Cestode

There are three groups of medically important helminths; Cestodes (tapeworms), Nematodes (roundworms) and Trematodes (flukes).These parasites live in both the body spaces (gut lumen, bile ducts, lungs, oral cavity, etc.) and in tissues (blood, muscles and skin).

Cestodes

The cestodes (or tapeworms) form a group of worms, exhibiting two unmistakable morphological features; they all possess flat, ribbon like bodies and lack an alimentary canal. Adult tapeworms usually inhabit the alimentary canal of their hosts (though they occasionally are found in the bile or pancreatic ducts) and attach themselves to the mucosa by means of a scolex. Despite the lack of a digestive system they do absorb food from the hosts intestine; thereby providing the tapeworms a habitat that is associated with high nutritional levels, feeding the tapeworms high growth rate.  Larvae on the other hand show a wide range of habitat preferences, being found in almost any organ of both vertebrate and invertebrate hosts. Though most larval species show a preference for a particular site.

Tapeworms (Taenia species)

The tapeworms are hermaphroditic and require an intermediate host. The adult tapeworms found in humans have flat body, white or grayish in color. They consist of an anterior attachment organ or scolex (suckers or grooves and  has rosetellum, which has 1 or 2 rows of hooks situated on the center of the scolex) and a chain of segments (proglottids) also called strobilla. The body consists of a chain of segments or proglottids, which can be immature, mature or gravid; the latter of which contain a fully developed uterus packed with eggs.  Therefore, each tapeworm is made up of a ‘string of individuals’ having a complete set of reproductive organs in progressive degrees of sexual maturity and budding off from a body attached to the host tissue by a head or scolex.

Adult tapeworms inhabit the small intestine, where they live attached to the mucosa. Tapeworms do not have a digestive system. Their food is absorbed from the host’s intestine.

Taenia species

v  Taenia solium (pork tapeworm)

v  Taenia saginata (beef tapeworm)

v  Hymonolepis nana (dwarf tapeworm)

v  Echinococcus granulosis (Dog tapeworm)

v   Taenia saginata and Taenia solium

Genus Taenia has at least 32 recognized species, of which Taenia saginata and Taenia solium are the two species of medical importance, which cause infections in man. T. saginata has a cosmopolitan distribution, but is more common in developing countries where hygiene is poor and the inhabitants have a tendency of eating raw or insufficiently cooked meat. T. saginata is the most common adult tapeworm found in man. T solium is virtually extinct in Europe and the USA. A new third species, Taenia asiatica (also known as Asian Taenia) has been identified to cause human infection in China.

Taenia saginata

•      Taenia saginata is known as the beef tape worm.

•      It is the commonest large tapeworm of human causing intestinal taeniasis.                     

•      The infection is acquired orally by ingesting beef infected with the larvae (Cysticercus bovis) of the parasite.  

The disease is relatively common in Africa, some parts of Eastern Europe, the Philippines, and Latin America. This parasite is found anywhere where beef is eaten, even in countries such as the United States, with strict federal sanitation policies. In the US, the incidence of infection is low, but 25% of infected cattle are still sold. The total global infection is estimated to be between 40 and 60 billion. It is most prevalent in Sub-Saharan Africa and Middle East.

Habitat and Morphology

Adult

It is a white, ribbon like, flattened and segmented worm measuring 4 meter to 10 meter, even 15 meter in length. An adult worm consists of a head (scolex), a  
neck and body (strobila), consisting a chain of segments. The scolex is quadrate in shape and measures less than 2 mm in diameter. It bears 4 cup shaped muscular suckers, which may be pigmented. A short and fragile neck measuring 3 mm to 7mm in length follows the scolex. The gravid segments are rectangular, measure 20 mm by 5mm to 7mm and are present in the posterior part of the worm. Typically these gravid segments break off from the strobila and are passed in the faeces singly and often simultaneously. In the gravid proglottid, the uterus contains up to 15 side branches filled with eggs.

Egg

The eggs are round or oval and measure 33micrometer-43micrometer in diameter. They are bile stained.

Fig: Eggs of  Taenia saginata

Life cycle

The life cycle of Taenia saginata

The life cycle of T. saginata is indirect and digenetic, involving cattle and humans with an interim of living in the environment. Humans as the definitive host harbour adult worms which release infective eggs into the environment. Cattle as the intermediate host pick up the viable eggs from contaminated vegetation.

Intermediate host

Cattle acquire the embryonated eggs called the oncospheres when they eat contaminated food. Oncospheres enter duodenum, the anterior portion of small intestine. The oncospheres hatch in the duodenum under the influence of gastric juices. The embryonic membranes are removed, liberating free hexacanth ("six hooked") larvae. With their hooks they get attached to the intestinal wall and penetrate the intestinal mucosa into the blood vessels. The larvae can move to all parts of the body via general circulatory system, and finally settle in skeletal muscles within 70 days. Inside the tissue they cast off their hooks and instead develop a protective cuticular shell, called cyst. Thus it becomes a fluid-filled cysticercus. Cystercus can also form in lungs and liver. The inner membrane of the cysticercus soon develops numerous protoscolices (small scolices) that are invertedly attached to the inner surface. The cysticercus of T. saginata is specifically named cysticercus bovis to differentiate from that of T, solium, cysticercus cellulosae.

