T. solium, the pork tapeworm: Beware the carrier
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Humans are the only definitive host for the tapeworms Taenia saginata and Taenia solium. T. saginata, or the beef tapeworm, is very long (up to 15 feet or more), but generally poses little danger when it develops in humans and will not be discussed further.
T. solium is potentially dangerous to humans in that the eggs of the tapeworm, if ingested, can result in cysticercosis, a potentially serious disease, often involving the central nervous system. Eggs of T. saginata, if ingested, will not cause disease in humans.
Taenia asiatica is a tapeworm discovered relatively recently that infects pigs in the Far East. Humans are the definitive host for this tapeworm. Its role in producing disease has not been clarified. It has been suggested by some that T. asiatica eggs may be capable of producing cysticercosis in humans.
Life cycle of T. solium
Pigs become infected by ingesting eggs of T. solium, which have been excreted by humans carrying the worm in their intestine. Larva, hatching from the ingested eggs, burrow into the intestinal wall, and after being carried in the circulation, develop into larval cysts (cysticerci) in the striated muscle of the pig. Humans become infected by eating the raw or undercooked muscle containing the cysticerci. After ingestion, each of the cysticerci develops into a scolex (the head of the tapeworm), which attaches to the mucosa of the small intestine. The tapeworm grows from the scolex by adding segments (proglottids), each of which contain many eggs.
Some eggs leak into the stool. Proglottids are periodically passed in the stool; the eggs are released after passage. T. solium tapeworms have an average of 1,000 proglottids, each containing thousands of eggs. It takes 2 to 3 months for the tapeworm to grow and start releasing proglottids and eggs in the stool. The tapeworm can live for years. Mature proglottids are motile and easily visible to the naked eye.
The T. solium tapeworm (usually up to 9 feet long) per se is of little consequence to the human host. However, if the eggs being passed in the stool are ingested by fecal-oral autoinoculation, the larval forms resulting from the eggs hatching can penetrate into the wall of the intestine. They are then carried in the circulation and can develop into cysticerci in muscles, eyes, the central nervous system and elsewhere in the body. The result is cysticercosis with manifestations that depend on where the larval form develops. These cysticerci may persist for many years without provoking an inflammatory reaction or antibody production. The cysticerci also may degenerate, producing inflammation, or they may become calcified.
In addition to fecal-oral autoinoculation — most importantly — the infected carrier of the tapeworm can infect others by fecal contamination of ingested material and cause cysticercosis.
It is important to stress that humans can develop cysticercosis only by ingesting eggs of T. solium; ingesting inadequately cooked pork can result only in tapeworm infection and not cysticercosis.
Epidemiology of T. solium
T. solium infection and cysticercosis are rare in the United States except in immigrants or visitors from Mexico, South and Central America, Eastern Europe, India and parts of Africa and Asia where sanitation is poor (ie, pigs have access to human feces) and people eat raw or undercooked pork. Travelers to these parts of the world can become infected with the tapeworm by eating raw or undercooked pork. In addition, travelers may develop cysticercosis from food contaminated with the feces of an infected food handler. Infection of a person in the U.S. also may occur from an infected food handler residing in the U.S. who is from a country where T. solium infection is common. For example, cases of cysticercosis were reported in Orthodox Jews who strictly avoid porcine products; they became infected by food handlers from Latin America. In countries where pigs are not raised, T. solium is rare. Similarly, cysticercosis occurs primarily in the countries where T. solium is found because the disease occurs only when eggs are ingested from the feces of a human carrying T. solium.
That being said, at least 2,000 cases of neurocysticercosis are diagnosed each year in the U.S.; and about 2% of patients presenting with seizures in U.S. emergency rooms are found to have neurocysticercosis.
Symptoms
T. solium tapeworm
The tapeworm itself usually does not produce symptoms except for the occasional passage of motile proglottids through the anus. Tapeworms can occasionally cause symptoms because of their size. Some of the reported symptoms include anorexia, abdominal pain and weight loss. Rarely, proglottids can cause symptoms by obstructing the biliary system, the pancreatic ducts or the appendix.
Cysticercosis
The major disease that can result from infection with T. solium is cysticercosis. Although cysticerci may develop anywhere, the most common and most serious manifestations occur when they develop in the central nervous system or eye. However, this author did see an autopsy of a young man who died of an arrhythmia and had cysticercosis involving the heart.
Neurocysticercosis is frequently asymptomatic. The cystercerci may be parenchymal, which is most common, or extraparenchymal (in ventricles, subarachnoid space or the spinal cord). When symptoms occur, most often the first symptom with parenchymal disease is a seizure, or with extraparenchymal disease, symptoms are related to increased intracranial pressure. In most low-income countries, cysticercosis is a major cause of adult-onset seizures. It has been reported that neurocysticercosis is responsible for about 29% of seizures in endemic areas. Other symptoms of neurocysticercosis include headache, vomiting, impaired thinking, visual disturbances and other manifestations of brain or spinal cord involvement, such as cranial nerve abnormalities. Subcutaneous cysticerci can develop and be palpable.
Diagnosis
Tapeworm
Taenia eggs may be found in the stool. Because Taenia eggs are passed intermittently, at least three stool samples should be examined microscopically on three different days. All Taenia eggs look alike, so the presence of eggs will not differentiate between the different Taenia tapeworms. Proglottids that are also passed intermittently have a specific appearance, which can be used to identify T. solium. Patients who pass proglottids should be asked to bring them in to the laboratory for examination. The proglottids of T. solium contain 7 to 13 primary lateral uterine branches, whereas T. saginata proglottids have 12 to 30 primary lateral branches. T. asiatica proglottids are similar to T. saginata proglottids.
