Taenia solium, the pork tapeworm, belongs to the cyclophyllid cestode family Taeniidae. It is found throughout the world and is most common in countries where pork is eaten. It is a tapeworm that uses ( Homo sapiens) as its definitive host and pigs and boars (family Suidae) as the intermediate or secondary hosts. It is transmitted to pigs through human feces that contain the parasite eggs and contaminate their fodder. Pigs ingest the morula, which develop into larvae, then into , and ultimately into infective tapeworm cysts, called cysticerci. Humans acquire the cysts through consumption of uncooked or under-cooked pork and the cysts grow into adult worms in the small intestine.
There are two forms of human infection. One is "primary hosting", called taeniasis, and is due to eating under-cooked pork that contains the cysts, resulting in adult worms in the intestines. This form generally is asymptomatic; the infected person does not know they have tapeworms. This form is easily treated with anthelmintic medications which eliminate the tapeworm. The other form, "secondary hosting", called cysticercosis, is due to eating food, or drinking water, contaminated with faeces from someone infected by the adult worms, thus ingesting the tapeworm eggs, instead of the cysts. The eggs go on to develop cysts primarily in the muscles, and usually with no symptoms. However, some people have obvious symptoms, the most harmful and chronic form of which is when the cysts form in the brain. Treatment of this form is more difficult but possible.
The adult worm has a flat, ribbon-like body which is white and measures long, or more. Its tiny attachment, the scolex, contains suckers and a rostellum as organs of attachment that attach to the wall of the duodenum. The Strobilation, consists of a chain of segments known as proglottids. Each proglottid is little more than a self-sustainable, very lightly ingestive, self-contained reproductive unit since tapeworms are .
Human primary hosting is best diagnosed by microscopy of eggs in faeces, often triggered by spotting shed segments. In secondary hosting, imaging techniques such as computed tomography and nuclear magnetic resonance are often employed. Blood samples can also be tested using ELISA of enzyme-linked immunosorbent assay.
T. solium deeply affects developing countries, especially in rural settings where pigs roam free, as clinical manifestations are highly dependent on the number, size, and location of the parasites as well as the host's immune and inflammatory response.
After a short neck is the elongated body, the strobila. The entire body is covered by a covering called a tegument, which is an absorptive layer consisting of a mat of minute specialised microvilli called microtriches. The strobila is divided into segments called proglottids, 800 to 900 in number. Body growth starts from the neck region, so the oldest proglottids are at the posterior end. Thus, the three distinct proglottids are immature proglottids towards the neck, mature proglottids in the middle, and gravid proglottids at the posterior end. A hermaphroditic species, each mature proglottid contains a set of male and female reproductive systems with numerous testes and an ovary with three lobes.
If released early enough in the digestive tract and not passed, fertilised eggs can mature using upper tract digestive enzymes. The tiny oncosphere larvae, activated by exposure to host enzymes and bile salts, penetrate the intestinal wall and migrate in the blood stream or lymphatics to reach sites where they can develop into Cysticercus.
As a hermaphrodite, it reproduces by self-fertilisation, or cross-fertilisation if gametes are exchanged between two different proglottids. Spermatozoa fuse with the ovum in the fertilisation duct, where the are produced. The zygote undergoes holoblastic and unequal cleavage resulting in three cell types, small, medium and large (micromeres, mesomeres, megameres). Megameres develop into a syncytial layer, the outer embryonic membrane; mesomeres into the radially striated inner embryonic membrane or embryophore; micromeres become the morula. The morula transforms into a six-hooked embryo known as an oncosphere, or hexacanth ("six hooked") larva. A gravid proglottid can contain more than 50,000 embryonated eggs. Gravid proglottids often rupture in the intestine, liberating the oncospheres in faeces. Intact gravid proglottids are shed off in groups of four or five. The free eggs and detached proglottids are spread through the host's defecation (peristalsis). Oncospheres can survive in the environment for up to two months.
Humans are also accidental secondary hosts when they are colonised by embryonated eggs, either by auto-colonisation or ingestion of contaminated food. As in pigs, the oncospheres hatch and enter blood circulation. When they settle to form cysts, clinical symptoms of cysticercosis appear. The cysticercus is often called the metacestode.