Definitive host

Humans contract infective cysticercus by eating raw or undercooked meat. Once reaching the jejunum the inverted scolex becomes evaginated to the exterior under stimuli from the digestive enzymes of the host. Using the scolex it gets attached on the intestinal wall. It takes about 5 to 12 weeks for the larva to mature into adulthood. Adult worm can live to about 25 years in the host. Usually only a single worm is present at time. However, multiple worms are also reported. In each mature proglottid self-fertilisation produces zygotes, which divide and differentiate into embryonated eggs called oncospheres. With thousands of oncospheres, the oldest gravid proglottid detach. Unlike in other Taenia gravid proglottids are shed individually. In some cases the proglottid ruptures inside the intestine, and the eggs are released. The free proglottids and liberated eggs are removed by peristalsis into the environment. On the ground the proglottids are motile and will shed eggs as they move. These oncospheres in external environment can remain viable for several days to weeks in sewage, rivers, and pastures.

Pathogenesis

Humans develop a tapeworm infection by eating raw or undercooked beef. The cysticercus becomes activated, attaches to the wall of the small intestine by the scolex, and becomes a mature tapeworm. This maturation process takes 10-12 weeks after that adult tapeworms live in the human small intestine causing intestinal disturbances and intestinal obstruction. Humans pass gravid eggs in feces; a single tapeworm produces an average of 50,000 eggs per day and may live 25 years. There is no known secretory products as its virulence determinants. Survival in the intestine is the key to virulence. It survives on host’s nutrition. As it has no digestive system, nutrition is host’s nutrition . Another aspect of virulence is the long survival period, that creates persistent and unresolved intestinal motility disorder of the host sufficiently linked to develop psychological outburst. Segments creeping out of anus cause: irritation, itching & anexiety and may cause appendicitis or cholangitis. These mature eggs contaminate pastures and barnyards, where cattle and pigs ingest them. Upon reaching the alimentary canal of infected animals, the embryos are released, penetrate the gut wall, and enter the circulation.

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Clinical Symptoms

T. saginata infection is usually asymptomatic. However heavy infection often results in weight loss, dizziness, abdominal pain, diarrhoea, headaches, nausea, constipation, or chronic indigestion, and loss of appetite. There can be intestinal obstruction in humans and this can be alleviated by surgery. The tapeworm can also expel antigens that can cause an allergic reaction in the individual. It is an also rare cause of ileus, pancreatitis, cholecystitis and cholangitis.

Laboratory Diagnosis

Microscopy (Detection of eggs in faeces )

The basic diagnosis is done from a stool sample. Feces are examined microscopically  to find parasite eggs. The eggs look like other eggs from the family Taeniidae, so it is only possible to identify the eggs to the family, not to the species level. Since it is difficult to diagnose using eggs alone, looking at the scolex or the gravid proglottids can help identify it as Taenia saginata. Differentiation of the species from other species of Taenia, such as T. solium and T. asiatica, is notoriously difficult because of their close morphological resemblance, and their eggs are more or less identical. Detection of eggs can be done  in perianal region by a swab.

Proglottids  identification (Segments may creep out of anus)

Proglottids sometimes trickle down the thighs of infected humans and are visible with unaided eye, so can aid with identification. Observation of scolex help distinguish between T. saginata, T. solium and T. asiatica.

When the uterus is injected with India ink, its branches become visible. Counting the uterine branches enables some identification (Taenia saginata uteri have 12 or more branches on each side, while other species such as Taenia solium only have five to 10).

Antibody detection

May prove useful especially in the early invasive stages, when the eggs and proglottids are not yet apparent in the stools.

Other methods

PCR detection of ribosomal 5.8S gene. T. saginata’s uterus stems out from its center to form 12 to 20 branches, but in contrast to its closely related Taenia species, the branches are much less in number and comparatively thicker; in addition, the ovaries are bilobed and testes are twice as many.

Eosinophilia and elevated IgE levels are chief hematological findings. Also Ziehl–Neelsen stain can be used to differentiate between mature Taenia saginata and Taenia soium, in most cases Taenia saginata will stain while Taenia solium will not, however the method is not strict.

Treatment

Taenaisis is easily treated with praziquantel (5–10 mg/kg, single-administration) or niclosamide (adults and children over 6 years: 2 g, single-administration after a light breakfast, followed after 2 hours by a laxative; children aged 2–6 years: 1 g; children under 2 years: 500 mg).

Albendazole is also highly effective for treatment of cattle infection.

Prevention

•  Adequate cooking (56°C for 5 minutes) of beef viscera destroys cysticerci.
• Refrigeration, freezing (-10°C for 9 days) or long period salting is lethal to cysticerci.
•  Inspection of beef and proper disposal of human excreta are also important measures.

Taenia solium

•      Taenia solium is the pork tape worm, which causes intestinal taeniasis, an infection similar to that caused by T. saginata.

•      It is an intestinal zoonotic parasite found throughout the world, and is most prevalent in countries where pork is eaten.

•      In addition to this, the larva (Cysticercus cellulosae) of the worm also causes a serious disease in man known as cysticercosis.

