Hookworm teeth
Credit: AJ Cann, flickr
It started about seven years ago. I thought it was just a fleeting fascination but the seeds had been sown for a lifelong affair – I was hooked. I would tell anyone who would listen. Always something of a chatterbox, I would wax lyrical about their variety, ingenuity and rapid ability to adapt. It was time to face the truth – I was in love with parasites.
My friends were very tolerant. At least, they were until I started to regale them with parasites’ lifecycles over breakfast. Dan stuck it out the longest but the hookworm was the final straw. I’m not sure if it was the burrowing through the bottom of the foot, the bursting out of the alveoli into the lungs or the sucking of blood from the intestinal wall that did it. I do know that the result was a slammed door and a half eaten bagel in the bin.
What is it about parasites that fascinate me so much? They are just so damn clever. By definition, parasites have an unequal relationship with their host, a bit like the ‘friend’ who always turns up just in time for dinner but never cooks in return. Parasites come up with equally cunning ways to scavenge resources such as nutrients or water, but to make the most of their host will also use their bodies as somewhere to live and breed. Over time, however, the hosts develop their own strategies to avoid or get rid of their sponging guests.
This leads to a continual battle of one-upmanship and it’s usually those brilliantly devious parasites who are one step ahead. For example, hairworms live and breed in fresh water but they grow inside grasshoppers, gorging on their innards. When they’re ready to leave – that is, just before they suck the grasshopper completely dry – they produce proteins that change the grasshopper’s behaviour, fooling it into entering the water where the now fully-grown hairworm can emerge, swim away from its drowning victim and start the cycle all over again.
Humans have hosted parasites for as long as we can remember. Tapeworm infections are mentioned in the records of Hippocrates and Galen. Today, the World Health Organization estimates that a quarter of the world’s population is infected with parasitic worms, be they tapeworms, flukes, roundworms or some combination of the three. The worms themselves are rarely lethal, but this means that infections can last for decades. The chronic burden of harbouring growing worms severely impacts on quality of life. Their constant siphoning of resources can lead to blindness, anaemia and stunted growth. Infection rates are highest in parts of the world where access to food and nutrition is already limited, which only exacerbates the problems.
Ingenious humans
Despite having lived with parasites for millennia, we humans have slipped behind in the battle of wits. Current treatments for parasitic infections are rather hit and miss. But now, scientists at the Wellcome Trust Sanger Institute have sequenced the DNA of four species of tapeworm, shedding light on exactly why some treatments do not work. For instance, mefloquine is a drug that successfully treats schistosomiasis but is ineffective against tapeworms. It targets specific enzymes in the nervous system but the DNA sequencing showed that tapeworms do not produce much of these enzymes themselves. As such, drugs like mefloquine that disrupt them have little effect.
As well as working out what isn’t working, the researchers have been trying to identify new targets, and the very adaptations that have enabled worms to successfully parasitise humans may turn out to be their weakness.
One approach is to stop the tapeworms from breeding in humans. In biological terms, humans and worms are fundamentally pretty similar, so to be effective, any such treatment needs to be able to halt the worms’ rapid proliferation while minimising damage to any surrounding human organs. Killing growing cells while minimising side-effects is the same challenge faced when treating cancer and when the team compared likely drug targets in worms, many of them overlapped with the targets of existing cancer therapies. Indeed, some drugs developed to kill parasitic worms have been shown to slow down the growth of cancerous tumours and it is hopeful, therefore, that the reverse will be true as well. Identifying potential treatments that are already approved for use in humans should make it much faster to get new drugs for treating parasite infections rather than developing them from scratch.
It might just also signal the end of my seven-year crush on parasites, I see an ingenious human scientist on the horizon…
Reference:
Tsai, I et al. (2013). The genomes of four tapeworm species reveal adaptations to parasitism Nature, 496 (7443), 57-63 DOI: 10.1038/nature12031
Filed under: Infectious Disease Tagged: DNA sequencing, hookworm, parasites, parasitic worms, Research, tapeworm, Wellcome Trust Sanger Institute
