Lurking in the lumen of your intestine could be an unsuspected surprise. Many organisms are known to colonize the human digestive system, yet one of the most well-known is the tapeworm. Obtained by consuming undercooked meat, adult tapeworms are usually harmless. However, these animals exhibit a complex life-cycle, including a juvenile-phase commonly known as larval cysts. Such cysts occasionally infect humans, which in some serious cases cause neurological problems, blindness, epilepsy and even death. The symptoms of larval cyst infections can take several years to develop before possible diagnosis, and confirmed cases arise in both developed and developing countries. Currently, there are no fully effective methods of treatment, and the World Health Organization has classified two tapeworm species on their list of 17 Neglected Tropical Diseases that Need Action. Despite this, new research published last month in the journal Nature reveals how a combination of techniques used to decode the genetic sequences of certain tapeworm species, together with drugs that are currently used for targeting cancer, could help scientists find an effective treatment without the expensive and laborious procedures typically employed when developing new drugs.
My Personal Experiences
This research brings back memories of a former first-hand experience with the world of parasites:
Two years ago, I travelled across Ethiopia; while doing so, I met a fellow traveler who I have since stayed in contact with. Roughly two months after leaving East Africa, she sent me an email: “I have a souvenir from Ethiopia!” I thought for a moment what it could be: some jewelry; a wooden carving; a clay lip-plate from the Mursi tribe; a book? After my initial enquiry, I received the following response: “No, I have a Taenia!” From my former studies, I know that Taenia is the genus that many tapeworm species belong. Additionally, in Ethiopia it is the local custom to share food – so what she ate, I also ate. Thus, it did not take too much detective work to realize that I had a good chance of having an extra Ethiopian souvenir, as well. Indeed, the next day I discovered that I was also a host, and that my lower digestive tract provided an environment that was occupied by a worm. Remedies for treating adult tapeworms are available from most pharmacies; nevertheless, I have to admit that the experience gave me feelings that were a strange mix of both disgust and fascination.
Tapeworms are specialized, parasitic representatives of a basal group of animals known as flatworms, which comprise the Phylum Platyhelminthes. A typical adult tapeworm consists of a head-like structure, known as a scolex, which bears two rows of hooks and four suction cups, used to anchor the animal to the inner intestinal lining. Trailing behind are a series of segments, known as proglottids, which are flattened and slightly ridged, providing the organism with a large surface area to volume ratio, necessary for absorbing nutrients from the surrounding intestinal contents. This long, flat chain of segments – which can be up to 20 meters long in a human and over 30 meters long in certain whale species – gives the animal its common name due to their resemblance to a length of tape. The posterior segments specialize by developing eggs and sperm (the animal is hermaphrodite), and once ripe, they break off and leave the host via the animal’s feces.
Out in the open, it is possible that the eggs are consumed by an herbivorous or omnivorous animal, such as a cow, sheep or pig, which is known as the intermediate host. If so, the ingested eggs pass along the new host’s digestive system, through the intestinal wall, and into the circulatory system, where they are subsequently transported by the bloodstream to the brain, liver, muscular tissue, or other region; a process that initiates the next phase of development, and the formation of a cyst. At this stage, the cyst lays dormant, waiting for its current host to be eaten by a definitive host, such as a human, and thus allowing the tapeworm to complete its life-cycle by growing into a sexually mature adult within the secondary host’s intestine. Undeniably, the complex life-cycle of such an atomically simple animal – that is dependent on two hosts – is quite extraordinary.
Humans are usually infected with adult tapeworms as a result of eating undercooked and infected meat; such infections are especially common in tropical Africa and South East Asia. Eating uncooked fish can also result in tapeworm infections; accordingly, Finland and Japan have some of the highest incidents of adult fish tapeworm infections due to the high consumption of raw fish. Occasionally, though, larval cysts are also consumed by humans, which can migrate to the brain or other vital organs, and sometimes lead to severe problems. Adult tapeworms are readily treated with medications. Yet there are few effective drugs for treating cysts. However, pioneering research in medical science and genetics could change this.
Recently, an international team of researchers, based at the Wellcome Trust Sanger Institute in the United Kingdom, have mapped the genetic codes of four tapeworm species that frequently rely on humans as host animals. A main aim of this procedure and the subsequent analysis was to look for specific genetic weaknesses that could be used as targets by drugs that are used for treating cancerous growths. This is because the larval cysts of tapeworms are comparable in structure to cancer tumors. Thus, if comparisons are found between both tapeworm and human genomes, appropriate candidate drugs that are currently developed for targeting specific gene sequences in cancer growths, could be used to target similar genetic make-ups in tapeworm cysts. In other words, researchers are looking for the tapeworm genetic equivalents of the targets that are used by some cancer treating drugs, which suppress cell division and prevent DNA replication. If such findings prove successful, several years of medical research and development could be saved, which is more economical than developing completely new treatments.
Further research concerning the treatment of tapeworm cysts involved utilising existing knowledge on the biology of tapeworms and combining such understanding with results from the tapeworm genomes. During their evolutionary history, the parasitic life-style that characterises all tapeworm species has led to their inability to produce cholesterol and other fats, which are necessary for the development of larval cysts. Rather, tapeworms obtain and subsequently modify required fats from their hosts. The most active genes in a tapeworm are those involved with the processes that are necessary for acquiring fats from their host, and which synthesise proteins that bind fat molecules together or lead to the development of proteins that build fatty-acids. Therefore, the disruption of these proteins, and the genes that enable their manufacture, could prove to be an efficient treatment.
After having a personal experience with tapeworms, I now realize that I was fortunate in that I was infected with an adult. (Needless to say, the realization that I was providing a habitat for a long, intestinal parasite was quite a memorable one). However, for other people around the world who are infected with the more serious larval cysts, and especially those people who live in developing nations, the promising advancements in genetic sequencing and subsequent drug development techniques (combined with improved education in personal hygiene and cooking methods) are important and economical steps forward in helping to treat people and ultimately save lives.