Introduction:
Tapeworms (Taenia solium and Taenia saginata) belong to the class Cestoda of phylum Platyhelminthes. Tapeworms are flat, parasitic animals with complex life cycles that infect animals, including humans. They are exclusively endoparasites.
Comparison and contrast of Taenia solium and Taenia saginata in terms of morphology:
Differences between Taenia solium and Taenia saginata:
Differences in features of adult worm
| S.N. | Features | Taenia saginata | Taenia solium |
| 1 | Length | 5-10m | 2-3m |
| 2 | Scolex | Large quadrate | Small and globular |
| a | Rostellum and hooks | Absent (unarmed) | Conspicuous round rostellum present with two rows of alternating round and small dagger-shaped hooks (20-50 in number) |
| b | Pigmentation of sucker | Suckers may be pigmented | Suckers are not pigmented. |
| 3 | Neck | Long and narrow | Short and half as thick as the head |
| 4 | Strobila | ||
| a | Proglottid number: | 1000-2000 | Below 1000 |
| b | Size of the gravid segment | 20 mm x 5 mm (Gravid is four times as long as it is broad) | 12 mm x 6 mm ( Gravid is twice as long as it is broad) |
| c | Explusion | Singly expelled by the breaking away of the gravid segment | Passively expelled in chains of 5 or 6 from the ruptured wall of the uterus |
| d | Uterus | 15-30 lateral branches on each side; thin and dichotomous | 5-10 lateral branches on each side; thick-lipped genital pore present alternating between the right and left side of adjacent segments |
| e | Vagina | Present ( Prominent Vaginal sphincter) | Absent |
| f | Accessory ovarian lobe | Absent | Present |
| g | Testes | 300-400 follicles | 150-200 follicles |
Figure 1. Carmine-stained scolex of Taenia saginata (A) and Taenia solium (B).
(A) shows the scolex of T. saginata lacking hooklets, while (B) displays the scolex of T. solium with a rostellum and two rows of hooklets. Both images highlight the suckers used for attachment to the host’s intestinal wall.
Differences in the features of eggs
| Feature | Taenia saginata | Taenia solium |
| Egg | Up to 100,000 eggs present per proglottid | Upto 50,000 eggs present per proglottid |
Difference in features of larva
| Feature | Taenia saginata | Taenia solium |
| Metacestode of T. saginata is called cysticercus bovis (Beef bladder worm), which is a fluid-filled vesicle, seen as a shiny white dot in infected beef. | The metacestode of T. solium is called cysticercus cellulosae (pork bladder worm), which is a fluid-filled vesicle, seen as a thick white spot. |
Similarities between the morphology of Taenia solium and Taenia saginata:
- Adults of both T. solium and T. saginata are ribbon-like, dorsoventrally flattened, and creamy white.
- Both have no body cavity and have rudimentary excretory and nervous systems.The bodies of both T. solium and T. saginata are differentiated into Scolex, Neck, and Strobila.
- Scolex of both species have four cup-like muscular suckers that aid in attachment to host cell intestinal epithelia.The necks of both T. solium and T. saginata are unsegmented which grow continuously, and proliferate proglottids by transverse fission or asexual budding.
- In both, Strobila is the main bulk of the body, and the segments of Strobila are called proglottids.
- In both, proglottids differentiate into gravid proglottids (which have a branching uterus with fertilized eggs), mature proglottids (which are hermaphrodites bearing both male and female organs), and immature proglottids (which lack reproductive organs).
- Eggs of both T. solium and T. saginata are 30- 40 μm in diameter and are indistinguishable.
- Eggs of both have a thin hyaline embryophore around them, which disappears after release. The inner embryophore is radially striated.
- A completely formed embryo (oncosphere) with three pairs of hooklets (Hexacanth embryo) is present in both eggs.
- Eggs of both T. solium and T. saginata are yellow brown as they are bile-stained and can’t float in salt solution.
- Larvae of both T. solium and T. saginata are called Cysticercus, which are ovoid, milky white, opalescent fluid-filled vesicles.
Similarities between the lifecycle of Taenia solium and Taenia saginata:
- Both Taenia solium and Taenia saginata cause zoonotic disease and cause infestation of the intestinal tract with adult worms, termed Taeniasis.
- Both species have a complex two-host life cycle.
- In both species, the definitive host is humans who harbor the adult Tapeworm.
