TAENIA SAGINATA (Beef tapeworm)

GEOGRAPHICAL DISTRIBUTION

Infection is common in countries where beef is often eaten raw or under-cooked such as African countries, Brazil, Argentina, USA, former Russian federation countries and Europe.

LOCATION IN HOST

The worm is found attached to the mucosa of the small intestine (ileum) with its scolex. The rest of the worm is free in the lumen of the intestine.

MORPHOLOGY

They are flat, ribbon-like worms. Their bodies are formed of chains of segments (proglottids) of varying sizes and shapes. At the anterior end is the scolex or the ‘holdfast’ organ which is used for the attachment of the worm to the intestinal mucosa.

The scolex is 1-2mm and is pear shaped. There are four suckers on the scolex but no rostellum or hooks. The fully grown worm is exceptionally long, sometimes reaching 20 meters. Usually it measure about 5 meters and has about 1000-2000 segments. The mature segments measure approximately 12 mm wide and 10 mm long. In the mature segments both male and female reproductive organs are found. On the lateral margin are the genital pores, one on each segment. In gravid segments (the last segments of the strobila) the uterus occupies the whole space with primary and secondary uterine branches (twenty uterine branches at one side).

LIFE CYCLE

Humans are the only definitive host of the parasite and cattle are the only intermediate host.

The gravid segments are dislodged and are passed in the faeces of the infected person. The detached gravid segments also have their inherent movement and may actively move down the lumen and out of the anus and sometimes along the thighs.

When the segments are deposited on the ground the eggs are scattered on the soil. Eggs deposited on vegetation can live for months to years until they are ingested by cattle.

Eggs measure 30-45 µm. Each egg has an outer vitelline membrane which is usually lost in the faeces. The inner, thick wall is made up of keratin blocks and appears striated. This is called the embryaosphore. Inside is the embryo and now it is called the onchosphere. It has six embryonic hooklets and because of this it is also known as the hexacanth larva.

The egg when ingested by cattle, the onchosphere hatches out in the duodenum. With the help of the hooklets it penetrates the intestinal mucosa and enters the venous capillaries (or mesenteric lymphatics) within half an hour. Then they reach skeletal muscles. There they lose their hooklets and grow into infective cysticercus bovis larva in about 10-12 weeks. The infective cysticercus bovis is ovoid, white in colour and measures 8*5 mm. It has an invaginated scolex and four suckers but no hooks. Humans acquire the infection when infected beef containing cysticercus bovis is eaten raw or under-cooked.

PATHOGENESIS AND PATHOLOGY

The scolex does not cause much damage to the mucosa. Sometimes inflammation may be present. Several cases of intestinal obstruction and perforation have been reported. The symptoms are often due to metabolic products of the worm.

CLINICAL FEATURES

Often the infection is asymptomatic except noticing whitish segments in faeces or experience of segments actively moving out of the anus. Vague abdominal symptoms such as pain, nausea, weight loss and anorexia may be present. In some the symptoms are relieved by ingestion of food.

DIAGNOSIS

The diagnosis is made by the detection of eggs or proglottids in the stool. Eggs may also be present in the perianal area.

1. The gravid segments passed in the faeces could be injected with Indian ink to show the lateral uterine branches of the uterus. In Taenia saginata there are more than 13 primary branches of the uterus(compare with Taenia solium).

2. If the scolex is passed following treatment, the absence of hooks distinguishes it from the Taenia solium.
   

3. If the gravid segments are ruptured eggs may be found in faeces (taenid egg).

EPIDEMIOLOGY

This is common infection where beef consumption is high. Infection often occurs when cattle are young. Faeces containing eggs pollute pastures. The role of birds transmitting eggs from sewage to pastures is also possible.

PREVENTION AND CONTROL

Meat infection is the important public health measure. Cycticerci in muscle are usually killed when frozen at minus 10ºC for 10 days or heating thoroughly above 56ºC is also effective.

TREATMENT

A single dose of praziquantel (10 mg/kg) is highly effective. Niclosamide can also be given. The dose is 2gm for adults and lower doses for the children. The tablets are chewed and washed down with a little water.