Cysticercosis is an infectious disease caused by the larval stage of the pork tapeworm (Taenia solium), known as Cysticercus cellulosae, which parasitizes tissues. Humans are typically the definitive host, but if they accidentally ingest eggs, they become an intermediate host and develop cysticercosis.
When the central nervous system is affected, it is called neurocysticercosis (NCC). NCC is a leading cause of adult-onset epilepsy worldwide, and the WHO considers it a major public health concern. The CDC has designated it as a neglected parasitic infection.
When the eye or orbit is affected, it is called ocular or orbital cysticercosis (OOC). OOC is considered a preventable cause of blindness.
Endemic areas include Southeast Asia, the Indian subcontinent, Central and South America, and Africa, where sanitation is poor. In the United States, there are approximately 1,000 new hospitalizations due to NCC each year, and 10% of emergency visits for seizures in the Southwest are attributed to cysticercosis. Most cases in the US are among immigrants from Central and South America.
The distribution of cysticercosis by site of infection is reported as follows:
In India, the ocular adnexa (orbital adnexal tissues) are the most common site of involvement. In Europe and the United States, the posterior pole of the eye is the primary site of infection.
It is prevalent in areas with poor sanitation, such as Southeast Asia, the Indian subcontinent, Central and South America, and Africa. In the United States, cases are increasing, particularly among immigrants from Central and South America, with approximately 1,000 new NCC hospitalizations per year.
Symptoms vary depending on the location of the cysticercus.
When NCC is present, it may cause seizures/epilepsy, stroke due to cerebral infarction, cognitive decline, and neuropsychiatric disorders.
Extraocular muscle cysticercosis presents with nonspecific symptoms such as recurrent swelling, eyelid edema, and eye movement disorders, and may be misdiagnosed as pseudotumor leading to steroid administration1).
Findings vary depending on the site of infection.
Anterior Segment
Anterior chamber cysticercosis: Rare. It is presumed to enter via the anterior chamber angle.
Pupillary block: Glaucoma may occur due to anterior chamber obstruction by the cyst.
Slit-lamp findings: A cyst of 1.5 to 6 disc diameters shows undulating movements under light stimulation.
Posterior segment
Subretinal cysticercus: Most commonly in the inferotemporal quadrant. It is presumed to reach via the short ciliary arteries.
Intravitreal cysticercus: Migrates from the choroidal circulation across the retina into the vitreous cavity.
Retinal detachment: Exudative or rhegmatogenous retinal detachment occurs as the cysticercus moves.
Optic disc edema: Due to compression of the optic nerve. Rare.
Subconjunctival and orbital
Subconjunctival cysticercus: Recognized as a raised red to yellow nodular mass.
Proptosis and eyelid swelling: Findings of orbital cysticercosis. May be accompanied by ocular muscle palsy.
Extraocular muscle thickening: Most commonly affects the superior rectus. Cysticercus and scolex can be confirmed by ultrasound1).
If symptoms suggest brain lesions, a complete neurological examination is necessary. Papilledema, cranial nerve palsy, stroke signs, and meningeal irritation signs may be present.
Cysticercosis develops when humans ingest eggs of the pork tapeworm. There are three routes of infection.
After ingestion, the protective capsule of the eggs is dissolved by gastric acid. After maturing into larvae, they travel through the bloodstream to tissues such as the brain, eyes, and striated muscle.
Risk factors include the following1).
The route of infection is not limited to pork consumption. Infection can also occur through ingestion of water, vegetables, or fruits contaminated with eggs. Therefore, even strict vegetarians are at risk of infection in endemic areas.
Diagnosis of NCC and OOC involves a combination of clinical, serological, and radiological tests.
Imaging tests are most useful for definitive diagnosis. The characteristics of each modality are shown below.
| Modality | Findings | Characteristics |
|---|---|---|
| Ultrasound (B-scan) | Well-defined cyst + hyperechoic scolex | Inexpensive, no radiation, useful for treatment monitoring |
| CT | Low-density mass + central high-density area (scolex) | Excellent for detecting calcified lesions |
| MRI | Low-signal cyst + high-signal scolex | Most useful for NCC evaluation. Higher scolex detection rate than CT |
Ultrasound is recommended for initial evaluation. Repeated examinations every two weeks are useful for assessing treatment response1). In a study of 161 cases, MRI was shown to be superior to CT in detecting the scolex1). When the cysticerci die, contrast enhancement may be present but the scolex may not be identifiable.
Standard treatment for orbital cysticercosis and NCC is as follows:
Surgical intervention is performed according to the site of infection.
If the cysticerci are killed by drugs, the cyst contents are released into the vitreous cavity, causing a severe inflammatory reaction. Because of the risk of leading to PVR, glaucoma, cataract, and phthisis bulbi, surgical removal by vitrectomy should be performed first. For details, see the section on “Standard Treatment.”
When humans ingest eggs of Taenia solium, gastric acid dissolves the protective capsule of the eggs. The released larvae pass through the intestinal wall and travel via the bloodstream to tissues throughout the body. In the ocular region, they are thought to reach the posterior segment via the short ciliary arteries.
The progression after cysticercus parasitism is classified into three stages1).
In the posterior segment, the cysticercus migrates from the choroidal circulation across the retina into the vitreous cavity. This process can cause retinal tears, leading to rhegmatogenous retinal detachment. Exudative retinal detachment occurs due to the inflammatory reaction associated with the presence of the cysticercus.
The route of entry into the anterior chamber is still debated, but one theory suggests it occurs via the anterior chamber angle.
The inflammatory reaction upon parasite death is the main cause of ocular complications, and controlling this reaction is key to treatment strategy.
Soman et al. (2021) reported a case of left superior rectus muscle cysticercosis in a 25-year-old woman who had been treated conservatively for recurrent cellulitis for 2 years. Ultrasound B-scan identified the cysticercus and scolex, and MRI confirmed the diagnosis. Symptoms completely resolved after 4 weeks of albendazole 15 mg/kg. No NCC was observed1).
When a cysticercus is present in the posterior segment, the following complications may occur.
The prognosis is best for patients who start treatment early and whose imaging findings normalize after the initial infection.
In patients who develop seizures due to NCC, the seizure recurrence rate reaches 49% at 4 years and 68% at 6 years. Long-term management with antiepileptic drugs is often necessary.
In some US states, cysticercosis is reportable to health authorities.