•      T. solium is the only cestode for which man acts as both the definitive host (harbouring the adult worm) and the intermediate host (harbouring the larva of the parasite).

•      The tape worm causes cysticercosis,especially neurocysticercosis, a serious disease in humans.

Taenia solium is found worldwide. Because pigs are intermediate hosts of the parasite, completion of the life cycle occurs in regions where humans live in close contact with pigs and eat undercooked pork. Taeniasis and cysticercosis  is most common in Latin America, Southeast Asia and Africa. It is particularly prevalent in rural areas   where domestic pigs are allowed to roam freely. It is diminishing in eastern and  southern Europe, and is very rare in Muslim countries.

Morphology

The characterstics of eggs are Spherical and brown in colour, measures 31-43μm in diameter. The thin, outer transparent shell, when present, represents the remnant of the yolk mass; it causes the eggs to clump together.The inner embryophore is brownthick walled and radially striated. It contains an oncosphere(14 to20μm in diameter) with 3 pairs of

hooklets.

•      The eggs of T. solium and T. saginata are morphologically similar.

Adult

Adult tapeworms develop, (up to 2 to 7 m in length and produce less than 1000 proglottids, each with approximately 50,000 eggs) and reside in the small intestine for years as much as 25 years. The adult worm has a scolex, neck and strobila consisting of segments. Scolex is round,measures 1mm in diameter, has four suckers and is armed with a conspicuous rostellum. The neck is short, about 5mm to 10mm in length. The strobila or body consists of 800-1000 segments or proglottids which are i) immature ii) mature and iii) gravid. The immature segments are broader than long. The mature segments are wider than long. The gravid proglottids are longer than broad (12mm by 6mm). The gravid segments look grayish-black and transparent when fully developed.

Life cycle

T. solium is a digenetic helminth and its life cycle is indirect. It passes through pigs, as intermediate hosts, into humans, as definive hosts. From humans the eggs are released in the environment where they ingestion by another host. Humans as the definitive hosts are directly infected from contaminated meat.

Definitive host

Humans are infected by the larval stage called cysticercus (cysticercus cellulosae) from a measly pork. A cysticercus is oval-shaped containing inverted scolex (specifically "protoscolex"), which pops out externally once inside the small intestine. This process of evagination is stimulated by bile juice and digestive enzymes of the host. Using the scolex it anchors to the intestinal wall. It grows in size using nutrients from the surrounding. Its strobila lengthens as new proglottids are formed at the neck. In 10–12 weeks after initial infection, it becomes adult worm. As hermaphrodite it reproduces by self-fertilisation, or cross-fertilisation. A single gravid proglottid can contain more than 50,000 embryonated eggs. Gravid proglottids often rupture in the intestine liberating the eggs in faeces. The intact gravid proglottids are shed off in groups of 4 or 5. The free eggs and detached proglottids are released into the environment through peristalsis. Eggs can survive in the environment for up to two months.

Intermediate host

Pigs ingest the eggs from human faeces or vegetation contaminated with human excreta. The embryonated eggs enter the intestine where they hatch into motile oncospheres. The embryonic and basement membranes are removed by the host's digestive enzymes (particularly pepsin). Then the free oncospheres get attached on the intestinal wall using their hooks. With the help of digestive enzymes from the penetration glands, they penetrate the intestinal mucosa to enter blood and lymphatic vessels. They move along the general circulatory system to various organs, and large number are cleared in the liver. The surviving oncospheres preferentially migrate to striated muscles, as well as the brain, liver, and other tissues, where they settle to form cysts called cysticerci. A single cysticercus is spherical measuring 1–2 cm in diameter and contains invaginated protoscolex. The central space is filled with fluid like a bladder, and hence it is also called bladder worm. Cysticerci are usually formed within 70 days and may continue to grow for a year.

Humans are also accidental primary hosts when they are infected by embryonated eggs, either by autoinfection or ingestion of contaminated food. As in pigs, the oncospheres hatch, enter blood circulation, and have predilection for brain tissue and other soft muscle tissues. When they settle to form cysts, clinical symptoms of cysticercosis appears. The cysticercus is often called metacestode. If they localize in the brain, serious neurocysticercosis follows.

Pathogenesis

 Ingestion of T. solium eggs or proglottid rupture within the host intestine can cause larvae to migrate into host tissue to cause cysticercosis. The severity of cysticercosis depends on location, size and number of parasite larvae in tissues, as well as the host immune response. Viable cysticerci (larval form) in most organs cause minimal or no tissue reaction, but death of the cysts in the CNS can elicit an intense tissue response. Infection in the brain (cerebral cysticercosis) may result in severe symptoms due to mass effect and inflammation induced by degeneration of cysticerci and release of antigens. 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. Due to this, dementia or hypertension can occur due to perturbation of the normal circulation of cerebrospinal fluid.

The neurocysticercosis may present with seizures, signs of increased intracranial pressure, hydrocephalus, focal neurologic signs, altered mental status, or aseptic meningitis. Cysticerci may also infect the spinal cord

Clinical manifestation (taeniasis)

Most individuals with taeniasis are either asymptomatic or have mild-to-moderate complaints. Abdominal pain and nausea are reportedly more common in the morning and are characteristically relieved by eating small amounts of food. Children are more symptomatic than adults and often manifest change of appetite, both increased and decreased. Symptoms in infants are more pronounced and consist of vomiting, diarrhea, fever, weight loss, and irritability.