These symptoms could continue until the tapeworm dies from the course of treatment but otherwise could continue for many years, as long as the worm lives. If untreated it is common that the infections with T. solium last for approximately 2–3 years. It is possible that infected people may show no symptoms for years.
In more severe cases, dementia or hypertension can occur due to perturbation of the normal circulation of cerebrospinal fluid. (Any increase in intracranial pressure will result in a corresponding increase in arterial blood pressure, as the body seeks to maintain circulation to the brain.) The severity of cysticercosis depends on location, size and number of parasite larvae in tissues, as well as the host immune response. Other symptoms include sensory deficits, involuntary movements, and brain system dysfunction. In children, human eye cysts are more common than in other parts of the body.
In many cases, cysticercosis in the brain can lead to epilepsy, seizures, lesions in the brain, blindness, tumour-like growths, and low eosinophil levels. It is the cause of major neurological problems, such as hydrocephalus, paraplegy, meningitis, , and even death.
Stool tapeworm antigen detection: Using ELISA increases the sensitivity of the diagnosis. The downside of this tool is it has high costs, an ELISA reader and reagents are required and trained operators are needed. A studies using Coproantigen (CoAg) ELISA methods are considered very sensitive but currently only genus specific. A 2020 study in Ag-ELISA test on Taenia solium cystercicosis in infected pigs and showed 82.7% sensitivity and 86.3% specificity. The study concluded that the test is more reliable in ruling out T. solium cystercosis versus confirmation.
Stool PCR: This method can provide a species-specific diagnosis when proglottid material is taken from the stool. This method requires specific facilities, equipment and trained individuals to run the tests. This method has not yet been tested in controlled field trials.
Serum antibody tests: using Western blot and ELISA, tape-worm specific circulating antibodies have been detected. The assays for these tests have both a high sensitivity and specificity. A 2018 study of two commercially available kits showed low sensitivity with patients diagnose with NCC (neurocysticercosis) especially with calcified NCC versus patients with cystic hydatid disease. Current standard for serologic diagnosis of NCC is the lentil lectin-bound glycoproteins/enzyme-linked immunoelectrotransfer blot (LLGP-EITB).
Guidelines for diagnosis and treatment remain difficult for endemic countries, most of them developing with limited resources. Many developing countries diagnosed clinically with imaging.
In neurocysticercosis, most patients under cysticidal therapy will have significant improvement in seizure control. A combination of praziquantel and albendazole is more effective in treating neurocystercosis. A 2014 double blind randomized control study showed increased parasiticidal effect with albendazole plus praziquantel.
A vaccine to prevent cysticercosis in pigs has been studied. The life-cycle of the parasite can be terminated in their intermediate host, pigs, thereby preventing further human infection. The large scale use of this vaccine, however, is still under consideration.
During the 1940s, the preferred treatment was oleoresin of aspidium, which would be introduced into the duodenum via a Rehfuss tube.
The secondary host form, human cysticercosis, predominates in areas where poor hygiene allows for mild fecal contamination of food, soil, or water supplies. Rates in the United States have shown immigrants from Mexico, Central and South America, and Southeast Asia bear the brunt of cases of cysticercosis caused by the ingestion of microscopic, long-lasting and hardy tapeworm eggs. For example, in 1990 and 1991 four unrelated members of an Orthodox Jewish community in New York City developed recurrent seizures and brain lesions, which were found to have been caused by T. solium. All had housekeepers from Mexico, some of whom were suspected to be the source of the infections. Rates of T. solium cysticercosis in West Africa are not affected by any religion.
Neurocystiscercosis is noted at around one-third of all epilepsy cases in many developing countries. Neurological morbidity and mortality remain high in lower-income countries and high amongst developed countries with high rates of migration. Global prevalence rates remain largely unknown as screening tools, immunological, molecular tests, and neuroimaging are not usually available in many endemic areas.
Description
Life cycle
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Definitive host
Intermediate host
Diseases
Signs and symptoms
Taeniasis
Cysticercosis
Diagnosis
Prevention
Treatment
Epidemiology
See also
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