Dwarf Tapeworm, Hymenolepis nana

The dwarf tapeworm (Hymenolepis nana) is a species of tapeworm that is classified in the Platyhelminthes phylum. It once held three other scientific names including Vampirolepis nana and Taenia nana. It is found throughout the world but occurs most often in temperate regions. As its common name implies, the dwarf tapeworm is small, reaching an average body length of 1.5 inches. The head, or scolex, holds a retractable beak like organ that has twenty to thirty hooks and four string suckers, known as a tetrad. Its body is slender, but the segments are wider than they are long. These segments, or proglottids, begin near the neck of the worm and are young, but they mature as they occur further down the body. This species, like all tapeworm species, is hermaphroditic. Mature segments contain three testes and an ovary. Once an egg is made within the segment, it can be released in the feces, but it most often remains in the body where it can mature without ever leaving the initial host.

The dwarf tapeworm can infect many hosts, but is one of the most common tapeworms to infect humans. The life cycle of this worm begins when a host consumes eggs within infected feces. This is most often accomplished when the host consumes infected food. Once the eggs travel through the stomach to the small intestine, they hatch and develop into oncospheres. The oncospheres penetrate though the mucosa lining of the intestine and travel through the lymphatic system to the villi, where they will stop and develop into cysticercoids. The worm is protected within this cyst, but its head and tail protrude from it. After five to six days of development, the worms break free from the cysticercoids and attach themselves to the lumen in the small intestines where they mature.

Although the dwarf tapeworm most often lives and reproduces in one host, it is capable of developing into larvae within an intermediate host like a small beetle or flea. This shows that, like other species of parasitic worm, the dwarf tapeworm may have once been dependent upon an intermediate host for survival. Once the worm has matured inside its host, it will feed on the nutrients, specifically carbohydrates that pass through the intestine. It is thought that this species does not have a preferred diet or required nutrients, so it will feed on whatever the host is digesting.

The dwarf tapeworm does not typically cause harm in adult hosts, but children can be in danger if infected by the species. This most often occurs when the infection is heavy and large numbers of larvae are burrowing into the intestinal wall and consuming too many nutrients. In most cases, this will not cause death, but if the human host is already weak, the resulting loss of nutrients caused by the infection could greatly endanger the host. Heavy infections often occur when the infection has comprised several generations of worm. When symptoms of this type of infection occur, they are usually caused by an allergic reaction to the excretions of the tapeworm. A host with an infection of over two thousand worms may experience symptoms including abdominal pain, diarrhea, loss of appetite, irritability, loss of sleep, restlessness, and enteritis. Symptoms like nausea, vomiting, headache, anorexia, hives, dizziness, increased appetite, and behavioral changes are rare but do occur. Some infected children may experience epileptic seizures.

A dwarf tapeworm infection can be treated by using the medicines Niclosamide and Praziquantel. Praziquantel is most often used because of its efficiency in eradicating an infection. This drug is able to kill the worm quickly at any level of development. By using proper sanitation with food and water sources, it can be relatively easy to prevent an infection of the dwarf tapeworm. This includes insect and rodent control and proper disposal of waste.

Image Caption: Three adult Hymenolepis nana tapeworms. Credit: Georgia Division of Public Health/Wikipedia