Tapeworms have a life cycle that most people would probably describe as hideous. An adult tapeworm lives in the intestine and releases segments of itself called proglotiids, packed full of infectious eggs. In humans, the long, flat, ribbon-like adults may cause unpleasant abdominal symptoms and nutritional deficiency as they steal food from inside the intestine.
If infectious tapeworm eggs are accidentally ingested, through poor sanitation or unwashed food, they can pose a much more serious health problem. When this happens early larval forms of the worm work their way into the body, infiltrating muscles and vital organs. Here they hide out in pearly round cysts (of their own making), resistant to attack by the human immune system. The young larvae’s plan is to wait for the host to die so that they can be eaten and develop in to adult worms. Meanwhile the cysts grow; over time some can grow larger than the size of a baseball. The cysts may also multiply themselves, spreading the infection. The fluid-filled cysts can cause serious health risks. For example, the hatched larvae of the pork tapeworm, Taenia solium, tend to grow into cysts in the brain, causing seizures, meningitis, dementia or death.
Sadly, this dramatic horror is all too real for many people across the world. Treatments can involve surgical removal of cysts, however, often this is not a possible option and drugs currently used for treatment of tapeworm cysts are not very effective.
A recent paper comparing the genomes of four tapeworm species sheds light on previously unknown characteristics of these monstrous organisms. As well as being interesting in terms of biology, the findings will hopefully lead to better treatment strategies for the unlucky victims of tapeworm infection.
From their genomes, several genes have been identified that tell us how the worm steals its energy from the host, scavenging for fats and cholesterol. In fact, the results showed that they lack genes that allow them to make some of their own essential nutrients, indicating just how dependent they are on their hosts for life.
One particular gene was identified that is likely to make a great target for drugs because it is so essential. In other animals there are two separate genes, but in tapeworms there is just the one. The enzyme, called thioredoxin glutathione reductase (TGR), is involved with detoxifiying of the worm, and without it the worm would die.
The larval cysts of tapeworms have been compared to cancer because they show uncontrolled growth and invasion of tissue, and are hard to kill without also damaging surrounding organs. In the tapeworm genomes the scientists found several genes that are also the targets of cancer drugs, and crucially these are active in the hard-to-kill cysts. This means that cancer drugs, already licensed for use in humans, could be repurposed to treat tapeworm cysts without the expensive and lengthy process of developing an entirely new drug.
Tapeworms are by their nature secretive and cunning organisms, hiding inside of us. But their genomes have revealed their weaknesses – they had better watch out.
Tsai and Zarowiecki et al., Nature 496, 57–63 (04 April 2013) doi:10.1038/ nature12031