When I was an undergraduate at UC Santa Barbara I had the fortunate opportunity to take a parasitology course with Dr. Armand Kuris. During this intensive course we learned that parasites face unique challenges in life, and that our typical day is drastically different from that of a parasite. For example, we may wake up everyday and go to work or school, and come home for a nice dinner before going to sleep. For most of us, these activities are not exceptionally hostile experiences.
Consider the life of a parasite. A parasite must first encounter a host, which can be quite challenging if hosts are scarce or patchy, or not around at opportune times. Some infective stages only have a few hours to find a host before they perish in the environment. Next, the parasite must be compatible with it’s new host. Just because a parasite may find a potential home doesn't mean it will be able to live there. For example, have you ever been the unfortunate recipient of "swimmer's itch"? Perhaps you went for a dip in a lake or pond, only to be covered with small, intensely itchy bumps later on? That uncomfortable rash was caused by parasitic worms called trematodes (see below) that burrowed into your skin and tried to take up residence in your body. However, you were not a compatible host (these trematodes often use birds as their final hosts), and your immune system was able to quickly destroy the little invaders.
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Another thing to keep in mind is that once a parasite successfully locates and enters an appropriate host, their new residence will soon try to kill them. Imagine living in a house that is alive and constantly assaulting you with the sole intention of ending your life. A parasite's life can be tough, and only a fraction of them may survive to reproduce.
How is one to thrive given so many obstacles? Parasites have evolved a variety of ways to locate hosts and once inside, many can combat or even evade the host immune system. They have conquered the host-time to celebrate! Well, not really. What if the parasite needs to leave it’s cozy home it worked so hard to obtain to complete its life cycle? This is the harsh reality faced by many parasites. Some must travel to mating locales that the host does not frequent, while others require transmission to additional hosts for their survival and reproduction. Still, others must find proper accommodations for their offspring inside an unwilling provider. This is when things get bizarre and where the science fiction writers come for ideas. This is why I am in graduate school.
Parasites that encounter these challenges will often hijack their hosts, manipulating their physiology, appearance, and/or behavior in ways that benefit the parasites. This phenomenon is termed “host manipulation”, and it has been identified in a wide variety of host-parasite relationships. Manipulations may be subtle (such as an increase in host activity levels) or spectacular (such as dramatic changes in host appearance and/or behavior). Although these manifestations are highly variable, the end result is often the same: the parasite prospers at the expense of the host. I have highlighted some cool examples below.
Zombie snails
Snails get infected with the Leucochloridium spp. parasite after inadvertently ingesting their eggs as they graze on contaminated vegetation. Once ingested, the trematode eggs release larvae called miracidia that develop into sporocysts which then form broodsacs that engorge the snail's eyestalks. These broodsacs contain hundreds of infective stages called cercariae that must get to a bird to complete the lifecycle. How will they get from a snail into a bird? The infested eyestalks begin to resemble caterpillars or grubs, and the broodsacs within pulsate when exposed to light! Birds searching for a meal may mistake these bags of worms for yummy insect larvae and go in for the kill.
Mind control by wasps
The Jewel Wasp (Ampulex compressa) mother has her work cut out for her. She must find a suitable host and somehow lure it back to her burrow where it will be held hostage, serving as a safe haven and living meal for her future offspring. What host would agree to this? Remarkably, this is not a problem for Jewel Wasps are they are powerful manipulators of the mind! When a female is ready to lay eggs, she locates an unsuspecting host for her young, usually a cockroach. She ambushes the roach, delivering a venomous sting into a specific region of it's brain. The neurotoxic venom does not paralyze the roach, but rather it makes the host submissive and seemingly undisturbed by the turn of events. The roach does not fight back, nor does it attempt to escape. Instead, it stands still while the wasp clips its antennae, drinking hemolymph ("insect circulatory fluid") from the open ends. Like leading a dog on a leash, the wasp grasps the host's antenna and leads it into her burrow. Once inside she will lay an egg on the zombified roach and the hatched larva will burrow into it's body, feasting on the internal organs. After five days or so, the larva will pupate and later exit the hollowed out carcass as an adult wasp.
Suicidal crickets
It is thought that "horsehair worms" (Phylum Nematomorpha) got their name from an old superstition that supposed the worms arose from horse hairs that fell into watering troughs. We now know that these worms do not arise from horse hairs, but are instead free-living adults that enter pools of water in search of mates for reproduction. As juveniles, the worms are obligate parasites that require an insect or crustacean for food and development (adults do not feed). However, a challenge arises once they mature and are ready to mate because their terrestrial hosts do not frequent aquatic habitats. How does one lead a cricket to water? Research suggests the worms chemically manipulate the central nervous system of their hosts, causing them to actively seek out watery areas. When the host eventually finds water it will jump in, stimulating the worm to exit the body, often killing the host in the process.
Additional reading sources:
1. D.G. Biron, L. Marche, F. Ponton, H.D. Loxdale, N. Galeotti, L. Renault, C.
Joly, and F. Thomas. 2005. Behavioural manipulation of a grasshopper
harboring a hairworm: a proteomics approach. Proc. R. Soc. B. 1577: 2117-2126.
2. C. Combes. 2001. Parasitism: the ecology and evolution of intimate interactions.
University of Chicago Press.
3. R. Gal and F. Libersat. 2010. On predatory wasps and zombie cockroaches:
investigations of "free will" and spontaneous behavior in insects. Commun.
