Parasitic worms: infecting several hosts in succession is not as risky as it seems
Researchers at the Humboldt-Universität zu Berlin compiled infection rates from different worm species and found that a worm’s odds of infection improve through the life cycle.
Parasitic worms lead dangerous lives. Take for example the human fish tapeworm – it infects tiny plankton as first host, which are eaten by a second host, small fish, which are eaten by a third host, bigger fish like trout, which are finally eaten by a mammal, like people. Only then, in the gut of a mammal, does the worm reproduce. Such a complex life cycle is treacherous because every time the worm infects a new host, it might be digested or killed by the host immune system.
But a new study suggests worm life cycles might not be as risky as we think.
Researchers at the Humboldt University of Berlin compiled infection rates from hundreds of experiments with different roundworm, tapeworm, and thorny-headed worm species. They found that a worm’s odds of infection improve through the life cycle. Worms have higher infection rates in the second host than in the first host and higher still in the third host. This increase seems to be caused by worm growth; bigger worms had better infection rates. So, our fish tapeworm has a higher probability of infecting humans, the last host, than plankton or fish, the earlier hosts, because it grows in plankton and fish before infecting humans as a larger larva.
This result also sheds light on another mystery: why worms grow so much in little hosts. Our example tapeworm can grow to be about 10% of the body mass of its plankton first host. Growing this big can take a couple weeks, which is a long time in the life of a plankton, so there is a fair chance the host dies before the worm finishes growing. The payoff to this aggressive growth is that the worm is better able to infect the next host. Thus, growth in little hosts, although risky, seems to substantially improve the chances that worms finish their hazardous life cycles.
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