The most common serious complication of adult tapeworm infection is appendicitis. Other reported complications include obstruction of bile ducts, pancreatic duct and tapeworm growth in ectopic locations (eg, middle ear, adenoid tissue, uterine cavity). A mild eosinophilia of 5-15% may occur in 5-45% of patients; higher levels are rare.

Cysticercosis and neurocysticercosis (NCC)

In cysticercosis, the cysticerci are most often located in subcutaneous and intermuscular tissues, followed by the eye and then the brain. The CNS is involved in 60-90% of patients (ie, NCC). Most patients have more than one cyst; as many as 200 cysts have been reported.

NCC symptoms include 3 characteristic syndromes: convulsions and/or seizures, intracranial hypertension, and psychiatric disturbances, which may occur separately or simultaneously.Onset can be insidious (eg, elevated intracranial pressure) or abrupt (eg, floating cysticerci suddenly block cerebrospinal fluid [CSF]).

Other possible infections

• Ocular cysticercosis: The subretinal space, vitreous, and conjunctiva are the most frequent sites of infection. Common manifestations of infection include severe pain and blurred or lost vision.
• Muscular and dermatologic cysticercosis: Cysticerci in muscles often are associated with NCC. Any muscle mass may be involved and appear as acute myositis. However, most patients are asymptomatic. Subcutaneous nodules may be evident.
• Coenurosis: Patients with cerebral coenurosis can present with seizure or intracranial hypertension. Ocular coenurosis manifests as a red and painful.

Laboratory Diagnosis

Microscopy Examination

Microscopic identification of eggs and proglottids in feces is diagnostic for taeniasis, but is not possible during the first 3 months following infection, prior to development of adult tapeworms. Repeated examination and concentration techniques will increase the likelihood of detecting light infections.

 Nevertheless, identification of Taenia is not possible if solely based on microscopic examination of eggs, because all Taenia species produce eggs that are morphologically identical. Eggs of Taenia spp. are also indistinguishable from those produced by cestodes of the genus Echinococcus (tapeworms of dogs and other canid hosts).

Microscopic identification of gravid proglottids (or, more rarely, examination of the scolex) allows species determination.

Figure : Mature proglottid of T. saginata, stained with India ink. Note the number of primary uterine branches (>12)

Separation of T. saginata and T. solium is best accomplished by examination of mature proglottids.  Taenia saginata has 12-30 primary lateral uterine branches, while T. solium has 7-13 primary lateral uterine branches. Visualization of the branches can be improved by clearing the specimen in lactophenol followed by India ink injection into the lateral genital pore.

Antibody detection

May prove useful especially in the early invasive stages, when the eggs and proglottids are not yet apparent in the stools.

Other methods

     -  Biopsy from suspected nodule .

     - Ziehl- Neelsen stain differentiates between eggs of T. solium & T. saginata.

     - Gravid segments should be inspected.

     - Cysticercosis, is diagnosed radiologically.

-   CT and/or MRI testing for patients with CNS  symptoms

Treatment

Praziquantel is the medication most often used to treat active taeniasis, given at 5-10 mg/kg orally once for adults and 5-10 mg/kg orally once for children. Praziquantel is cysticidal and can cause inflammation around dying cysts in those with cysticercosis, which may lead to seizures or other symptoms.

 Niclosamide is an alternative, given at 2 g orally once for adults and 50 mg/kg orally once for children. After treatment, stools should be collected for 3 days to search for tapeworm proglottids for species identification

Prevention and control

The best way to avoid getting tapeworms is to not eat undercooked pork. Moreover, a high level of sanitation and prevention of faecal contamination of pig foods also plays a major role in prevention. Infection can be prevented with proper disposal of human faeces around pigs, cooking meat thoroughly and/or freezing the meat at −10°C for 5 days.

For human cysticercosis, dirty hands are attributed to be the primary cause, and especially common among food handlers.Therefore, personal hygiene such as washing one's hands before eating is an effective measure.

Taenia asiatica

v  Taenia asiatica, commonly known as Asian taenia or Asian tapeworm, is a parasitic tapeworm of humans and pigs.

v  It is one of the three species of Taenia infecting humans and causes taeniasis.

v  T. asiatica causes intestinal taenisis in humans and cysticercosis in pigs. There is a suspicion that it may also cause cysticercosis in human.

v  Like other taenids, humans are the definitive hosts, but in contrast, pigs, wild boars, as well as cattle can serve as intermediate hosts.

T. asiatica is generally neglected in the global elimination context of human taeniasis cysticercosis.  T. asiatica remains ignored mainly for 2 reasons; first, its supposedly non-cosmopolitan character, being restricted to Asian countries, and second, its close molecular similarities to T. saginata suggesting that T. asiatica probably does not cause human cysticercosis since T. saginata eggs do not infect  humans.

Based on the appearance of eggs and proglottids, the diagnosis was first believed to be Taenia saginata, the beef tapeworm. However, on further investigation including patient history,the worm probably is T. asiatica (also known as the Asian Taenia species). Discovered only in 1980s from Taiwan and other East Asian countries as an unusual species. The rate of a prevalence is estimated to be up to 21% in Asian taeniasis which is documented in nine countries in Asia, including Taiwan, South Korea, Indonesia, the Philippines, Thailand, south-central China, Vietnam, Japan and Nepal.