Integ. Biol. 5: 458-461.
4. R. Gal and F. Libersat. 2010. A wasp manipulates neuronal activity in
sub-esophageal ganglion to decrease to decrease the drive for walking in its
cockroach prey. PLoS ONE 5 (4): e10019.
4. K.D. Lafferty and A.K. Morris. 1996. Altered behavior of parasitized killifish
increases susceptibility to predation by bird final hosts. Ecology 77: 1390-1397.
5. J. Moore. 2002. Parasites and the behavior of animals. Oxford University Press.
6. J.C. Shaw, W.J. Korzan, R.E. Carpenter, A.M. Kuris, K.D. Lafferty,
C.H. Summers, and O. Overli. 2009. Parasite manipulation of brain monoamines
in California killifish (Fundulus parvipinnis) by the trematode Euhaplorchis
californiensis. Proc. R. Soc. B. 276: 1137-1146.
Some people doing cool work on host manipulation:
Dr. Janice Moore and colleagues:
http://www.biology.colostate.edu/faculty/pillbug
Dr. Robert Poulin and colleagues:
http://www.otago.ac.nz/Zoology/staff/otago008915.html
Dr. David G. Biron and colleagues:
http://gemi.mpl.ird.fr/OPM/Biron/biron_A.htm
Dr. Frederic Thomas and colleagues:
http://gemi.mpl.ird.fr/OPM/Thomas/thomas_A.htm
Dr. Kevin D. Lafferty and colleagues:
http://www.lifesci.ucsb.edu/eemb/labs/kuris/
The Jewel Wasp (Ampulex compressa) mother has her work cut out for her. She must find a suitable host and somehow lure it back to her burrow where it will be held hostage, serving as a safe haven and living meal for her future offspring. What host would agree to this? Remarkably, this is not a problem for Jewel Wasps are they are powerful manipulators of the mind! When a female is ready to lay eggs, she locates an unsuspecting host for her young, usually a cockroach. She ambushes the roach, delivering a venomous sting into a specific region of it's brain. The neurotoxic venom does not paralyze the roach, but rather it makes the host submissive and seemingly undisturbed by the turn of events. The roach does not fight back, nor does it attempt to escape. Instead, it stands still while the wasp clips its antennae, drinking hemolymph ("insect circulatory fluid") from the open ends. Like leading a dog on a leash, the wasp grasps the host's antenna and leads it into her burrow. Once inside she will lay an egg on the zombified roach and the hatched larva will burrow into it's body, feasting on the internal organs. After five days or so, the larva will pupate and later exit the hollowed out carcass as an adult wasp.
Suicidal crickets
It is thought that "horsehair worms" (Phylum Nematomorpha) got their name from an old superstition that supposed the worms arose from horse hairs that fell into watering troughs. We now know that these worms do not arise from horse hairs, but are instead free-living adults that enter pools of water in search of mates for reproduction. As juveniles, the worms are obligate parasites that require an insect or crustacean for food and development (adults do not feed). However, a challenge arises once they mature and are ready to mate because their terrestrial hosts do not frequent aquatic habitats. How does one lead a cricket to water? Research suggests the worms chemically manipulate the central nervous system of their hosts, causing them to actively seek out watery areas. When the host eventually finds water it will jump in, stimulating the worm to exit the body, often killing the host in the process.
Additional reading sources:
1. D.G. Biron, L. Marche, F. Ponton, H.D. Loxdale, N. Galeotti, L. Renault, C.
Joly, and F. Thomas. 2005. Behavioural manipulation of a grasshopper
harboring a hairworm: a proteomics approach. Proc. R. Soc. B. 1577: 2117-2126.
2. C. Combes. 2001. Parasitism: the ecology and evolution of intimate interactions.
University of Chicago Press.
3. R. Gal and F. Libersat. 2010. On predatory wasps and zombie cockroaches:
investigations of "free will" and spontaneous behavior in insects. Commun.
Integ. Biol. 5: 458-461.
4. R. Gal and F. Libersat. 2010. A wasp manipulates neuronal activity in
sub-esophageal ganglion to decrease to decrease the drive for walking in its
cockroach prey. PLoS ONE 5 (4): e10019.
4. K.D. Lafferty and A.K. Morris. 1996. Altered behavior of parasitized killifish
increases susceptibility to predation by bird final hosts. Ecology 77: 1390-1397.
5. J. Moore. 2002. Parasites and the behavior of animals. Oxford University Press.
6. J.C. Shaw, W.J. Korzan, R.E. Carpenter, A.M. Kuris, K.D. Lafferty,
C.H. Summers, and O. Overli. 2009. Parasite manipulation of brain monoamines
in California killifish (Fundulus parvipinnis) by the trematode Euhaplorchis
californiensis. Proc. R. Soc. B. 276: 1137-1146.
Some people doing cool work on host manipulation:
Dr. Janice Moore and colleagues:
http://www.biology.colostate.edu/faculty/pillbug
Dr. Robert Poulin and colleagues:
http://www.otago.ac.nz/Zoology/staff/otago008915.html
Dr. David G. Biron and colleagues:
http://gemi.mpl.ird.fr/OPM/Biron/biron_A.htm
Dr. Frederic Thomas and colleagues:
http://gemi.mpl.ird.fr/OPM/Thomas/thomas_A.htm
Dr. Kevin D. Lafferty and colleagues:
http://www.lifesci.ucsb.edu/eemb/labs/kuris/
