I’m always looking for examples of parasite infection modifying host behavior. Most interesting are cases where the modified behavior enhances parasite transmission. Of this type, there are several examples.
- Ant-Fluke (Youtube video)
Meet Dicrocoelium, a fellow parasite.
This lancet fluke’s lifestyle is a migration through the fluids of three hosts; a cow, a snail and an ant.
Hungry snails eat the dung of the infected cows and swallow inadvertently the sequestered fluke eggs. Once hatching in the snail’s intestine, they burrow through the gut wall and into a digestive gland. Within this gland, flukes reproduce a second generation - spewed back into the world by the tormented snail as balls of slime, each sticky drop, a seething mass of flukes.It is in the third host, the ant host, where we observe mind control. Foraging ants come across the sparkling orbs of snot, sensing the source of moisture that they are, they quench their thirst on the slick beverage. Entering the third hypersea reservoir, the flukes undulate through the ant’s fluids, most form cysts in the abdomen, but some home in on the nerve clusters that control the mandibles in the ants head.
The temperature drops into the coolness of evening, and the infected ant feels compelled to leave its brethren, forsaking them to climb a grass stalk spire to its apex - the pastures emergent canopy. Preparing to make itself a sacrifice, it anchors itself to the flimsy blade, attached firmly by its mandibles.
It waits motionless throughout the night to be devoured by the primary host, the cow. Herds like ruminating clouds pass over the ant, blotting out the stars, the hoof falls reminiscent of distant thunderclaps. If the ant survives until morning the flukes relinquish their control allowing the ant to scurry back to join its fellow workers in the gloom and away from the solar furnace which would be death to both the host and the backseat driving parasite. By day the ant is a regular Joe indistinguishable from any other ant, but when night falls, again it makes its ascent into ‘munch range’ over and over until eventually consumed, drowned in cud, bursting open as a swarm of flukes within the cows stomach. The flukes complete the cycle by penetrating the bovines liver, becoming adult egg producers.
- Snail-Fluke (Youtube video)
- Humans, toxoplasmosis, and promiscuity.
- And now- Whirling disease.
It’s kinda neat- the parasite, Myxobolus cerebralis, infects the poor fish, and makes it swim in circles until it dies.. Wow, imagine that.. You, go, now- run in circles until you die. This bug recently spurred the slaughter of 80K fish in Maryland..
Seems likely that a parasite that has evolved the “ability” to cause CNS impairment and gross modification in behavior has some function… I wonder why this bug causes fish to swim in circles.. Any ideas?
My Holy Grail- To find evidence of an STD that modifies behavior so as to enhance transmission. I always think about syphilis, and its neurological effects, but the evidence guts isn’t there… YET
1 response so far ↓
1 Anonymous // Feb 19, 2007 at 1:15 am
As I understand it syphilis is no longer infectious when it gets to the tertiary stage, so the neurological changes are unlikely to increase fitness of the parasite. However, there are a number of examples of behavioural changes etc. with STDs:
Dourine in horses anecdotally makes stallions more sexually active
Some STDs of ungulates interrupt the oestrus cycle and cause females to be permanently on heat (See Lockhart et al (1996), Biol. Rev. 71: 415-471).
There are some interesting if inconclusive examples from insects as well. If I may quote from a particularly magnificent publication on the subject:
“McLachlan (1999) showed that male midges (Paratrichocladius rufiventris) infected with the mite Unionicola ypsilophora were more likely to be in mating pairs than uninfected males. As discussed earlier, the mites rely on female midges to return them to water to complete their life cycle. If they find themselves on a male midge, therefore, they are effectively dead unless their host mates with a female, allowing the mites to transfer to the female midge, giving a clear selective advantage to mites that somehow increase the probability of their male hosts mating. Whether this qualifies as a true adaptation by the parasite is not clear yet because not enough is known of the mechanism by which the increase in infected males in mating pairs occurs.
Raina et al. (2000) found that Hz-2V infected female corn earworm moths Helicoverpa zea produce two to three times more sex pheromone than uninfected female moths, possibly enhancing their ability to attract male moths, although they also reported that these animals vigourously resisted mating. Abbot & Dill (2001) found that male Labidomera clivicollis beetles infected with the mite Chrysomelobia labidomera were more likely to displace other males from mating pairs, which again could be interpreted as being adaptive manipulation of the host by the parasite to increase transmission. Webberley et al. (2002), by contrast, found that infection of Adalia bipunctata with Coccipolipus hippodamiae did not have any effect on the mating behaviour of the host. Taken together, these studies suggest that STDs are capable of manipulating their host’s behaviour to increase transmission. More detailed studies are needed, however, to determine whether these behavioural changes are real adaptations by the parasite or whether they are simply by-products of the pathology the parasite causes (Poulin, 2000; Moore, 2001).” (Knell, R.J. and Webberley, K.M., 2004, Biol. Rev. 79: 557-581).
Cheers
Rob Knell
Leave a Comment