Morphological structures

Adult

The body of Taenia asiatica is yellowish-white in colour, about 350 cm long and 1 cm broad, divided into the anterior scolex, followed by a short neck and a highly extended body proper called strobila. It is an acoelomate animal with no body cavity or digestive system. The scolex bears four simple suckers as attachment organs to the intestinal wall of the host. The strobila is composed of a chain of ribbon-like segments called proglottids. There are more than 700 proglottids in the strobila, but less than 1000 (~900), while T. saginata in comparison have more than 1000 proglottids. The proglottids are distinguishable into mature and gravid proglottids. The gravid proglottids are full of fertilised eggs.

Egg

Eggs of  Taenia asiatica is similar to the egg of Taenia solium. The eggs are round or oval and measure 33micrometer-43micrometer in diameter. They are bile stained.

Fig: Egg of Taenia asiatica

Life cycle

The life cycle of T. asiatica is indirect and digenetic, and is completed in humans as the definitive host, and the intermediate host is mostly pigs (including wild boar in Taiwan), and possibly cattle on rare occasion. The complete life cycle is shortest among human taenids. Humans ingest the infective larvae called cysticercus from raw or undercooked meat, or viscera of pigs. The adult worm inhabits the small intestine where it gets attached to the mucosa using its suckers and rostellar hooklets. Upon sexual maturity it undergoes self-fertilisation. Fertilized eggs are released through the faeces along with the gravid proglottid which gets detached from the strobila. The number of proglottids released per day may vary from 0 to 35. Cysticercus grows into adult in about 2.5 to 4 months, by the time gravid proglottids are found in faeces.

T. solium eggs infect only omnivorous intermediate hosts (pigs, humans, and dogs). In nature, T. asiatica eggs, just as T. solium, do not infect herbivorous hosts since they only hatch in omnivorous intermediate hosts (pigs and wild boars). Pigs and wild boars ingest the infective embryo while grazing. The digestive enzymes will break the thick shell of the egg and allow formation of the zygotes called "oncospheres". These oncospheres then penetrate the mucous layer of the digestive tract and enter the circulation of the host. This is where the young larval stages form a pea-sized, fluid filled cyst, also known as “cysticercus”, which migrate to visceral organs like liver, serosa and lungs in pigs, and liver in cattle. In contrast to T. saginata, the larval development is short, taking about four weeks. Cystecerci have a predilection for liver.

Pathogenesis

The adult worms live in the lumen of the intestine and may cause demage in it and migrant to duodenum. Due to which, multiple erosions and active bleeding from ulcers in the stomach and duodenum. They acquire nutrients from the intestine, leading to malnutrition of the host.  T. asiatica rarely cause neurocysticercosis in humans.

Clinical symptoms

T. asiatica causes intestinal taenisis in humans and cysticercosis in pigs. In pigs cysticercus has a tendency to produce in vital organs such as liver and lungs. In intestinal taenisis, infection may lead to cause ulcer with bleeding.

In neurocysticercosis symptoms include severe headache, blindness, convulsions, and epileptic seizures, and can be fatal. Neurocysticercosis is the most frequent preventable cause of epilepsy worldwide.

Laboratory Diagnosis

Microscopy Examination

The basic diagnosis is examination of a stool sample to find the parasite eggs. However, there is a serious limitation as to the identification of the species because the eggs of all human taenids look the same. Even with the proglottids it is extremely difficult to identify T. asiatica from other taenids because of their striking resemblances. The species and T. saginata are frequently confused due to their morphological similarities and sympatric distribution.

serology

To date the most relevant diagnosis of taeniasis due to T. asiatica is by enzyme-linked immunoelectrotransfer blot (EITB). EITB can effectively identify it from other taenid infections.

others

 Identification often requires histological observation of the uterine branches and PCR detection of ribosomal 5.8S gene. The presence of rostellum on the scolex, a large number of uretine branches (more than 57) and prominent posterior protuberances in gravid proglottids, and wart-like formation on the surface of the larvae are the distinguishing structures.

Treatment

Niclosamide (2 mg) is very effective against experimental infection in human. In general cestode infections are treated with praziquantel and albendazole. Atrabine is quite effective but indicates adverse effects in humans. The commonly used drugs for tapeworms, benzimidazoles are relatively ineffective. Praziquantel at a single dose of 150 mg is the most effective medication against T. asiatica without causing side effects.

Differences between taenia species T. saginata, T. asiatica, T. solium

Hymenolepis nana

Dwarf tapeworm  (Hymenolepis nana, previously known as Vampirolepis nana, Hymenolepis fraterna, and Taenia nana) is a cosmopolitan species though most common in temperate zones, and is one of the most common cestodes of humans, especially children.