- In both species, the usual cycle of transmission in man involves infection by ingestion of “mealy” inadequately or improperly cooked meat (pork in T. solium & Beef in T. saginata).
- In both T. solium & T. saginata, the definitive host cycle begins in the intestine when ingested Cysticerci from “measly” meat are digested. Then protoscolex evaginates and attaches itself to intestinal well by the help of suckers and hooks.
- In both species, worms grow by reproducing their cells at the neck level, developing proglottids which mature as they become more distal to the scolex.
Then they subsequently develop into sexually mature adult worms within 60-70 days, i.e., both have a prepatent duration of about 10 weeks. - In both T. solium & T. saginata, mature proglottids undergo self-fertilization within segments & fertilized eggs are then intermittently excreted from segments into the large intestines & then expelled in feces. Sometimes gravid proglottids containing eggs are also passed out.
- Intermediate host (pig/cattle) gets infected by ingestion of eggs passed in human feces. In intestine, hexacanth larva hatch out of egg & attach to intestinal mucosa by suckers and hooks of scolex.
- Both T. solium & T. saginata evade the mucosa & vessels of the submucosa & lead the extraintestinal phase as they are carried by circulation to muscles, brain, or other tissues.
Then eggs of both lose hooklets & enlarge & develop into fluid-filled cysts within 9-10 weeks. - Metacestodes so formed are called cysticerci in both T. solium & T. saginata.
Difference in the life cycle of T. solium & T. saginata:
- Taenia saginata was first distinguished from its close relative Taenia solium by Goeze in 1782.
- Although it is difficult to estimate the life cycle of T. solium is most likely only a few years long and significantly different from that of T. saginata, which has a longer lifecycle.
- Intermediate host of T. solium is both pig & man, but that of T. saginata is only cattle.
- Transmission in man begins by ingestion of “measly” inadequate or improperly cooked pork in case of T. solium, but ingestion of “measly” inadequate or improperly cooked beef in case of T. saginata.
- Attachment in human jejunum is facilitated by hooks, and after fertilization of the egg, gravid segments are expelled singly by breaking away of the gravid segment in T. saginata, but expulsion is made passively in chains of 5-6 from the ruptured wall of the uterus in the case of T. solium.
- Eggs of T. saginata are not infective to humans, but eggs of T. solium are infective to humans.
- The number of eggs per proglottid in T. saginata is 100000, while it is 50,000 in T. solium
- Humans can be infected with cysticercus via feco-oral contamination in case of T. solium, but human cysticerci are not observed in T. saginata.
- In case of T. solium, humans can also be infected in the same way as pigs by consumption of food or water contaminated with eggs or by internal autoinfection.
- After ingestion, the oncosphere is released in the intestine & extra-intestinal phase can be exhibited as they invade the intestinal mucosa & carried via circulation to different tissues & develop into cysticercus cellulose in the CNS, eye, and subcutaneous tissue.
- In a nutshell, T. solium can cause human Cysticercosis while T. saginata cannot.
Figure: Life cycle of Taenia solium
The diagram depicts the transmission and development of Taenia solium in humans and pigs. Humans acquire infection by consuming undercooked pork containing cysticerci, leading to intestinal tapeworms. Autoinfection or ingestion of eggs can result in cysticercosis in humans, particularly affecting the central nervous system (CNS), eyes, and muscles. Pigs become infected by ingesting eggs, developing cysticerci in their muscles.
Figure: Life cycle of Taenia saginata
This figure illustrates the life cycle of T. saginata (beef tapeworm), showing transmission between humans (definitive host) and cattle (intermediate host). Humans acquire infection by consuming undercooked beef containing cysticerci. In the human intestine, the parasite develops into an adult worm. Eggs passed in feces are ingested by cattle, where they develop into cysticerci in the muscle tissue, completing the cycle.
References:
- Sastry AS, Bhat S. Essentials of Medical Parasitology. 2nd ed. New Delhi: Jaypee Brothers Medical Publishers; 2018.
- Centers for Disease Control and Prevention (CDC). Scolex of Taenia saginata and Taenia solium [Photographs]. DPDx Image Library & Public Health Image Library (PHIL), ID# 5262. Available from: https://phil.cdc.gov/
- Paniker CKJ, Satish G. Paniker’s Textbook of Medical Parasitology. 7th ed. New Delhi: Jaypee Brothers Medical Publishers; 2017. p. 154.