Various immunological and molecular assays have been developed to detect the antigens or eggs of T. solium in stools, but these tests are not routinely available for diagnosis. Similarly, serum antibody tests have been developed to identify carriers of T. solium but are available only in certain research laboratories.
Cysticercosis
Cysticerci that become calcified over time may be seen on X-ray, but CT scans and MRIs are the best tools for visualizing the cysticerci, especially in neurocysticercosis. MRIs are superior to CT scans in visualizing noncalcified cysticerci. If there is an inflammatory reaction in the brain due to degeneration of a cysticercus, the imaging shows a contrast-enhancing ring around the cysticerus. Ophthalmologic examination should be performed to look for cysticerci in any patient with suspected cysticercosis.
Serological tests are available but may be negative when there are relatively few cysticerci. The serological tests fall into two categories: the enzyme-linked immunoelectrotransfer blot (EITB), and commercial enzyme-linked immunoassays (ELISAs). The ELISAs are inferior in terms of false-negative and false-positive tests. The EITB should be used, if possible. A positive serological test with a negative MRI of the central nervous system may indicate the presence of cysticerci elsewhere in the body.
Treatment
Tapeworm
The drug of choice for treatment of the tapeworm is praziquantel given as a single oral dose of 5 to 10 mg/kg. Niclosamide is also effective but is not available in the U.S. It must be noted that anthelmintic drugs can destroy cysticerci and provoke an intense inflammatory reaction. Therefore, it is important to determine if cysticercosis is present and to administer corticosteroids with the anthelmintic drug if it is present or even withhold treatment depending on the location of the cysticerci (eg, in the eye).
Cysticercosis
When cysticerci produce symptoms, surgical removal should be considered. If complications such as hydrocephalus occur when a cysticercus obstructs the outflow of cerebrospinal fluid, a neurological procedure such as placement of a shunt may become necessary. Anticonvulsant drugs will be necessary when seizures occur, as will corticosteroids for increased intracranial pressure.
Antiparasitic treatment must be considered with great caution. Viable cysticerci generally provoke little inflammation in the surrounding tissue. With death of the larva, there is intense inflammation, which in the central nervous system can cause severe symptoms or, in the eye, blindness. Antiparasitic therapy is not recommended in patients with elevated intracranial pressure or diffuse cerebral edema. In these patients, measures to reduce the pressure should be used, such as corticosteroid therapy or surgery for hydrocephalus.
Decision-making and use of antiparasitic drugs are complicated for neurocysticercosis, as detailed in the 2017 guidelines from the Infectious Diseases Society of America and the American Society of Tropical Medicine and Hygiene. They state, “Note that diagnosis and management of patients with neurocysticercosis can be challenging even with expert guidelines. Due to this complexity, clinicians with little experience with this disease should have a low threshold for consultation with an expert in the disease.” Expertise is necessary to determine whether to use these drugs and which regimen to use. If treatment is given to kill the larva, corticosteroids must also be administered, often over the long term. Testing for tuberculosis is recommended before embarking on the possible path to long-term steroid administration. If the decision is made to use antiparasitic treatment, albendazole, often with praziquantel, is the regimen of choice.
Prognosis
The T. solium tapeworm causes few or no symptoms. Although cysticercosis is usually asymptomatic, depending on the number and location of the cysticerci, it can be life-threatening. Large numbers of cysticerci are generally more problematic than relatively few cysticerci.
The prognosis in neurocysticercosis depends mainly on the location of the cysticerci. Extraparenchymal cysticerci, such as those in the ventricles or meninges, carry a worse prognosis than parenchymal cysticerci. Development of hydrocephalus, intracranial hypertension or cerebral infarction caused by cysticerci can result in death.
Prevention
Good sanitation that prevents pigs from contacting human feces will prevent pigs from becoming infected. Inspection of pork can reveal cysticeri, which are visible to the naked eye, allowing that carcass to be discarded. Good hand-washing habits will decrease the chance of carriers infecting others. However, the best way to avoid T. solium tapeworm infection is to avoid undercooked pork. Cooking to a temperature of 145°F will kill the larva, as will freezing. There is no sure way of avoiding cysticercosis.
- References:
- CDC. Parasites: Cysticercosis. https://www.cdc.gov/parasites/cysticercosis/index.html. Accessed March 29, 2020.
- CDC. Parasites: Taeniasis. https://www.cdc.gov/parasites/taeniasis/. Accessed March 29, 2020.
- Emedicinehealth. Symptoms and signs of cysticercosis. https://www.emedicinehealth.com/cysticercosis/symptom.htm. Accessed March 29, 2020.
- Eom KS. Parasitol Int. 2006;doi:10.1016/j.parint.2005.11.022.
- Jeon HK, et al. Korean J Parasitol. 2013; doi:10.3347/kjp.2013.51.1.51.
- Medical Care Development International. Taeniasis. https://mcdinternational.org/trainings/malaria/english/dpdx5/html/Frames/S-Z/Taeniasis/body_Taeniasis_mic1. Accessed March 29, 2020.
- O’Neal SE, Flecker RH. Emerg Infect Dis. 2012;doi:10.3201/eid2106.141324.
- Schantz PM, et al. N Engl J Med. 1992; doi:10.1056/NEJM199209033271004.
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- For more information:
- Donald Kaye, MD, MACP, is a professor of medicine at Drexel University College of Medicine, associate editor of the International Society of Infectious Diseases’ ProMED-mail, section editor of news for Clinical Infectious Diseases and an Infectious Disease News Editorial Board Member.
Disclosure: Kaye reports no relevant financial disclosures.
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