Hymenolepis nana has been called the dwarf tapeworm and has a worldwide distribution. The fact that an intermediate host is not required in the life cycle was determined in the late 1800s. In 1921, Saeki demonstrated direct cycle of transmission of H. nana in humans; transmission without an intermediate host. H. nana is the most common cestode in humans with infection  prevalence highest among children encountered  worldwide and in warm arid climates with poor sanitation facilities. Although the parasite has a wide distribution, it is more prevalent in warm areas. It exists in Egypt, Sudan Portugal, Spain, Sicily, India, Japan, South America, Cuba and parts of Eastern Europe. The infection is most commonly seen in children, although adults are also infected.

Morphology

Adult

The entire worm is small, measuring up to 40mm in length by a maximum of 1mm in diameter. The size of the strobila is usually inversely proportional to the number of worms present in the host. The minute scolex (0.32mm in diameter) is rhomboidal, has four suckers (80μm in cross section) and a short rostellum, armed with 20-30 spines in one ring, capable of invagination into the apex of the organ. Beginning with short, narrow, immature proglottids, those more distal in position become increasingly broader.

Eggs of Hymenolepis nana  which measure 30 to 47 µm in diameter. The oncosphere is covered with a thin, hyaline, outer membrane and an inner, thick membrane with polar thickenings that bear several filaments. The rostellum remains invaginated in the apex of the organ.are immediately infective when passed with the stool and cannot survive more than 10 days in the external environment  . 

LifeCycle

Infection is usually acquired by the ingestion of H. nana eggs, primarily from human stool. The eggs hatch in the stomach or small intestine, and the liberated larvae, or oncospheres, penetrate the villi in the upper small intestine. The larvae develop into the cysticercoid stage in the tissue and migrate back into the lumen of the small intestine, where they attach to the mucosa. The adult worms mature within several weeks. They are very small compared with eggs of the Taenia species and measure up to 40 mm long. The more worms present, the shorter the total length of each worm.  Under heavy infections, autoinfection may occur when some of the eggs regurgitated into stomach hatch in the upper small intestine. H. nana can also develop in beetles, fleas or other insects to the cysticercoid stage. If the host accidentally ingests any of these intermediate hosts, the liberated cysticercoid larva attaches to the intestine and develops into an adult worm, this mode of infection is probably not common.

Fig: Life cycle of H. nana

 Pathogenesis

Hymenolepis nana lodges itself in the intestines and absorbs nutrients from the intestinal lumen. In human adults, the tapeworm is more of a nuisance than a health problem, but in small children, many H. nana can be dangerous. Usually it is the larva of this tapeworm that causes the most problem in children. The larva will burrow into the walls of the intestine, if there are enough tapeworms in the child, severe damage can be inflicted. This is done by absorbing all the nutrition from the food the child eats. Usually a single tapeworm will not cause health issues.

Hymenolepis nana usually will not cause deaths unless in extreme circumstances and usually in young children or in people who have weakened immune systems. In some parts of the world, individuals that are heavily infected are a result of internal autoinfection. H. nana's larval stage occurs either inside an auto infected host's intestinal villus or an intermediate rat host

Clinical manifestations

Symptoms in humans are due to allergic responses or systematic toxaemia caused by waste products of the tapeworm. Light infections are usually symptomless, whereas infection with more than 2000 worms can cause enteritis, abdominal pain, diarrhea, loss of appetite, restlessness, irritability, restless sleep, anal and nasal pruritus. Rare symptoms include anorexia, increased appetite, vomiting, nausea, bloody diarrhea, hives, extremity pain, headache, dizziness and behavioral disturbances. Occasionally epileptic seizures occur in infected children.

An infection with H. nana may cause no symptoms even with a heavy worm burden. Heavy human infection can be attributed to internal autoinfection in which the eggs hatch in the intestine and follow the normal life cycle to the adult worm. This autoinfection feature of the life cycle can lead to complications in the compromised patient.  Heavy human infection can be attributed to internal autoinfection in which the eggs hatch in the intestine and follow the normal life cycle to the adult worm. This autoinfection feature of the life cycle can lead to complications in the compromised patient.

Laboratory Diagnosis

Microscopy examination

Egg: The identification of H. nana eggs in stool specimens. The eggs are most easily seen on a direct wet smear or a wet preparation of the concentration sediment. H.nana  eggs are frequently spherical or ovoid with a thin hyaline shell and measure 30-47 μm in diameter. The oncosphere with its 3 pairs of hooklets lies in the center of the egg and is separated from the outer shell by sizeable space.

Since the eggs of H. nana and H. diminuta look very much alike, identification to the visual identification of the polar filaments seen in H. nana eggs.

Adult worms: The adult worm is rarely seen in the stool. The scolex has four suckers and a short rostellum with hooks. The worms are very small compared with worms of the Taenia species and measure up to 40 mm long. The more worms present, the shorter the total length of each worm.  The adult worm is rarely seen in the stool.

Fig: Egg and adult worm of Hymenolepis nana

Fig: Egg and adult worm of Hymenolepis nana

Serology

Different serology tests like Enzyme-linked immunosorbent assay (ELISA), immunodiffusion (ID), immunoelectrophoresis (IEP) (10,11), double diffusion (DD), immunoprecipitation (IP) and indirect immunofluorescent antibody test (IFAT)  are used in the diagnosis of Hymenolepis spp. infections. This test is used to differetiate the species of Hymenolepis tapeworm.

