of lacunar infarcts due to small-vessel vasculitis, or symptoms of large-vessel infarcts due to cysticercal erosion into major arteries or severe inflammation of those arteries.
  • Patients with spinal cysticerci typically present with radicular symptoms, but rarely with motor or sensory deficits traceable to a spinal level.
  • Patients with ocular cysticerci report visual changes.
• Spinal cord involvement
  • Radiculopathy
  • Myelopathy
• Other
  • Meningoencephalitis
  • Stroke (in young adults)
  • Subcutaneous nodules
  • Ocular cysts

Physical

Meningoencephalitis may manifest as pyrexia, altered senses, seizures, increased ICP, multiple cranial nerve involvement, or even brainstem or cerebellar involvement.

Both parenchymal and extraparenchymal disease can cause elevated ICP. Signs include hyperreflexia, papilledema (a late sign), and the Cushing reflex (a preterminal event).

• Parenchymal disease
  • Physical examination findings are usually normal.
  • Patients who have had seizures may have typical manifestations of the postictal state, with somnolence, an altered level of consciousness, and poor memory.
  • Focal neurologic deficits are unusual and suggest alternative diagnoses, such as tuberculoma, tumor, or, rarely, extraparenchymal neurocysticercosis.
• Extraparenchymal disease
  • Ocular cysticerci are visible upon ophthalmologic examination.
  • Rare spinal cysticerci manifest as sensory or motor deficits or back tenderness.
  • Patients with neurocysticercosis of the cisterns may present with lacunar infarcts or large-vessel infarction with associated upper motor neuron signs.
  • Subcutaneous cysts may be palpable as fluid-filled nodules that resemble sebaceous cysts.

Causes

• Extraneural cysticercosis: Extraneural infection with T solium typically involves the eye, muscle, or subcutaneous tissue. It is not known whether oncospheres actively migrate to those organs or passively enter tissues during high blood flow. 
• Neurocysticercosis: Infection of the CNS with T solium and associated host inflammation.

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