Taenia saginata

Cattle acquire the embryonated eggs, the oncospheres, when they eat contaminated food. Oncospheres enter the duodenum, the anterior portion of the small intestine, and hatch there under the influence of gastric juices. The embryonic membranes are removed, liberating free hexacanth ("six-hooked") larvae. With their hooks, they attach to the intestinal wall and penetrate the intestinal mucosa into the blood vessels. The larvae can move to all parts of the body by the 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 the cyst. Thus, they become fluid-filled cysticerci. Cysterci 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.[3][6]

Taenia saginata - oncosphere

Humans contract infective cysticerci 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 attaches to the intestinal wall. The larva mature into adults about 5 to 12 weeks later. Adult worms can live about 25 years in the host.[3] Usually, only a single worm is present at a time, but 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 proglottids 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 shed eggs as they move. These oncospheres in an external environment can remain viable for several days to weeks in sewage, rivers, and pastures.[3][6][7]

T. saginata infection is usually asymptomatic, but heavy infection often results in weight loss, dizziness, abdominal pain, diarrhea, headaches, nausea, constipation, chronic indigestion, and loss of appetite. Intestinal obstruction in humans can be alleviated by surgery. The tapeworm can also expel antigens that can cause an allergic reaction in the individual.[8] It is also a rare cause of ileus, pancreatitis, cholecystitis, and cholangitis.[9] Taenia saginata has been reported as a cause of gallbladder perforation if left untreated in some cases.[10] Taenia saginata adult worms can live in the host for up to 25 years and most infections will last 2 to 3 years without treatment.[11]

The Taenia saginata remains asymptomatic due to the fact the organism does not present cysticerci in humans. Therefore, there is no presence of cysticercosis in humans either. Typically, cysticercosis is a parasitical tissue infection which infect the brain and muscle tissues. However the Taenia saginata can cause taeniasis, which is an infection. Taeniasis causes weight loss, pain and blockages in the intestines which can potentially become life-threatening.[12]

The basic diagnosis is done from a stool sample. Feces are examined 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.[6] 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 (T. saginata uteri have 12 or more branches on each side, while other species such as T. solium only have five to 10).[2]

Differentiation of the 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. Identification often requires histological observation of the uterine branches and PCR detection of ribosomal 5.8S gene.[13] The uteri of T. saginata stem out from the 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.[14]

Eosinophilia and elevated IgE levels are chief hematological findings. Also Ziehl–Neelsen stain can be used to differentiate between mature T. saginata and T. solium, in most cases T. saginata will stain while T. solium will not, but the method is not strictly reliable.[15]

Adequate cooking at 56 °C (133 °F) for 5 minutes of beef viscera destroys cysticerci. Refrigeration, freezing at −10 °C (14 °F) for 9 days or long periods of salting is lethal to cysticerci. Inspection of beef and proper disposal of human excreta are also important measures.[8]

Taeniasis 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).[8] Albendazole is also highly effective for treatment of cattle infection.[16] During the 1940s, the preferred treatment was oleoresin of aspidium, which would be introduced into the duodenum via a Rehfuss tube.[17]