Treatment

The 2 drugs that have been described for the treatment of hymenolepiasis are praziquantel and niclosamide. Praziquantel, which is parasiticidal in a single dose for all the stages of the parasite, is the drug of choice because it acts very rapidly against H. nana. Although structurally unrelated to other anthelminthics, it kills both adult worms and larvae. In vitro the drug produces vacuolization and disruption of the tegument in the neck of the worms, but not in more posterior portions of the strobila. Praziquantel is well absorbed while taken orally, it undergoes first-pass metabolism and 80% of the dose is excreted as metabolites in urine within 24 hours.

Prevention

v  Preventing fecal contamination of food and water in institutions and crowded areas is of primary importance.

v  General sanitation and rodent and insect control (especially control of fleas and grain insects) are also essential for prevention of H. nana infection.

v  Repeated treatment is required for Hymenolepis nana at an interval of 7-10days.

Hymenolepis diminuta

Hymenolepis diminuta is a small tapeworm commonly found in rats and mice.  Hymenolepis diminuta, also known as rat tapeworm, is a species of Hymenolepis tapeworm that causes hymenolepiasis. H. diminuta is prevalent worldwide, but only a few hundred human cases have been reported. Few cases have ever been reported in Australia, United States, Spain, and Italy. In countries such as Malaysia, Thailand, Jamaica, Indonesia, the prevalence is higher. It has a world wide distribution in these hosts but is infrequently found in humans, with only sporadic cases being reported. 

  

Morphology

It has slightly bigger eggs and proglottids than H. nana and infects mammals using insects as intermediate hosts. The adult structure is 20 to 60 cm long and the mature proglottid is similar to that of H. nana, except it is larger. The ova are large, ovoid and yellowish with a moderately thick shell.  They contain an oncosphere with six hooklets and a clear area between the oncosphere and the shell. They measure 70-85µm by 60-80µm.

Life cycle and Transmission

The life cycle of H. diminuta requires an intermediate arthropod host e.g. earwigs, larval fleas and various beetles.  Human infection occurs by the accidental ingestion of an infected arthropod, which contains the cysticercoids. The cycle begins as arthropods ingest the eggs. Arthropods are then able to act as the intermediate host. When ingested, the eggs develop into cysticercoids. Rodents can become infected when they eat arthropods. Humans, especially children, can ingest the arthropods as well and therefore become infected via the same mechanism. As the definitive host (rats) eats an infected arthropod, cysticercoids present in the body cavity transform into the adult worm. The resulting eggs are then passed through the stool.

Clinical manifestation

Human infection results from eating such foods as dried  fruits and precooked breakfast cereals in which the infected grain insects larvae are present. Eating an intermediate host of H. diminuta. H. diminuta is often asymptomatic. However, abdominal pain, irritability, itching, and eosinophilia are among the existing symptoms in a few of the reported cases.The symptoms associated with H. diminuta infections are few if any.  

Laboratory Diagnosis

Diagnosis is based on recovery and identification of the characteristic ova in a formol-ether concentrate of feces.  Adult worms and proglottids are rarely seen in stool samples.

Treatment

The treatment for this condition is a single dose of praziquantel, repeated in 10 days.

Prevention

Good hygiene, public health and sanitation programs, and elimination of rats help prevent the spread of hymenolepiasis.

Echinococcus granulosus

·       Echinococcus granulosus, also called the Hydatid worm or Hyper Tape-worm or Dog Tapeworm.

·       it causes cystic echinococcosis, also known as hydatid disease.

·      Hydatid disease’ and ‘hydatidosis’ are terms that should be restricted to infection with the metacestode, and ‘echinococcosis’ to infection with the adult stage.

·      Infection with Echinococcus may be naturally transmitted between humans and other animals and therefore claims membership of the most significant group of communicable diseases, the zoonoses. ‘

Cystic echinococcosis (CE) is caused by infection with the larval stage of Echinococcus granulosus. CE is found in Africa, Europe, Asia, the Middle East, Central and South America, and in rare cases, North America. The disease is most commonly found in people involved in raising sheep, as a result of the sheep's role as an intermediate host of the parasite and the presence of working dogs that are allowed to eat the offal of infected sheep.

Morphology

The adult tapeworm ranges in length from 2 mm to 7 mm and has three proglottids ("segments") when intact an immature proglottid, mature proglottid and a gravid proglottid. Like all cyclophyllideans, E. granulosus has four suckers on its scolex ("head"), and E. granulosus also has a rostellum with hooks. 

Hydatid cyst

Characteristics of Hydatid cyst

 Size: Measures about 22-25μm in thickness, gives rise to brood capsules with scolices, secretes hydatid fluid and forms the outer layer.

•       Outer layer-ectocyst

•       Inner germinal layer-endocyst

Characteristic features of hydatid fluid 

•       Clear colorless fluid

•       Specific gravity(1.005-1.010)

•       Slightly acidic

•       Contains sodium chloride, sodium sulfate, sodium phosphate and sodium and calcium salts of succinic acid.

•       Antigenic

•       Highly toxic

•       Hydatid sand.

Life cycle

E. granulosus requires two host types, a definitive host and an intermediate host. The definitive host of this parasite are dogs and the intermediate host are most commonly sheep, however, cattle, horses, pigs, goats, and camels are also potential intermediate hosts. Humans can also be an intermediate host for E. granulosus, however this is uncommon and therefore humans are considered an aberrant intermediate host. The parasite is transmitted to dogs when they ingest the organs of other animals that contain hydatid cysts. The cysts develop into adult tapeworms in the dog.

The adult Echinococcus granulosus (3 to 6 mm long) resides in the small bowel of the definitive hosts, dogs or other canids. Gravid proglottids release eggs that are passed in the feces. Infected dogs shed tapeworm eggs in their feces which contaminate the ground.  Sheep, cattle, goats, and pigs ingest tapeworm eggs in the contaminated ground; once ingested, the eggs hatch and develop into cysts in the internal organs. After ingestion by a suitable intermediate host (under natural conditions: sheep, goat, swine, cattle, horses, camel), the egg hatches in the small bowel and releases an oncosphere that penetrates the intestinal wall and migrates through the circulatory system into various organs, especially the liver and lungs. In these organs, the oncosphere develops into a cyst that enlarges gradually, producing protoscolices and daughter cysts that fill the cyst interior. The definitive host becomes infected by ingesting the cyst-containing organs of the infected intermediate host. After ingestion, the protoscolices evaginate, attach to the intestinal mucosa, and develop into adult stages in 32 to 80 days. The most common mode of transmission to humans is by the accidental consumption of soil, water, or food that has been contaminated by the fecal matter of an infected dog. Echinococcus eggs that have been deposited in soil can stay viable for up to a year.  

Pathogenesis

The damage produced is mainly mechanical. The young cyst that develops from embryos lodged in vital centers may soon interfere with functions of the organ with damaging, even fatal results. Benign cyst may be asymptomatic or it may produce physical burden to the patient. The severity depends on the type of tumor and organ or tissue where it first becomes implanted. Anaphylactic reactions develop.

Clinical manifestations

Many hydatid cysts remain asymptomatic, even into advanced age. The parasite load, the site, and the size of the cysts determine the degree of symptoms. Theoretically, echinococcosis can involve any organ. The liver is the most common organ involved, followed by the lungs. These 2 organs account for 90% of cases of echinococcosis. Organs affected by E granulosus are the liver (63%), lungs (25%), muscles (5%), bones (3%), kidneys (2%), brain (1%), and spleen (1%).

• In the liver, the pressure effect of the cyst can produce symptoms of obstructive jaundice and abdominal pain. With biliary rupture, the classic triad of biliary colic, jaundice, and urticaria is observed. Passage of hydatid membranes in the emesis (hydatid emesia) and passage of membranes in the stools (hydatid enterica) may occur rarely.
• Involvement of the lungs produces chronic cough, dyspnea, pleuritic chest pain, and hemoptysis. Expectoration of cyst membranes and fluid is observed with intrabronchial rupture.
• Headache, dizziness, and a decreased level of consciousness may signify cerebral involvement. Specific neurologic deficits may occur depending on the location of the cyst in the brain.

Secondary complications may occur as a result of infection of the cyst or leakage of the cyst. Minor leaks lead to increased pain and a mild allergic reaction characterized by flushing and urticaria. Major rupture leads to a full-blown anaphylactic reaction, which is fatal if not treated promptly. A rupture into the biliary tree can lead to obstruction by the daughter cysts, producing cholangitis. Rupture into the bronchi can lead to expectoration of cyst fluid.

Laboratory diagnosis

Clinically, Slowly growing cystic tumor with history of close contact to dogs which can be indentified by

X-ray imaging: (esp. in the lungs, & calcified cysts)

    - Rounded solitary, or multiple, sharply contoured cysts of 1-15 cm. in diameter.

    - Internal daughter systs, give a car wheel appearance.

    - Thin crescent or ring shape calcifications.

Computarized tomography (CT) & ultrasonography

   - Detects uncalcified cysts.

   -  special value in the follow up of treated cases.

Finding hydatid material:  After surgery, in sputum or urine.

Serological tests:

    - IHA, or better by ELISA

Molecular diagnosis:

    - DNA analysis or PCR.

Intradermal test of Casoni:

    - May give false +ve results in 18% of cases.

    - May give allergic reaction

High eosinophilia

Treatment

Humans should avoid handling fecal matter of canines and avoid consuming infected animals and home slaughtering animals. If a human, becomes infected, there are a variety of methods for treatment. The most common treatment in the past years has been surgical removal of the hydatid cysts . However in recent years, less invasive treatments have been developed such as cyst puncture, aspiration of the liquids, the injection of chemicals, and then re-aspiration.

 Benzimidazole-based chemotherapy is also a new treatment option for humans.

Prevention

—  In order to prevent transmission to dogs from intermediate hosts, dogs can be given anthelminthic vaccinations. In the case of intermediate hosts, especially sheep, these anthelminthic vaccinations do cause an antigenic response meaning the body produces antibodi avinash response however it does not prevent infection in the host.

—  Clean slaughter and high surveillance of potential intermediate host during slaughter is key in preventing the spread this cestode to its definitive host. It is vital to keep dogs and potential intermediate host as separated as possible to avoid perpetuating infection.

—  According to mathematical modeling, vaccination of intermediate hosts, coupled with dosing definitive hosts with anthelminths is the most effect method for intervening with infection rates.